ELSEVIER Sensors and Actbators A 45 (1994) 179-182

Influence of topology on the response of lateral magnetotransistors

H. Trujillo, A. Nagy, J.C. Cmz Mi.iectmnks Research Centu, PO Bar 8016, Havana 8, Cuba

Received 1 February 1994; in revised form 27 July 1994, accepted 19 August 1994

Abstract

Experimental comparative results on the response to magnetic induction of lateral bipolar dual-collector magnetotransistors with diierent distances between the emitter contact and the baseemitter junction are presented. The reduction effect upon an induced Hall voltage across the junction due to the base contact is also considered. The experimental results con&m that in devices with a far-off emitter contact the emitter injection modulation (EIM) is increased, giving rise to a sensitivity increase of about 20% at 10 mA emitter current for the investigated devices.

Keywords: Lateral magnetotransistors; Topological influence

1. Introduction

Several types of semiconductor magnetic sensors have been reported [ 11, with different galvanomagnetic mech- anisms related to their performance. Magnetotransistors (MTs), whose response to a magnetic induction has been explained in terms of emitter injection modulation (EIM) [2], carrier deflection [3], or both [4], are out- standing among them.

According to EIM, when a magnetic induction B impinges upon a lateral structure with two collectors (Fig. 1) a Hall voltage AV,, appears across the emit- ter-base junction, changing its forward polarization, so that half of it is more polarized and the other half is less polarized. This gives rise to a collector-current difference.

Id l-l Fig. 1. Topology of device types 4, 7 and 10.

Bidimensioaal simulated results have been published recently [5], which sustain this theory.

In this paper comparative results of the response of lateral MTs with different geometries are presented. The results may be explained in terms of EIM.

2. TheonWal considerations

It is well known [6] that the presence of ‘current’ ohmic contacts to a Hall plate of length L and width fl leads to a reduction of the induced Hall voltage through a function G(L!L), such that for OJL N 1 a short-circuit of the Hall voltage is almost obtained.

In this case, in the presence of an orthogonal magnetic induction B, the current carriers undergo a deflection given by: $f?=p,,B, where p,, is the Hall mobility.

In lateral bipolar MTs, the above phenomenon would also be present, therefore the distances of the base and emitter contacts from the emitter-base junction ( W, +d and a, respectively) should be taken into account (see Figs. 1 and 2).

For emitter and base contacts relatively close to the emitter-base junction, a change A& of collector current will be produced due to the carrier-deflection mech- anism:

(1)

Else.vier Science %A. SSDI 0924-4241(94)00838-9

180 H. T&lo ei al. I Sensors and Actuators 45 (1994) 179-182

Fig. 2. Topology of device type 13.

Table 1 Device dimensions

Device WB (fit@ w (run)

4 20 loo 6 7 40 80 6

10 78 80 6 13 20 80 116

where I,, is the emitter current, IV; the base width and W the emitter length. The other symbols were defined previously.

For sufficiently separated contacts a Hall voltage is developed, so that EIM prevails. For the intermediate cases the MT response will be due to the simultaneous presence of both mechanisms.

If the emitter contact is placed far away from the emitter-base junction, an enhancement of the sensitivity due to EIM should be expected.

3. Device structures

Lateral bipolar.NPN MTs,with diierent base widths and different distances between the emitter contact and emitter-base junction were built. The topology of the device types 4,7 and 10 is shown schematically in Fig. 1 and that for the device type 13 in Fig. 2.

Table 1 gives some characteristic dimensions (in the mask) of these devices.

They were fabricated on p-type (100) substrates with 4-7 ohm cm resistivity. The collectors and emitter regions were phosphorus diffused with N,= 6~ l@* cm-3 and Xj-4 pm.

4. Expecime&al :I

The device characterization was realized through the measurement of the parameters B and So, where /3 is

the common emitter-current gain and S, is the magnetic sensitivity to a change AB of magnetic induction, defined as

S, = Az,l(l,AB) (2)

The measuring circuit shown in Fig. 3, with its previously reported inherent advantages [7,8], was employed.

Fig. 3. Measuring circuit.

Table 2 Measurement results for device types 4, 7, 10 and 13 (B -0.13 T)

IO (W Parameter Device type

4 7 10 13

5 B Cm 0.54 0.3 0.49 ? &X10* (T-y 184.3 251.8 255.5 287.9

10 B 0.75 0.43 0.26 0.36 10 S,X lo* (T-‘) 239.7 276.9 305.4 347.3

f 47

Fig. 4. Exparimental @ vs. W, values: 0, I,=5 mA; X, I,= 10 mA; 0, extrapolated values (E-O.13 T).

H. Tmjillo et aL I Sensors and Achutors 45 (1994) 179-182 181

Fig. 5. Experimental So vs. Wa values; 0, I,=5 mA; X, I,,- 10 mA; 0, interpolated values.

Table 2 gives mean measured results for samples of each type of device at two current levels.

5. Discussion

The results of Table 2 show that in device types 4, 7 and 10 with similar topologies, /I decreases and the sensitivity increases when the base width or current level is increased.

The base-width increase gives rise to a decrease of emitter injection efficiency. This is followed by an increase of the base current, which gives rise to a sensitivity increase.

In addition, a greater IV, gives a larger distance from the base contact to the emitter-base junction. This favours EIM and depresses carrier deflection. The overall response to both mechanisms is such that the S,, increase due to EIM surpasses the decrease due to carrier deflection.

The response of device type 13 (with the emitter contact far from the emitter-base junction) requires a

more careful analysis. In spite of its nominal IV,= 20 w, it has ~3 = 0.49, a value intermediate between those values corresponding to device types 7 and 10, and notwithstanding it has the greatest sensitivity at the same current level.

In Fig. 4 experimental p versus IV, values for device types 4, 7 and 10 at two I0 current levels are plotted. From this Figure, the apparent or effective base width (IV& for device type 13 is = 47 pm at B=O.49 and Z,= 5 mA. Analogously IV,,=52 km at /3= 0.36 and I,=10 mA.

For these base widths there should be sensitivities between those of sensor types 7 and 10, as shown in Fig. 5 (interpolated, values).

The greater sensitivity of sensor type 13 is thus attributed to the beneficial effect of a greater distance of the emitter contact from the emitter-base junction, enabling an increase of G(LYL).

The effective base width is analysed in connection with Fig. 6. To each coordinate x’ there belongs a base width, IS&, and thus a current density emitted by the bottom of the emitter [8], J&‘) such that

we+.?

I” = s

J”(X’) dr’ (3) WB

Assuming a uniform current density, Eq. (3) may be written as

1” = (LI~)KkI (4)

Combining Eqs. (3) and (4), it follows that

we+9

WB&= s

(olr,y,&) dx’ (5) WB

The analytical calculation of the effective base width is complex due to high-level injection conditions in the base region and other multidimensional effects. Thus it cannot be compared with the experimental values.

Fig. 6. Diagram used in the calculation of the effective base width.

182 H. TmjYllo et aL I Senrors and Actuators 45 (1994) 179-182

The reduction of the Hall voltage due to the base contact would be more pronounced if the base contact and collector regions in Fig. 1 are interchanged.

A small distance of the base contact from the emitter-base junction implies a reduction of the mod- ulation AV,,. The effects reported in Ref. 9 about the occurrence of Hall voltages smaller than the ones predicted by EIM theory may be justified in this way.

6. Conclusions

The design of lateral MTs may be improved by placing the emitter contact far away from the emitter-base junction, giving rise to an increase of sensitivity as a function of current level.

It is inferred that the emitter and base contacts produce a reduction of the Hall voltage in MTs like the well-known one occurring in Hall plates.

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Biographies

Hector Trujillo graduated in electronic engineering from ISPJAE, Havana, Cuba in 1969. He received an M.Sc. in microelectronics in 1976. He has been a professor of the Electrical Faculty of ISPJAE since 1970. He has also worked as an assistant researcher at the Microelectronics Research Center (CIME), Ha- vana, Cuba since 1970.

Agnes Nag graduated in physics from Havana Uni- versity in 1971 and received an M.Sc. in microelectronics in 1978. She has been an assistant professor at the Electrical Faculty of ISPJAE since 1977. She has worked as an assistant researcher at CIME since 1972.

Juan Carlos Crux graduated in electrical engineering from ISPJAE, Havana, Cuba in 1981, and received an M.Sc. in microelectronics in 1991. He has been a professor at the Electrical Faculty of ISPJAE since 1984. He has also worked as an assistant researcher at CIME since 1984.