sic based electronics for high temperature applications...sic based electronics for high temperature...

SiC Based Electronics for High Temperature Applications

Alton Horsfall and Nick Wright

School of Electrical, Electronic and Computer Engineering,Newcastle University

Acknowledgements

• Dr Jon Goss

• Konstantin Vassilevski, Irina Nikitina, Rajat Mahapatra, Dan Brennan• Rupert Stevens, Hassan Habib, Simon Barker,

Ben Furnival, Omid Mostaghimi, Amit Tiwari, Kartheek Nagareddy, Hua Chan• Engineering and Physical Sciences Research Council,

The Royal Society, DTI, Research Council UK, ONE North East• BAE Systems Submarine Solutions, Rolls Royce,

Semelab, Dynex, Raytheon Semiconductors, QinetiQ

Introduction• Background to SiC

• SiC Devices

• Diodes

• FETs

• SiC Circuits : Wireless Sensor Nodes

• Applications

• The Route Forward

Inside SiC

• Each carbon atom has four silicon neighbours

• Tetrahedral configuration (like diamond)

• The C – Si bond is far stronger than Si – Si

Inside SiC

• The bonds are formed by the sharing of electrons between adjacent atoms (covalent bonding)

• To allow a current to pass, we need to use these valence electrons

• We need to make them mobile(conduction electrons)

Inside SiC

• To do this, we apply energy to the SiC

• The amount of energy required to break a bond and free an electron is 3.26eV (the bandgap, Eg)

• Eg for Si is 1.12eV

• Eg for GaAs is 1.43eV

Inside SiC

• The energy given to the SiC is related to the temperature

• Higher bandgap means higher maximum operating temperature (theoretical maximum is >900C)

• The high energy required to break the C – Si bond also makes SiC chemically and biologically inert

Image from SiCrystal AG

Radiation Tolerance• Radiation damage arises from

the creation of Frenkel pairs (vacancy and interstitial)

• The high strength of the C – Si bond means it takes more energy to achieve this

• e – h creation energy of 7.78eV (Si 3.6eV)

• Fewer defects created for a given exposure leading to anincreased radiation tolerance

Radiation Damage

• Minor change in detector characteristics after a simulated earth - jupiter return trip

!

R.C. Stevens, et al, Proceedings of ECSCRM 2010

Material Properties

Comparison with Traditional Silicon


• SiC Devices

• Diodes

• FETs


• Applications


SiC Schottky Diodes

n 4H-SiC

n+ 4H-SiC

5nm Cr / 600nm Au

Boron implanted guard ring: Activated impurities: 1.5!1013cm-2

NiSi Ohmic Contact 5nmCr / 600nm Au

50nm thermal grown SiO2 Ti (NiSi, Mo)

Schottky Contact

Designed for 1.2, 3.3 and 6.6kV operationAchieved over 95% of theoretical breakdownDirect replacement for Si PiN diodes with lower switching

loss I.P.Nikitina, et al, Mat. Sci. Forum, Vols. 556-557 (2007) pp. 873

SiC Schottky Diodes

• Recent innovation in the use of an epitaxial field stop ring to reduce the leakage current and offer an improved yield

!

!

60 80 30 30 80

20

1 2 3 4

5 11 6 7 8 9 10

K. Vassilevski, et al, Proceedings of ECSCRM 2010

SiC Schottky Diodes

• Operation stable to 900C (Mo top contact)

• Simulation model which can accurately describe the behaviour in both forward and reverse operation from a single set of parameters

C. Dimitriu, et al, Semicon. Sci. Tech, Vol. 20 (2005) pp. 10

SiC Schottky Diodes

• Lower on state resistance than silicon - Smaller

• Insignificant switching losses

• Improvement in efficiency

CPWR-TECH1

Table 1: Key electronic properties of Si, GaAs, and 4H-SiC

Property Silicon GaAs 4H-SiC

Band gap, Eg (eV) 1.12 1.5 3.26

Electron mobility, µn (cm2/Vs) 1400 9200 800

Hole mobility, µp (cm2/Vs) 450 400 140

Intrinsic carrier concentration, ni (cm-3) at 300 K 1.5x1010 2.1x106 5x10-9

Electron saturated velocity, vnsat (x107cm/s) 1.0 1.0 2.0

Critical breakdown electric field, Ecrit (MV/cm) 0.25 0.3 2.2Thermal conductivity, ! (W/cm•K) 1.5 0.5 3.0-3.8

electron mobility than 6H-SiC. Table 1compares the key electronic properties of4H-SiC to Si and GaAs. The higherbreakdown electric field strength of SiCenables the potential use of SiC SBDs in600-2000 V range. Specific benefits of SiCelectronic properties are:

! The 10x higher breakdown electric fieldstrength of SiC reduces the specific on-resistance compared to the Si and GaAsSBDs. This is illustrated in Fig. 1. At600 V, a SiC SBD offers a Ron of 1.4m"-cm2, which is considerably, lessthan 6.5 m"-cm2

for a GaAs SBD and73 m"-cm2

for a Si SBD. This meansthat the SiC SBD will have a muchsmaller foot-print.

! The higher bandgap results in muchhigher schottky metal-semiconductorbarrier height as compared to GaAs andSi, resulting in extremely low leakagecurrents at elevated junctiontemperatures due to reduced thermionicelectron emission over the barrier.

! The very high thermal conductivity ofSiC reduces the thermal resistance of thedie.

Ron

(moh

m-c

m2 )

VB (Volt)

SiGaAs

SiC

200 400 600 800 1000 12000

2

4

6

8

10

VB (Volt)

.

Fig. 1 Specific on-resistance of Si, GaAsand 4H-SiC SBDs as a function of thebreakdown voltage.

The power electronic systems operating inthe 600-1200 voltage range currently utilizesilicon PiN diodes, which tend to store largeamounts of minority carrier charge in theforward-biased state. The stored charge hasto be removed by majority carrierrecombination before the diode can beturned off. This causes long storage andturn-off times. The prime benefits of theSiC SBD lie in its ability to switch fast (<50ns), with almost zero reverse recovery

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input capacitance is about 240 pF, whichdrops to 90 pF at 100 V and saturates to 50pF above 300 V. This capacitance iscomparable to low voltage Si SchottkyDiodes.

0

1

2

3

4

5

6

7

8

9

10

11

0 25 50 75 100 125 150 175 200TC - Case Temperature ( degree C )

Forw

ard

Cur

rent

(Am

ps)

Fig. 4 The current derating curve for the 10A/600 V SiC SBD.

0

50

100

150

200

250

300

350

400

1.0 10.0 100.0 1000.0

VR (V)

C (p

F)

Fig. 5 The reverse capacitance vs. voltagecurve for the 10 A/600 V SiC SBD.

The turn-off characteristics of the 10 A/600V 4H-SiC SBD are compared with a SiFRED at different temperatures (Fig. 6).The SiC diode, being a majority carrierdevice, does not have any stored minority

carriers. Therefore, there is no reverserecovery current associated with the turn-offtransient of the SBD. However, there is asmall amount of displacement currentrequired to charge the Schottky junctioncapacitance (< 2 A) which is independent ofthe temperature, current level and di/dt. Incontrast to the SiC SBD, the Si FREDexhibits a large amount of the reverserecovery charge, which increasesdramatically with temperature, on-currentand reverse di/dt. For example, the Qrr ofthe Si FRED is approximately 160 nC atroom temperature and increases to about 450nC at 150°C. This excessive amount of Qrrincreases the switching losses and places atremendous burden on the switch and diodein typical PFC or motor control applications.

-10

-8

-6

-4

-2

0

2

4

6

8

10

-1.0E-07 -5.0E-08 0.0E+00 5.0E-08 1.0E-07 1.5E-07 2.0E-07Time (s)

Cur

rent

(A) SiC 10 A/600 V SBD

TJ = 25, 50, 100, 150C

600V, 10A Si FRED TJ = 25C TJ = 50C TJ = 100C TJ = 150C

Fig. 6 Turn-off switching waveform of the10 A / 600 V SiC SBD in comparison to SiFRED (IXYS DSEI 12-06A).

In a switching application, the diode will besubjected to peak currents that are greaterthan the average rated current of the device.Fig. 7 shows a repetitive peak forward surgecurrent of 50 A at 25°C for the 10A / 600 VSiC SBD. This 60 Hz half sine wavemeasurement indicates a repetitive peakcurrent of 5X the average.

SiC Schottky Diodes

• Lower on state resistance than silicon - Smaller

• Insignificant switching losses

• Improvement in efficiency

CPWR-TECH1

frequency was 90 kHz, and the gateresistance at the MOSFET was 50 !. Thecurrent rating of the MOSFET was higherthan the average rating to accommodate thereverse recovery current of the diode, and tomaintain a high efficiency of the circuit.Under full load condition, a 600 ! resistorwas utilized, while at half load condition,1200 ! was used. Voltage and currentmeasurements were taken on both theMOSFET as well as the diode, in order toestimate the power losses in thesecomponents. The input and output powerwas also measured to calculate theefficiency of the circuit. Under full loadconditions, the temperature on the MOSFETcase was measured with and without anexternal fan on the device. After all thesemeasurements were taken using the ultrafastSi diode, they were repeated using Cree’s 4A, 600 V SiC SBD (CSD04060).

HalfLoad Si HalfLoad SiC -- FullLoad Si FullLoad SiC0

100

200

300

400

500

Ene

rgy

Loss

es (m

icro

Joul

es)

MOSFET Turn ON MOSFET Turn OFF Diode Turn ON Diode Turn OFF

Fig. 11 Comparison of switching losses inPFC with Si and SiC diodes.

Fig. 11 shows the comparison of theswitching energy losses per switching cyclein the MOSFET and Diode under half loadand full load conditions. Further, the turn-ON and turn-OFF losses within each deviceare separated. Under half load conditions,the total switching losses decrease by about25% from 266 µJ to 200 µJ when the Sidiode is replaced by SiC SBD. The 50%

decrease during Diode turn OFF losses, and27% decrease during MOSFET turn ON areprimarily responsible for this overallreduction in losses when a SiC SBD is usedin the circuit as compared to when a Sidiode is used. The MOSFET turn OFFlosses and Diode turn ON losses are similarwhen Si and SiC Diodes are used in thiscircuit.

Under full load conditions, Diode turn OFFlosses decrease by 44%, MOSFET turn ONlosses decrease by 39%, and Diode turn ONlosses decrease by 29% when a SiC diode isused in this circuit as compared to a Sidiode. The MOSFET turn OFF lossesremain similar in both cases. An overalldecrease of 27% in switching losses ismeasured when the circuit uses a SiC diodeas compared to a Si diode. It is worth notingthat diode turn ON losses are significantlylower as compared to Si PiN diodes underfull load conditions because of a slower turnON process in a PiN diode as compared to aSBD under higher current operation. Theseresults also show that the dominantreduction in switching losses occurs due tothe small reverse recovery losses in the SiCdiode as compared to the case with Sidiodes.

86

88

90

92

94

96

98

100

Silicon Carbide

Silicon

Silicon Carbide

Silicon

Full Load ConditionHalf Load Condition

Effi

cien

cy (%

)

Fig. 13 Comparison of overall efficiency ofPFC with silicon and SiC diodes.

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done at the rated device forward current,with the device junction temperature held at200°C. The power cycle consisted of 7minute on/off cycles (3.5 min. on / 3.5 minoff) with the on-current set to the devicerated current, a maximum junctiontemperature of 175°C and a junctiontemperature delta of greater than 100°Cduring the cycle.

POWER FACTOR CORRECTION(PFC)

One of the largest applications for SiCSchottky rectifiers in the near future is in thecontinuous conduction mode (CCM) powerfactor correction (PFC) circuit. In traditionaloff-line AC-DC power supplies used incomputer and telecom applications, the ACinput sees a large inductive (transformer)load which causes the power factor to besubstantially lower than 1. A PFC circuitallows the AC input line to see near-unitypower factor, as required by new legalrequirements. As shown in Fig. 10,chopping the full wave rectified input with afast switch (MOSFET), and then stabilizingthe resulting DC waveform using a capacitoraccomplishes this function. When theMOSFET is ON, it is necessary to preventthe current to flow from the output capacitoror the load through the MOSFET. Hence,when the FET is ON, the Diode is OFF, andvice versa. During the switching transientwhen the Diode is turning OFF and theMOSFET is turning ON, the reverserecovery current from the Diode flows intothe MOSFET, in addition to the rectifiedinput current. This results in a large inrushcurrent into the MOSFET, requiring asubstantially large sized MOSFET, than thatrequired if the Diode had no reverserecovery current. This large MOSFETrepresents a substantial cost in this circuit.These switching losses also limit thefrequency of operation in the circuit, and

hence its cost, size, weight and volume. Ahigher frequency would allow the size of thepassive components to be correspondinglysmaller. Many fast silicon rectifiers alsoshow “snappy” reverse recovery, whichresults in a large EMI signature, which arealso unacceptable to the new Europeanrequirements. A fast rectifier with smoothswitching characteristics will allow for highefficiency PFC circuits, which also complywith new legal requirements.

Brid

ge R

ectif

ier

FET

DiodeL

Cout

AC

Inpu

t

DC

Out

put

Fig. 10 A simplified schematic of a powerfactor correction (PFC) circuit.

A 4H-SiC diode is such a rectifier. Thisnear-zero reverse recovery SiC Schottkyrectifier offers low switching losses whilestill showing comparable on-stateperformance of conventional siliconrectifiers. Due to the majority carriertransport properties of these rectifiers, theyshow only a capacitive current during theirturn-off transient, which flows through thepower MOSFET. In order to measure thebenefit of these high performance rectifiers,a 250 Watt PFC test circuit was comparedwith an ultrafast Silicon Diode as well asSiC SBD.

This test circuit used a 14 A, 500 VInternational Rectifier MOSFET (IRFP450),and a 6 A, 600 V ultrafast IR Si PiN diode(HFA08TB60). The input voltage was keptat a constant 120 V RMS, and the outputvoltage was 370 V DC. The operating

O. Mostaghimi, et al, Proceedings of ECSCRM 2010

Other SiC Diodes

• JBS diodes are also available commercially - Infineon ThinQ generation 2

• Better for higher voltages (3.3kV +) and higher current densities

Semicond. Sci. Technol. 23 (2008) 125004 A Perez-Tomas et al

0 2 410-2

10-1

1

101

50

0 2 4 0 2 4 6 Forw

ard

Cur

rent

Den

sity

, JF [A

/cm

2 ]

2000

0.4

4

40

400

Forw

ard

Cur

rent

, IF [A

]

T = 25 °CT = 100 °CT = 200 °CT = 300 °C

10

SBD2

Forward Voltage, VF [V]

JBS

T

PiN

T

T

T

°°°°

Figure 4. Forward I–V characteristics of the fabricated 3.3 kV classSiC diodes in the temperature range of 25–300 !C.

metal resulting in a patch contact with different local barrierheight [11]. The presence of these inhomogeneities resultsin deviations from the ideal behavior (from the text-bookthermoionic emission theory) of the electrical characteristicsof SBDs. For example, the ideality factor, which is equalto 1 for an ideal diode, has usually a value greater thanunity (along with a significant leakage current level). Despitethese deviations, manufactured Schottky contacts are normallyreported with a repetitive turn-on voltage of around 1 V, anideality factor ranging from 1.2 to 2, and a contact resistancelower than 10"5 ! cm"2. In the case of a PiN diode, the turn-on voltage is established by the built-in potential of the P+

anode/N" drift p–n junction, eventually depending on theband-gap energy. Due to the wide band gap of SiC, the turn-on voltage of SiC PiN diodes is higher than that of a Si–PiNdiode (#1 V). However, because of the thinner drift regionand a higher doping level, we achieve a greatly reduced on-resistance when compared with high power Si–PiN [12].

Figure 4 shows the evolution of the forward electricalcharacteristics versus temperature in the 25–300 !C rangefor SBD2, JBS and PiN. On-state characteristics have beenmeasured at each temperature interval (25 !C) and summarizedin the figure. As can be inferred from figure 4, the drift regionresistance for a thick/low doped 3.3 kV class epitaxial layeris very important in the forward operation of the SBD diodes.For current densities (JF ) lower than 2 A cm"2 (25 !C), theI–V characteristic of the SBD2 diode is established by thethermoionic emission over the metal/semiconductor barrier.The value extracted for the barrier height was " = 1.17 eV,with an ideality factor of 1.36, in accordance with valuespreviously reported in the literature [13, 14]. For larger currentdensities, the drift region resistance becomes dominant. Thisgreatly increases the voltage drop from currents as low as50 mA (2 A cm"2) (SBD2). This fact explains the two regionsof operation with temperature, observed for SBD (figure 4).For the lower anode voltage, the SBD follows the thermoionictheory. For larger current densities, the resistor componenttakes precedence. The drift region resistance can be definedas [15]

Rdrift(T ) =Lepi

qµn(T )NDA, (1)

where µn is the electron mobility, q is the electronic charge,ND is the drift doping value, A is the active area, T

is the temperature and Lepi is the N" drift length. Inthis situation, the forward voltage drop of the SBD diodesincreases with temperature because of the drift region electronmobility reduction with increasing temperature. Due to theconductivity modulation from the bipolar operation mode, thedrift resistance of PiN has a reduced effect in the 0.01–50 Acurrent range. Up to 10 A, the voltage drop is lower than4 V in all the temperature range. In any case, the maximumvariation in the on-state voltage drop versus temperatureremains below 0.5 V. Because of the SiC band gap narrowingwith temperature, the p–n built-in voltage decreases and thecarrier lifetime significantly increases with temperature. Asshown in figure 4, the PiN diode turn-on voltage decreaseswith increasing temperature, due to the reduced built-inpotential. In addition, the contact resistance also diminisheswith temperature. However, for higher temperatures (T >

200 !C) and for higher current densities (JF > 500 A cm"2),an intersection in the I–V curves becomes evident, as inferredfrom figure 4. This crossover in the I–V characteristics isexplained as a result of a competition between the drift-regionlifetime and contact resistance enhancement and reductionof the carrier mobility in the substrate, with increasingtemperature [12]. Hence, the forward characteristics ofthe diodes are very stable with temperature, resulting in anoticeable thermal runaway protection, required for parallelingof diodes in power electronics applications.

As mentioned before, JBS diodes have been developedto combine the advantages of both Schottky and PiN diodes.Hefner et al [16] summarized the properties of JBS diodes as

(a) low voltage drop in the on-state like the SBD.(b) low leakage in the off-state like the PiN.(c) fast switching characteristics like the SBD.(d) good high temperature characteristics.

As suggested from figure 4, the I–V characteristics fromthe JBS are like stretched out SBD forward characteristicsdue to bipolar operation mode. At 25 !C, the I–V curveof a JBS diode is that from SBD with slightly improvedon-resistance, especially for higher anode bias (larger than6 V). With increasing temperature the JBS diode becomesmore bipolar, effectively reducing the drift resistance from10 A cm"2 but maintaining the low turn-on voltage. At roomtemperature, SBD and JBS exhibit basically the same turn-on voltage, since the JBS conduction mechanism is merelyunipolar (figure 5). The anode metal for SBD and JBS isbased on Ni. An optimized Ti/Ni bi-layer was deposited, andannealed at 700 !C for 2 min, as anode metal for SBDs. Ni(annealed at 700 !C for 2 min) was also chosen as anode metalfor JBS, resulting in Schottky contact to n-type epi and ohmicto P+ regions. As mentioned before, the larger turn-on voltageof PiN is due to the larger SiC p–n junction built-in voltagewhen compared to the metal/SiC junction. Temperatureincrease results in a reduction of the turn-on voltage, for SBD,JBS and PiN. The SBD (and JBS) shift with temperatureis well explained by the thermoionic emission theory[7]. Barrier inhomogeneities have been reported [11, 13]

4


Figure 5. Comparison of the on-state characteristics of the SBD,JBS and PiN 3.3 kV class SiC diodes at 25 !C and 300 !C.

Figure 6. Evolution of the on-resistance versus temperatureextracted from the diodes for different current densities.

to cause further variations with temperature of the barrierheight and ideality factor, which are not predicted by thethermionic diffusion theory. For PiN, the p–n built-in voltagedecreases with temperature and correspondingly, the turn-onvoltage. This shift is well illustrated in figure 5.

The differential on-resistance, extracted at 25 !C and300 !C, versus forward current density is presented in figure 6.At 25 !C, for JF < 10 A cm"2, SBDs and JBSs presentessentially the same on-resistance, which is lower than thatfrom PiN because of its larger turn-on voltage. This is due tothe fact that only the Schottky component of JBS is activated.In the forward current density range of 10–100 A cm"2, JBSdiodes are the devices displaying lower on-resistance. This isdue to the thinner epi layer and the beginning of the bipolaractivation, for larger anode bias. There, the SBD on-resistanceis basically the drift layer resistance. For JF >100 A cm"2,PiN diodes exhibit the lower on-resistance, due to the highlyeffective conductivity modulation in the drift region. JBSbipolar conduction is activated, but still smaller than PiN. SBDresistance is much higher than that from bipolar devices forsuch current density. The JBS bipolar activation is enhancedwith temperature. At 300 !C, the on-resistance extractedfrom the JBS rectifier is basically the same as from a PiNdiode, in the 10–2000 A cm"2 current density range. In

Figure 7. Schematic of the step-down dc–dc converter used forevaluating the dynamic losses of the diodes.

Figure 8. Turn-off current waveforms for the 3.3 kV SiC diodes inthe temperature range of 25–300 !C with inductive load.

contrast, the SBD on-resistance increases significantly withtemperature.

4. Dynamic characteristics

The reverse recovery properties of SiC diodes under inductiveload switching were investigated by means of a step-downdc/dc test converter. The circuit is depicted in figure 7.The controlled switching device is a 1200 V-45A Si-IGBT(IRG4PH50KPBF) from International Rectifier [17]. Thediode switching conditions were: IF in the range of 2–15 A,an applied voltage V of 500 V and a dI/dt of 210 A µs"1.The converter topology has been chosen to investigate theSiC diode response for most of the hard switch conditionsin high power 3.3–6.6 kV applications, such as in aninverter leg of an induction three-phase electric motor [18].The turn-off current waveforms were measured with aTektronix current probe P6021 (100 MHz bandwidth). Highfrequency (100 kHz) capacitance measurements have beencarried out with a computerized Keithley 590 CV Analyzer.Figure 8 shows the turn-off current waveforms for SBD1, JBSand PiN diodes at 25 !C and 300 !C (IF = 6A). Junctioncapacitance measurement (figure 9(a)) is a common procedurefor extracting the doping value of the epi, for Schottky andPiN rectifiers. Besides, the capacitance value extracted couldbe incorporated as a model parameter in electronic circuitsimulators, such as Spice, for the simulation of the switchingcharacteristics. Because for the SiC SBD diodes switching

5


40 30 20 10 0102

103

104

0 100 200 3000.1

1

101

102

103

Rev

. Rec

over

y C

harg

e / A

rea,

Q' rr

[µC

/cm

2 ]

Junc

tion

Cap

acita

nce

/Are

a, C

' [pF

/cm

2 ]

Reverse Voltage [V]

(a) (b)

dI/dt = 210A/µsV = 500V I

F = 6 A

SBD1JBSPiN

Temperature, T [ oC]

Figure 9. (a) Reverse junction capacitance of the tested diodesmeasured at 25 !C. (b) Reverse recovery charge and on-state voltagedrop versus temperature at 6 A for the 3.3 kV SiC diodes.

is dominated by junction capacitance, there is virtually notemperature dependence of the reverse recovery waverformover the range 25–300 !C (figure 9(b)). In contrast, duringthe static forward mode of a PiN diode, a surplus of minoritycarriers at the edge of the space charge region is obtained,due to their finite lifetime. This surplus concentration (knownas reverse recovery charge) must be removed by electron–hole recombination, after switching to blocking conditions.This recombination takes a finite amount of time known asthe reverse recovery time (trr). In any case, 4H-SiC 3.3 kVPiN diodes exhibit much smaller reverse recovery charge (Qrr)compared with their Si counterparts [5, 12]. This is due tothe much smaller carrier lifetime and thinner drift layer of SiCPiN devices. The JBS diode presents reduced reverse recoverycharge depending on the anode bias and the temperature, i.e.,if the bipolar mode of operation is activated. For PiN andJBS diodes working in the bipolar mode of operation, thereverse current peak (Irr) and reverse recovery charge increasewith temperature, because of the carrier lifetime increasingwith temperature. As inferred from figure 9(b), the amountof charge stored in the drift region is an order of magnitudelower for JBS (0.024 µC) when compared to PiN (0.72 µC),at room temperature. On the other hand, the on-resistanceof both rectifiers is comparable at 6 A (250 A cm"2), being22 m! cm2 for JBS and 18 m! cm2 for PiN. These resultssuggest that the JBS topology exhibits the best trade-offbetween reverse recovery charge and on-resistance for lowto moderate forward current density. Figure 10 shows the3.3 kV SBD, JBS and PiN Ron versus Q#

rr (Qrr per unitarea) diagram, for different temperatures and current densities.SBDs are included in the diagram for comparison. Theswitching properties of SBD diodes are established by thejunction capacitance and hence, there is no minority chargestored during reverse operation. This is also valid for JBSdiodes working at low temperatures and low current densities.

5. Reliability: forward voltage drift

In the previous sections, the electrical characteristics of freshSBD, JBS and PiN diodes were compared. SBDs clearly

0.1 1 10 100 5001

10

100

500

25

80

600 A/cm2

250

25 oC100 oC200 oC300 oC

250

SBD

PiN

JBS

Diff

. On-

Res

ista

nce

Ron

[m!

·cm

2 ]

Reverse Recovery Charge /Area, Q'rr

[µC/cm2]

Figure 10. On-resistance versus reverse recovery charge per unitarea for different temperatures and forward current densities.

underperform JBS and PiN diodes in terms of current handlingbecause they are just majority carrier devices. However,severe forward voltage drift, where the diode voltage dropmay increase by several volts during a forward stress, is oneof the main factors hampering bipolar devices commercialdevelopment. This degradation mechanism is due to thegeneration of stacking faults under a forward stress [6]. Duringa forward stress, bipolar current flow in the drift layer provokeselectron–hole recombination. This recombination providesenergy to the lattice, allowing basal plane screw dislocationto generate stacking faults. Electrons are trapped in thesestacking faults. The effective area of the diode is then reducedbecause the area under the fault is depleted of plasma. Alarger amount of applied voltage is now required to maintainthe same forward current level, causing the forward voltagedrift. Recent innovations have reduced the epilayer basal planedensity below 10 cm"2, turning these problematic defectsinto relatively benign threading edge dislocations [10]. ThePiN diode starting material has been treated with similartechniques to reduce this basal plane density. Prior stressI–V characteristics for SBD, JBS and PiN are shown in figure11. The turn-on voltage of SBD and JBS is around 1 V. SBD1area (25 mm2) is ten times larger than JBS and PiN (2.6 mm2).The voltage drop of SBD1 is for this reason strongly reduced.JBS bipolar activation occurs at around 6 V. From this voltage,the slope from the I–V curve is progressively analogous tothat from a PiN diode. The PiN diode exhibits a turn-onvoltage of around 2.3 V, displaying the steepest slope. Then,demanding stress tests were conducted in our PiN diodes anddiscussed elsewhere [19]. For PiN diodes, manufactured onenhanced starting material, the dc stress (ST) was 20–60 hat 10 A (25 !C–225 !C). The diodes were characterized inthe forward mode after the dc stress. A remarkable 20% ofthe characterized PiN rectifiers (24 diodes in total) showed nodegradation at all. However, a voltage shift ("VF), in the rangeof 1–2.2 V, was evidenced for the rest of the diodes (figure 11).Analogously, forward voltage drift was also observed for mostof our JBS diodes. The dc stress for JBS (and for SBD) diodeswas 50 h at 8 A (25 !C). In this case, the epitaxial layer of JBSdiodes was not especially treated for stacking fault reduction.

6


• SiC Devices

• Diodes

• FETs


• Applications


JFETs• 1200V Forward

Blocking• Electron flow

from source to drain controlled by the depletion region around the gate implants

Source Gate

Drain

P+

500oC 28oC

!

JFETs

• Recent development in self aligned JFET source fingers to offer a simplified process

!

K. Vassilevski, et al, Proceedings of ECSCRM 2010

JFETs

• Commercial devices now starting to become available - however only rated for low temperature operation• Only available for normally off

operation - Cascode structures

MOS Devices

• Optimisation of the ohmic contact process with the oxidation step for MOS formation

• Highly stable high K/SiO2 interface to 500C

!

!

!B.J.D. Furnival, et al, Proceedings of ECSCRM 2010 M.H. Weng, PhD Thesis, Newcastle University, 2007


• SiC Devices

• Diodes

• FETs


• Applications


Wireless Sensor Node

• Silicon carbide technology coupled with high temperature passive components

Sensor Module

• Radiation detection • Pollutant detectionM.H. Weng, et al, Meas Sci Tech, Vol. 19 (2008) 024002M.H. Weng, et al, IEEE Sensors, Vol. 7 (2007) pp. 1395

Data courtesy of ICS, BAE Systems Submarine Solutions

Energy Harvesting

• Thermoelectric device• Increasing power density from a thermal gradient

Output Voltage

0

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0.45

0.5

0 0.5 1 1.5 2 2.5

∆T

Self OscillatingBoost Converter

• Boost the low voltage level to enable circuit operation

0

2

4

6

8

10

12

1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9 1.95 2

IncreasingT

Energy Harvesting

• Continuous wireless transmission from lowest vibration amplitude

15

Sleep for Tsleep

Wake Up

Sample Vref

Vref < Vtest?Yes

Activate & read sensor

Deactivate sensor

500kHz Clock, activate transmitter

Transmit Vref and Vsens

Deactivate transmitter31KHz clock

Vtest = Vlower

Vtest = Vupper

Fig. 3: Operation algorithm, light area indicates low power 31kHz operation and dark area

indicates 500kHz operation

August 25, 2010 DRAFT

14

Fig. 1: System diagram of the final sensor system

(a) (b)

Fig. 2: Circuit topologies of (a) rectifier and (b) multiplie


18

Sample Period

Transmit Period

Stor

ed V

olta

ge/V

2.05

2.10

2.15

2.20

2.25

2.30

2.35

2.40

2.45

Time/s0 10 20 30 40 50 60

2.28

2.29

31.5 32.0

Dual Clock Frequencies8MHz Clock only

Fig. 6: Charge/discharge waveform for the stored voltage with dual clock operation at 100mg

acceleration


S. Barker, et al, Submitted to IET Wireless Sensor Systems

Energy Harvesting

• Piezoelectric energy harvesting operable at 300C

Voltage/V

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Temperature/K300 350 400 450 500 550 600

Fig. 6. Change in Schottky diode voltage drop with temperature

that the energy harvester output and so will provide a suitablerectification method.

To investigate the effects on increased temperature on thewhole system, the piezoelectric energy harvester was drivenat 400mg and connected to the input of the full wave rectifier.The output power of the rectifier was measured when loadedwith a 1MΩ and with the measured matched loads shown infigure 5. Figure 7 shows the change in rectified power withtemperature, it shows that the power significantly decreaseswith temperature from 320µW at room temperature, to 80µWat 573K, into the matched load. The reason for the lowcorrelation between the matched load data and the data fitis due to the error in the matched load determination, for thisreason the test was also run with a 1MΩ load also. At roomtemperature the power delivered to the 1MΩ load is 50µW,this decreases to 7µW at 573K which is proportionally thesame as the matched load scenario.

Pow

er/μ

W

0

100

200

300

TemperatureK300 350 400 450 500 550 600

Power MatchedPower 1MΩ

Fig. 7. Power delivered to both matched and 1MΩ loads change withtemperature

To determine if the full wave rectifier is fully operational atthe elevated temperatures and not simply acting as a half waverectifier, the output wave form of the rectifier can be analysed.The unsmoothed output from a full wave rectifier should havea frequency twice that of the input waveform, and no part ofthe signal below 0V. Figure 8 compares the output from therectifier at room temperature to 573K, when the piezoelectric

energy harvester is driven at resonance. The output wave-form at room temperature has frequency of 285Hz and at 573Ka frequency of 87Hz, these are twice the input frequency andshow that the rectifier is fully operational.

Amplitude/V

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Time/s0 2 4 6 8 10 12 14 16 18⋆10-3

300K573K

Fig. 8. Change in Schottky diode voltage drop with temperature

IV. CONCLUSION

This work has shown that high temperature energy har-vesting, incorporating silicon carbide electronics and a PZTenergy harvester, can operate at 300C. The data shows thatalthough the peak output voltage from the piezoelectric energyharvester decreases at elevated temperatures, it is still capableof producing a useable voltage. The output voltage from thepiezoelectric device at 573K is much lower than the roomtemperature full wave rectifier voltage drop. However, as thetemperature increases the voltage drop of the SiC Schottkydiodes decreases at 2mV/K, so at 300C the voltage drop of asingle diode is 0.1V. This decreased voltage drop means thatthe system is able operate at elevated temperatures and producea rectified output. This output can be used to power a hightemperature communications or sensor system for extendedperiods of time when coupled with high temperature capacitivestorage elements, as such this is a first step to developing ahigh temperature vibration based energy harvesting system.

REFERENCES

[1] S. E. Saddow and A. Agarwal, “Advances in Silicon Carbide Processingand Applications,” Semiconductors Materials and Devices Series.

[2] F. Nava, E. Vittone, P. Vanni, G. Verzellesi, P. Fuochi, C. Lanzieri, andM. Glaser, “Radiation tolerance of epitaxial silicon carbide detectors forelectrons, protons and gamma-rays,” Nuclear Instruments and Methods inPhysics Research Section A: Accelerators, Spectrometers, Detectors andAssociated Equipment, vol. 505, pp. 645–655, 2003.

[3] N. Wright and A. Horsfall, “SiC Sensors: A review,” Journal of PhysicsD: Applied Physics, Jan 2007.

[4] S. Barker, B.Miao, D.Brennan, N. G. Wright, and A. B. Horsfall, “HighTemperature Storage for Energy Harvesting in Hostile Environments,”IEEE Sensors 2009.

[5] D. Brennan, Materials Science Forum, pp. 963–956, March 2010.[6] J. Thomas, M. Qidwai, and J. Kellogg, “Energy Scavenging for Small-

scale Unmanned Systems,” Journal of Power Sources, Jan 2006.[7] S. Barker, B. Miao, D. Brennan, K. Vassilevski, N. Wright, and A. Hors-

fall, “Silicon Carbide based Energy Harvesting Module for HostileEnvironments,” ICSCRM 2009, pp. 1–4, Oct 2009.

[8] R. Sood, Y. Jeon, J. Jeong, and S. Kim, “Piezoelectric micro powergenerator for energy harvesting,” Proc. of Solid-State Sensor and ActuatorWorkshop.

S. Barker, et al, Proceedings of IEEE Sensors 2010, Hawaii

Until now no investigation has been carried out into howtemperature affects a PZT energy harvester and how the outputpower from a silicon carbide rectifier changes.

This paper presents the results from a Piezo Systems Incpiezoelectric energy harvester heated from room temperatureto 300C and how the output from a silicon carbide full bridgerectifier changes also.

II. EXPERIMENTAL

The experimental setup comprises of an LDS V101 shakerdriven from an audio amplifier whose signal is produced by aLabVIEW program combined with a DAQ card. The systemacceleration is measured at the base of the vibration shaftwith an ADXL103 analogue accelerometer, this allows theinitial acceleration to be set and, if need be, for the system tocontrol the acceleration at a constant level. The piezoelectricdevice is mounted on a custom made high temperature carriagewhich is attached to the end of the vibration shaft, the metalelectrodes on the energy harvester are also removed at theclamp so that they add no extra parasitics to the system. Theenergy harvester output can be switched between the externalvariable load or to the input to a full wave rectifier, which is inturn connected to the load. The output voltage seen across theexternal load is buffered by a Keithley 6517A electrometer,with a specification input impedance of 200TΩ, before beingrecorded. The system is mounted on top of an oven which canmaintain a stable temperature set point up to 600C whichallows the energy harvester and the full bridge rectifier to betested from room temperature to 300C. The vibrations aredelivered to the energy harvester via a steel shaft connectingthe inverted shaker to the energy harvester through a hole inthe oven roof.

III. RESULTS AND DISCUSSION

A. Piezoelectric characteristicsMost mechanical and electrical systems exhibit resonance

and in most cases this is detrimental to the system (eg.bridges, buildings). However, in the case of vibration energyharvesting, this can be used to dramatically increase theoutput voltage from the device. At resonance, the peak tipdisplacement of the bi-morph will be much greater, and sowill significantly increase the stress in the piezoelectric layers.By using this phenomena, piezoelectric devices can producevery high voltages (>50V is not unrealistic) and so make itthe ideal energy harvesting technique to be used with a wideband gap semiconductor, which inherently have higher built involtages. As resonance is such a important factor in the outputproduced by a piezoelectric energy harvester, it is importantsee how this changes with temperature. Figure 1 shows howthe resonant frequency of the energy harvester changes withtemperature. The test was conducted from room temperatureto 300C in 25C increments at 3.9ms−2 (400mg).

It is clear that the resonant frequency of the device de-creases with temperature. By modelling the system as a simplemechanical cantilever, equation 1 can be used to extractthe Youngs modulus, where f is the open circuit resonant

Volta

ge/V

2

4

6

8

10

Frequency/Hz40 60 80 100 120 140

300K Vpiezo/V323K Vpiezo/V348K Vpiezo/V373K Vpiezo/V398K Vpiezo/V423K Vpiezo/V448K Vpiezo/V473K Vpiezo/V498K Vpiezo/V523K Vpiezo/V548K Vpiezo/V573K Vpiezo/V

Fig. 1. Frequency sweep change with temperature for a piezoelectric energyharvester

frequency of the harvester, l is the cantilever length, mt is theattached tip mass, mc is the cantilever mass, w is the widthand t is the overall thickness of the device.

Y = 2πfoc4l3(mt + (0.24mc))

wt3(1)

Equation 1 makes the assumption the the brass shim ofthe bi-morph has no impact on the mechanical characteristicsof the system. This assumption seems valid as the calculatedvalue for the room temperature Youngs modulus is withinclose range of that in the literature [8]. Figure 2 shows thedecrease in Youngs modulus with temperature, indicating thatthere are two linear regions separated by an inflexion at 450K.This could be the elastic limit of the device, indicating thatbeyond 450K the cantilever will not return to its formerstiffness. This is supported by equation 1 which suggests alinear relationship between resonant frequency and Youngsmodulus.

Youn

gs M

odul

us/P

a

0

10

20

30

40

50

60

70⋆109

Temperature/K300 350 400 450 500 550 600

Fig. 2. Youngs modulus variation with temperature

Besides the decrease in resonant frequency, the other notablechange in figure 1 is the change in the peak output voltageat resonance. Given that most piezoelectric materials have aCurie point at which they revert to a non-piezoelectric state,it would be expected that the output voltage would steadilydecrease until this point. However, the characteristics shownin figure 1 look not only to be governed by the piezoelectric

Signal Conditioning

• Low frequency AC coupled amplifier

Communications

• Temperature stable FSK Transmitter to 300C

• Optoelectronic components to >1GHz at 300C


• SiC Devices

• Diodes

• FETs


• Applications


Case study 1 : Monitoring exhaust gas

• Conventional Lambda sensors cannot function from a cold start and this is the time when around 95% of the pollution is generated

• Need a high response speed - engines typically 8000 rpm - 133Hz

• Sensors need to be able to run higher than this, so we need a submilli second response

Petrol Engines

• SiC sensors have demonstrated an ability to spot individual cylinders when mounted in the exhaust manifold

SPETZ et al.: CURRENT STATUS OF SILICON CARBIDE BASED HIGH-TEMPERATURE GAS SENSORS 563

Fig. 3. Response of a Schottky diode placed in the manifold of afour-cylinder petrol engine run at 2400 r/min. Four different response curvesare shown, where one cylinder has a fat air fuel mixture (excess fuel).From top to bottom: cylinders 1, 3, 4, and two have excess fuel. Decreasingvoltage across the device indicates excess fuel. Sensor temperature 700 C.Composition of the device: Pt (100 nm);TaSi (10 nm); SiO (1 nm); 6HSiC (from [22]).

III. EXPERIMENTAL

A. Sample Preparation

Silicon carbide, 6H and 4H SiC wafers, n-type (nitrogen

doped to 1–9 10 cm ) with a 5–10 m n-type epilayer

(3 10 cm – 3 10 cm ) are used [18] as well as

some p-type 4H wafers [19]. Standard RCA cleaning proce-

dures were used [6] before a wafer was oxidized in dry oxygen,

1250 C for 3.5 h, and annealed in argon for another 30 min

at the same temperature. The formed oxide was etched off to

prepare a smooth SiC surface [20] and then the wafer was

reoxidized following the same procedure. The obtained oxide

thickness was measured by ellipsometry to be 100–150 nm.

An ohmic back contact was formed by 200 nm TaSi and

400 nm Pt. The Pt coating of TaSi served as protection of

the back contact from oxygen in subsequent high-temperature

measurements in air. The TaSi formed an ohmic contact with

the heavily doped substrate and at the same time increased

the adhesion to the substrate. After removal of the oxide by

diluted HF, gate electrodes were deposited, 10 nm TaSi or

1 nm Ta and 100 nm Pt, through a shadow mask of copper.

All metals were deposited by DC-magnetron sputtering at a

substrate temperature of 350 C. A schematic drawing of the

device as prepared is shown in Fig. 1.

B. Measurements

The Schottky diodes were mounted on ceramic rods with

buried heaters. Due to the high temperature only special

materials like ceramic glue, quartz, and platinum could be

used. Platinum ribbons are surface welded to the gate contacts.

The heated rods are mounted either directly in the exhaust pipe

or in a stainless steel holder for, in total, ten rods.

The temperature was measured by a chromel-alumel ther-

mocouple close to the sensor on the heated rods. A comput-

erized gas mixing system was used for the supply of gases.

The total gas flow was 200–300 ml/min, and the gas pressure

over the sensor was about one atmosphere. All gases were of

99.99% purity or better.

IV. RESULTS AND DISCUSSION

A. Cylinder-Specific Monitoring of Combustion Engines

The SiC based MISiC sensors, as well as special types of

the metal oxide sensors, SrTiO , have been demonstrated for

cylinder-specific control of car engines [21], [22]. The fast

response [7] and the sensitivity to reducing gases at very high

temperatures [6] enabled cylinder-specific monitoring, which

was tested in an engine test bench using Schottky diode sensors

operated at about 700 C (see Fig. 3) [22].

Speed of response in the order of milliseconds is not easy

to estimate, since changing of the gas atmosphere over the

sensor in milliseconds is not trivial. Gerblinger et al. [23] used

automotive injection valves into a 6-ml chamber to measure

the response time of SrTiO sensors to below 10 ms. Tobias

et al. used moving gas outlets to estimate the time constants

of the response of MISiC sensors [7]. Two gas outlets were

moved back and forth under the sample at a typical speed of

20 Hz. The pulse response of the sample was monitored on an

oscilloscope screen, and from this the time for the sensor signal

to reach steady state in hydrocarbons and oxygen, respectively,

was estimated to be less than 5 ms.

Thus the MISiC sensors fulfill the requirements, as men-

tioned in the introduction above, from the car industry of

operation temperature and time constants for the gas response,

but continuous operation for 160 000 kilometers in car ex-

hausts requires further development and knowledge about the

devices.

B. Long-Term Stability of Schottky Diode Devices

The advantage of Schottky diodes is the simple processing

and that they can be operated by very simple electronics. To

meet the requirements from the car industry, however, the

long-term stability of SiC Schottky diodes at high temperatures

has to be improved.

Since the current in a Schottky diode depends exponentially

on changes in the barrier height, , a change of the metal

work function will change the forward current, , of the diode.

For

(2)

where is the area of the diode, the effective Richardson

constant, the ideality factor, and other constants have the

obvious meanings. However, it is not obvious that a change in

the metal work function, as introduced, e.g., by a change in the

gas ambient, will show up as a change in the barrier height.

Surface states, e.g., dangling bonds, are known to introduce

pinning of the Fermi level of Si Schottky diodes and changes

in the metal work function will not change the barrier height

any longer [24]. The termination of the silicon surface by an

oxide layer gave reproducible gas sensitive Schottky diodes

[25], [26]. An interfacial oxide layer will make the Schottky

Authorized licensed use limited to: Newcastle University. Downloaded on September 8, 2009 at 05:05 from IEEE Xplore. Restrictions apply.

I. Lundstrom et al. / Sensors and Actuators B 121 (2007) 247–262 255

Fig. 12. The sensor signal, the voltage at a constant current of 100 mA, of anSiC sensor operated at 550 !C and placed in the exhaust pipe of a petrol enginewhere the branches from the cylinders join. Cylinder 1 has excess fuel whilecylinders 2–4 have close to stoichiometric air to fuel ratio [42,55].

cavity surrounding the sensor, the sensor signal at high tempera-tures varies between two levels only based on the ratio betweenhydrocarbon and oxygen atoms in the gas mixture independentof the type of hydrocarbon [46,47]. The conditions in the gasphase is described by the so called !-value:

! = (mair/mfuel)real

(mair/mfuel)stoich

with ! = 1 at stoichiometric conditions. Three different cases forthe behavior of the sensor are identified, reaction rate limita-tion, diffusion limitation or injection limited conditions [46]. Atconditions when injection limitations prevail. The surface willin practice have only two states or phases, excess hydrogen orexcess oxygen and the phase transition will ideally take place atstoichiometric conditions. However, the air to fuel ratio at theswitch point depends on the catalytic activity, the temperature,size of the sensor and the geometry of the sensor housing.

We have found that the process can be manipulated by forexample large catalytic areas close to the sensor surface [46,47].The first experimental set up utilized capacitor devices with arather large catalytic gate mounted on a Pt foil in an oven, thatis with heated gases, while the second used transistor devicesmounted on ceramic heater and non-heated gases. While theswitch point for the former case is close to ! = 1 for a temperatureabove 300 !C, it was 1.08, that is, oxidizing conditions, for thelatter case even at 700 !C. The reason for this deviation wascarefully studied and it was possible to experimentally adjustthe switch point to 1.00 through an increase of the catalyticactivity in terms of a larger catalytic surface area, a temperature"400 !C and a lower flow rate.

5.3. Reactions at the oxide–semiconductor interface

At temperatures above 500 !C the diffusion rate of hydro-gen atoms through silicon dioxide is rather high. The effect ofthis has been studied both in capacitor devices and Schottkydiodes for 6H and 4H SiC of both n and p-type [42,48,49].

Fig. 13. The CV characteristics of a Pt/TaSix/Pt/SiO2/SiC device operated in20% O2/Ar and 5000 ppm H2/Ar, respectively, at about 750 !C [48].

It has been suggested that the hydrogen atoms may reversiblydecorate surface states on the SiC surface, which for capacitorsbased on n-type SiC result in a decreased inversion capacitanceas shown in Fig. 13. The activation energy for forming the hydro-gen surface state complex was estimated to 0.9 eV and for thereverse hydrogen release process 2.8–2.9 eV [48]. The breakingof the hydrogen surface state complex thus takes a temperatureabove 650 !C to occur at a reasonable speed, and for the case ofchanging from 6H to 4H SiC with slightly higher bandgap, thetemperature had to be raised even further. Also the small sig-nal conductivity of the inversion layer was investigated for the4H capacitor devices and showed to decrease in the hydrogenambient as compared to a hydrogen free ambient (N2, Ar or O2)[49]. It was suggested that the decoration of surface states by thehydrogen atoms increases the minority carrier generation time.Schottky diodes with a very thin interfacial oxide layer, whichis necessary for the gas response [42], were also investigatedaccording to this phenomena. Schottky devices were annealedat 500 or 600 !C in either hydrogen or oxygen ambient for up to15 h. Subsequently the temperature was lowered in the annealingambient to 400 !C and the IV curves and gas detection proper-ties were investigated. The IV curves shifted to lower voltagesin forward bias by up to 1 V after the hydrogen anneal, that isthe sensor baseline, the voltage at a constant forward current,decreases. The change in baseline was reversibly shifted due torepeated anneal at 600 !C in hydrogen or oxygen (or N2/Ar).The gas response to intermittent pulses of hydrogen or oxy-gen at 400 !C was not affected by the different positions of thebaseline, suggesting that the formed complexes during hydrogenanneal that moves the baseline are different from the complexes,which are involved in the detection of hydrogen [42]. It shouldalso be noted that the leakage current of reverse bias, of the IVcurve was also recorded and it changed to a larger leakage cur-rent after the hydrogen anneal. In conclusion the investigationsabove indicate the importance of defect free SiC material forhigh temperature operation of field-effect devices. It should alsobe pointed out that in gas sensors based on an SiC transistor with

The authors tested a technique developed by Krause et al. [127] that involves theuse of a sputtering process at a high pressure, 50 mTorr. This produces films withlarge grains and good adhesion properties, which has the advantage of enablingmetal patterning to be performed by using the lift-off process. These films also havea favorable porosity and thus perform very well as ammonia sensors, as described inSection 2.2.2 [52].

2.6.3 Mounting

For testing in both laboratory and industrial applications, the sensor chip producedby the authors is mounted by gluing onto a ceramic heater, which is attached to a16-pin holder while maintaining an air gap underneath [Figure 2.19(b)]. This allowsthe sensor chip to be heated to 600°C, whereas the holder below stays at 200°C. A Pt100 element is also glued to the heater and used for temperature control.

To carry out measurements in exhaust gases, the 16-pin holder is mounted in aspecially designed tube [Figure 2.19(a) [128], which was improved recently [52].The exhaust gases are cooled while passing through the tube, which makes itpossible to make measurements with the sensor chip at a constant temperature thatis somewhat below the exhaust gas temperature.

Hunter et al. have mounted their sensors by gluing the SiC sensor chip onto athin membrane realized by spin-on glass, with a heater and temperature detectorunderneath. This mounting technique enabled operation of the sensor at 600°C witha heater power of close to 1W [4].

Micromachining in SiC is developed by several groups, for a review see [129].Solzbacher et al. have constructed a microhotplate based on a SiC-membrane and aHfB2 thin-film heater, where the active part of the membrane is separated from thesurroundings by six SiC microbridges. The heater is designed for operation tem-peratures up to 700°C and can be operated at 400°C with a power of 35mW [130,131]. Boston Microsystems also offers a monolithic microhotplate in SiC [132].

2.6.4 Device Operation

It is advantageous to operate the FET devices in a constant current mode using afeedback system to compensate for the voltage change caused by the gas molecules.

58 High-Temperature SiC-FET Chemical Gas Sensors

(a) (b)

Figure 2.19 (a) The tubing mounting used for engine exhaust measurements. The flow direction ofthe gas is indicated by the arrows. To the left a schematic drawing of the device is shown, and to theright a photograph of the real device without the inner tube. (b) A sensor chip mounted togetherwith a ceramic heater and a Pt-100 element on a 16-pin holder. (From: [128]. © 2003 Elsevier B.V.Reprinted with permission.)

Mounting system for SiC devicesin exhaust manifolds

I. Lundstrom, Sensors and Actuators B, Vol. 121 (2007) pp. 247

Case Study 2 : Volcanic Eruptions

• Over 500 million people live within range of a volcano

• Emissions of sulfur increased by an order of magnitude prior to the eruption of Mt Pinatubo in 1991

• Major disruption to air travel in Europe in 2010 when Eyjafjallajokull erupted in Iceland (cost of $200M per day)

• Current technologies are short term, or require human intervention

• Move to a self powered, ubiquitous sensor network which can identify changes to people sat some distance away

Case Study 2 : Volcanic Eruptions

• The extent of the ash cloud on 15th of April 2010

Current technology

• Optical absorption

• Very power hungry and liable to theft

• In fumarole measurements at 400C, large quantities of CO2,H2O as well as SO2, HCl, HF and H2S

SiC sensors: a review

typical constituents of a diesel exhaust are given in table 1. Theresponse time of the sensor is less critical in this applicationthan in the monitoring of individual cylinders in the petrolengine as the sensor is mounted after the catalytic converter.The detection of NOx is also possible, but the Pt gate materialsshows a cross sensitivity to hydrocarbons and would requirearray technology to allow a concentration to be extracted froma mixture [74].

Whilst large boiler and commercial premises assesstheir pollution using Fourier transform infrared (FTIR)spectroscopy, the cost of these systems is too high for smallinstallations, such as domestic central heating boilers. Ithas been shown that silicon carbide sensors can be used todetermine the mode of operation for a boiler operating onwood pellets [49]. This showed that the CO concentrationin the exhaust varies between 80 and 500 ppm, with an NOvariation between 60 and 100 ppm, which correlated wellwith the measured oxygen concentration. Most interestingly,this application has shown that silicon carbide technologycan offer the reliability required for deployment in extremeenvironments, with the sensors operating for over six monthsat 300 !C with no sign of failure [53].

3.2. Space/Earth observations

With almost half a billion people worldwide living in theshadow of a volcano, the ability to predict an eruption wouldsave a large number of lives. For example, 80 000 people wereevacuated from the slopes of Mt Pinatubo in the Phillipines,prior to the 1991 eruption. It is known that the constitutionof the gas emitted from vents around the volcano changesprior to an eruption as the dynamics of the magma belowthe surface changes. The critical measurement is the carbon–sulfur ratio, based on the relative quantities of each element inthe emitted gases. The sulfur flux at Mt Pinatubo increased ten-fold immediately prior to the eruption in 1991 [75]. Typicalemissions from a volcano vary from location to location, butgenerally contain large quantities of H2O and CO2, with tracelevels of SO2, HCl, HF and H2S at temperatures around 400 !C.Monitoring of the gases emitted is either by optical absorptionmeasurements [76] (limited by the background levels of thegases of interest) or by collecting the gas and analysing in alaboratory by means of mass spectrometry [77]. The opticalsystems used are often based on the absorption of light froma deuterium source (shown in figure 8) and these systemsrequire a substantial level of power to operate. By deployinga SiC sensing array into the vent itself, it will be possible todetect trace levels of gas emitted from the volcano in real time.In this application, reliability of the sensor will be critical.With a high water vapour content, the gas is highly corrosiveand the packaging used for the sensor and any monolithicallyfabricated amplifier electronics will be severely tested.

With the remoteness of some volcanic sites, the ability tomonitor the emissions for scientific purposes dominates overthe concern for public safety. The emission of large quantities(often several mega tonnes per year from a crater) of SO2

has a potentially significant effect on the global atmosphere[78], which is as yet unknown. The deployment of a longterm monitoring station at one of these sites will require thedetection system to have a low power budget, which is ideally

Figure 8. Spectroscopic measurements of H2S/SO2 ratios, using aspectometer and deuterium bulb at Solfatara, Italy. The optical pathis denoted by a dashed line. From [74].

suited to the use of solid-state detectors, rather than opticalor mass spectrometry based systems. Wireless networkingbetween nodes spread across a volcanic crater would allowa more complete model of the dynamics of the magma inthe caldera, rather than relying on extrapolation from a singlelocation.

Space and planetary exploration offers the ultimate test forextreme environment electronics. With temperatures rangingon the surface of planets ranging from 750 to under 20 K, aradiation dose in excess of 1 Mrad and corrosive atmospheres,the desire to perform scientific investigations on extraterrestrialplanets is extremely challenging. One of the current targetsis to map the atmosphere of Venus as part of the EuropeanSpace Agency ‘Cosmic Visions’ programme [79]. The limitedcurrent knowledge of the composition comes from earth basedspectroscopy data and a small number of Russian probes,which successfully reached the surface in 1970 and survivedfor less than 3 h [80]. The measurements taken with theseprobes show that the majority of the atmosphere comprisesCO2, with traces of N2, H2O, O2, H2S, SO2, H2SO4 and CO.

By using a series of microprobes dropped from a balloonfloating in the upper atmosphere, it is planned to mapthe variation of the Venus atmosphere both vertically andlaterally [81]. Dropping the probes as ballast will alsomaintain the buoyancy of the balloon and enable it to monitorthe behaviour of the upper atmosphere for longer. Themicroprobes are designed to be self-contained systems withradio communications back to the balloon, which will samplethe atmosphere down to the surface. The use of solid-statesensors, with no moving parts and a low mass and powerconsumption, is ideal for this deployment.

The radiation tolerance of SiC sensors will also be usefulin the investigation of the chemical composition of the Joviansystem, one of the targets for Cosmic Visions [80]. The highmagnetic field around Jupiter acts to create a region of intenseradiation flux. Within this region, lies Io, which apart fromEarth is the only location in the solar system to show evidenceof volcanic activity. Another potential target is Titan, one ofSaturn’s moons, which has an atmosphere comprising mainlynitrogen. Silicon carbide has the ability to provide not justthe detectors for radiation and gas composition but also the

6351

Case Study 3 : Nuclear Storage

• Currently 19 nuclear plants producing electricity in the UK

• Generating 20% of our electricity requirement

• All but one will close by 2023

• decontamination / clean up is potentially a big market in the near / medium term

• Expect 25 to 30 GWe of new capacity, a lot of which will be Gen IIIb (possibly 35% of UK requirement)

• Image from news.bbc.co.uk

Decommissioning / Waste• Classified into three types

• Low Level Waste – dry storedActivity below 4 million Bq/kg (Alpha)Activity below 12 million Bq/kg (Other)

• Intermediate Level WasteDoes include materials from decommissioningNot currently segregated by half life

• Activation of metals from neutron flux

• 55Fe 2.73yr, 60Co 5.271yr, 63Ni 100yr

• Segregation requires the ability to perform energy spectroscopy measurements in hostile environments

Decommissioning / Waste

• High Level Waste – Based on thermal conditionsnot generated during decommissioningmostly fission products (99Tc, 135Cs, 79Se) and actinides (237Np, 241Am, 242Cm ) Generate 12,000 tons per year

Decommissioning / Waste

• High Level Waste – Based on thermal conditionsnot generated during decommissioningmostly fission products (99Tc, 135Cs, 79Se) and actinides (237Np, 241Am, 242Cm ) Generate 12,000 tons per year

Time of peak radioactivity for HLW

End of ‘ingrowth’, where the actinides have decayed into more stable elements and continuous monitoring is not as critical

The future• Higher burn fuel : Extract 60GWd per ton of Uranium

(currently 40GWd – up from 8GWd in Magnox stations)

• Increase in the amount of hydrogen generated, which is then absorbed by the fuel rod cladding (by about 40%)

• Storage issue for HLW – monitor gas build up for the first 50 years

• Increased radioactivity in waste fuels (50% higher than current)

• Will increase the peak level of radioactivity in storage by 100% and shift the peak to 2080

• Need to store barrels further apart

• Data from New Scientist 12th April 2008 pp. 8


• SiC Devices

• Diodes

• FETs


• Applications


The Route Forward• Wide bandgap semiconductors offer the possibility of putting

power electronic systems in to a variety of hostile environments

• Of these, silicon carbide offers the most mature processing technology and hence the shortest module development time

• The reduction in switching loss enables higher efficiency, smaller passives and higher operating frequencies in comparison to silicon devices

• Applications include hybrid electric vehicles, oil and gas exploration, civil nuclear ...

• More collaboration between research and application is needed to maximise the potential benefits

The Route Forward• Commercial, validated production now

available for power and signal level devices

• Production quality 6” wafers demonstrated September 2010 -66% price drop per cm2 in comparison with 4”

Images courtesy of Cree Inc and Raytheon Semiconductors Limited

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