Comparison of light triggered and electrically triggered thyristor turn-on

As part of an EPRI (Electric Power Research Institute) funded research program on a directly light triggered (LT) thyristor for HV dc application, an existing 53-mm 2600-V 1000-A electrically fired device was suitably modified to be turned on with an incident photo-pulse of 20 nJ, the basic problem being the retention of a 2000 V/µsdV/dtcapability. The price paid for high sensitivity and highdV/dtcapability was found to be a device inherently more susceptible todV/dtfailure. In the efforts to cope with this problem a number of computer-type models were developed to assist in predicting turn-on in both electrically and light fired devices with one or more amplifying stages. At the same time, devices were fabricated which could be either light or electrically fired. Both the model and experiment point to faster turn-on of the light fired device and an increased requirement for careful design.     

Development of a 2.6-kV light-triggered thyristor for electric power systems

The status of a present Electric Power Research Institute, Inc. (EPRI) funded research program on a directly light-triggered thyristor for high-voltage direct current (HVDC) application is reviewed. An existing 53-mm, 2600-V, 1000-A electrically fired device was suitably modified to be turned on with an incident photopulse of 30 nJ, the basic problem being the retension of a 2000-V/µsdV/dtcapability. Design tradeoffs betweendV/dtand gate sensitivity are discussed as well asdi/dtproblems encountered in turning on a 2-in device with such a small gate signal. Special experiments and design and analysis computer programs have helped in assessing temperature excursion during turn-on and led to improved design with adi/dtcapability approaching that of the original electrically gated device.     

A 2.5-kV static induction thyristor having new gate and shorted p-emitter structures

An SI thyristor with new gate and shorted p-emitter structures (DTT-SI thyristor) is proposed to realize a high-voltage high current high-speed device having a low forward voltage drop. Investigations using fabricated 2.5-kV 100-A DTT-SI thyristors and numerical analyses show that the DTT-SI thyristor has a good trade-off between the forward voltage drop and switching characteristics when the channel width is 8-10 µm and the maximum impurity concentration is about 1 × 10¹⁷to 4 × 10¹⁷cm^-3. The typical fabricated DTT-SI thyristor has a 2.5-kV forward blocking voltage with a 58-V reverse gate bias voltage, a 1.4-V forward voltage drop with a 100-A anode current, a 2- µs turn-on time, adi/dtcapability higher than 4000 A/µs, and can interrupt a 900-A anode current with a 3.5-µs turn-off time and a 5.6 gate turn-off gain on application of a 100-V reverse gate bias voltage.     

High-power dual amplifying gate light triggered thyristors

The feasibility of directly light triggering a high power phase control thyristor is investigated. Work is described on an optically triggered gated 53-mm diameter 2600-V 1000-A thyristor which is similar to an electrically gated production version. Test results describing the response of this thyristor to various optical signals are presented. Our work has shown that this cell can be directly triggered by light at an equivalent gate current which is a factor of three below its present dynamic gate requirements and still largely retain all its blocking and dynamic characteristics. This improvement is obtained by the use of a second very sensitive amplifying gate stage which is responsive to light. All wafer processing of the light sensitive thyristor was carried out on standard production lines. Tests made on static dV/dt, di/dt, blocking voltage, and leakage current on light sensitive devices all closely match parameters of the standard electrically fired equivalent cell.     

Fabrication and optical-switching results on the integrated light-triggered and quenched static induction thyristor

An integrated structure of the light-triggered and light-quenched (LTQ) static induction (SI) thyristor is introduced and is fabricated by the combination of the SI thyristor and SI transistor process technology. The device consists of a buried-gate light triggered (LT) SI thyristor and a p,channel surface gate static induction photo-transistor (SIPT). An anode voltage VAKof 500 V at an anode current IAKof 1 A (600 A/cm²: channel current density) is optically switched with a triggering power ofP_{LT} = 11mW/cm²(92 µW) and a quenching power ofP_{LQ} = 11mW/cm²(110 µW) in a turn-on time of 0.7 µs and a turnoff delay time of 1.0 µs. The integrated LTQ SI thyristor is a novel type of self-turn-off power device that is turned on and off by optical means.     

Low-loss high-speed switching devices, 2300-V 150-A static induction thyristor

Focussing attention to the performance of high-speed high off-state voltage and large current provided in the buried-gate-type static induction (SI) thyristor, a 2300-V 150-A low-voltage-drop high-speed medium-power SI thyristor was developed. Irrespective of the magnitude of switching current, the SI thyristor has the characteristics of fast turn-on time and less on-gate current compared to that of the GTO thyristor. The characteristics of this SI thyristor obtained as the result of manufacturing this prototype were such that the forward blocking voltage was 2300 V at a gate reverse voltage of -5 V, the reverse blocking voltage was 2350 V, and the forward voltage drop was 1.4 V at an anode current of 150 A and 2.2 V at an anode current of 450 A. The switching characteristics were such that the turn-on time was 1.5 µs when an anode current IAof 150 A becomes ON, turnoff time was 2.5 µs at IA= 100 A and 3.6 µs at IA= 200 A. This SI thyristor is able to break the anode current of 1000 A at a gate current of 95 A. Performance exceeding 1100 A/µs was confirmed for the di/dt capability and even for dv/dt, and these normally can be operatable even at 100 times higher current compared with maximum average current.     

Controlled turn-on thyristors

In a one or more amplified stage thyristor design it is possible to control the peak current level of all but the final stage with impedance built into the p-base zone. This impedance reduces both the current and the duty cycle of the protected amplifying stage effectively protecting it from undesirable temperature rises during turn-on. A further bonus and perhaps equally important is the fact that the amplifying stage and its current control impedance can be used to reduce and essentially fix the voltage level at which the following stage turns on. This results in a lower voltage, lower stress turn-on of the following stage, and a device essentially protected from di/dt turn-on failure. This paper describes several aspects of controlled turn-on in the context of a 2.6- and 6-kV light triggered thyristor. In particular we discuss selection of the resistor value, the problem of unwanted current control resistor modulation by device current as well as some factors affecting the proper wattage of such resistors. We also discuss the role current control resistors can play in controlling avalanche current from known locations on the device.     

High dI/dT light-triggered thyristors

Directly light-triggered, 4000- and 6000-V thyristors were designed, fabricated, and tested to obtain high performance in dI/dt, dV/dt, and photosensitivity. Built-in resistors protected both auxiliary stages during high dI/dt turn-on. The novel use of etched moats 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/µs at 60 Hz was measured on a 6000-V thyristor, and 850 A/µs was safely triggered with only 1.8 mW of light. The dV/dt immunity of the photogate structure measured 4000 V/µs, rising exponentially to 80 percent of 4000 V, VDRM. 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 mechanism responsible for occasional burn-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 dI/dt performance with minimal light power and for nondestructive dV/dt triggering.     

Progress in light activated power thyristors

Light activated power thyristors would have considerable advantages in intermediate- and high-voltage circuits, as power and trigger circuits could be electrically separated by use of glass fiber cables. Besides high-voltage capability, such devices must have turn-on delay times, dv/dt capabilities, and di/dt stabilities which are comparable to conventionally fired thyristors. The necessary trigger power, however, has to be kept low enough to enable firing with GaAs light emitters, which are available now or will be in the near future. The dv/dt sensitivity is an essential limitation for the reduction of the minimum necessary trigger power. Optimizing of the thyristor emitter shunts results in an already acceptable compromise, but much better results can be obtained by a gate structure which actively compensates dv/dt fault triggering. Our test devices show good turn-on behavior. A short survey on different GaAs-light sources and the coupling problem is given.     

The dV / dt capability of field-controlled thyristors

A detailed analysis of thedV/dtcapability of field-controlled thyristors is presented. It is demonstrated for the first time thatdV/dtinduced turn-on can occur in these devices due to gate debiasing as a result of capacitive gate current flow if a large series gate resistance is present in the circuit. A theoretical analysis of thedV/dtcapability is presented based upon this mechanism which predicts that thedV/dtcapability will decrease inversely with increasing gate series resistance at low values and become independent of the gate series resistance at very high values. The quantitative calculations of thedV/dtcapability that have been made by using this theory are in very good agreement with measurements taken on asymmetrical field-controlled thyristors fabricated from wafers of various thickness. The results obtained in this study allow the conclusion that thedV/dtcapability of field-controlled thyristors are superior to that of conventional thyristors.     

The di/dt capability of thyristors

This paper describes an experimental investigation of the di/dt failure mechanism of thyristors. The location of the initial turn-on region and the spread of the "on" region were observed on a specially designed thyristor having many monitoring electrodes. The turn-on process was studied for triggering by gate, by breakover, and by dv/dt. In many cases it was found that turn-on occurred at almost the same region, whether it was triggered by breakover or by dv/dt. This area coincided with the final holding position in the turn-off process. The di/dt capability of the thyristor was measured. It was found that the capabilities were almost the same for the three triggering methods. The destruction temperature in the di/dt test was estimated from the area of the burn-out spots and the energy dissipation.     

Improvement of thyristor turn-on by calculation of the gate-cathode characteristic before injection

The ohmic part of the gate-cathode characteristic of thyristors has been calculated numerically. Normal and amplifying gate structures as well as emitter shorts are included. By a simple extension of the model, the case of the dv/dt and breakover turn-on has also been treated. It is thus possible to calculate the minimal control current and voltage for a cathode side thyristor geometry and to optimize the device with respect to the turn-on process.     

A thyristor protected against di/dt failure at breakover turn-on

The principle and the operation of a thyristor that can be turned on by exceeding its breakover voltage are described. The principle uses the concept of an auxiliary thyristor amplifying the small breakover current to a large gate current for the main thyristor. In this arrangement the breakover turn-on has to occur first in the auxiliary thyristor. This is ensured by a doping of the n-base of the auxiliary thyristor which is higher than that of the n-base of the main thyristor. Time resolved infrared photographs of the breakover turn-on are presented. Also, infrared photographs of the breakdown radiation from p-n-p structures are used to give a survey on the starting silicon which already contains the inhomogeneous doping.     

A low-loss high-speed switching device: The 2500-V 300-A static induction thyristor

An anode-emitter shorted-type 2500-V 300-A buried-gate static induction (SI) thyristor was fabricated and resulted in a very-high-speed turn-on time of 2.0 µs and a turn-off time of 3.1 µs, both at 1000 A, and in very low-loss performance due to the reduction of the tailing current. The switching loss and the conduction loss of the high-power SI thyristor is for the first time evaluated in this paper. Snubber-circuitless operation is demonstrated for the first time for the high-power SI thyristor.     

Functional integration of the light-triggered static induction thyristor and the static induction phototransistor

An integrated structure of the Light-Triggered and Light-Quenched Static Induction (LTQ SI) thyristor is introduced and is fabricated by the combination of the SI thyristor and the static-induction-transistor (SIT) process technology. The device consists of a buried-gate light-triggered (LT) SI thyristor and a p-channel surface-gate static induction phototransistor (SIPT). The analog voltage VAKof 250 V at the anode Current IAKof 2 A (600 A/cm²: channel current density) is optically switched with a triggering power ofP_{LT} = 8.8mW/cm²(150 µW) and a quenching power ofP_{LQ} = 8.8mW/cm²(88 µW) in a turn-on time of 1.2 µs and a turn-off delay time of 1.2µs. The integrated LTQ SI thyristor is a novel type of the self-turn-off power switching device which is turned on and off by optical means.     

A high-power gate-controlled switch (GCS) using new lifetime control method

A high-power gate-controlled switch (GCS) with high switching speed was developed using a new method for controlling minority-carrier lifetime where both iron and gold were doped into the device. An improved temperature dependence of the forward voltage drop of the device was obtained because each of the forward voltage drops determined by iron and gold has opposite temperature dependence. The lifetime was controlled reproducibly by two-step diffusion of lifetime killers, that is, iron diffusion at high temperature and gold diffusion at lower temperatures afterwards. The relation between the forward voltage drop and the lifetime was theoretically analyzed and the agreement between the theory and experimental results was fairly good. The GCS of 0.15-cm²active area has the ratings of blocking voltage of 1500 V, available turn-off current of 160 A, forward voltage drop of 3 V at anode current of 100 A, and turn-off gain of 9. The turnoff time and turn-on time of less than 2 µs could be obtained. Thedv/dtanddi/dtare 1000 V/µs and 500 A/µs, respectively. The operation of 50 kHz at 100 A/1000 V could be realized with the inductive load of 50 µH by the GCS. The SIPOS (SemiInsulating POlycrystalline-Silicon) passivation was applied to the GCS in order to obtain the high reliability.     

Observation of turn-on action in a gate-triggered thyristor using a new microwave technique

The turn-on action by the p-base and n-emitter gates in a thyristor was studied by a new microwave technique. The initial conducting area, the lateral distribution of gate current flowing through the junction, and the time variation of excess carrier density injected into the n-base by the gate current were determined by measuring the reflection of microwave energy, vertically incident upon a small area (0.2 × 0.2 mm²) of the n-emitter layer. The new microwave technique has proved to be useful in designing new gate structures and in studying the operation of new devices.     

High-voltage high-power gate-assisted turn-off thyristor for high-frequency use

The fast switching thyristor with an integrated rectifier-diode connected in antiparallel to the cathode-emitter junction of auxiliary thyristor has been made. This thyristor has the ratings of blocking voltage 1200 V, average current 400 A. The turn-off time of less than 6 µs can be obtained by applying -10 V gate bias. It has the interdigitated gate structure and the high-frequency current rating more than 500 A at 10 kHz.     

Behavior of thyristors under transient conditions

The capability of gate-triggered thyristors to withstand steep wavefront, high-current pulses (i.e., di/dt capability) is a function of both junction temperature and frequency of operation. Localized internal heating occurs during turn-on and may lead to thermal runaway. The conditions required for this to occur have been determined by destructively testing many devices. The initial conducting area of a thyristor largely determines di/dt capability, which is not necessarily related to the size of the device but is a function of the design of the gate region. Gate drive is very important for determining the di/dt capability of a thyristor having a conventional gate design. Two devices which have been designed to increase the initial conducting area are discussed. One of these devices, if improperly designed, can lose its effectiveness with high gate drive. This characteristic can be studied by observing the reverse recovery current immediately following short forward current pulses.     

金属氧化物半导体控制晶闸管的di/dt优化

金属氧化物半导体控制晶闸管(MCT)相比于绝缘栅双极型晶体管(IGBT)具有高电流密度、低导通压降和快速开启等优势,在高压脉冲功率领域具有广阔的应用前景.作为脉冲功率开关,MCT开启过程对输出脉冲信号质量有很大影响.采用理论分析并结合仿真优化重点研究了MCT开启瞬态特性.通过对MCT开启过程进行详细地理论分析推导,给出了MCT开启过程中阳极电流和上升时间的表达式.结合Sentaurus TCAD仿真优化,将MCT开启过程中电流上升速率(di/dt)由40 kA/s提升至80 kA/s,极大地改善了器件开启瞬态特性.最后,总结提出了提高器件开启瞬间di/dt的设计途径.