ELSEVIER Sensors and Actuators A 67 ( 1998) 96-101

A - PHYSICAL

Magnetic-field sensor based on a thin-film SO1 transistor

Pere Losantos a,*, Caries Can6 a, Denis Flandre b: Jean-Paul Eggemont b ’ Cewe NacionaJ de Microe!ecrrbnicu CNM-CSIC, Campw UAB, Belluferm, E-U8193 Spain

b UnivwsitP Catholique de Louvain, Louva/n-In-Nerve. B&ium

Abstract

This paper presents a magnetic sensor on thin-film SOI-SIMOX that takes advantage of a previous bipolar structure, the VCBM (voltage- controlled bipolar MOS transistor) to improve magnetic response with low power consumption. The buried oxide avoids substrate currents, while keeping a high relative sensitivity, up to 50 % T- ‘, of the sensor. The paF!er introduces the structure and theoretical operation regions for both electric and magnetic features. Experimental results on two devices validate the previous analysis, presenting the main figures of merit. Finally, th? total device efficiency parameter is introduced. Q 1998 Ejsevier Science S.A. All rights reserved.

Keywords: Magnetic senmr~; SOI; Bipolar; VCBM

1. Introduction

Several silicon magnetic sensors have been reported in the literature and are well summarized in Ref. [ 1 I. Some clas- sifications may be made; we chose the one based on electrical effects in such a way thatthree groups may appear, presenting the following features:

0 Substrate Hall-like sensors, with two current supplies plus two Hall voltage measurement contacts. Fabricated by any standard simple technology, they may be orthogonal or paralIe1 to the magnetic field. They have relatively low sen- sitivity. As they are surrounded by some signal treatment circuits, the whole technology (CMOS or bipolar) is used.

0 MOS channel-effect sensors (MAGFET-Hall, split- drain, etc.) belong to the orthogonal sensors group and take advantage of the Iow thickness of the channel to enhance the magnetic sensitivity. As only one carrier is present and the channel mobility is lower than the bulk one, these devices show low relative sensitivity though high absolute sensitivity and low consumption.

l Bipolar-effect sensors may be included in both parallel and orthogonal groups, MOS or bipolar fabrication; they present high sensitivity and offset and large power consump- tion due to parasitic transistors.

The goal of this work is to achieve bipolar-effect magnetic performance without losing the low-consumption behaviour of MOS devices. To this end, several papers on simulated

* Corresponding author. Tel.: +343-580-26-25; Fax: +343-580-14-96; E-mail: pere@cnaes

0924-4247/98/$19.00 0 1998 Elsevier Science S.A. M rights reserved. PllSO924-4247(97)01771-S

and experimental structures have been published [ 2,3] pre-m senting different approaches to this question, such as bulk micromachining, buried layers and thick-film silicon-on- insulator (SOI). To our knowledge, in all cases the sensor was a parallel one, but in one case [4] a thick-film SO1 orthogonal one has been published.

This paper presents an orthogonal thin-film SOI magnetic sensor that achieves a relative magnetic sensitivity of 50% T- ’ (bias dqendent) with collector current in the range of 50 p,A. The total consumption of the device is also introduced as a way to measure the device efficiency, not only in terms of the collector-current splitting but also taking into account

the total current, which is scarcely specified in the bipolar magnetic-sensor literature.

2. Device description and analysis

2. I. Device description

SOI technologies were born in the late 1970s and nowadays present a mature state of the art. The goal is to isolate the structural silicon region of the wafer from the active ones by an oxide. SIMOX stands for ‘separation by implanted oxy- gen’ and consists in the formation of a buried layer byimplan- tation of oxygen ions beneath the surface of a siIicon wafer. A first classification can be made depending on the full (FD) or partial (PD) depletion of the film, usually known as well as thin- or thick-film SOL

P. Losantos et al. /Sensors and Actuators A 67 (1998) 96-101 97

The device presented here is an SO1 structure to be used as a magnetic-field sensor and inspired by a classical device [ 51: the voltage-controlled bipolar MOS (VCBM) transistor introduced in 1987 by Colinge. The original VCBM was an SO1 MOSFET structure that could be operated in both MOS and bipolar regimes, resulting in an enhancement of the cur- rent capability for short-channel devices. The present device, shown in Fig. 1, takes advantage of the previous structure to measure magnetic field with a good sensitivity, low power consumption and other related SO1 performances. The main modification concerns the base length, which is enlarged to improve the magnetic response, and the introduction of a second collector in order to measure the current deviation. The bipolar emitter (n+ ) behaves as a source for the MOS structure, collectors (n+ ) are the drains, and the base (p) is the silicon film under the gate. The whole structure is sur- rounded by the SIMOX buried and field oxides that eliminate any substrate current. A brief electrical analysis is now given in order to make it easier to understand the magnetic behaviour.

2.2. Electrical behnviour

Depending on the gate and base voltage, the device can operate in the four following modes:

Pure MOS regime: this is the most evident regime, and takes place while V,,,, < 0.7 V ( emitter-base (EB) junction diode is off), the gate voltage V,,,, > V,, (typically 0.5 V) ,

Vdmin > 0 and Vsource = 0. There is no base current (holes) injected from the base contact. Emitter current equals collec- tor current.

Pure bipolar regime: this regime appears for gate voltages 0 > V,,,, > V,, ( V,,,), the base-collector (BC) junction is reverse biased and the base-emitter (BE) is forward biased (V,, > 0.7). Under such conditions the film is fully depleted (thin-film SOI) though the channel is not yet well formed and electrons injected from the source-emitter may reach the two collectors with low recombination ratio.

High-recombination regime: as W,, Z+ L,, a back-to-back diode behaviour with high recombination current takes place when the BC junction is reverse biased, BE is forward biased

Collector

Fig. 1. MOS-bipolar thin-film SOI transistor.

and V,,,, < 0. The accumulation layer close under the gate oxide ensures high recombination for electrons entering the base, providing large base hole current ( lemi = I,,,,) and very low collector electron current, ZcO,.

MOS-bipolar mixed regime: for Vgat,> V,, ( Iffilm) and Ibase > I,, Z, being the current to forward activate the p+--p- n+ base-emitter diode.

2.3. Magnetic behaviour

As in this case we are dealing with a confined structure [ 61, two galvanomagnetic effects, Lorentz direct deflection and indirect deflection, may take place under the action of the orthogonal magnetic field. Concerning magnetic behav- iour, only two of the four electrical modes are of interest:

Pure MOS mode: electrons flowing through the inversion channel will deflect because of the Lorentz force over a flow- ing charge. This is the same effect we would measure in a split-drain MOS device.

High-recombination regime: device analysis in this case is somewhat more difficult, since at least the two aforemen- tioned effects may appear. Electrons injected from the emitter diffuse into the base and recombine with majority holes, leading to an exponential decrease of electron current:

Zp(x)a[exp(qU,,lkT)-l]exp(-x/Z,) (1)

Therefore, a non-uniform concentration of majority holes appears that creates an electric field Ebase. This field will induce two effects: on one hand, minority carriers that nor- mally diffuse will drift as well, thus being subjected to the Lorentz direct deflection. On the other hand, majority-carrier drift will generate an increasing Hall voltage from the EB junction to the base contact, which will deviate the emitter current mainly to one collector under the so-called indirect deflection. As the Hall voltage is almost zero near the EB junction (because there is a maximum of carrier concentra- tion), no emitter injection modulation is expected.

3. Experimental

In order to demonstrate the feasibility of this orthogonal sensor, two devices have been designed and fabricated at the Microelectronics Department of the UniversitC Catholique de Louvain (UCL), Belgium. The structure (Fig. 1) is an n- channel MOS-SO1 with polysilicon gate dimensions WXL=60 kmX50 km for the larger one (#A) and WX L = 60 km X 20 km (#B) for the smaller, where L is the source<mitter to drain-collector distance. There is also a p+ film contact that plays the role of the bipolar base contact, and a substrate contact not shown. The technology is the standard at UCL, 3 p,rn length CMOS (54 nm gate oxide thickness) with one polysilicon and one metal layer, SIMOX buried oxide (380 nm) and thin film (thick- ness = 100 nm) .

98

1 .E-05

1 E-06

I.E-07

3

- 1 .E-08

1 E-09

l.E-10

l.E-11

P. Losantos et al. /Sensors and Actuators A 67 (1998) 96-101

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 Vgate M

_“- .._.~ --.. -c Icivb=O.5 & Ict vb=l -IbVb=S

L

Ic lb Vb=l _____ le Vb=O.5 *. le vb=l _-_- _-“.““”

Fig. 2. Collector, emitter and base currents for device #A.

3.1. Electrical characterization

The VCBM is a five-terminal device that can be operated in several ways. In order to clarify the plots shown from now on, we shall use the name VCBM for the set-up where gate and film are short-circuited in such a way that gate-voltage sweeps will also be base-voltage sweeps. In Fig. 2 a gate- voltage sweep versus collector, emitter and base currents is shown, with base voltage as the second variable at V,,, = 0.5, 1 .O and 1.5 V. Fig. 3 plots the base-voltage sweep, with gate voltage as the second variable with two values, V,,, = - 5 V and V,,,, = V,,,. From these plots, we can draw the following conclusions:

Pure MOS behaviour: for V,,, = 0.5 V, the bipolar effect is not present and the characteristic MOS curve can be appre- ciated in Fig. 2 for ZcO,, with its sub-threshold slope.

High-recombination regime: up to V,, (V,,,,), ICd is 0.0 llemi, which is almost equal to Zkae. The threshold voltage decreases with increasing V,,,, as expected because of the body effect on SO1 technologies. When the inversion voltage is reached, the base current decays dramatically and we enter the pure MOS mode.

Mixed bipolar-MOS regime: in Fig. 3 we realize that for V,,,, < 0.7 V there is a slight shift for the VCBM mode with respect to the MOS regime (V,,,, = 5 V), and the bipolar current adds up to the MOS one.

Pure bipolar mode: it can be appreciated in Fig. 4, where an hfe (/? gain) measurement is carried out in two set-up configurations ( V,,,, = - 5 V and V,,,, = If,,,,,) by means of a Gummel measurement. The lower x-axis represents I,,,, while the upper x-axis shows V,,,, which equals V,,,, in the VCBM case. A great difference can be appreciated between

I.505

z

l.E-06

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0 0.5 1 1.5 2

Vb M Fig. 3. VCBM behaviour, showing collector-current increase.

the two curves in Fig. 4. In the case of the VCBM set-up, a maximum p gain of nearly 40 has been measured for device #B and 15 for device #A at V,,,, = 0.55 V, hfe > 1 being for all the sweep range. After this point, the hfe value decays because of the channel influence.

3.2. Magnetic-jield response

Some VCBM figures of merit are now presented, following standard definitions for bipolar magnetic sensors:

(7.1

P. Lmantos et al. /Sensors and Actuators A 67 (1998) 96-101 99

Vgate 100

IO

l.COE-10 1 DIE-09 1.0x-08 l.oOE-07 l.COE-06 l.COE-05 l.WE-04

k [Al

Fig. 4. Hfe plot for devices #A and #B, and for VCBM and high-recombination mode.

(3)

where S, stands for absolute sensitivity, SricO’ for collector relative sensitivity, Zc,, Zc2 are collector currents, and B the applied orthogonal magnetic field.

Fig. 5 shows the magnetic response for both #A and #B devices. According to the previous analysis, the larger mag- netic sensitivity S, (#A= 7 p,A T- ‘, #B = 5 p,A T- ’ for V,,,= 1.5 V, V,,,, = 1.5 V) corresponds to the intersection point V,,,, = V,,, where maximum recombination and chan- nel current are achieved, thus presenting the two deflection effects. Concerning the relative sensitivity S,‘“’ and since the MOS relative sensitivity is low, we expect the largest value at the point of lowest Z,,,, just before sub-threshold current

Fig.

begins to flow. This point is around V,,,, = 0, and depends on the base voltage, as was shown in Fig. 2. The relative sensi- tivity Sri“” is in the range 50% T- ’ for device #A, 38% T- ’ for device #B and for V,,,, = 1.5 V, V,,,, = 0 V, increasing as V,,, does.

3.3. Device sensitivity

The main drawback for bipolar transistors is the high con- sumption required to achieve high sensitivity. A substrate parasitic transistor can drive currents at least 10 times the collector current, though this value is not usually presented in the literature. We can introduce here the device efficiency &emi:

45 , I O.E+oO

-----1 E d -3.E-06 9

Y6.EGp -3 -2 -1 0 1 2 3

Vgate M

5. Relative (S,‘“’ = S,,) and absolute (S,) sensitivity plots for devices #A and #B at B = 0.45 T

loo P. Losantos et al. /Sensors and Actuators A 67 (1998) 96-101

l.E-04

l.E-05

F l.E-06 9

8

l-E-07

0.7 1.2 1.7 2.2 2.7

VbM Fig. 6. Relative (S,““= S,,), absolute (S,) and emitter (Sn(:mi = S,) sensitivities for device #B.

(4)

which represents magnetic performance over total current consumption. As shown in Fig. 6, applying the largest V,,, means achieving a higher relative sensitivity (If,,,, = 0 V) , but as the emitter-current dependence with V,, is exponential, this means substantially increasing the consumption. Fur- thermore, the device efficiency is no longer increasing but slightly decreasing up to V,,,,, =3 V. We can, however, choose the best operation point to fit the signal-conditioning circuitry requirements in terms of output current.

Finally, both devices have presented similar non-lineari- ties, around 7% in the range - 0.5 to 0.5 T, and an equivalent offset of 40 mT. We have to point out, anyway, that the technology is not well optimized in terms of misalignement and that the offset figure could be lowered by improving this feature. All measurements have been performed with a self- made electromagnet up to 1 T, HP4145 semiconductor parameter analyser and room temperature.

4. Conclusions

A new application of a known SO1 modified structure has been presented showing some interesting results. The main one is that a high magnetic relative sensitivity (up to 50% T- ’ for device #A) at low device current (40 p+A) with good linearity is achieved. The largest relative sensitivity is obtained not in the bipolar but in the back-to-back diode operation mode, because of the high recombination current that arises due to the large base length. Concerning absolute sensitivity, its maximum is achieved for V,,, = V,,,,, where the largest hfe gain value is found. Finally, a device sensitivity parameter is introduced to point out that though the relative sensitivity increases with increasing V,,,, in terms of total

consumption, it is not worth doing this because no device sensitivity enhancement is achieved, and it can only be useful in order to fit post-processing circuitry requirements.

Acknowledgements

This work was supported by the SO1 HCM-UE network project CHRX-CT-93-0203.

Refeerences

[ l] C.S. Roumenin, Solid State Magnetic Sensors, Handbook of Sensors and Actuators Series, Elsevier, Amsterdam, 1994.

[2] R. Castagnetti, Integrated magneto-transistors in bipolar and CMOS technology, Dissertation ETH No. 10751, Zurich ( 1993).

[3] C. Riccobene, Multidimensional analysis of galvanometric effects in magneto-transistors, Dissertation ETH No.1 1077, Zurich ( 1995).

[4] R. Gottfried-Gottfried, et al., CMOS-compatible magnetic field sen- sors fabricated in standard and in SOI technologies, Sensors and Actu- alors A 25-27 ( 1991) 753-757.

[ 51 J.-P. Colinge, An SOI voltage-controlled bipolar-MOS device, IEEE Trans. Electron Devices 34 ( 1987) 845.

[6] Ch.S. Roumenin, Bipolar magnetotransistor sensors. An invited review, Sensors and Actuators A 24 ( 1990) 83-105.

Biogriaphies

Pere Lmantos Viiiolas was born in Madrid (Spain) in 1969. He received the B.Sc. degree in physics in 1993, and the M.Sc. degree in 1995 from the Universitat Autdnoma de Barcelona. He is currently working at the Centre National de Microclectronica (CNM) in Barcelona towards a PbD. degree. His main research topics are magnetic sensors and actuators.

P. Losantos et al. /Sensors and Actuators A 67 (1998) 96-101 101

Caries Cnne’ was born in Girona, Spain, in 1960. He has a Ph.D. in telecommunication engineering. In 1986 he joined CNM and has been a senior researcher since 199 1, working on the areas of CMOS technologies and their compatibility with sensors, for microsystems integration. Currently he is the head of the Silicon Technologies and Microsystems Department at CNM.

Denis Flandre was born in Belgium in 1964. He received the electrical engineering and Ph.D. degrees from the Universite Catholique de Louvain, Louvain-la-Neuve, Belgium, in 1986 and 1990, respectively. His doctoral research was on the modelling of silicon-on-insulator (SOI) MOS devices for characterization and circuit simulation. In 1985, he was a summer student trainee at NTT Headquarters, Tokyo, Japan. From October 1990 to September 1991, he was with the CNM, Barcelona, Spain, working on the characterization and numerical simulation of SO1 MOS processes and devices. He is now at the Laboratoire de Microelectronique (DICE), Lou- vain-la-Neuve, Belgium, as a research associate of the National Fund for Scientific Research (FNRS, Belgium) and

an invited lecturer at the Universite Catholique de Louvain giving courses on ‘Integrated analog circuit design’. He is currently involved in the development of digital and analog SO1 MOS circuits for special applications, more specifically high-speed, low-voltage, low-power, microwave, rad-hard or high-temperature electronics. Dr Flandre is co-recipient of the 1992 Biennial Siemens - FNRS Award for an original contribution in the fields of electricity and electronics. He has authored or co-authored more than 100 technical papers or conference contributions. He is a member of the Advisory Board of the EU Network of Excellence for High-Tempera- ture Electronics (HITEN).

Jean-Paul Eggemont was born in Braine-1’ Alleud, Belgium, in 1970. He received the electrical engineering degree from the Universite Catholique de Louvain (UCL), Louvain-la- Neuve, Belgium, in 1992. Since 1992, he has been working towards the Ph.D. degree at the Microelectronics Laboratory, UCL, where he is involved in the development of analog SO1 CMOS circuits for high-temperature and high-frequency applications.