BMFET versus BJT in reverse bias safe operations

Microelctronics Journal 27 (1996) 243-250 Copyright O 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0026-2692/96/$15.00

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0026-2692(95)00093-3

BMFET versus BJT in reverse bias safe operations G. Busatto and L. Fratelli Department of Electronics Engineering, University of Naples, "FedericoH", Via Claudio 21, 1-8012'5, Naples, Italy. Tel: +39 81 7683539. Fax: +39 81 5934448

E-mail: [email protected]

An experimental comparison between the RBSOA performances ofa BMFET and two familiesof power BiTs is presented. All the measurements were performed by means of a non-destructive tester which allowed the acquisition of a homogeneous set of measurements, keeping stray parameters under control. The behaviour of the devices is put in relation to the test conditions and it is shown that the size of the safe region of operation is a function of the geometrical parameters of the device, among which there are metallization lay-out and geometry of its elementary cell. Moreover, superior RBSOA performance of BMFET due to geometry of base diffusion, which was predicted by simulation, is experimentally confirmed. 1. Introduction he conductivit3r modulation o f the drift T region o f a vertical J F E T is used to make power bipolar devices, such as the BSIT [1, 2] and the bipolar m o d e J F E T (BMFET) [3, 4], which show very fist switching performances together with on-re,;istance m u c h lower than that observed on tlhe other m o d e m power devices: BJT, I G B T and M C T [5]. These devices can be considered good competitors for the other bipolar devices, particularly in highvoltage, m e d i u m - p o w e r applications. O n the other side, performances in reverse bias assume a paramount: relevance in the overall

evaluation o f a power device, as the p h e n o m e n o n o f secondary breakdown can strongly limit the employment o f the device in switching applications. T h e study o f the reverse bias safe operating area (RBSOA) of power BJTs has recently been extended to new generation devices like cellular BJTs [6, 7], which show some improvement in R B S O A , compared with BJTs o f classical design. Recendy, the R B S O A o f the B M F E T has also been investigated [8], which proved the very good reverse bias performances of such a device. However, the data available in the literature show a dependence o f the p h e n o m e n a involved during device secondary breakdown on the unavoidable parasitic elements o f the tester used for the measurements [9], therefore the comparison between device performances becomes awkward if it is not performed on homogeneous measurements. T h e aim of the paper is to present an experimental comparison between the behaviour of the two devices during the switching on inductive loads in both safe and unsafe conditions and to show the different behaviours

243

G. Busatto and L. Fratelli/BMFET versus BJT

with relation to the testing conditions. This study allows not only a comparison of device performances, but also gives insights on the physical basis of the phenomena linked to reverse bias secondary breakdown and to parameters of technological origin, which influence their behaviour.

2. The experimental set-up and tested devices

A simple non-destructive R B S O A tester was used to measure the switching characteristics of the devices during the turn-off under inductive load conditions. In this tester, a protection circuit was able to remove the energy from the device as secondary breakdown occurred, thus avoiding damage to it and allowing an entire set of measurements to be performed on the same device sample. The basic schematic of the R B S O A tester is reported in Fig. 1. The circuit is suitable for measuring up to a collector current of 20 A and a collector voltage of 1500 V. Special care has been taken in the design, for reducing the value of stray parameters to a controlled value. These parameters are expressly: (1) equivalent capacitance which is parallelled to the collector and emitter of the device under test; (2) forward recovery time of diodes in the clamping branch; and (3) stray inductance of the same branch.

....... fSl

"

m

DI Vcc

L

Fig. 1. Basic schematic of non-destructive tester used to perform the measurements.

244

In the quiescent condition, the switch $1 is open and the base of the device under test is reverse biased. Diode D2 is reverse biased all the time $2 is open, as the dc voltage of the generator VCLAMP is always greater than the collector voltage of the device under test. Then switch $1 is closed, and the transistor under test (TUT) is saturated, forward biasing the gate, so that the inductor is charged by a Vcc dc voltage generator. As the inductor current reaches the value for the test, the T U T base is reverse biased, and at the same time switch $1 is opened, in order to insulate the T U T from the generator; from then on, current flows into the loop composed of the freewheeling diode D1, the inductor and the T U T , so that current remains almost constant during storage time. Finally, switch $2 is turned on with a manually adjustable delay from the start of the test. It acts like a crowbar, so that D2 becomes forward biased and the output port of the T U T is shunted: so, if some instability occurs, destructive dissipation is avoided. The delay is initially set so that the crowbar is turned on during storage time of the T U T . Then delay is progressively increased and the test repeated; at the end, the crowbar operates just after secondary breakdown occurs in the T U T , removing energy. Leakage current is systematically measured after tests, in order to exclude degradation of electrical characteristics of the device, which could affect results of subsequent measurements.If VCLAMP is set at a value which is lower than the maximum voltage which can be withheld by the device, this branch operates as an effective clamping branch, limiting device voltage. However, unclamped tests give more information than clamped ones, as they are less influenced by interaction with the circuit, so only unclamped tests will be presented in this paper. In such a circumstance, VCLAMP is used to reverse bias diode D2, during all the time the

Microelectronics Journal, Vol. 27, Nos 2-3

crowbar branch is inactive. The small capacitance of the reverse biased diode is put in series with the series of the output capacitance of the three power MOSFETS, which physically constitute switch $2. That way the equivalent capacitance parallelled to the output port of the T U T is reduced to wdues of the order of 50 pF. All the tests were performed on a single sample of each device, so avoiding the spread of the results due to differences of technological origin among the various specimens. The behaviour of two different families of BJTs with different lay-out geometries [10] was compared with the BMFET. All these devices show comparable blocking voltages and current handling capabilities. The BJTs of the first family are switching devices with a classical lay-out design. In the following, they are referred to as SWBJTs (switching BJTs). The other BJT belongs to a more modern switching device family, specifically designed to show improved RBSOA performance. They have a 120 #m wide cell and they are referred to as INB3Ts (interdigitated BJTs). Finally, the BMFET is intrinsically a cellular device with the external contacts obtained by a double layer metallization technology. In Table 1, the geometrical and electrical parameters of tested devices are summarized. It is worth noting that the devices' die sizes are different so that the comparison will be performed in terms of mean current density, computed as Ic/(chip area).

3. Discussion

In Fig. 2 the turn-off waveforms for a BMFET under unclamped inductive load conditions are reported in the case of safe commutations for different values of the extracted base current. The test conditions are:/Cin (collector current at voltage rise) = 4 A, IBv (forward base current)=0.8A and I B p , (reverse base current) = 0 . 6 A , 2A and 4A, respectively. The figure shows that, under these conditions, the voltage rises up to a value called the sustaining voltage, VSUST, and remains almost constant for a long interval while the device is avalanching. During this time, the collector current, reported in the figure only for the case of IBR=0.6A, decreases almost linearly from Icin to zero. When the collector current approaches the value of IBR, the emitter current, not reported in the figure, becomes zero and the device behaves like a vertical PIN diode with the voltage across it limited by its primary breakdown, WIN. Figure 2 also shows that, at fixed Icin, the value assumed by the sustaining voltage increases by >Z ., 1ooo ~ s00 o =

I IBR=4A

- =. .

I .

.

I .

Z

.

8 =

e~

-

2~ -1 08

200

-~ 0 8 .A

-0.5

I

I

I

0

0.5

1.0

1.5

T I M E [}.ts]

Fig. 2. Current and voltage waveforms during the turn-off ofa BMFET on inductive unclamped load: safe turn-off.

TABLE 1 Devicesparameters Device

Chip area (cm2)

Blockingvoltage (V)

Cell pitch (/am)

Emitter width (/am)

BMFET SWBJT INBJT

0.25 0.25 0.40

~1050 940 890

50 -120

4 220 80

245

G. Busatto and L. Fratelli/BMFET versus BJT

changing the extracted base current, InR, from 0.6A up to 4A, and VSUST=VwN when IBR=4A. This behaviour indicates that the sustaining condition can be considered as a quasi steady-state phenomenon characterized by the dynamic equilibrium between avalanche generated pairs of carriers and holes extracted through the base terminal [11]. As IBv. increases, a greater number of pairs must be generated to balance holes extracted; hence, the peak electric field increases and generally the collector voltage also does. Besides the safe behaviour of Fig. 2, the bipolar power transistors show many unsafe conditions which can be summarized in the waveforms of Figs. 3-5. All of these latter waveforms are characterized by the presence of a secondary breakdown phenomenon, i.e. a very fast collapse of the collector voltage to a relatively low value depending on the test conditions. As secondary breakdown occurs, collector current evolution depends only on external circuit: in an inductive circuit, it slowly increases and may cause the destruction of the device within a few tenths of nanoseconds, if a protection circuit is not used. In the plots of Fig. 3, the secondary breakdown takes place after a sustaining condition is reached. In Fig. 3a, which refers to a BMFET, it takes place right after the collector current becomes equal to the base current and the collector voltage reaches VVIN; the test conditions are: I c i n = 6 A , I B F = I . 2 A and IBR= 2.5 A. The behaviour of Fig. 3b was observed on the INBJT under the test conditions: I c i n = 6 A , IBF = 1.2A and IBv.= 1.0A. It is characterized by a secondary breakdown which takes place during the sustaining phase when the collector current is still larger than IBv.. In the waveform of Fig. 4, the secondary breakdown develops before a sustaining condition is reached and, hence, when the

246

(a) 1200 ~" 1000 ~ 800 "~ 600 ~8 "~ 200 0 -200

0

200 400 Time [ns]

600

800

0

200 400 Time [ns]

600

800

(b) 1200

E looo ~ 8oo

o -200

Fig. 3. Voltage waveforms during the turn-off of (a) a BMFET and (b) an INBJT, on inductive unclampedload: secondarybreakdownduring sustainingvoltagephase. collector current (not reported in figure) is still approximately equal t o I c i n - The behaviour refers to a SWBJT in the test conditions: Icin= 4A, IBv = 0.8A and Imp= 2.0A. The waveform of Fig. 5, obtained on the BMFET under the test conditions Icin= 6 A, IBF = 1.2 A and IBv. = 5.0 A, is similar in shape to the one of Fig. 4. The difference is that the secondary breakdown voltage is equal to VViN. This latter situation is less dangerous than the previous ones: in fact, when the device operates in an inductive clamped circuit, the external circuit clamps the voltage before WIN is reached. The kind of behaviour exhibited by the devices is a strong function of the operating conditions,

Microelectronics Journal, Vol. 27, Nos 2-3

(a)

1200

E

I000 800

© >

600 400 PIN

200 ©

0 -200

0

200

400

600

800

TIME [ns]

Fig. 4. Voltage waveforms during the tum-offofan INBJT on inductive unclamped load: secondary breakdown during voltage rise. 1200 ~" 1000 800 0>

600

"~

400

~ m

200

0 rj

0 -200

0.4

0.6

0.8

1.0

(b)

_/ 0

0.2

1/(reverse currentgain) - IBR/Ic~.

48 gE 40

~ 32 •~r- 24 200

400

600

800

"~ J6 r-

TIME [ns]

Fig. 5. Voltage waveforms during the tum-offofa B M F E T on inductive unclamped load: secondary breakdown at

0

0.2

0.4

0.2

0.4

0.6

0.8

1.0

0.6

0.8

1.0

VpIN.

namely collector current and reverse base current; instead the: value of forward base current, IBv, does not affect device behaviour, provided that it is enough to saturate the device; hence, all the measurement were performed keeping IC/IBF = 5. In Figs. 6a, 6b and 6c the maps of the behaviour, as a function of the test conditions, are presented for BMFET, INBJT, and SWBJT, respectively. The abscissas correspond to the reverse base current normalized to Ic (IB~ Ic = 1~fiR, fie being the reverse forced current gain), and the ordinates are the collector current densities computed with respect to the chip area. Each region of the figures, coloured with a different

(c)

48 eq

40

~ 32 ~

24

r-

0

IB~/Ici.

Fig. 6. Maps ofbehaviours for (a) BMFET, (b) INBJT and (c) SWBJT as a function of collector current and

llflp.

= I~R/Sc.

247

G. Busatto and L. Fratelli/BMFET versus BJT

grey tone, indicates a different behaviour described by the corresponding inset. In particular, the regions marked with (a) correspond to the safe conditions, whereas the regions marked with (b), (c) and (d) are related to the behaviours of Figs. 3, 4 and 5, respectively. It is worth noting that the regions marked with (b) for the BMFET always correspond to the behaviour of Fig. 3a, whereas within region (b), the behaviour of INBJT and SWBJT gradually varies from that in Fig. 3a to that in Fig. 3b, as IB~dlc= 1~tip, increases. Moreover, the duration of the sustaining condition within region (b), for INBJT and SWBJT, is a function of the reverse base current and reduces as IBR increases. A full description of all mechanisms is reported in [12]. The figures show that the behaviour of all devices becomes less safe as either the collector current increases or IBR increases. The performance of the BMFET is much superior to the BJT's in all the ranges of the mean current densities. In effect, the maps of the two BJTs are very similar in shape even though the width of region (c) of the SWBJT is much larger than that of the INBJT. For both of them the safe region (a) is confined to very low values of IBR and the region where the behaviour becomes of the kind (d) shrinks as Ic increases, being wider for INBJT. Moreover, neither of the BJTs shows a totally safe behaviour, even at a current density as low as 5 A/cm 2. Instead, the behaviour of the BMFET is totally safe up to a current density of 24 A/cm 2 and also remains safe for higher current densities but for low values oflBp,/Ic. The width of region (d) in the BMFET is almost independent of the current density and the behaviour becomes of kind (d) for IB~Ic > 0.6. Finally, the behaviour of the kind (c), which primarily limits the RBSOA of the device in an inductive clamped circuit, is shown by BMFET for currents higher

248

than 30 A/cm 2, whereas it is observed on INBJT for currents higher than 10 A/cm 2. The comparison between Figs. 6a, 6b and 6c also shows that the shape of region (c) for the BMFET is geometrically similar to the corresponding SWBJT and INBJT. Such a shape is consistent with a current crowding phenomenon which can excite the instability and trigger the secondary breakdown in the device [13]. The lower border of the region is similar to a hyperbole, which is consistent with the fact that, as the collector current increases, a lower IBRJlc, and then a lower crowding, is required to trigger the secondary breakdown. Many causes conspire to cause the crowding of current inside a device, i.e. metal lay-out, pad lay-out, the geometry and physics of the elementary cell, and so on. It is hard to identify the contributions of each cause to the total crowding, particularly regarding the lay-out as it is related to many geometrical parameters, so a comparison between the devices based on their lay-out becomes very difficult. Ifa comparison is made by focusing attention on the elementary cell of the devices, the BMFET shows interesting properties which can be invoked to explain its better performances. In Fig. 7, for the sake of comparison, the elementary cell of the BMFET (a) is sketched together with one-half of the elementary cell of the INBJT (b) on the same scale; the geometry of the SWBJT is not reported as it is even larger than the latter. At first glance, the cell-pitch of the BMFET is about one-third that of the INBJT and the width of its emitter is much smaller, as is outlined in Table 1. Moreover, the geometry of the BMFET base region helps to reduce crowding. In effect, its small base underneath the emitter is obtained by lateral diffusion resulting in a lightly doped region which ensures a good emitter efficiency; the remaining part of the base is

Microelectronics Journal, Vol. 27, Nos 2-3

Emitter N"

4. Conclusions

An experimental comparison between the R B S O A performances o f a B M F E T and two families o f power BJTs was presented along with a variety o f exhibited behaviours. O n the basis o f their dependence on the test conditions, it was proposed that the size o f the safe region o f operation is a function o f some geometrical parameters o f the device, including metallization lay-out. It was also proposed that the particular shape o f the base region o f the B M F E T helps in limiting crowding o f the current inside the elementary cell and then makes R B S O A performance o f the B M F E T m u c h superior to that o f the BJT.

a),- BMFET Emitter N"

References b)

-

INBJT

Fig. 7. Comparison between BMFET elementary cell (a) and one half of the [NBJT elementarycell (b). heavily doped in order to optimize the transport parameters inside tl:le base [14]. Hence, the B M F E T shows an internal base resistance m u c h lower than the INBJT's, where the doping o f the base cannot be so high in order to avoid unacceptable emitter efficiencies. The above considerations were confirmed by numerical 2D simulations [15] and can be invoked to explain the better performances o f the BMFET. /

O n the other hand, while the B M F E T emitter width is only 4/lm, its length is 200/lm, so that its aspect ratio becomes very unfavourable and crowding o f the current can still take place in the direction perpendicalar to the cell. This mechanism was proposed in [8] to be the main cause o f secondar/ breakdown and was attributed to an improper design o f the metallization lay-out. Then, for the BMFET, the lower border region (c) can be improved further by a proper design ofmetallization.

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G. Busatto and L. Fratelli/BMFET versus BJT

[9] D. W. Beming, Semiconductor measurement technology: a programmable reverse-bias safe operating area transistor tester, N I S T Special Publication 400-87, August 1990. [10] L. Fratelli, G. Busatto, P. Spirito and G. Vitale, Analysis of second breakdown limits in RBSOA of bipolar transistor, Proc. 5th EPE Conf., Vol. 2, Brighton, UK, Sept. 1993, pp. 101-106. [11] L. Fratelli and G.F. Vitale, On the reverse bias safe operating area of power bipolar transistor during inductive turn-off, Solid-State Electron., 37(2) (1994) 275-287. [12] G. Busatto, G. Vitale and L. Fratelli, Instabilities in modem bipolar transistor during the turn-off under

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inductive loads, Proc. MIEL'95, Nis, Serbia, Sept. 1995, pp. 391-396. [13] B.A. Beatty, S. Krishna and M.S. Adler, Second breakdown in power transistor due to avalanche injection, IEEE Trans. Electron Devices, ED-23(8) (1976) 851-857. [14] S. Bellone, G. CocomUo, G. Fallica and S. Musumeci, Current gain enhancement effect by gate doping in Bipolar Mode Field Effect Transistor (BMFET), Electron Device Lett., EDL-37(1) (1990) 303-305. [15] L. Fratelli, G. Vitale and P. Spirito, Analysis of stress in power bipolar devices during inductive turn-off, Pr0c. ISPSD '93, Monterey, CA, May 1993, pp. 171-176.