74 IEEE ELECTRON DEVICE LETTERS, VOL. EDL-6, NO. 2, FEBRUARY 1985

Analysis of Insulated Gate Transistor Turn-Off Characteristics

B. J. BALIGA

Abstract-A model based upon a MOSFET driving a wide-base p-n-p transistor is presented for analysis of the turn-off behavior of n-channel insulated gate transistors. This model is found to provide a very good quantitative explanation of the shape of the collector current waveform during turn-off. Verification was accomplished using insulated gate transistors (IGT's) fabricated with two voltage ratings and a variety of radiation doses. This analysis allows the separation of the channel (electron) and minority carrier (hole) current flow in the IGT for the first time.

I. INTRODUCTION HE INSULATED gate transistor (IGT) is a recently

Tdeveloped power semiconductor device with a high input impedance and high forward conduction current density in the on-state [I], [2]. This device was created by the functional integration of the existing MOS and bipolar technologies to achieve the best attributes of both, A cross section of the device structure is shown in Fig. 1. In this device structure, the lower p+-region injects a high concentration of minority carriers (holes) into the n-type drift region when the device is biased in the on-state by an applied positive gate bias. This injected minority carrier density is typically 100 to 1000 times higher than the doping level of the n-type epitaxial drift region and reduces its resistance during forward current flow. In the on-state, the device current-voltage characteristics resemble those of a p-i-n rectifier. Consequently, it has been shown that the IGT operates at a forward conduction current density 20 times that of an equivalent MOSFET and 5 times higher than that of the power bipolar transistor.

The equivalent circuit of the IGT is shown in Fig. 2. It can be modeled as two coupled transistors (one p-n-p and the other n-p-e) which form a thyristor and a MOSFET that shunts the n-p-n transistor. To obtain gate-controlled device characteristics, it is important to defeat the parasitic thyristor from turning on. This is achieved by using the emitter shorts, as illustrated in Figs. 1 and 2(a). This letter discusses the turn-off characteristics of an insulated gate transistor under gate-controlled power switching conditions. Under these conditions, the upper n-p-n transistor can be assumed to be inactive, and the equivalent circuit can be modified, as shown in Fig. 2(b). This circuit consists of an n-channel MOSFET driving the base current of a p-n-p transistor. Since the p-n-p transistor must be designed to support high

Manuscript received October 29, 1984; revised November 26, 1984. The author is with General Electric Company, Corporate Research and

Development, Schenectady, NY 12345.

EHANNEL EPITAXIAL

LAYER

J2

P+ SUBSTRATE

Fig. 1 . Cross section of insulated gate transistor structure showing channel (I,) and minority carrier ( 4 ) currents.

, ' E t Y E , I I

rr ' c t d c

(a) (b)

Fig. 2. Equivalent circuits for IGT. (a) Including parasitic n-p-n transis- tor. (b) Excluding parasitic n-p-n transistor.

voltages, it has a large base width which results in a low current gain. Consequently, the electron and hole currents indicated in Fig. 2(b) are comparable in magnitude.

11. TURN-OFF CHARACTERISTICS AND MODEL Under forced gate turn-off conditions, the IGT current

waveform has been found to consist of two segments, as shown in Fig. 3(a). In this case, the gate turn-off circuit had a very low series resistance (< 10 a), It can be seen that the collector current drops abruptly to a fraction of the

0741-3106/85/0200-0074$01 .OO O 1985 IEEE

BALIGA: TRAKSISTOR TURN-OFF CHARACTERISTICS 7 5

c BP

COLLECTOR CURRENT

0

0

(b)

Fig. 3 . (a) Photograph of typical observed collector current (5-A peak) waveform of the IGT. Note the abrupt decrease in collector current at the end of the (25-V) gate pulse. The time scale is 1 1 s per division, (b) Illustration of collector current waveform with parameters defined for analysis.

steady-state value when the gate voltage is reduced to zero and then decays slowly. It has been hypothesized that this initial abrupt drop in collector current is due to the injection of electrons from the drift region into the p+-substrate [3]. This occurs at the early stages of turn-off due to the high concentration (typically 1017 per cm3) of electrons at the episubstrate interface. When the electron concentration decreases, the injection efficiency for electrons into the p f - substrate becomes low and further collector current decay occurs via recombination.

Another explanation for the observed collector current waveform has been postulated [ 2 ] without verification. This alternative analysis was based upon the equivalent circuit shown in Fig. 2(b). In this circuit, the IGT current consists of If,, the bipolar current based upon minority carrier transport through the base of the p-n-p transistor, and I,, the majority carrier current flowing through the channel of the MOSFET. When the gate voltage is reduced to zero during device turn-off, the electron current I,, flowing through the inver- sion channel of the MOSFET, is abruptly shut off. This would account for the observed abrupt drop in the IGT collector current during turn-off. Following this event, the stored minority carrier charge in the n-base continues to decay by recombination. During this phase, holes are collected by the reverse-biased upper junction of the IGT in the form of the current I h .

The validity of this model for IGT turn-off behavior can be

verified by examining the effect of collector voltage and n-base lifetime upon the turn-off waveforms. In order to perform this analysis, a simple analytical model for the gain of the wide-base p-n-p transistor is provided below. In this model,

I, = (1 - a)lc (1)

I h = arc (2)

where a is the common base current gain of the p-n-p transistor. Due to the large base width of this transistor, its current gain is controlled by the base transport factor:

CY= a ~ = llcosh (F) where W is the undepleted n-base width and L is the carrier diffusion length. The undepleted n-base width is strongly dependent upon the reverse voltage across the collector of the p-n-p transistor, i.e., the upper junction (Jl) of the IGT. It can be calculated by using

where d is the epitaxial layer thickness, is the dielectric constant, q is the electron charge, N is the doping concentra- tion of the epitaxial layer, and Vis the voltage across the IGT at turn-off. The carrier diffusion length L in (3) is dependent upon the lifetime at the high injection levels prevalent in the IGT during current conduction. This diffusion length can be obtained from the high-level lifetime (7):

L = G 7 ( 5 )

where D is the high-level (ambipolar) diffusion coefficient. The high-level lifetime (7) depends upon the processing conditions and can be controlled by using electron irradiation [4]-[6]. It can be directly obtained from the turn-off waveform by measurement of the time taken for the tail of the collector current to decrease from its initial value I' to ( l /e)I ' , as illustrated in Fig. 3(b). Using the above model and equations, it is possible to calculate the initial drop in current AIc as a function of the collector voltage ( VcE) at turn-off, the epitaxial layer thickness (d), and the lifetime (7).

111. EXPERIMENTAL VERIFICATION In order to verify the proposed model based upon the

MOSFET driving the p-n-p transistor, measurements of the IGT turn-off waveforms were conducted using a variety of devices under a wide range of collector voltages. The devices used in this study are listed in Table I. They consist of devices, with an active area of 6 x cm2, fabricated with two different epitaxial layer thicknesses and doping levels to achieve forward blocking capability of 300 and 600 V. Devices from each of these two categories were then subjected to 3-MeV electron irradiation at doses ranging from

16 IEEE ELECTRON DEVICE LETTERS. VOL. EDL-6, NO. 2, FEBRUARY 1985

A

B

C

D

E

F

G

H

1

J

K

L

-__ Breakdown

Vollase

(VI

300

300

300

600

600

600

300

300

300

600

600

600

TABLE I DEVICE PARAMETERS

90 1 . 3 ~ IO" 2

90 1 . 3 ~ 1 0 ~ ~ 4

58 4x10" 0.5

5 8 4 x 1 0 L 4 8

55 4riO" 16

90 1 . 3 ~ 1 0 ' ~ , 0.5

i I I

4 Iligh-lcvei

I Lifetime

I

!

0.9

0.5

1.9

1 .z

0.65

2.4

0.34

0.24

2.6

0.38

0.25

Alnbipoiar

Diffusion

Length

(gm)

40.0

30.0

22.4

43.6

34.6

25.5

49.0

18.5

15.4

51.0

19.5

15.8

0.5 to 16 MR to reduce the lifetime (7). Forced gate turn-off measurements of the collector current waveform were then performed for each of these devices using a steady-state collector current of 5 A under a broad range of collector voltages. The collector voltage at turn-off was changed while keeping the collector current at 5 A by changing the load impedance. Under these measurement conditions, the collec- tor voltage at turn-off rose abruptly to the specified value due to the inductance in the load. This is representative of typical turn-off conditions in power circuits in which the IGT is used.

The variation of the abrupt drop AIc (normalized to the steady-state collector current Icss) with increasing collector voltage is shown in Fig. 4(a) for the 300-V devices and Fig. 4(b) for the 600-V devices. In each case, curves for three devices, subjected to radiation doses of 1, 2, and 4 MR, are presented. The lifetime, obtained from the decay of the collector current during turn-off, for each of these devices is provided in Table I. From this lifetime, the ambipolar diffusion length can be calculated, and is also listed in Table I. Using this diffusion length and the epitaxial layer parame- ters given in Table I, the theoretical curves for (AIc/Iw) have been calculated using the equations provided in the previous section, and are included in Fig. 4 for comparison with the experimental data. According to the model, as the collector voltage increases, the gain ( a ) of the p-n-p transistor increases due to a reduction in the undepleted base width (W). This results in a reduction in the base drive current I,, required to support the collector current IC; this is exhibited as a reduction in AIc. The model also predicts that when the lifetime (7) is reduced, the gain (a) of the p-n-p transistor will decrease. This results in an increase in the base drive current (I,) required to support the collector

0 9 I i

0 8

0.7

0.6

0.5

0 4

0.3

0.2

(AI, 11, ,J

- 0 9 -

0 .8

- 0.7

-

0 6 -

0 5 - ( A I c / I c ss)

0.4 - 0 3

0 2 -

-

100 200 300 FORWARD BLOCKING VOLTAGE (VOLTS!

(a)

0 1

0

- 1MR (C

I I I I 0 100 200 300 400

FORWARD BLOCKiNG VOLTAGE (VOLTS)

(b) Fig. 4. Comparison of measured data and theoretically calculated curves

of the drop in collector current. (a) 300-V devices fabricated using 58-pm epitaxial layers. (b) 600-V devices fabricated using 90-pm epitaxial layers.

current, IC, which in turn manifests itself as an increase in AIc. Both of these features are observed in the experimental data. In fact, extremely good agreement between the theoret- ically calculated AI, and experimentally observed data is apparent in Fig. 4.

Further verification of the model was obtained by examin- ing the turn-off characteristics of devices irradiated to various doses, as illustrated in Fig. 5 . As the electron irradiation dose increases, the lifetime in the n-base is reduced, as provided in Table I. This leads to a reduction in the p-n-p transistor gain, which in turn manifests itself as an increase in AI, during turn-off. This behavior is clearly observed in Fig. 5 among the measured devices. Once again the close agreement between the model and experimental data allows its verification.

IV. CONCLUSIONS This paper provides verification of a model for the turn-off

characteristics of insulated gate transistors based upon a MOSFET driving a p-n-p transistor. The model has been experimentally verified using devices designed with two different epitaxial layer parameters over a broad range of collector voltages and n-base lifetime values. Using this

BALIGA: TRANSISTOR TURN-OFF CHARACTERISTICS 71

0 9

0 8

0 7

0.6

0.5

0 4

0.3

- THEORY 0 DATA

FORWARD BLOCKING VOLTAGE = 180V STEADY STATE COLLECTOR CURRENT 5 AMPERES

0 2 4 6 8 10 ‘ 2 14 16 18 2 0 ELECTRON IRRADIATION DOSE !,MEGARADS!

Fig. 5 . Effect of electron irradiation upon the abrupt drop in collector current for 300-V and 600-V IGT’s. The dots are measured data points and the solid line was calculated using the theoretical analysis.

model and the turn-off waveforms of the IGT, it is possible to experimentally obtain a separation of the channel (electron) current and the minority carrier (hole) current for the first time. In addition to providing an improved understanding of

the turn-off process in the IGT, the ability to separate the channel current and the minority carrier current provides an insight into the device physics which can lead to improve- ments in device design and performance.

REFERENCES B. J. Baliga, M. S . Adler, P. V . Gray, R. P. Love, and N. Zommer, “The insulated gate rectifier,” in Int. Electron Devices Meeting, Abstr. 10.6, pp. 264-267, 1982. J. P. Russel, A. M. Goodman, L. A. Goodman, and J. M. Neilson,

IEEE Electron Device Lett., vol. EDL-4, pp. 63-65, 1983. “The COMFET-A new high conductance MOS-gated device,”

B. J. Baliga, M. S . Adler, R. P. Love, P. V . Gray, and N. Zommer, “The insulated gate transistor-A new three terminal MOS-Controlled bipolar power device, ‘’ IEEE Trans. Electron Devices, vol. ED-31, pp. 821-828, 1984. B. J. Baliga, “Fast-switching insulated gate transistors,” IEEE Electron Device Lett., vol. EDL-4, pp. 452-454, 1983. M. F. Chang, G. C. Pifer, B. J. Baliga, M. S. Adler, and P. V. Gray, “25 amp, 500 volt insulated gate transistors,” in Int. Electron DevicesMeeting, Abstr. 4.4, pp. 83-85, 1983. A. M. Goodman, J. P. Russell, L. A. Goodman, C . J. Neuse, and J. M. Neilson, “Improved COMFETs with fast switching speed and high current capability,” in Int. Electron Device Meeting, Abstr. 4.3, pp. 799-82. 1983.