Sensors and Actuators A, 39 (1993) 55-57 55

Diode-connected magnetotransistors

H. Tru j i l l o , A . N a g y , M. M i c h e l e n a , P. R o d r i g u e z a n d R . U g a r t e Microelectronics Research Centre, P.O. Box 8016, Havana 8 (Cuba)

(Received August 21, 1992; in revised form March 17, 1993; accepted April 2, 1993)

Abstract

The magnetic response of diode-connected magnetotransistors (DCMTs) is investigated experimentally and compared with that of magnetotransistors (MTs). Two different measuring circuits are also considered, and it is found that the sensitivity depends on the measuring circuit. The DCMT can achieve a greater magnetic sensitivity than the MT.

1. Introduction

Plenty of work [1, 2] has been devoted to the device structures and characterization of magnetotransistors (MTs) whose sensitivities range from 0.6 to 106%/% requiring different levels of technological complexity and biasing circuitry [2].

Micromagnetodiodes are also reported [3] whose principle of operation is based on double-injection phenomena and the presence of two surfaces with different recombination velocities.

Some experimental results about diode-connected magnetotransistors (DCMTs) are presented in this pa- per and compared with those of MTs. The effects of two measuring circuits on the measured sensitivity are also considered.

2. Experiments and results

Orthogonal lateral bipolar NPN MTs with two base contacts were built according to the topology illustrated in Fig. 1. The silicon substrate serves as an active base region for the device. The substrate employed was of P-type with (100) crystalline orientation and 4--6 12 cm resistivity. Emitter and collector N + regions were phos- phorus diffused with Xj = 4/zm and N D ~ 6 × 1018 cm -3. The device layout is shown in Fig. 1.

Sensitivity measurements were done with a calibrated permanent magnet of 0.13 T. Two different measuring circuits were employed. They are shown in Fig. 2 (circuit A, as used by Zieren and Duyndam [4] for signal processing of vertical magnetotransistors (VMTs)) and Fig. 3 (circuit B).

Sample devices were measured as MTs (bases con- nected to ground in Figs. 2 and 3) or as DCMTs (bases

Fig. 1. Layout of the device. Scale 1000 tzm=2 ~m (500:1).

short-circuited to collectors, dotted lines in the same Figures).

The measured mean values are shown in Tables 1 and 2, where the values of the magnetic sensitivity are computed as

M So = IoAB (1)

where M is the difference between the currents at each side of the structures, Io is the emitter current

Elsevier Sequoia

56

R

m

Fig. 2. Measur ing circuit A.

A' ~ s

Uo

----- t./o m- 4,t/2

[ . . . .

I J 4./'c

,,

at/e . . . . 7

i

Fig. 3. Measur ing circuit B.

T A B L E 1. Circuit A. Compara t ive results

MT D C M T (ma)

So Rel. sens. So Rel. sens. (1 +1//3) (T -l) Sosrr/SoMT (T -1) SODCMT/SoMT

2 0.79 0.018 1 0.045 2.50 2.26 5 0.61 0.024 1 0.066 2.75 2.56

T A B L E 2. Circuit B. Compara t ive results

10 MT D C M T (mA) So ( T - ' ) So (T - I )

2 0.0065 5 0.0166 0.0033

.9O

~o.

7 0

60"

5"0,

~o

AUo(,.v)

So .= 0 .o i , '2 / r

_ So _- O, O/46/T

5" IO

U~a (vo h's)

Fig. 4. Output voltage change AUo vs. UcB: 0, Rc = 3.6 k~; x, Rc=ll kl].

and z3d3 is the change of magnetic induction in Tesla (see Figs. 2 and 3).

Figure 4 illustrates the change AUo (mV) versus the collector base voltage, UcB, for an MT sensor, using circuit B for two values of the collector resistance Rc.

3. Discussion

Due to the low/3 values of the lateral MTs studied, the diode connection yields a greater change of current M. For bases wired to ground the change in base current is not useful, while when wired to collectors, both changes A/c and Ma are added. This implies that (see Fig. 2)

M = Mc(1 + 1//3) (2)

and also

SODCMT = Somt(1 + 1//3) (3)

This explains the results shown in Table 1, where the sensitivity of the diode connection surpasses that of the MT according to eqn. (3) to a first approximation.

Because these devices operate by current changes, a better approach could be obtained using the small- signal current gain, hfe instead of/3. Since the devices

are operated at high-level injection, hf~ </3 and a better fit between theory and practice is expected.

High values of B or offset voltages may change expression (3). The results of Table 2 show the influence of the measuring circuit. If these results are compared with the corresponding ones of Table 1, a lower sen- sitivity is observed in this case. Circuit B does not give adequate insulation between both sections of the dif- ferential device, because in spite of the insulation between the collectors, certain dynamic resistive cou- pling due to the Early effect still remains.

Moreover, for circuit A SODCMT~>SoMT, while the opposite is true for circuit B. This unexpected result may be explained taking into account the parasitic resistance between the bases, which is active only for the DCMT using circuit B.

Therefore, considering the MT and the DCMT as independent devices, a suitable circuit should be selected to obtain in each case the optimum sensitivity.

In Fig. 4 the output voltage change due to the presence of a magnetic field of 0.13 T upon an MT biased with Io = 5 mA is plotted using circuit B for different values of UcB. Using a greater Rc value increases the output voltage. However, this implies a greater potential im- balance at the base region, which forces a greater interbase current change, consequently lowering the measured sensitivity.

The effect is more pronounced, as was noticed ex- perimentally, if the collectors are left floating and measurements are taken with circuits A and B at equal current levels.

4. Conclusions

The results presented in this paper are valid irre- spective of the galvanomagnetic mechanisms deter- mining the measured sensitivities.

Circuit A, forcing a near-zero voltage between the collectors, decreases the resistive coupling between the electrodes, enabling a greater sensitivity to be developed by the differential sensor.

DCMTs give a greater sensitivity than MTs (circuit A), differing from usual magnetodiodes in that their functioning does not imply a high level of double injection and also that they are symmetric with respect to the direction of induction B.

References

57

1 H.P. Baltes and R.S. Popovi6, Integrated semiconductor mag- netic field sensors, Proc. 1EEE, 74 (1986) 1107-1132.

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

3 A. Mohaghegh, S. Cristoloveanu and J. de Pontcharra, Double injection phenomena under magnetic field in SOS films: a new generation of magnetosensitive microdeviees, IEEE Trans. Electron Devices, ED-28 (3) (1981) 237-242.

4 V. Zieren and B.P.M. Duyndam, Magnetic field sensitive multicollector NPN transistors, IEEE Trans. Electron Devices, ED-29 (1982) 83-90.

Biographies

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

Agnes Nagy graduated as a physicist from Havana University in 1971, and received an M.Sc. in micro- electronics in 1978. She has been an assistant professor at the Electrical Faculty of ISPJAE since 1977, and has worked as an assistant researcher at CIME since 1972.

Margarita Michelena graduated in electronic engi- neering from ISPJAE, Havana, Cuba in 1973, and received a Ph.D. in technical sciences in 1990. She has worked as an assistant professor at the Electrical Faculty of ISPJAE since 1978, and as an assistant researcher at CIME since 1976.

Pedro Rodriguez graduated as a physicist from Havana University in 1980, and received an M.Sc. in micro- electronics in 1988. He has been an assistant professor at the Electrical Faculty of ISPJAE since 1980, and an assistant researcher at CIME since 1984.

Rend Ugarte graduated in electronic engineering from ISPJAE, Havana, Cuba in 1987. He has worked at COPEXTEL, Havana, Cuba, since 1987.