power semiconductor device physics part 1 & 2 : prof. j.p. chante december 02 th, 2002 ñin...
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Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
DrainGate
P-substrate
N+ N+
Bulk
Source
In order to determine the electrical characteristics (threshold voltage) of the transistor, it is easier to study the MOS capacitor first.
Gate
Silicon
Bulk
The ideal Mos Capacitor
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
The MOS capacitor is a poly-Si/SiO2/Si structure.
Gate
P-typeSilicon
Bulk
P-type silicon substrate
Insulating layer SiO2
Back-side metallization
Gate-contact : polysilicon
The ideal Mos Capacitor
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
VGB < 0
When negative voltage is applied to the gate with respect to the semiconductor :
Potential is applied between insulating layer and semiconductor Electric field appears toward the gate majority carriers (holes) are attracted to the surface of the p-type semiconductor
The semiconductor near the surface becomes more p-type : accumulation
The ideal Mos Capacitor
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
VGB > 0
When positive voltage is applied to the gate with respect to the semiconductor :
Electric field appears toward the drain The positive voltage will induce a negative charge to appear near the surface
of the p-type semiconductor The semiconductor near the surface becomes less p-type :
depletion
The ideal Mos Capacitor
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
VGB = Vth the threshold voltage Vth is defined as the applied voltage when the electron concentration is two times bigger than the initial hole concentration
the region near the surface in this case has conduction properties of n-type material the n-type surface layer is formed not by doping but instead by inversion of the originally p-type
material due to the applied voltage this inverted region separated from the underlying p-type material by a depletion layer is the basis of
MOSFET operation
The ideal Mos Capacitor
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
VGB > 0 and more than Vth
With further increase in positively applied voltage, and higher than the threshold voltage Vth :
electric field remains toward the bulk electric field magnitude is high in the semiconductor minority carriers (electrons) are attracted towards the surface when the electron concentration is bigger than the hole concentration, a thin n-type layer is created the semiconductor near the surface becomes n-type :
inversion
The ideal Mos Capacitor
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Field-effect transistors (FET) operation is based on an electric field effect (established by a voltage applied to the control gate terminal)
FETs are also called unipolar transistor since the current is conducted by only one type of carrier
MOSFET stands for Metal-Oxide-Semiconductor FET even though all advanced VLSI processes uses polysilicon gate rather than metal gate
Properly bias of transistor is : source and bulk are short-
circuited and grounded gate to source voltage is VGS
drain to source voltage is VDS
The Mosfet Transistor
DrainGate
P-substrate
N+ N+
Bulk
Source
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
The MOSFET is a normally off device. With a small negative VGS, more holes will be attracted to
the surface underneath the gate. Source and bulk are grounded and then the source-
bulk junction is in equilibrium state drain voltage is positive and then drain-bulk junction is
reverse biased No current path exists.
The Mosfet Transistor (cont’d) : VGS < 0 and VDS > 0
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
With a small positive VGS, holes will be pushed away from the surface underneath the gate. Source-bulk junction is in equilibrium state Drain-bulk junction is reverse biased
No current or small current exists between drain and source.
The Mosfet Transistor (cont’d) : VGS > 0 and VDS > 0
(VGS < Vth )
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Electrons will begin to accumulate, forming a conduction channel.
Small VDS has no influence on VGS bias and then the channel is uniform
Current path exists between drain and source
VGS influences the electron concentration in the conduction channel : as VGS increases, the concentration
increases, on-state resistance decreases linear variation of the current versus drain
voltage VDS
The Mosfet Transistor (cont’d) : VGS > 0 and small VDS > 0
(VGS > Vth )
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
As VDS increases, drain-bulk junction is highly reverse biased and then SCR is stretched
the potential difference between the gate and the drain decreases. The channel formed will no longer be uniform and begin to tapper off near the drain end. The voltage VDS is noted VDSsat
DrainGate
P-substrate
N+ N+
Source
Bulk
N+
+
VDS>0
+
V GS>V TH
The Mosfet Transistor (cont’d) : VGS > 0 and high VDS > 0
(VGS > Vth )
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Eventually the channel at the drain end will disappear as VDGVTH.
The drain current will not shut off abruptly, but instead will remain at the same level called IDSsat.
The pinched channel can be considered as a choke point. This point moves toward the source as VDS increases.
Current is due to the electrons flow in the conduction channel, due to the electric field (Drain toward Source).
The Mosfet Transistor (cont’d) : VGS > 0 and high VDS > 0
(VGS > Vth )
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
MOSFETs have two regions of operation : the triode the saturation regions.
Mosfet Operation (cont’d)
vD S > vGS V TvD S< vGS V T
Saturation region(active region)
Triode(linear region)
V GS increease
VGS V T+1
VGS V T
vD S
iD
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
The threshold voltage can be changed by deliberately adjusting the doping concentration near the surface of the channel.
In the extreme, a channel can be formed (by ion-implantation) without an applied voltage. This type of MOSFET is called depletion mode device.
Mosfet Operation (cont’d)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Drain current is proportional to the channel width Z, and inversely proportional to length L,
Current in the transistor is limited to few mA L should be as short as possible, and also the doping level
of the channel (in order to have a small Vth) :
source gate drain
LZ
N+ N+
P
bulk
SCR of drain-bulk junction can reach source-bulk junction even for small VDS voltage
Maximum voltage of VDS is limited
This structure is not suitable for power
Mosfet Operation (cont’d)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
High voltage device requires a low doped and thick layer. High current device needs numerous basic cell in parallel vertical structure is the solution
epitaxial
layer
source m etallization
polysilicongate
P w ell
drain m etallization
source cell
channel
N-
N+
The Mosfet transistor structure
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
High voltage device requires a low doped and thick layer. High current device needs numerous basic cell in parallel vertical structure is the solution
Conduction channel is formed in p-type region underneath the gate.
N-type low doped layer allows to achieve high breakdown voltage Electrons reach the n-type layer and then flow vertically toward
the drain Low doped region is resistive and then it is necessary to reduce
current density
The Mosfet transistor structure
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
As VDS increases, SCR stretches in n-type and p-type zone.
Conduction channel is pinched-off Electric field in SCR sweeps
the electrons toward the drain
The Mosfet transistor structure
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Source
Gate
P+N+ N+
P
N-
R P
N+
RN
Drain
The Mosfet transistor equivalent circuit
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Advantages and drawbacks of MOSFET Advantages
MOSFET transistor is unipolar deviceconduction is conducted by majority carriers lack of storage charge involves a high switching speed
MOSFET transistor is fast
Control is made through a voltage applied to the gate (capacitor)during steady state, voltage is sufficientduring transient switching, a dynamic current is required to load and unload the input capacitorControl energy is necessary only during the switchings
MOSFET transistor is easy to control
The Mos-Bipolar Power Devices
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Drawbacks
Conduction assumed by majority carriers requires electric fieldvoltage drop in the layersthis voltage drop increases as :
breakdown voltage is highercurrent density is higher
losses are important in a power MOSFET
Conduction losses are important in MOSFET
The Mos-Bipolar Power Devices
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Advantages and drawbacks of Bipolar Drawbacks
Conduction with minority carriers implies storage chargeduring transient switching, this charge needs to be loaded and unloadedswitching time is slow in bipolar transistor
Bipolar transistor is relatively slowcontrol of the device requires currentcontrol requires energy during all conduction periods
Control of Bipolar transistor requires energy
The Mos-Bipolar Power Devices
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Advantages and drawbacks of Bipolar Advantages
Conduction is assumed by both type of carriersBasic principle of conduction is diffusion of carriers
electric field in the layers is lowsmall forward voltage dropsmall conduction losses
Conduction losses are small in bipolar transistor
In order to profit by advantages of both types of transistor :
MOS-Bipolar Power Devices
The Mos-Bipolar Power Devices
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Top structure is identical to MOSFET Substrate is p-type
two kinds of IGBT :
with buffer layerhomogeneous base
Insulated Gate Bipolar Transistor (IGBT)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Forward bias of transistor is : emitter and bulk are short-circuited
and grounded gate to emitter voltage is VGE
collector to emittter voltage is VCE > 0
In forward mode, J1 is forward and J2 is reverse
n-type buffer layer allows to reduce the thickness of N- layer and avoid punch-through of J1
IGBT with buffer layer
Insulated Gate Bipolar Transistor (IGBT)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Top structure is identical to the previous one
punch through is avoided thanks to a thick n-type layer
P+-layer is very thin
IGBT with homogeneous base
Insulated Gate Bipolar Transistor (IGBT)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
For both types of IGBT, a positive voltage VGE will involve a conduction channel in p-type layer underneath the gate
As VCE is higher than Vbi of J1, then MOS part of the device will inject electrons in n-type layer
N-type layer is the base of a PNP transistor, where J1 is collector-base and J2 is emitter-base junction
MOS part of the device supplies the base current of the bipolar PNP transistor Then junction J1 injects holes in n-type layer
Holes diffuse in n-type layer and are collected by J2
Insulated Gate Bipolar Transistor (IGBT)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Em itter
Gate
P+N+ N+
P
N-
R P
P+
Collector
N-
Insulated Gate Bipolar Transistor (IGBT)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Insulated Gate Bipolar Transistor (IGBT)
High input impedance and high current gaIn
Turn off by zero gate voltage (remove the conducting channel)
Faster switching speed than BJT and can operate in medium power up to 20 kHz
Improved input and output capacitances
C
E
G
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Insulated Gate Bipolar Transistor (IGBT)
Turn off of the device can be divided in two parts : gate to emitter voltage VGE decrease will induce
the break of conduction of the MOS transistor
• bipolar transistor has then a non connected base
bipolar transistor will remove the storage charge either by collector current or recombination
• The decrease of the current is slower with a time constant depending on the lifetime of the minority carriers
Effect of the minority carrier lifetime on the current queue
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Insulated Gate Bipolar Transistor (IGBT)
Structure and circuit used for the turn off60 µm
30 µm
8 µm
1014 cm-3
10 µm/ 1016 cm-3
10 µm/ 1018 cm-3
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Insulated Gate Bipolar Transistor (IGBT)
Doping concentration in the device
P-type buried layer
Base
Buffer layer
Co
nce
ntr
atio
n [
cm-3]
depth [µm] width [µm]
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Switching off of the IGBT : t = 0
Storage charge in the base is important (in the range of 1016 cm-3)
Electron distribution Hole distribution
Co
nce
ntr
atio
n [
cm-3]
Co
nce
ntr
atio
n [
cm-3]
depth [µm]width [µ
m]
depth [µm]width [µ
m]
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
MOS part of the device stops supply the base current of the bipolar PNP transistor
SCR will stretch in the base by sweeping the carriers
Co
nce
ntr
atio
n [
cm-3]
Co
nce
ntr
atio
n [
cm-3]
depth [µm]width [µ
m]
depth [µm]width [µ
m]
Switching off of the IGBT : t = 80 ns
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
As reverse voltage increases, SCR is stretched Storage charge has drastically decreased
Co
nce
ntr
atio
n [
cm-3]
Co
nce
ntr
atio
n [
cm-3]
depth [µm]width [µ
m]
depth [µm]width [µ
m]
Switching off of the IGBT : t = 1 µs
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Switching Characteristics
Ideal Switch : No power Limit (unlimited breakdown voltage and forward
current) Zero turn-on and turn-off times (infinite frequency of operation) No power dissipation (no on-resistance and no leakage current)
Practical switch : Limited power handling capabilities (max voltage and max
current) Delayed turning on and off (limited frequency of operation) On-resistance and off-leakage current (power dissipation)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Ideal Switching Characteristic Curves :
v sw V off
V on time
isw
Ioff
Ion
time
p ( t)
time
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Non-Ideal Characteristic Curves :
v sw
V off
V ontime
isw
Ioff
Ion
time
p ( t)
time
P maxP min
• Different losses should be considered :
• conduction losses
• off losses
• switching losses
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Power Diode
V F
V BR
IF
Is
iD
vD
vD
iD
ON
OFF
I-V characteristic
Typical I-V characteristic Ideal I-V characteristic
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Thyristor
Forward currentcarrying(ON)
Forward voltageblocking(OFF)
Reverse voltageblocking
ON
iA
vA K
Typical I-V characteristic Ideal I-V characteristic
+
vAK
_
iA
ig
Anode (A)
Cathode (K)
iA
ig1
vA K
Forward blockingregion
Latching currentHolding current
Forward breakovervoltage
ig3>i g2>i g1
ig=0ig1ig1ig1ig1
Max reversevoltage
Reverse blockingregion
vA K
Reverseavalanche region
I-V characteristic
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Bipolar Junction Transistor (BJT)
I-V characteristic
iC
vC E
Cut-OFF(OFF-state)
Saturation(OFF-state)
Active region Increasingbase
current iC
vC E
ON-state
OFF-state
Ideal switch characteristics
Typical I-V characteristic Ideal I-V characteristic
+
vC E
_
iC
iB
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Power MOSFETI-V characteristic
vD S > vGS V TvD S< vGS V T
Saturation region(active region)
Triode(linear region)
V GS increease
VGS V T+1
VGS V T
vD S
iD
Typical I-V characteristic
+
vD S
_
Drain (D)
Source (S)
Gate (G)
Power semiconductor device physics Part 1 & 2 : Prof. J.P. Chante December 02th, 2002
Thermal model of the system
S em i-co nd uc teur
A l O32 D issip a teur
Q
C1
2
R 2
C 3C2 +2
R 3R 1
R c
T aQin
C1 C2+2
C 3
2
C1
6C2
6
C 3
6
Heat sinkPower Chip
Thermal flux
Substrate
In order to estimate the maximum junction temperature