Optimization of lateral magnetotransistors with integrated signal amplification

Sensors

and Actuators,

91

11 (1987) 91 - 98

OPTIMIZATION OF LATERAL MAGNETOTRANSISTORS INTEGRATED SIGNAL AMPLIFICATION J BURGHARTZ Instltut

WITH

and W VON MUNCH

fir Halble~tertechmk

der Unwersttat

Stuttgart,

O-7000 Stuttgart

(F R G )

(Recewed January 28, 1986, m rewed form May 21, 1986, accepted August 21, 1986)

Abstract This paper describes the optmuzatlon of a lateral magnetotranslstor (LMT), combmed with a signal amphfler The base width, the posltlon of the base contact and the substrate dopmg level are important design clnterla An mtegrated clrcult compnsmg a dual-collector lateral magnetotranslstor and a dtiferentlal amphfler yields a sensltlvrty of 10 V/T

1. Introduction Lateral magnetotranastors (LMTs) have been mvestlgated previously because of then- high sensltlvlty as compared to transducers based on Hall generators [ 1 - 61 If very small magnetic fields are to be measured, the transducer must be combmed with a signal amphfler. It IS the purpose of this paper to derive a figure-of-ment for lateral magnetotranslstors In addltlon, the optlmlzatlon of lateral magnetotranslstors by suitable choices of the base width, the posltlon of the base contact and the substrate doping level wrll be discussed These conslderatlons are servmg as guldelmes for the design of an integrated clrcult compnsmg a dual-collector LMT and a drfferential amphfier The conflguratlon of a dual-collector LMT IS shown m Fig 1 The device 1s operated with a constant base current IB m the milliampere range A magnetic field apphed parallel to the s&con surface causes a vanatlon of the collector currents Icl and IcZ m opposite duectlons, as shown m Fig 2. The relative vanatlon of the collector current,

for each of the two collectors 1s proportional Hence a relative senwtlvlty

f&(B)

(2)

K= I,(O)B

0250-6874/87/$3

to the magnetic mductlon B

50

@ Elsevler Sequola/Prmted m The Netherlands

BlCl

E

C2 82

n-substrate

(a) Fig

(b) 1 Basuz conflguratlon

of a dual-collector

LMT

(a) cross-section,

(b) top view

-10

c

I

0

1

I

1

I

WI

I

I

I

7%

Fig 2 Relative varlatlon of the collector with magnetic mductlon J3 (1, = 10 mA)

currents 1~1 and 1~2 of a dual-collector

LMT

can be defined I, (0) 1s the collector current m the absence of a magnetic field. The current amphflcatlon of the LMT 1sgwen by

=cw

P= -y--

(3)

B

2. Denvatlon of the figure-of-ment The basic clrcult diagram of a dual-collector LMT connected to a differentlal amphfer 1sshown m Fig 3. Usmg eqns (2) and (3), the output voltage U, = 2RAtc = RKPI~B

(4)

can be calculated. It 1s obvious that a high feedback resistance R unproves the output signal level For an mtegrated version of the clrcult an upper lunlt of R = 100 kS2 seems to be reasonable m view of reproduclblllty and stability problems A near-zero d c bias of the dlfferentral amphfler can be obtamed by proper choice of the load resistors Rc, one of these resistors IS adJustable As seen from eqn. (4), the output signal voltage 1s proportional to the base current In practice, however, it 1s desxrable to operate the clrcult at a

93

Fig

3 Clrcult diagram of a dual-collector

LMT

connected

toa

dlfferentlal

amplifier

low current level. A high base current may cause chip heatmg and, consequently, an mstablllty of the operatmg pomt Hence, it 1s useful to define an efficiency VA q =

(5)

(I, + 2Ic)B

of the lateral magnetotranslstor. (It 1s assumed that the power consumption of the dlfferentlal amplifier E negh@ble ) Combmmg eqns (3), (4) and (5), one obtams

(6)

=RS2

where a=

KP 1+P

-

1s the figure-of-merit

(7) of the LMT

3. Technology The devices under mvestlgatlon were produced by standard photohthographic and dlffuslon processes. Lateral magnetotranstirs of the pnp type were generated with the followmg process steps (1) Wet oxldatlon at 1000 “c, 300 mm. (2) Boron predeposltlon (BzHs) at 1000 “C, 20 mm. (3) Boron drwe-m at 1100 “C, 50 mm (4) Phosphorus predeposltlon (PO&) at 855 “C, 40 mm. (5) Phosphorus tive-m 1050 “c, 30 mm (6) Contact evaporation (alummlum, 1 pm) and smtermg at 480 “C, 10 mm With the above process parameters, Junction depths of about 2 pm were obtamed Substrates w&h (100) onentatlon and donor concentrakons between 6 X lOl4 cmm3 and 1 5 X 1016 cmm3 were used

94

The npn devices were produced m the same manner, except for shghtly mcreased drffuslon times (Junction depths approximately 2 5 r,lm) The substrate dopmg levels were m the range 10” cmV3 to 1016 cmm3 It 1s obvious that the device performance mamly depends on the base wxdth w and the posltlon of the base contact, z e , the distance b between the collector region and the base contact region Devices with 8 pm < w < 600 pm and 8 pm =Gb G 540 I_cmwere produced The length of the devices was I = 190 flrn 4. Experimental

results

Figure 4 shows the current ampllficatlon 0, the sensltlvlty K and the figure-of-ment SZ for pnp devices as a function of the base width w It 1s obvious that /3 and K are varymg m opposite ways with mcreasmg base width The figure-of-ment 52 exhibits a maxunum near w = 20 pm The influence of the posltlon of the base contact on the device performance IS less pronounced, 1 e , the figure-of-ment degrades only slightly If the distance b IS increased from 8 pm to 100 pm (Fig 5) The expenmental results concernmg the mfluence of the substrate dopmg level are shown m Figs 6 and 7 It 1s obvious that a low substrate doping level favours the optlmlzatlon of lateral magnetotranslstors, ze ,

15

30

+T

P 10

20

05

10

w=72~ 0

0

Fig 4 Amphflcatlon 0, relative sens1tlvlt.y K and figure-of-merit S2 as functions of the base width w (pnp transistors, ND = 6 X lOI cmm3) Fig 5 Current amphflcatlon j3, relative sensltlvlty K and figure-of-merit S2 as functions of the dtstance b between collector and base contact (pnp transistors, ND = 6 X 1Ol4 cmm3)

95

6

2 I-

OI”“’ 10

lo0

w wn

loo0

10

L

m$iiim

Fig 6 Figure-of-merit !2 as a function of the base width w, with the substrate doping level as parameter (pnp and npn transistors) Fig 7 Figure-of-ment C! as a function of the distance b between collector and base contact, with the substrate doping level as parameter (pnp and npn transistors)

devices exhlbltmg a high figure-of-ment can be obtamed with a wide range of base mdths Devices of the npn type are superior to pnp devices due to then higher current amphflcation and figure-of-merit (Fig 6) Apart from the optlmlzatlon by proper choice of the geometrical conflguratlon, it 1s necessary to select a suitable operating point It has been established that both the relative sensitlvlty K and the current amphficatlon /3 can be unproved by an increase of the base current Noise problems, on the other hand, are a hmltmg factor when small magnetic fields are to be detected A base current 5 mA G IB < 10 mA seems to be a reasonable compromrse for the devices described here The following design parameters are recommended for npn devices Iv, = 1015 cme3, w = 30 I_cm, 8 pm < b < 40 pm

5. Integrated magnetic field transducer The magnetic field transducer consists of a current source, a dualcolIector LMT, a CMOS differential amphfier and a bipolar output stage The complete cucult d-am 1s shown m Fig 8 The substrate 1s n-type, (100) onented, with a donor concentration ND = 4 X 1Ol4 cmm3 Accordmg

96 R6 1

“A

-u. 3

Fig 8 &cult

diagram of the mtegrated magnetic field transducer

to Fig 6 it 1s expected that a properly desqned lateral magnetotranslstor wti exhibit a figure-of-ment !i2 around 4%/T. The magnetic field transducer was produced by process steps that are sunllar to those described m Section 2 Two addltlonal ion nnplantatlon processes are necessary to generate the channel regions of the n-channel field-effect transistors and the resistors In essence, the followmg process steps are employed (1) Wet oxldatlon at 1000 “C, 300 mm (2) Boron Implantation, 125 keV, 4 X 1013 cmF2 (3) Boron drive-m and annealmg at 1200 “C, 350 mm (4) Boron predeposltlon at 1000 “C, 20 mm (5) Boron drive-m at 1100 “c, 30 mm (6) Phosphorus predeposltlon at 855 “C, 40 mm (7) Phosphorus dnve-m at 1050 “C, 10 mm. (8) Dry oxldatlon at 1000 “C, 150 mm (9) Boron lmplantatlon, 120 keV, 9 X 10” cmw2 (10) Annealmg at 750 “C, 30 mm (11) Contact evaporation (alummmm 1 r,cm) and smtenng at 480 “C, 15 mm The posltwe supply voltage +U, 1s fed to the amphfler stages through an integrated forward-biased diode Dl m order to keep the substrate potentml slightly above the hq&est potential of the amphfler For reasons of symmetry, a diode D2 1s also mserted mto the negative supply lme. The prehmlnary design of the mtegrated clrcult features adjustable resistors Rl and R3, t e , these resistors can each be selected from five different values Thus, the operatmg point of the lateral magnetotrans=tors can be balanced by proper choice of R3 The reslstor R7 symbolizes the resistance of the bulk matenal between the diode Dl and the lateral magnetotranslstor In order to facllltate an adJustment of the output offset voltage, one of the load resistors IS dlvlded mto several segments Special attention has to be mven to the posltlon of mJectmg and reversebiased p-n Junctions wlthm the clrcult The diodes Dl and D2, for mstance, are placed at a distance of several dlffuslon lengths from reverse-biased Junctions

97

6. Results and dlsrcusslon The transducer cvcult accordmg to Fig 8 1s operated w&h a base current of 5 mA for the dual-collector lateral magnetotranslstors. The mtnnslc amphflcatlon of the four-stage CMOS differential amphfler IS 100 dB, approximately The output impedance of the clrcult 1s about 10 S2. Figure 9 shows the output voltage U, as a function of the magnetic mductlon B An overall sensltlvlty of 10 V/T can be deduced from this plot The total current requved by this clrcult 1s 8 mA. Accordmg to the defmltlon gwen by eqn. (5), an efficiency of 1250 V/A T has been achieved As indicated by the dashed line m Fig 9, a slightly unproved balancmg of the sensor bridge unll be necessary to cancel the offset voltage

Fig

9 Output voltage VA of the magnetic field transducer

vs magnetic mductlon

B

Acknowledgements Fmanclal support by the Deutsche Forschungsgememschaft 1s gratefully acknowledged The authors thank Mrs S Opltz, Dlpl.-Ing (FH) A. Marz and Mrs U Schaller for the device fabncatlon, Dip1 -1ng Ch. Horst asslsted m cucult design

References M Mltmkova, T V Persiyanov, G I Rekalova and G Shtyubner, Investlgatlon of the character&ics of slhcon lateral magnetotranslstors with two measuring collectors, Sov Phys Semtcond 12 (1978) 26 A W Vmal and N A Masnarq Magnetic transistor behavior explained by modulatlon of emitter inJection, not carrier deflection, IEEE Electron Devzce Lett , 3 (1982)

203 V Zleren, explained

Devace

S Kordlc and S Mlddelhoeck, Comment on ‘Magnatlc tram&or behavior by modulation of emitter qectlon, not carrier deflection’, IEEE Electron Lett, 3 (1982) 394

4 R S Popovlc and H P Baltes, An mvestlgatlon of the sensltlvlty of lateral magnetotransistors, IEEE Electron Deutce Lett , 4 (1983) 51 5 R S Popovlc and H P Baltes, Dual-collector magnetotranslstor optlmlzed wrth respect to qectlon modulation, Sensors and Actuators, 4 (1983) 155 6 A W Vmal and N A Masnarl, Operatmg prmclples of bipolar transistor magnetic sensors, IEEE Tram Electron Deuces, 31 (1984) 1486

Blographes Joachzm Burghartz received the Dip1 -1ng degree from the RhemlschWestfahsche Techmsche Hochschule Aachen, Germany, m 1982 Since then he has been working at the Instltut fur Halbleltertechnlk, Urnverslty of Stuttgart, on lateral magnetotranslstors Wuldemar van Munch obtamed his Dlpl -Phys and Dr phinat degrees from the Unlverslty of Frankfurt, Germany He has worked m the research and development laboratones of the Deutsche Bundespost at Darmstadt and of IBM Germany at Boblmgen He was Professor at the Unlverslty of Hanover from 1969 to 1978 Smce then he has been Professor at the Electrical Engmeenng Department of Stuttgart Umverslty