On the reverse bias safe operating area of power bipolar transistors during inductive turn-off

~ ) Pergamon

Solid-State Electronics Vol. 37, No. 2, pp. 275-287, 1994 Copyright © 1994 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0038-1101/94 $6.00 + 0.00

O N T H E R E V E R S E BIAS S A F E O P E R A T I N G A R E A O F P O W E R B I P O L A R T R A N S I S T O R S D U R I N G I N D U C T I V E

T U R N - O F F

L. FRATELLI and G. F. VITALE

Department of Electronic Engineering, University of Napoli, via Claudio 21, Napoli, Italy

(Received 24 May 1993; in revised form 27 July 1993)

Abstract--The turnoff transient of power bipolar transistors, under inductive load, has been analyzed by a 2D numerical simulator in the region where impact ionization is present, with the purpose of investigating the factors which limit their reverse bias safe operating area (RBSOA). Results show that the geometry of the device elementary cell, as well as its doping concentration profile have a strong effect on the maximum voltage the device can withstand at high currents and on the stress the device undergoes during turnoff. The steady-state (or sustaining) voltage locus, calculated while the device is avalanching, allows to understand a basic reason for the decrease of maximum voltage at high currents, which is observed in the RBSOA of some devices, and allows to predict the shape of turnoff voltage waveforms, that are experimentally observed in recently developed devices. The use of power density maps allows to identify the basic differences among the different device structures, in terms of power concentration resulting from current crowding and impact ionization effects.

D E /r, dc

&c

~c N~ P po q u

~o v~ Voe, sus

Vs Xdp

/~r

NOTATION

ambipolar diffusion length electric field semiconductor band gap average collector current density: ratio of terminal collector current to collector area of elementary cell collector current density in a one dimensional (1 D) model conduction current density epilayer donor density volume power density peak free hole density in the base, 1D model electronic charge net recombination-generation rate open-emitter, collector to base breakdown voltage collector voltage sustaining voltage: stable collector voltage in avalanche condition, with base reverse-biased electron saturated drift velocity boundary between the drift and plasma regions, 1D model reverse gain: ratio of collector current to reverse base current semiconductor dielectric permittivity

l. INTRODUCTION

Reverse Bias Safe Operating Area (RBSOA) represents a critical constraint in the operation of power devices, when they are turned off under inductive load. Bipolar Junction Transistors (BJT) are known to suffer a severe limitation in their RBSOA, due to the so-called "second Breakdown" (SB) which causes their maximum voltage to decrease rapidly as current increases, thus drastically reducing their performance. Second Breakdown which, in principle,

does not affect the operation of unipolar devices, is a major drawback of BJT as compared with power MOSFET.

It is generally recognized that, while thermal effects determine the BJT failure under forward bias[l], avalanche injection, described for the first time by Hower and Reddi in [2], is the primary cause of failure in reverse bias. The mechanism of avalanche injection[2], is the following: the combined effects of space-charge limited current and impact ionization, cause a current-controlled negative differential resistance in the output characteristics of the BJT. Negative resistance causes current filamentation which in turn causes a strong localized heating and eventually the device failure at a voltage substantially reduced with respect to the constant- power value. Avalanche injection is an extremely fast phenomenon, in that it is produced by an internal positive feedback of electrical nature. It causes the device collapse in times of the order of a few nanoseconds.

While avalanche injection requires fairly high current densities to develop, actually it limits the operation of BJT also at relatively low collector currents, owing to the non uniform distribution of current across the device area. An elementary origin for a non uniform current distribution, is the layout of multicellular devices, which may cause a non homogeneous distribution of current among the paralleled elementary cells, due to potential drops along the emitter metallizations or on buses. Layout effects do not set any fundamental limit to a device's

275

276 L. FRATELLI and G. F. VITALE

performance (although they can be quite complicated to avoid) and will not be discussed here. Instead, if a single device cell is considered, non uniform current distribution originates from two-dimensional (2D) effects, which are responsible of lateral base depolarization and current crowding during BJT turnoff.

The correlation between device structure and second breakdown is difficult to investigate on experimental basis only, due to the destructive nature of the experiments and to the difficulties to separate the different phenomena which contribute to the onset of negative resistance. Moreover, in early power BJTs[3], failure always occurred during the voltage rise thus increasing the uncertainties of the results and at the same time confirming the concept of an intrinsic, unavoidable instability, associated with the turnoff at high currents.

More recently, BJTs have been realized that exhibit a full RBSOA also at high currents. In these devices, failure may occur after they have reached a stable, high-voltage plateau: the so-called "sustaining voltage" Vc ..... [4] thus indicating that avalanche injection in itself neither produces necessarily an instability nor causes the device failure directly. Nevertheless, dissipated energy is too small to be directly the origin of device collapse after V c ..... has been reached. It has also been demonstrated[5] that in modern BJTs V~ ..... can be very close to Vcbo, under suitable turnoff conditions, and that it is strongly affected by base drive circuit.

Numerical simulation is a very effective means for the understanding of these phenomena, as it allows to separate or couple effects of involved processes, in a way which is not feasible experimentally. After the fundamental paper by Hower and Reddi[2], in which it was demonstrated, on the basis of experiments and of a first-order model, that avalanche injection in BJT is basically the same effect which occurs in a n +nn +

diode, quantitative ID models of the current- controlled negative resistance characteristics of n +nn + structures were developed in [6,7]. Mank and Engl[8] demonstrated the 2D current constriction, as the BJT gets out of saturation at constant base-collector voltage. In [9], lateral current constriction and longitudinal effects were correlated by coupling two 1D models, in steady-state. In [10], the possibility of a collapse due either to avalanche injection or by high temperature effects was discussed, by using a model that includes both impact ionization and thermal effects on current transport. However, the assumption of a constant terminal voltage does not allow to extend these results to operation with inductive load.

Turgeon and Navon[ll] also included thermal effects in a 2D model along with inductive load conditions, to calculate the maximum device temperature while the device is avalanching. The zero-voltage base bias, used in this model, causes a moderate current crowding while, as discussed in [12], this effect has a primary role in determining

the conditions for second breakdown. In [12] it was also demonstrated that, under base reverse bias, current density decreases in going from base to collector. Thus the voltage at which second breakdown occurs is actually greater than predicted by I D models. The model in [12] did not include impact ionization and, therefore, was unable to calculate the maximum device voltage. The effects of more complicated circuit conditions have been included in [13] with the purpose of investigating the effects of base drive on current crowding, but the analysis was limited to voltages below the onset of impact ionization.

This paper is aimed at clarifying the behaviour of power bipolar transistors in inductive turn-off, with the help of the 2D device simulator TMA- MEDICI[14], which allows to calculate the turnoff transients, under realistic drive circuit conditions, by including a self-consistent model for impact ionization, as well as band-gap narrowing effects, spatially varying mobility and recombination.

A comparison is made between a classical BJT and devices which are either designed with a fine emitter linewidth or with the epilayer doping profile suitably tailored. Results show that the detailed analysis of the field-carrier interactions while the device is avalanching is sufficient to explain many of the recently observed phenomena, including the full RBSOA exhibited by BJTs of recent design even at high currents, and the dependence of sustaining voltage upon base drive conditions. If a map of the power density, within the elementary cell is obtained, it is also clear that a thermal effect can be the origin for BJT failure after the device has reached a stable avalanche condition[4]. As a result of this analysis, a better understanding of the reasons for the improved performance of "new" BJT structures is achieved, and a tool for the design of improved devices is supplied.

2. SIMULATION OF THE STRUCTURES

Figure 1 displays the doping profiles of all structures that will be discussed in this paper. The first structure is a standard 700V device, referred to in the following as: EBJT (Elementary BJT). Its cell width (distance between the centers of emitters) is 240#m, the epilayer is 50#m thick, and has an essentially constant doping concentration N D = 2 × 1 0 1 3 c m -3 except for a thin, I0/~m, 10~4cm 3 doped layer close to the epi-substrate interface.

The second structure, referred to as: BLBJT (Buffer Layer BJT), differs from the preceding one as it includes a 25 #m deep, 10 ~5 cm -3 doped buffer layer, as shown in the same Fig. 1. The cell geometry is the same as EBJT, the epilayer thickness has been adjusted to obtain a voltage rating similar to the other structures.

Reverse bias safe operating area 277

!

o=

o

Fig. 1.

l O W ' ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' =

,o,.- r / ] 10'" l / ]

,o" I..--4 4 /,-_'._.i I0 l~' CBJT 1

1 0 TM I , , , I , , , , ~ , [ , * , - 0 2 0 4 0 6 0 8 0 100

0,m)

Doping concentration profiles at the center of the cell, for the three device structures.

The third structure, indicated in the following as: CBJT (Cellular BJT) has a 50/~m wide cell. Epilayer thickness is 50pm, epilayer doping is constant (2 x 1013cm-3). No buffer layer is present into this structure, which is intended to understand the effect of a fine linewidth geometry.

Simulations have been carried out on one-half of the cell because of symmetry; turnoff transients corresponding to an unclamped RBSOA test, with inductive load, have been simulated. This mode of operation, in which the device undergoes maximum dissipation, as full current and voltage occur at the same time, enhances all phenomena related to device's stress during turnoff.

Figure 2(a) displays the voltage and currents at the terminals of the CBJT structure. Circuit is reported in Fig. 2(b). The initial conditions correspond to deep saturation for the device, at a current density, Jc = 80 A/cm 2. Then, a reverse voltage is applied to

BOO

& 600

?. 400

Z 200

e, 0

( a )

_ ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' _~ ;~ BO

" % " 60 ~

4 o ~

zo ~

0 u

" ' " . . Jo V,,

....... L ~ ............... ~ , ~

N t - - , - . - - - , , I , , , I , ~ 1 , , , I , ,

2 4 6 8 T i m e ~us )

10

(b)

T ' T '

/ !L

l V~r

Fig. 2. (a) Turnoff waveforms for a CBJT structure. Jc = 80 A/cm 2, initial fir = 5. (b) Circuit used in simulations.

the base contact by a driving circuit consisting of a 5 V voltage generator and a resistance such that the reverse gain f i r = 5 at the beginning of the turnoff. This is a standard way to bias the device in a RBSOA test.

After the storage time, the device experiences a very fast voltage increase at constant collector current and slightly decreasing base current, up to the point A where the so-called "sustaining voltage"[5,15] V c ..... =530V is reached. Rise time of collector voltage is about 50 ns in this phase of the transient.

After point A, the inductor discharge causes a linear decrease of the output current, while the base current is still constant, so B, decreases. This phase of the transient is relatively slow as compared to the first one. Its time evolution is determined by the inductance value. The sustaining voltage increases slightly in this phase while an increasing fraction of the collector current flows through the base terminal. At point B fir = 2 and Vc ..... = 600V. As fir= 1, at point C, all current is diverted into the base and V= .... equals the open-emitter collector to base breakdown voltage (Vcbo = 750 V). The final part of the voltage waveform, from point D onward, corresponds to the voltage decrease across the inductor with the device operating as a P - I - N diode.

In order to gain understanding of the basic phenomena involved with the inductive turnoff described above, in Section 3 the first phase of the transient (up to point A in Fig. 2) will be approximated by imposing a fixed collector current while the base circuit is still composed of a bias voltage and a resistor. In this way, the basic operation of the device is made clear, by avoiding the complications due to the interactions between the device and the output circuit. Then, the second phase of the transient, from point A to C, will be easily understood from the steady-state operation of the device, as shown in Section 4.

3 . T R A N S I E N T A T C O N S T A N T C U R R E N T

This Section discusses the transient analysis of EBJT structure for various (constant) collector currents. Figures 3 and 4 display the transient evolution of hole density and field, for a collector current density Jc = 80A/c m2 and fir = 5, at the beginning of the transient. During the transient the base current decreases slightly, with a corresponding slight increase of the effective fir which, however, has a second-order effect on transient evolution.

Figure 3(a, b) display the hole distributions for different times along two vertical sections of the device: on the symmetry axis [Fig. 3(a)] and at 70 gm from the center [Fig. 3(b)]. On the same plots, dashed lines indicate the net impurity concentration. Figure 3(c, d) display the magnitude of the electric field along the same sections and at the same times.

SSE 37/2--E


3

(a) 20k ' ' ' I ' ' ' I ' ' '

1819 ' '~; dopin.g. . . . 2:.

17 ~ " 356 ns i - -

15

13

0 20 40 60 (/~m)

) 3

( b )

zo)~,, , l i ' ' I ' l ,

19 ~ \ dop).nj[..., yE_

-

14 ..i _

12 I ~ I 0 20 40 60

(Urn)

2xlO 5

10

o

(c) , ' ' I ' ~ I ' '

k o r

20 40 (~m)

60

Z×10 5 ( d ) , , ) , , , I ' i

"~ 10 5

400 ns

356

334 n ~

0 ~ ~ 7 8 1 n s ' ~

20 40 60 (~,m)

Fig. 3. (a) Hole distributions along the emitter centerline and (b) at 70/~m from the center. (c) Electric field along the emitter centerline and (d) at 70 pm from the center. Con-

stant current transient, EBJT structure.

Figure 4 displays electron and hole distributions, along a horizontal line at a distance of 10 p m from the top surface of the cell where the highest current density occurs for this device [see Fig. 9(a)].

At the beginning of the transient, electrons and holes are evenly distributed across the cell, as seen from Fig. 4. As transient goes on, current crowding

effect develops: lateral voltage drop causes a partial reverse bias of the base and a lateral confinement of the electron-hole plasma toward the center of the emitter (see Fig. 4). At the same time, plasma concen- trates longitudinally toward the base junction, thus reducing the thickness of the current-induced base and further enhancing the base depolarization effect. This cumulative effect is clearly seen in Fig. 4 as a fast movement of the edge of the plasma region towards the center of the cell. Lateral confinement of the plasma causes most of the current to flow near the center of the emitter, so current density at the edge of the cell becomes smaller than the space-charge limited current density, and decreases with time as current crowds toward the center of the cell. For this reason, the longitudinal electric field at the edge of the cell, Fig. 3(d), changes its slope from positive to negative values during the transient, causing the field to peak at the base-epilayer junction.

In the middle of the cell, instead, current density is larger than space-charge limited current and increases with time, due to the lateral current constriction. Electric field slope is positive [Fig. 3(c)] and its peak is located at the epilayer-collector interface. Field peak increases with time and, as it becomes of the order of 10SV/cm, impact ionization becomes considerable: avalanche-generated holes are acceler- ated toward the base side of epilayer and accumulate at the base junction near the center of the cell only, where a plasma region develops again. Some reduction in the electric field slope takes place as avalanche-generated holes compensate partly the electron density within the space-charge region of the epilayer, but the peak field at the interface between epilayer and substrate, still increases with time, thus further increasing the hole generation rate. At the edge of the cell [Fig. 3(d)], field peaks at the base junction, and avalanche-generated holes are removed through the base without accumulating within the epilayer [Fig. 3(b)].

Eventually an equilibrium is reached between avalanche-generated and extracted holes, and the transient ends at a stable avalanching condition. This steady-state voltage is the sustaining voltage V~ .... and corresponds to point A in the curve of Fig. 2.

loll l ' III l I ' l ' I I' l [' ' l I l' ' I .......... holes ~ electJ 'oh$

17 ............. doping

g~ .1

14 0 20 40 60 BO 100 120

Dis tance (pro)

Fig. 4. Electron and hole distributions along a line at 10 ,um from the top surface. Constant current transient, EBJT

structure.

Reverse bias safe

$-- 19

o= 17 16

(a)

- - e l e c t r o n s I

. . . . . . . . . . . . h o l e s

I

k,

12 I ~ , I ~ , , I , 2 0 4 0

D i s t a n c e O.~m)

i

6 0

o .,**, -g

(b) ' I ' ' ' 1 ' '

0 I 0 2 0 4 0

D i s t a n c e ( /~m) 6 0

17

(c) '~ I ' ' 1 ' ' ' I ' ' ' 1 ' ' ' 1 ~ ' '

- - electrons . . . . . . . . . . . holes

- - 320 A / e m ' . . . . . . . . . . . . . doping

....... ")?:, .......................................

0 2 0 4 0 6 0 8 0 100 120 D i s t a n c e ( # m )

Fig. 5. (a) Electron and hole distributions along the emitter centerline. (b) Electric field along the emitter centerline. (c) Electron and hole distributions along a line at 10 pm from the top surface; Jc = 80, 320 A/cm 2, V= = 150 V and #, = 5.

V=.su s is uniquely determined by the device geometry and doping profile, and represents an absolute limit for BJT operation for each couple of J¢ and fir values.

From this point on, in real circuits the inductor discharges through the device and collector current goes to zero. Of course there is no guarantee that the device can withstand the energy involved in this process. As it will be seen in Section 5, power concentration may be so high that the device is destroyed during the transient before it reaches the sustaining voltage.

The mechanism of current shrinking toward the center of the emitter is a well known phenomenon, caused by reverse bias of a portion of the base junction, as a reverse current flows within the base. Being the result of lateral voltage drop within the base of the device, current crowding is a function of both the current extracted through the base and of base resistance. Figure 5(a) displays the distributions

operating area 279

of electrons and holes on the centerline of the cell during the transient, for two different currents at the terminals, J¢= 80 and 320A/cm 2, and at times corresponding to the same collector voltage /I= = 150 V. The corresponding field distributions are reported in Fig. 5(b). Figure 5(c) displays electron and hole densities along a line parallel to the base junction, at a depth of 10/~m from the top surface, where maximum current crowding occurs in this device [see Fig. 9(a)]. This Figure shows that the plasma region is wider for the higher current, indicating a smaller lateral voltage drop, despite the fact that a larger Jc, at constant fir, implies a larger base current. This result can be explained by observing that, during turnoff, the layer which is available for lateral current conduction includes both the metallurgical base layer and the "current- induced" base. As seen from Fig. 5(a, b), the space between source and drain can be roughly subdivided into two regions: a plasma region, which forms the current-induced base, and a drift region, the latter being in the space-charge limited regime. The variation of the plasma region with current can be understood with the help of a I D model, at the symmetry axis of the cell. The 1 D current density JLC, is constant and much higher than Jc, due to the uneven distribution of current over the cross section of the cell. If xdp is the boundary between the drift and plasma regions, the slope of field magnitude in the drift region is:

dE J L C - - qv~ N D - - ( 1 )

dx ~Vs

and diffusion current in the plasma region, where carrier distribution is roughly linear, is:

J e c = - 2 q D Po . (2) X d p

As Jec increases, the field slope increases, and this causes Xdp to increase for a fixed collector voltage. Also Po increases with Jec, so the current-induced base becomes thicker and more conductive as Jtc increases. The overall lateral voltage drop across the base layer, as Jc varies for fixed Br, is therefore the result of two competing phenomena: the increased base current, which tends to enhance base reverse bias, and the increased base thickness and conductivity, which tends to reduce the reverse bias. The latter effect prevails over the other, so that current crowding is reduced as Jc increases [see Fig. 5(c)]. Therefore it is not the increase of collector current, but rather the increase of base extraction at the origin of current confinement in a BJT.

If the size of current filament is defined as the lateral distance in which 90% of the current is concentrated, measured along the line of maximum crowding, at 10#m depth, the overall transient evolution of filament size versus collector voltage, displayed in Fig. 6, is obtained. Current shrinking

280 L. FRATELLl and G. F. VITALE

15o ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' \ " , 3 2 0 A / c m z . . . . . . EBJT

3~.~ to0 ~ .............

"" ............................. ':::2"~ 8 0 A / c m = •

o , , , , I , , , , I , = ~ l h , , , I , , , , 100 200 300 4 0 0 500

C o l l e c t o r v o l t a g e (V)

Fig. 6. Variation of filament width with collector voltage, during the transient.

toward the center of the cell is very fast at low currents (J~ = 80 A/cm 2) for both the EBJT and the BLBJT devices. The equilibrium size of filament (between 30 and 40/~m) is reached at a voltage well below the final value and also below the onset of impact ionization.

The variation of filament size is quite different at higher currents, J~ = 320 A/cm:, for both devices. The filament size is larger at the same voltage and its final size is reached about at the end of the transient, thus indicating a more effective carrier removal at higher currents, for a same fir" In all conditions, during the transient, filament size is larger for the BLBJT, anticipating a better performance for this device.

4 . STEADY-STATE ANALYSIS AND RBSOA

AS sccn in the preceding Section, the steady-state

voltage at which the constant-current turnoff transi-

ent ends, (the sustaining voltage), originates from the

equilibrium between avalanche generation and

current extraction through the base. Therefore, the

sustaining voltage is a function of the following

principal factors:

(I) Geometry of the cell and base doping, which

control the amount of current crowding within

the cell, and detcrrnincs the ability of the base

to remove minority carriers in an effective way.

(2) Base drive circuit which sets the rate of hole

removal in steady-state and therefore the

equilibrium hole generation rate.

o 0 0

' ' ',' I ' L 1 ' ' I i '

a =s ', ~s,=5 \ ~1 #, • , . '~ !~

", 't

t "... #,=2- \ ,6,

. . . . . . . . . . . EBJT "-. , ",,~ I r=2

B ~ "- t7 ; / . . . . . . . . cBrr '" 7 ' V J , , I ~ , , I , ~'s, /" , ,

200 400 600 800 Sustaining voltage V~s(V)

Fig. 7. Loci of sustaining voltage of the three device structures, for different fir values.

(3) Epilayer doping profile which affects amount of current crowding near the substrate and then the onset of space-charge limited current and the voltage for avalanche injection.

If the collector current is varied, by keeping fir to a constant value at the end of the transient, a locus of sustaining voltages is obtained on the Jc- VCe plane, which identifies the upper voltage boundary to the operation of the transistor. These limit curves are reported in Fig. 7, for the various device geometries. As reverse gain fir decreases, the sustaining voltage increases in all devices. This is due to the fact that a greater hole extraction from the base requires, at equilibrium, an increased hole generation, which in turn requires a higher electric field.

In the EBJT structure, the sustaining voltage decreases sharply as collector current increases. This is consistent with the reduction in the RBSOA at high current, observed experimentally in these devices, even if no other phenomena cause its premature

2x lO ~

10 5

o

0 0 60

(a) ' ' ' I ' ' ' I ' ' '

20 40 (,m)

6 (b) 2 × 1 0 i i i I I I I [ I I I

105

.2

2 0 4 0 60 (~rn)

(c) 2 x l O 6 ~ ] - - v - - - ' r - - - - - - r ]-

-~ 10 s

o W / / ~ \ \ ~ 16o ~ III \ \ \ ~ 2 4 o

N \\\3z0 B \~4o0

o ~ _ _ ~ 20 40 60

(~m)

Fig. 8. Steady-state electric field distributions, with collector current density as a parameter: (a) EBJT; (b) BLBJT; (c)

CBJT.

Reverse bias safe

failure. The reduction of sustaining voltage with increased current, in EBJT, is the result of field distributions in steady-state, shown in Fig. 8(a). In this device, in which severe current crowding effects take place, removal of minority carriers is poor, thus increase of the collector current causes the build-up of a plasma region over most of the epilayer, with a corresponding reduction in the field. The increase of impact ionization, needed to sustain the increased base current, is due to the narrow peak in the field at the substrate side, which does not compensate the former reduction, so the total voltage decreases as Jc increases.

The concept of tailoring the epilayer doping profile was recognized in [19] as a means to improve the BJT ruggedness. If the epilayer doping profile is modified, as in the BLBJT structure, by adding a buffer layer, the electric field plots become those of Fig. 8(b). As current increases, the low-doped part of the epilayer is brought into the high injection regime causing an increase of the field slope with current. At the same time, the buffer layer, that remains in low-injection in this device, ensures a reduced current crowding effect, so impact ionization is reduced and only a narrow plasma region builds-up near to the base junction. The corresponding field suppression is compensated by the increase of the field within the buffer layer, so the final result is a net increase of the sustaining voltage with current, as shown in Fig. 7. This is a necessary condition for a full RBSOA at high currents. It is worth pointing out that the narrow, relatively low doped layer, included in the EBJT structure, is not effective in increasing the sustaining voltage at high currents as it is easily brought into high injection.

From the previous discussion on base depolarization and current crowding effect, it turns out that an effective means to reduce this effect is to reduce the cell width, thus reducing the base depolarization by minimizing the path of base current toward the base contact. The CBJT device, which is characterized by narrow interdigitated cells, fulfils these requirements. The analysis of this structure shows significant improvements, as compared with the other structures, even though these improvements are obtained at the price of a more complicated technology.

The steady-state electric field distributions of the CBJT structure on a vertical section at the centre of the cell are shown in Fig. 8(c) for a range of Jc values. It is seen that a moderate minority-carrier accumulation takes place at the base junction in this device, and that the increase of the field peak at the substrate side does not compensate completely the field reduction at the base side, due to the absence of a buffer layer in this device.

The main improvement in the CBJT structure is in the moderate depolarization of the base, leading to a negligible current crowding effect, as shown in Fig. 9(c), where the current flow lines at the end of

operating area 281

the transient, for f i r = 5 are reported, for J¢ = 80 A/cm 2. For comparison, Fig. 9(b) displays the result for the BLBJT structure, while the plot for EBJT is reported in Fig. 9(a). As a result of the uniform distribution of current across the elementary cell, the local current density at the center of the CBJT structure is much smaller than in the other structures, for the same terminal current density. This is the reason why the field slope at the center of the cell, Fig. 8, is smaller in the CBJT than in the other structures.

Sustaining voltage locus for the CBJT structure is fairly vertical over a wide range of Jc values, exhibiting just a weak decrease at the highest currents (cf. Fig. 7). Thus also the CBJT structure is a good candidate for a full RBSOA at high currents.

The loci of sustaining voltage vs current, at constant fir, Fig. 7, can be used to understand the shape of turnoff waveform when the output circuit is inductive. After the sustaining voltage has been reached (point A in Fig. l) further increase of voltage is due to a decrease in collector current only, which causes a decrease of fir. This part of the transient, which is slow as compared to the first part, can be accurately described by the steady-state solutions, reported in Fig. 7, which predict an increase of the sustaining voltage as fir decreases. As fir = I the maximum device voltage, Vcbo, is reached.

5. POWER DENSITY ANALYSIS

As seen in the preceding section, some devices exhibit a V~.sus voltage close to the maximum device voltage and weakly dependent on current. This is only a prerequisite for a full RBSOA in these devices, as the possibility for a device to actually reach the limit curve in Fig. 7 for a particular biasing condition, depends also on power distribution[16,17]. This quantity supplies detailed information on stress suffered by the devices as it determines the local temperature rise within the device during turnoff. Power density maps have been calculated with the following expression[18], which includes thermal effects of recombination-generation process:

P ~- Jtc" E + qUEg. (3)

Current crowding effect and electric field are both rapidly varying across the elementary cell; so interpolation occasionally results in bumpy surfaces. This circumstance, however, does not affect the subsequent discussion, which deals with changes in the order of magnitude of power density.

Power density distribution for the EBJT structure, in Fig. 10, shows that, as transient evolves, power dissipation becomes concentrated into two major peaks, both located on the symmetry axis of the cell. The first peak is near the substrate whereas the second peak is at the level of the epi-base junction. In spite of the fact that current crowding is greater near to the base junction, the highest peak is the first one.


(a)

20 -

~ 4 0 I I / / / / / / \ \ \ -

.

li ll 22LZ - /

60

80 , , ~ , , , 0 20 40 60 80 100 120

Distance (microns)

(b) (c)

. . . . . . . . . . . . . . . . . /

. . . . . . i 20 20

i 4 0 f l ~40~

.~ 80[ ~5

60 60

. . . . . . . . 80 , ,I . . . . . . I . . . . 0 20 40 60 80 100 120 0 5 10 15 20 25

Distance (microns) Distance (microns)

Fig. 9. Steady-state current flowlines for: (a) EBJT; (b) BLBJT; (c) CBJT. Each tube carries 1/10 of the total current.

This happens because electric field near the substrate is significantly lowered thus balancing the higher current crowding. Both peaks increase with time and reach their maximum values at the final steady state [Fig. 10(b)], which was predictable, because both current crowding and electric field increase during the transient.

At the end of the transient, the power density at the peak is extremely large: 2.1 x 109W/cm 3 and such a power confinement is due to the interactions between field and carriers only, under avalanche injection conditions, independently of the contribution of thermal effects that are not included into the model. Such a high power density is probably sufficient to destroy the device during the transient, before a

steady-state condition is reached, or can trigger a temperature-induced instability.

The power density distributions, in steady-state, for the other device structures are reported in Fig. 11. Unlike the EBJT structure, the power density map of BLBJT has only one peak located at the base--collector junction. Moreover, the peak power in the BLBJT is about one-half that of the EBJT at the same current density (80 A/cm 2), despite the fact that the final voltage is higher. Thus the addition of a buffer layer in the epilayer of BLBJT is beneficial both because it prevents the degradation of the sustaining voltage at high currents, and because power concentration is reduced, which makes more likely that this device can actually reach its full


W/cm 3 (a)

W/cm 3 ? (b) 1.8 x 109

2.1 x 109

x-axis

y-axis

Fig. 10. Transient power density distribution for EBJT structure. Jc = 80 A/cm 2, fir = 5. (a) V~ = 150 V, (b) final steady-state.

Wtcm3 ~ (a)

x-axis

1.1 x 109

y-axis

Wlcm 3 (b)

Fig. 1 l(a,b). Caption on facing page.

284

Wlcm3 I (c)

3.7 x 10 7

x-axis

y-axis

W/cm 3 (d)

x-axis

8.3 x 107

y-axis

Fig. 11, Steady-state power density distributions for: (a) BLBJT at Jc = 80A/cm2, (b) BLBJT at Jc = 240 A/cm 2, (c) CBJT at J¢ = 80 A/cm 2, (d) CBJT at Jc = 240 A/cm 2. All maps are for fir = 5.

285

286 L. FRATELL! and G. F. VITALE

sustaining voltage. The power distribution does not change significantly at higher current densities as shown in Fig. ll(b).

Turning to the cellular (CBJT) device structure, Fig. 1 l(c, d) show that the peak power density is still located near the base-epilayer junction and that its value (3.7 x 107W/cm 3 at Jc=80A/cm 2) is about two orders of magnitude smaller than in the EBJT structure, for the same current density. The CBJT structure, in which the epilayer doping is constant, has a moderate current crowding effect just because of its small cell size; the power peak is so small in this device, because power is evenly distributed across the whole cell both in the direction perpendicular to the base, where the electric field has rectangular shape, and in the direction parallel to the base junction, which remains forward-biased over most of its length, thus preventing current crowding. In the CBJT structure too, the power distribution does not change that much at higher currents [Fig. ll(d)].

A summary of peak power densities for the three structures, as a function of collector current density and fir is displayed in Fig. 12. For all devices, peak power increases in a sublinear way with current density or even decreases slightly at the highest currents, as in the BLBJT. The peak power of the CBJT structure is two orders of magnitude smaller than in EBJT and at least 20 times smaller than the BLBJT at all currents. This makes the CBJT performance superior to the other structures for a full RBSOA. Increasing the current extraction from fir=5 to 2, causes an increase in the base depolarization and in current crowding and then an increased power density in all devices, with the CBJT structure still being by far the best of the three devices.

A virtually identical result is obtained if the CBJT cell is modified for a higher voltage rating. The curve labelled as: LBJT in Fig. 12 corresponds to a 60 pm wide cell with a 70 pm epilayer, leading to a 1000 V device. It is seen that there is no appreciable difference with the other cellular structure, in terms of peak power density, over the whole range of currents. This is due to the fact that the voltage rating of the device is proportional to the epilayer thickness, thus

B'-= 5" ,

~ I0 ~ vnM" -

o 10' g, , ~ l , , , I , , ~ I

0 200 400 2 600 Collector current density (A/crn)

Fig. 12. Steady-s ta te power densi ty peak values as a funct ion of Jc and fir. Points cor respond to the power densi ty maps

in Figs I0 and II.

making the power to increase roughly linearly with silicon volume. So if voltage rating increases, the power density remains constant, as it is an intensive quantity, provided the geometry of the cell ensures similar distributions of current and electric field.

6. CONCLUSIONS

In this paper, the behaviour of different BJT structures in inductive turn-off has been investigated, with the help of a 2D numerical simulator. Results show that the loci of sustaining voltage vs current, at constant fir, Fig. 7, can be used to calculate the shape of turnoff waveform after the sustaining voltage has been reached, as a quasi-steady state analysis applies.

Two conditions determine the shape of the RBSOA at high currents: the sustaining voltage locus and the power density. If sustaining voltage decreases as collector current density, Jc, increases, as in the EBJT structure, there is no chance of a full RBSOA, independently of the occurrence of other limiting factors. By properly designing the geometry and doping profile of the elementary cell, devices whose sustaining voltage does not degrade as Jc increases, can be realized. However, from the analysis of the power density maps of these devices, it has been shown that they can be submitted to quite different stresses for the same bias conditions. Therefore, it may happen that some devices will collapse during the transient, before they reach their upper voltage limit, set by the sustaining voltage, because of excessive power dissipation. The highly localized heating can either be a direct cause of failure or can trigger other kinds of instability.

If the cell is relatively wide (240/~m), the addition of a buffer layer gives the potential for a full RBSOA device along with an improved ruggedness. This is consistent with what is known experimentally.

Narrow interdigitated structures, with cell width as small as 50/~m, also have the potential of a full RBSOA because of the shape of their sustaining voltage. In addition, the peak power in these devices is drastically smaller than in the former device, with an improvement of more than one order of magnitude.

Very few devices featuring such a small geometry are available at present, and they have demonstrated the ability to attain a stable avalanching condition before failure or even to turnoff safely[4,5,15]. These devices may suffer from layout effects of uneven current distribution among cells, not taken into account in this paper, which may cause their actual performance to be worse than predicted. Probably the optimum design for turnoff performance of these narrow-cell devices will come out from a trade-off between these opposite requirements.

Acknowledgements--The au thors wish to t h a n k Professor Paolo Spirito for his useful suggestions and critical reading, and Dr Gianvito Persiano for his valuable assistance in simulation. This work was supported by CNR and CORIMME


REFERENCES

1. W. P. Bennett and R. A. Kumbatovic, 1EEE Trans. Electron Devices ED-28, 1154-1162 (1981).

2. P. L. Hower and V. Gopala Krishna Reddi, IEEE Trans. Electron Devices ED-17, 320 335 (1970).

3. H. B. Grutchfield and T. J. Moutoux, IEEE Trans. Electron Devices ED-13, 743-748 (1966).

4. D. L. Blackburn, Proc. EPE-MADEP, Firenze, Vol. 0, pp. 252-257 (1991).

5. H. Kerboua, D. Sebille and Miserey, Proc. EPE, Firenze, Vol. 1, pp. 160-163 (1991).

6. A. Caruso, P. Spirito and G. Vitale, IEEE Trans. Electron Devices ED-21, 578-586 (1974).

7. G. Vitale, Solid-St. Electron. 18, 1123-1130 (1975). 8. O. Mank and W. L. Engl, IEEE Trans. Electron Devices

ED-22, 339-347 (1975). 9. B. A. Beatty, S Krishna and M. S. Adler, IEEE Trans.

Electron Devices ED-23, 851-857 (1976).

10. K. Koyanagi, K. Hane and T. Suzuki, IEEE Trans. Electron Devices ED-24, 672-678 (1977).

11. L. J. Turgeon and D. H. Navon, IEEE Trans. Electron Devices ED-25, 837-843 (1978).

12. K. Hwang, D. H. Navon, T. Tang and P. L. Hower, IEEE Trans. Electron Devices ED-33, 1067 1071 (1986).

13. M. K. Johnson and P. A. Gough, Solid-St. Electron. 33, 259 272 (1990).

14. T M A - M E D I C I User's Manual. Technology Modeling Associates, Palo Alto, Calif. (1992).

15. D. L. Blackburn and D. W. Berning, Proc. IEEE Int. Electron Devices Meeting, pp. 297-301 (1980).

16. M. S. Adler, IEEE Trans. Electron Devices ED-25, 16 22 (1978).

17. M. Turowski, A. Napieralski and J. M. Dorkel, Proc. EPE-MADEP, Firenze, Vol. 0, pp. 363-368 (1991).

18. S. Selberherr, Analysis and Simulation o f Semiconductor Devices. Springer, Wien-New York (1984).

19. M. M. Shahidul Hassam and H. Domingos, Solid-St. Electron. 33, 1217 1221 (1990).

Documents

On the reverse bias safe operating area of power bipolar transistors during inductive turn-off