2192 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-34, NO. 10, OCTOBER 1987

High d / d T Light-’ riggered Thyristors

Abstract--Directly light-triggered, 4000- and 6000-V thyristors were designed, fabricated, and tested to obtain high performance in dZ/dt, dV /d f , and photosensitivity. Built-in resistors protected both auxiliary stages during high dI/dt turn-on. The novel use of etched ?oats to define the resistors was compatible with an optical gate structure that gives high dV/dt and good photosensitivity. No additional processing steps were needed to fabricate these devices, as compared to standard light-triggered thyristors. A record value of 1000 A j p s at 60 Hz was measured on a 6000-V thyristor, and 850 A/ps was safely triggered with only 1.8 mW of light. The dV/dt immunity of the photogate struc- ture measured 4000 V/ps, rising exponentially to 80 percent of 4000 V, V,,,. Thyristors triggered by dV/dt were not destroyed. A new model of resistor heating was combined with the first measurements of the current pulses through both built-in resistors to identify the mecln- anism responsible for occasional bnrn-out of the second resistor. The failure mechanism was conductivity modulation in the surface of the resistor during its microsecond on-time caused by thermally generated carriers. The test results confirmed the utility of built-in resistors for high d l /d t performance with minimal light power and for nondestruc- tive dV/dt triggering.

I. INTRODUCTION

F OR HIGH-POWER thyristors, the two chief advan- tages of light triggering are electrical isolation of the

gate and gate noise immunity. These advantages are sc attractive to circuit designers that it is a standard practice, to use optically activated gate circuits, even with elect& cally gated thyristors. Naturally, it is desirable to inte.. grate these functions by making a directly light-triggerec. high-power thyristor.

As is common with dl electron devices, the trend for. high-power light-triggered thyristors (LTT’s) is toward ever increasing performance criteria. Three important pa. rameters are: the minimum light-triggering power PLT, t h t :

critical rate of current rise dl/&, and the critical rate o F voltage rise, the dV/dt immunity. Of course, these pa- rameters place conflicting demands upon the device, and innovative designs are required to improve any one O F them without sacrificing the other two. Of particular im- portance to the present work are the demonstration by Hashimoto and Sat0 of a novel optical gate structure, which improves the photosensitivity versus d V / d t trade- off [l], and Temple’s use of resistors to improve the dZ/dt capability [2]. These discoveries have been synthesized. into a single light-triggered thyristor structure, whose de-

Manuscript received December 16, 1986; revised May 11, 1987. J. X. Przybysz is with the Westinghouse Research and Developmerkt

D. L. Miller, S. G. Leslie, and Y. C. Kao are with Powerex, Young;-

IEEE Log Number 8716123.

Center, Pittsburgh, PA 15235.

wood, PA 15697.

sign, fabrication, and performance are the subject of this paper.

Lower cost for the optical triggering system is the driv- ing force behind the efforts to improve the photosensitiv- ity of LTT’s. Improved sensitivity is obtained by reduc- ing the gate threshold current of the optical gate. However, these gates are more sensitive to faulty dVldt triggering because it takes less junction displacement cur- rent to turn them on. So, optical gate areas are made much smaller than electrical gates to minimize the dV/dt dis- placement current. In addition, in 1981, Hashimoto and Sat0 introduced a novel gate structure that gave an im- provement in the dV/dt versus photosensitivity trade-off by a factor of 2.4, as compared to a circular gate [l]. Another approach to improving the dV/dt versus photo- sensitivity trade-off, given by Ohashi et al., uses RC bal- lasting, wherein the dV/dt-induced voltage on the second amplifying stage is used to back bias the induced voltage on the first amplifying stage [ 3 ] .

While these approaches succeed in advancing the pho- tosensitivity and dV/dt capability, these light-triggered thyristors still lag behind the d I / d t capability of their electrically triggered counterparts. High rates o f current rise force large currents through very small conducting areas, resulting in meltdown of the optical gates. Ampli- fying gate structures improve the dZ/dt capability of elec- trically triggered thyristors, but even five stages of am- plifying gates fail to solve the dl/dt problem for light- triggered thyristors [4]. The fundamental obstacle is the finite turn-on’ delay time between the onset of conduction in the optical gate and the transfer of the load current to any succeeding stage. During these tenths of a micro- second, the circuit may force hundreds of amperes of load current through an optical gate that is only 1 mm in di- ameter.

As Temple proposed in 1981, a solution to this problem may be obtained by building current-limiting resistors into the thyristor, between the amplifying stages of the device (see Fig. 1) [2], [SI. He used annulus-shaped resistors, in a circularly symmetric geometry. But, the previously mentioned advances in the trade-off of photosensitivity versus dV/dt immunity cannot be incorporated in circu- larly symmetric designs.

All high-power light-triggered thyristors use an etched optical gate well to improve the light-to-photocurrent con- version efficiency. Hashimoto used this gate well etch to fabricate current-channeling moats, which restrict the flow of current to one direction. The efficacy of that procedure

0018-9383/87/1000-2!192$01 .OO 0 1987 IEEE

PRZYBYSZ et al.: HIGH d I / d t LIGHT-TRIGGERED THYRISTORS 2 193

9 A

K

(b) rpCurrent Channeling Moat

O K

WOR 2WR 27R 6 R 1.4R P

%1 RU RS2 RE RS3

WOR 2WR 27R 6 R 1.4R P

Fig. 1. Light-triggered thyristor with built-in current-limiting resistors. (a) Equivalent circuit. Photodiode depicts photogate. Resistors RL, and RL2 protect first- and second-stage auxiliary thyristors. Shunting resistors Rs,, RS2, and Rs3 determine gate sensitivities. (b) Cross section. High imped- ance under moat diverts lateral current flow to the right. Resistor values are for a 4-kV thyristor.

Fig. 2. Novel structure for built-in resistor. Moats are spaced apart a width W to channel lateral current through a length L, between electrodes.

to improve the photosensitivity while maintaining the dV/dt immunity had already been verified by Tada et al. [6], and it was desired to incorporate a similar structure in our light-triggered thyristors. By a natural extension of this process, it was possible to fabricate current-limiting resistors as shown in Fig. 2 . This novel technique for fab- ricating current-limiting resistors required no more pro- cessing steps than a standard light-triggered thyristor with an optical gate well.

The thyristors that were fabricated with this new design exhibited higher dZ/dt performance than any previously published values: as much as 1000 A/ps at 60 Hz. The effects of the current-limiting resistors were determined. These were the first measurements of the current pulses through both built-in resistors. These current-limiting re- sistors performed two important functions: 1) they limited the amount of current that flowed through the optical gate and the second stage during the turn-on delay, and 2) the additional resistance along this path of the current speeded up the transfer of the load current to the main cathode, thus limiting the duty cycle of these stages. Two addi-

tional benefits were gained by the incorporation of cur- rent-limiting resistors. First, there was a relaxation in the gate over-drive requirement, so that less optical gate power was required to safely control high dZ/dt loads. Second, it was possible to design the devices so that dV/dt triggering no longer resulted in destruction of the thyris- tor.

The desire to operate the thyristor over a wide voltage range, with high photosensitivity, resulted in a three-stage design. The design values of the interstage resistors de- pended on the gate sensitivities of succeeding stages, and vice versa. A new model for calculating the temperature rise in the resistors was used to design their physical size. This model worked well to explain the observed burn-out of resistors at the limits of device performance.

11. DEVICE DESIGN AND FABRICATION A. Selection of Resistance Values

The light-triggered thyristors described in this paper were three-stage devices consisting of two auxiliary cath- odes, also known as amplifying gates, in addition to the main cathode. As shown in Fig. 1 , the integrated device could be considered as a combination of a photodiode, three separate thyristors, and five resistors. Two of the resistors were current-limiting resistors. The other three represent the lateral resistance of the p-base under the cathode diffusions.

The choice of resistance values was made through a consideration of the desired dZ/dt capability, the input gate light power and the operating voltage range. This procedure will be illustrated by considering a thyristor that was designed to operate in the range from 12 to 4000 V, with a dZ/dt capability of at least 750 A/ps, and a min- imum light-triggering power of 5 mW.

When infrared light from a GaAlAs LED shines into the optical well of an LTT, it creates electron-hole pairs in the semiconductor. The forward-blocking junction of the thyristor acts like a p-n photodiode to separate these pairs and generate a photocurrent. With 5 mW of light power input to the thyristor package, light coupling and photocurrent conversion efficiencies should be large enough to generate at least 2 mA of photocurrent. The value of the first shunting resistor was chosen to be 600 Q, which should have a minimum gate triggering current ZGT of about 1 .O to 1.2 mA. Thus, the expected 2 mA of photocurrent should overdrive the photogate. Once the photogate turned on, the anode voltage would be applied across the first and second current-limiting resistors. This determined the electrical gate current that was supplied to the succeeding stages.

The shunt resistance of the main cathode is determined by the cathode shunt pattern along the turn-on line and by the turn-on line length. Since it was desired to have a 750 A/ps turn-on capability, a long turn-on line was re- quired. For this particular shunt pattern, it was calculated that the shunting resistance would be 1.4 Q, so the Z,, of the main cathode would be about 0.5 A.

In a high dZ/dt situation, it is customary to supply an

2194 IEEE TRKiSACTIONS ON ELECTRON DEVICES, VOL. ED-34, NO. 10, OCTOBER 1987

electrical gate current of at least ten times ZGn to ensure strong turn-on. Allowing another factor of four for pro- cess variations and for changes in the operating voltage, the first current-limiting resistor was chosen so that the photocathode could supply 20 A to the main cathode at 4000-V anode voltage, i.e., 200 62.

At the low voltage end of the operating range, 12 V acting across 200-62 resistor would supply only 60 mA to trigger the main cathode. But, the main cathode was ex- pected to have an ZGT of 500 mA, so a second auxiliary cathode was required. The shunting resistance of the sec- ond auxiliary cathode was chosen to be 27 Q, so that it would be strongly triggered by 60 mA. A second limiting resistor was placed in series with this second auxiliary cathode, primarily for the purpose of limiting its duty cycle during high-voltage turn-on. The design value for this second limiting resistor was 6 Q, which should allow 2 A of current to flow at 12-3 bias. This also allowed a safety factor of 4 for process variations.

This discussion illustrates how the values of the shunt- ing resistors and current-limiting resistors were chosen for the 4-kV light-triggered thyristor that is reported on in this paper. A similar design procedure was followed for the 6-kV LTT, whose results are also shown here.

B. Fabrication of Current-Limiting Resistors This paper introduces a novel fabrication process for

incorporating current-limiting resistors. Current channel- ing moats, sometimes called “trenches” [7], were etched into the silicon, simultaneously with the optical gate well. As shown in Fig. 2, two parallel current-channeling moats formed a conduction path of width W. Two aluminum electrodes spaced apart a length L between these moats defined a resistor. For a p-base diffusion with a sheet re- sistance of p , the value of the built-in current-limiting re- sistor was given by the formula

R = pL/W.

For example, the sheet resistance of the p-base was 50 Q per square for these devices. Parallel moats were spaced 4 mm apart and the electrodes were placed 1 mm apart to form 12-62 resistors. Two of these resistors were com- bined in parallel paths to make the desired 6-Q resistor for the 4-kV light-triggered thyristor (Fig. 3).

C. Resistor Wattage

choice of resistor area A = L X W. This section describes the factors that influenced the

Resistor area was minimized for two reasons: 1) Real Estate: Area devoted to the current-limiting re-

sistors subtracted from the overall current conducting ca- pability of the main cathode.

2) dV/dt Displacement Current: Large-area current- limiting resistors fed large amounts of dV/dt junction dis- placement current into succeeding stages, thus lowering their dl / ld t immunity.

The threat of overheating, due to ohmic dissipation in the current-limiting resistors, provided the impetus for

Fig. 3. Layout of optical gate for 4-kV light-triggered thyristor. Gate well is at center. Moats acted as barriers to lateral current flow. From central auxiliary cathode, current flowed through two 4004 resistors in parallel to second auxillary cathode, then through two 1 2 4 resistors in parallel to amplifying gate arms, to trigger main cathode.

large-area resistors. For a fixed value of resistance, a con- stant amount of heat was generated in the resistor during circuit turn-on. However, the temperature rise due to that heat generation varied inversely with the area of the re- sistor. Resistors were made physically large enough to keep the temperature rise within tolerable limits.

The heat generated during the 1 ,us of resistor on-time was fairly well localized to the heat-producing region. ‘Hence, there was a rapid temperature rise in the highly doped surface layer of the diffused resistor. As a rule of thumb, this temperature was kept below that at which the intrinsic carrier concentration would equal the dopant concentration. Otherwise, significant conductivity mod- ulation would occur, leading to a loss of resistance and thermal runaway.

The calculation of the temperature rise in the resistor during the period of current conduction was a problem in thermal diffusion. Heat was generated near the surface of the resistor during the 1-,us of current conduction. Then, the heat diffused through the bulk of the semiconductor during the thousands of microseconds of resistor off-time. This problem was mathematically similar to the calcula- tion of dopant distribution during a deposition and drive- in diffusion.

The voltage pulse across the resistor during the brief on-time generated a quantity of heat Q = j dt [ V ( t ) ] ’ / R . This heat was spread out over an area A = L X W, to an areal density ( Q / A ) (in joules per square centimeter). This quantity is analogous to the charge per unit area de- posited during a dopant deposition diffusion. Then, the

PRZYBYSZ et ai.: HIGH dl/& LIGHT-TRIGGERED THYRISTORS 2195

Fig. 4. Built-in resistor's transient thermal response to a microsecond cur- rent pulse, calculated in the new model. Danger of thermal runaway is greatest at the surface of the diffused resistor.

heat diffused into the silicon with a thermal diffusion con- stant D = 90 pm2/ps. This process is analogous to the redistribution of dopant impurities during a drive-in dif- fusion.

The calculated temperature rise in the resistor due to the Joule heating was

where x is the depth below the surface, C = 0.7 J/g"C is the heat capacity of silicon, and ,om = 2.33 g/cm3 is the density of silicon.

This analytical expression gives a singularity in surface temperature at time zero. But this temperature infinity was removed in the real case by two physical facts:

1) The ohmic heating was generated throughout the fi- nite thickness of the diffused resistor, as opposed to the Gaussian assumption of a spatial delta function for the heat deposition. The dopant profile of the resistor could be characterized by a diffusion length X = 10 pm.

2) There was a finite time scale. The heat pulse was actually generated over a period of about 1 ps, as opposed to the Gaussian approximation of an instantaneous heat input. Thus, the finite time also contributed a finite length scale X = 2 f i = 18 pm.

The combination of these two effects meant that the ini- tial heat pulse was actually spread out to a depth of about 21 pm (d102 + 18').

After taking these factors into account, we obtained a space and time dependence for the temperature excursions due to the ohmic heating as shown in Fig. 4.

To design the first light-triggered thyristor, estimated values of the pertinent quantities were substituted into the equation for the temperature rise. Subsequent devices could be designed from calculations based upon the mea- surements of actual light-triggered thyristors with built-in resistors. These calculations will be presented in the dis- cussion of results.

111. PERFORMANCE The ultimate goal for thyristors with built-in current-

limiting resistors is high dZ/dt turn-on capability. This section will show dZ/dt test results that were higher than any previously measured value.

Repetitive 60-Hz dZ/dt tests are most demanding, both for the device under test and for the test equipment. Be- sides the actual rate of the current rise, another important parameter is the anode bias at the beginning of turn-on. Fig. 5 shows the test waveforms for a 6000-V light-trig- gered thyristor with built-in current-limiting resistors. The anode voltage was set at 3000 V and the load current was permitted to rise at the rate of 1000 A/ps. Our 60-Hz dZ/dt tester was limited to a 3000-V maximum. The test was conducted at 60 Hz with the device permitted to self- heat, starting at room temperature and finishing at 115 "C after 15 min.

Low-repetition-rate (0.5 Hz) dZ/dt tests were con- ducted at 25°C on a tester which used an RLC circuit of 0.38 Q, 1 p H , and 1 pF. Fig. 6 shows the switching wave- forms of a 4000-V light-triggered thyristor biased at 3000 V and triggered with 20 mW of light power. In this test, the load current rose at the rate of 1600 A / p s . Without the test thyristor, the short-circuit dI /d t of this circuit was 2500 A/ps.

The record high dI /d t performance of these thyristors was due to the unique dynamics of the turn-on process in thyristors with built-in current-limiting resistors. To ex- amine this process, the voltages across the two current- limiting resistors were measured during turn-on.

For this measurement (Fig. 7), the device was triggered from an anode bias of 3000 V, the circuit current rose at the rate of 350 A/ps. Fig. 7(b) shows the fall of the anode-cathode voltage, with two distinctive rings as the first and second auxiliary cathode turned-on at 3.5 and 4 ps, respectively, after the application of the LED light pulse. Fig. 7(c) shows the voltage across the first current- limiting resistor. Note that at 3.6 ps, the voltage across the 2 0 0 4 resistor had risen to about 1750 V, which was nearly the full anode-cathode voltage. Hence, the voltage had been transferred from the plasma to the current-lim- iting resistor, in contrast to a conventional device where the voltage stress is localized to the turned-on plasma. The voltage across the first current-limiting resistor dropped very rapidly and was nearly back to zero in 1 ps. By this time, the load current had been transferred to the succeeding stages.

Fig. 7(d) shows the voltage drop across the second cur- rent-limiting resistor. Again, it can be seen that by 4.3 ps the 600-V drop across this second current-limiting resistor equaled the anode-cathode voltage on the device. Thus, the second current-limiting resistor protected the turn-on line of the second auxiliary cathode from excessive Joule heating. Furthermore, it can be seen that the total on-time of this second auxiliary cathode and second current-lim- iting resistor was about 1 ps. After this time, the full load current was transferred to the main cathode.

Admittedly, not every device tested achieved the high level of performance reported here. The observed failure modes were of two types.

1) Main Cathode Turn-on Line: This was the classic form of dI /d t failure, in which overheating during the dynamic plasma spreading period caused thermal run-

2196 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-34, NO. 10, OCTOBER 1987

dI/dt Test Waveforms ( 60 Hz)

dl’ldt = lo00 A / v s

1 ps/Div. Fig. 5 . High d l l d t turn-on of 6-kV light-triggered thyristor. Anode cur-

rent (1000 A/div.) and anode-cathode voltage ( 1000 V/div.).

dI/dt 1600 A l p s

1 p s/Div Fig. 6. High dI /d t turn-on of 4-kV light-triggered thyristor at 25°C and

0.5 Hz. Anode-cathode voltage (500 V/div.), anode-cathode current (500 A/div.), and light-emitting diode trigger current.

away at some random weak point along the periphery of the main cathode turn-on line. The corpses had a char- acteristic hole, burned down through the silicon from the cathode face to the molybdenum substrate.

2) Burn-Out of the Second Current-Limiting Resistor: The corpses were characterized by multiple burn-troughs , extending between the electrodes of the resistor, along the surface of the device.‘ Final failure was sometimes ac- companied by an anode-to-cathode burn hole associated ‘with one such trough.

Another striking feature of thyristors with built-in cur- rent-limiting resistors was the relaxation of the usual gate overdrive requirement. For example, Fig. 8 shows a 6000- V light-triggered thyristor that turned on at the rate of 850 A/ps with an input light power of only 1.8 mW. This was exactly the minimum light power required to trigger the device at 12-V bias. This low light power caused jitter in the turn-on delay. Consequently, the current and volt- age waveforms, as measured with a single-beam storage oscilloscope, may not be synchronized in Fig. 8.

Another example of safe turn-on with minimal light power is given in Fig. 9 for another 6-kV LTT. This de- vice was being operated in a 4OO-A/ps circuit at a pulse repetition frequency of 60 Hz. In this case, measurements were recorded on a dual-beam oscilloscope, thus assuring synchronization of the voltage and current waveforms. By

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1 pslDiv

9g. 7. Measurement of current pulses through built-in resistors during turn-on of a 4-kV thyristor at 25°C. (a) Trigger current through light- emitting diode. (b) Anode-cathode voltage (500 V/div. ). (c) Voltage across first current-limiting resistor (500 V/div. ). (d) Voltage across

A/div.). ( f ) Anode current (500 A/div.). second current-limiting resistor (500 V/div.). (e) Anode current (20

dI ldt 5 850 A/ps P. = 1.8 mw

L

Fig. 8. High dI /d t turn-on of 6-kV light-triggered thyristor at 100°C and 0.5 Hz with gating light power of only 1.8 mW. Anode-cathode current (500 A/dif. ), anode-cathode voltage ( 1000 V/div. ), and light-emit- ting diode trigger current. Voltage and current traces are not synchro- nized, due to variations in turn-on delay.

continuously turning down the LED, it was possible to

‘These bum-troughs are similar to those shown in Fig. 8-14 of “Ad- bring this thyristor to a minimal triggering state. Note that

vanced light-triggered thyristor,” by V. A. K. Temple, Electric Power the turn-on time was in Of 70 ps* In fact, the Research Institute’Rep. EL-3643, Aug. 1984. load current did not even begin to rise until after the LED

PRZYBYSZ et al.: HIGH d l / d t LIGHT-TRIGGERED THYRISTORS 2 197

dI/dt E 400 Alps

PRF= 60 Hz

Iv. DISCUSSION OF RESULTS Light-triggered thyristors with built-in current-limiting

resistors are capable of higher dZ/dt performance than their electrically triggered counterparts [8]. The 1000 A/ps measured in Fig. 5 was higher than previously pub- lished values for a 60-Hz dZ/dt test.

Built-in current-limiting resistors are more appropriate to light-triggered devices, since the gate potential of 1750 V, that was measured during turn-on in Fig. 7(c), could have severely damaged an electrical gating circuit. It posed no threat to a glass light pipe.

In Fig. 6, the built-in resistors held off the load current during the turn-on delay phase and partially discharged the circuit capacitance before the main cathode began to conduct. This effect poses a fundamental question about device ratings. Namely, should the dl/dt rating be based on the external circuit (2500 A / p s here) or on the circuit including the device under test ( 1600 A/ps)? In either case, this 4000-V light-triggered thyristor with integral current-limiting resistors has displayed an outstanding ability to withstand very high dZ/dt pulses.

In Figs. 5 and 6 , hundreds of amperes of load current flowed prior to the transition to full steepness. This dem- onstrates an essential point for the operation of these de- vices. The current-limiting resistors delay the full-circuit dZ/dt until the main cathode has turned on.

Directly light-triggered thyristors are complex devices. It is easier to fabricate a small, separate, light-triggered auxiliary thyristor, and to connect it and a discrete series resistor, between the anode and gate of a high-power elec- trically triggered thyristor [Z], [SI , [7] , [9]. Measure- ments of this combination show the effectiveness of the auxiliary thyristor [2]. An attempt to measure this effect in a high-power directly light-triggered thyristor with built-in current-limiting resistors was not as successful [8]. Circuit ringing produced a noise signal that was nearly as large as the effect being measured. However, the mea- surements of resistor voltage in Fig. 7 have now given a clear picture of the optical gate current during turn-on. Furthermore, these were the first measurements to also display the current through the second-stage amplifying gate.

Analysis of these data revealed two important effects: 1) The division of the anode-cathode voltage between

the plasma and the current-limiting resistor greatly re- lieved the turn-on stress to the auxiliary cathodes. The built-in resistor absorbed the turn-on stress for the auxil- iary cathodes.

2) The load current was rapidly transferred from the auxiliary cathodes to the main cathode. This short duty cycle was particularly important in limiting the tempera- ture rise in the current-limiting resistors.

These directly light-triggered thyristors used a three- stage design to ensure proper turn-on from 12 to 6000 V. No single current-limiting resistor could pass the proper gate current across the entire operating voltage range. This problem is likely to recur when a light-triggered auxiliary thyristor is used with a single discrete resistor. This com-

10 v slDiv Fig. 9. High dlldt turn-on of 6-kV light-triggered thyristor with minimal

light power at 25°C. Light power was so low that delay time was 70 p s and load current did not even start to rise until after the light-emitting diode was turned off. Anode-cathode voltage ( 500 V idif. ) , light-emit- ting diode trigger current, and anode-cathode current ( 1000 A/div. ).

dVldt Triggering 5 Q 15R

Fig. 10. Nondestructive dVldt-induced turn-on of 4-kV light-triggered thyristor. Anode-cathode current (20 A/div. ) and anode-cathode volt- age (500 V/div.).

had shut off. Still, the device was able to survive high dZ/dt turn-on at 60 Hz. With a continuous variation of the LED light power, only two results were observed un- der test conditions: either the device turned on safely, or, if the light power was below threshold, it did not turn on at all.

An example of safe dV/dt turn-on for a 4000-V light- triggered thyristor with built-in current-limiting resistors is shown in Fig. 10. This test was conducted by charging the capacitor of a 1 5 4 0.5-pF snubber, which was con- nected to the device under test. The device was able to survive the turn-on because the dV/d t trigger point was the second auxiliary cathode, which was protected by the second current-limiting resistor. At 6.5 ps after the start of the voltage rise, the device was conducting 120 A with an anode-cathode voltage of 750 V. This corresponds to 6 Q of resistance, the size of the current-limiting resistor.

The dV/d t immunity of 4-kV light-triggered thyristors, of the type shown in Fig. 3, was typically 1000 V I p s , measured exponentially to 3200-V peak. A wirebond across the turn-on line of the second-stage auxiliary cath- ode eliminated the dV/dt turn-on of that stage. Then, the dV/dt immunity of the photogate stage was measured as 4000 V/ps.

2198 IEEE “RANSACTIONS ON ELECTRON DEVICES, VOL. ED-34, NO. IO, OCTOBER 1987

bination may be unsuitable for phase control applicatio 2s because of an inability to fire at all phase angles.

The fundamental motivation for building in currer.t- limiting resistors is to protect the small sensitive auxiliary cathodes from the stresses of the high dZ/dt turn-on. A t the limits of performance, the devices that failed by a classic dZ/dt burn-out in the main cathode turn-on line demonstrated the value of this design principle. The dl:- vices that failed by surface melting in the resistor denl- onstrated the importance of understanding the resistor’s thermal response to the current pulse during turn-on.

The measurements of the voltage pulses across the cu :- rent-limiting resistors during turn-on were used to est[- mate their temperature rises, by the model presented in this paper. The 200-Q resistor had a surface area of 16 mm2. The voltage pulse was approximately Gaussiar with a peak of 1750 V and a full width at half maximum of 350 ns. Substituting these values gave A T ( x = 0, t == 0.5 ps) = 17.3”C. The 8-mm2 6-Q resistor sustained B pulse of 700-V peak and 0.6-ps width. These values gavz A T( x = 0, t = 1 ps ) = 223 “C. This meant that at ai3 operating temperature of 125 O C, the instantaneous sur- face temperature of the second current-limiting resistor was about 348°C. Silicon has an intrinsic carrier concen- tration of 10l6 cmV3 at 372°C [lo]. The resistor ap- proached a temperature at which conductivity modulation could cause a loss of resistance and thermal runaway. Un . der more severe test conditions, some devices developed burn tracks across the surface of this resistor. This anal- ysis suggests that this failure mode was due to excessive: heating in the surface layer of the built-in resistor, during the l-ps on-time of the second stage.

The design rules presented here for calculating the re- quired area of a current-limiting resistor differ from those presented in [5]. Those rules calculate the temperature rise by assuming that the heat is distributed throughout a vol- ume of silicon that includes the entire wafer thickness un- der a current-limiting resistor. But, it takes hundreds of microseconds before the heat diffuses through all of the silicon volume. The real danger existed in the first micro- second, during the current conduction period, when the instantaneous temperature approached temperatures at which the intrinsic carrier concentrations could over- whelm the background doping.

The high dZ/dt performance with minimal optical gat- ing power (Figs. 8 and 9) contrasted strongly with the destructive triggering that occurs in standard light-trig- gered thyristors as the optical power is reduced toward the minimum gating level. A standard light-triggered thyris- tor which triggers with 7 mW of light power will typically be used in a system that supplies 70 mW of light to turn on the device [ll]. High-power LED’s that can supply 70 mW of light power to the optical gate, after considerable losses in the fiber-optic system, are a work of art in them- selves [12]. The high cost of jumbo LED’s is a serious disadvantage in the application of directly light-triggered thyristors. In a sense, the elimination of the gate over- drive requirement could be considered as equivalent to an

improvement in the photosensitivity by a factor of about 10. The economies obtained through the use of a lower power and less expensive LED may be decisive in the decision to use directly light-triggered thyristors.

Yet another attractive feature of thyristors with built-in current-limiting resistors is the ability to design for non- destructive dV/dt triggering. In the conventional case, a single dV/d t turn-on event usually results in the loss of a thyristor. In Fig. 10, the action of the built-in resistor to limit the current through the dV/d t trigger point saved the thyristor from destruction. Each of the auxiliary cathodes and the main cathode had distinct values of dV/dt im- munity. The weakest link in the chain established the overall device rating. As a design principle, nondestruc- tive dV/dt turn-on capability may be obtained by arrang- ing for the weak link to occur in an auxiliary cathode that is protected by an adequate current-limiting resistor.

In conclusion, these results confirm the facts about light-triggered thyristors with built-in current-limiting re- sistors. They can perform at very high dZ/dt, as demon- strated by the record-high 1000 A/ps at 60 Hz. The gate overdrive requirement is eliminated, resulting in substan- tially lower costs for the light-gating circuit. These thyr- istors are not destroyed by dV/dt transients. Further- more, the novel use of current-channeling moats to define the built-in resistors was combined with an optical gate structure that has the added advantages of improved pho- tosensitivity and increased dV/d t immunity. All of these benefits were obtained with no additional processing steps, when compared to a standard light-triggered thyristor. Fi- nally, the’new model of the thermal transient behavior of built-in resistors, in conjunction with original measure- ments of the current pulse in the second-stage resistor, explained their occasional failure. Improved designs can now be made using this model.

ACKNOWLEDGMENT

The authors would like to thank L. S. Chen, L. R. Lowry, M. 3. Geisler, T. P. Nowalk, D. A. Walczak, J . B. Brewster, and E. S. Coleman for many helpful tech- nical discussions. We thank D . Peters, W. Van Mastrigt, G . Glenn, T. McAdams, E. Hohn, R. Fiedor, M. Stupar, D. Elder, S. Kalp, and J. Jerson for their technical as- sistance. We thank s. Fordyce, G. Markle, and G. Law for helping prepare the manuscript.

REFERENCES

[I] 0. Hashimoto and Y. Sato, “A high voltage, high current, light-ac- tivated thyristor with a new light sensitive structure,” in Proc. Power Electronics Specialists Con$, pp. 226-231, 1981.

[2] V. A. K. Temple, “Controlled thyristor turn-on for high d l / & ca- pability,” in IEDM Tech. Dig. , pp. 406-409, 1981.

[3] H. Ohashi, T. Ogura, and Y . Yamaguchi, “Directly light-triggered 8-kV-1.2 kA thyristor,” in IEDM Tech. Dig., pp. 210-213, 1983.

[4] V. A. K. Temple, “Development of a 2.6-kV light-triggered thyristor for electric power systems,” ZEEE Trans. Electron Devices, vol. ED-

[5] -, “Controlled turn-on thyristor,” IEEE Trans. Electron Devices,

[6] A. Tada, A. Kawakami, T. Miyazima, T. Nagagawa, K. Yamanaka,

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H. Mitlehner, “Light-activated auxiliary thyristox for high voltage 1985, he worked on the development of high-voltage light-triggered thyr-

55, 1985. applications,” Siemens Forsch.-u. EntwickL-Ber., vol. 14, .pp. 50- istors at the Westinghouse Electric Corporation Research and Development

Center, Pittsburgh, PA, and at the Westinghouse Semiconductor Division, V. A. K. Temple, “Turn-on reliability breakthrough in light triggered Youngwood, PA. Since 1986, he has continued this work at Powerex, thyristors,” in IEDM Tech. Dig., pp. 428-431, 1984. D. Silber, M. Fuellmann, and W. Winter, “Light activated auxiliaIy

Youngwood, PA.

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J . AppZ. P h y ~ . , V O ~ . 20, SUPPI. 20-1, pp. 99-104, 1981. tical transitions and photoionization of helium atoms. From 1981 through

*

Xi.

* Danald L. Miller (”82) received the B.Sc. de- gree in physics from the University of Missouri, Columbia, in 1973 and the M.S. and Ph.D. de- grees in physics from the University of Illinois at Urbana-Champaign in 1974 and 1979, respec- tively.

From 1973 to 1977, he was a National Science Foundation Graduate Fellow and from 1977 to 1978, he was a Harry J . Diffenbaugh Fellow. From 1979 to 1981, he was a member of the phys- ics faculty at the University of Nebraska, Lincoln,

Yu C. Kao (”61) received the B.S. degree from the Central Institute of Technology, China, and the M. S . degree from Carnegie-Mellon Univer- sity, Pittsburgh, PA, both in electrical engineer- ing.

He has 25 years of research and development experience with the Westinghouse Electric Cor- poration (1961-1985) in the area of high-voltage high-power semiconductor devices. He was re- sponsible for developing a technique for obtaining very high-voltage planar p-n junctions. Presently,

he is devoted to modeling the switching performance of thyristors and other four-layer switching devices and to the development of fast-switching de- vices for high-frequency applications. Since 1986, he has continued this work at Powerex, Youngwood, PA.