Plasma spread in high-power thyristors under dynamic and static conditions

The plasma spread properties of various thyristors were studied utilizing an infrared viewing technique. The plasma velocity occurring prior to equilibrium, and plasma spread conditions at equilibrium were determined. Velocity versus current density diagrams were generated for low-frequency, high-voltage and high-frequency, low-voltage devices. Effect of the initial turned-on line on plasma spreading is discussed.     

PIN photodetector under transient conditions: Simulation and experiments

An exact simulation of a Silicon PIN photodetector with transient light excitation is presented. The photodetector under consideration is designed to have good responsivity and fast response time in the wavelength range 0.6–0.95 μm which is of interest in fiber optic communication systems driven by GaAs and AlGaAs light sources. The photocurrent transient is computed for step and impulse light excitation using various values of reverse voltage, intrinsic layer thickness and doping. Good agreement is obtained between calculated and experimental waveforms.     

ESD作用下晶闸管dV/dt触发导通规律研究

针对电磁脉冲作用下晶闸管（silicon controlled rectifier,SCR）意外导通事故频发的问题,选择静电放电（electrostatic discharge,ESD）作为典型电磁脉冲源,对比分析了小电流SCR的ESD敏感度特性,确定了ESD作用下SCR的失效模式为门极电压作用失效、阴阳极断路.采用理论和试验相结合的方法得出了SCR意外导通的开启时间仅与阳极电压和器件本身特性有关的结论,并进一步通过方波电磁脉冲注入的对比试验,揭示了ESD作用下SCR因dV/dt触发导通的开启时间只与ESD注入电压有关:注入电压越高,开启时间越短.     

GTO thyristors

Major aspects of gate-turn-off (GTO) thyristors are discussed, including device modeling, design considerations, basic research on their switching phenomena, electrical characteristics, and applications. A device design is considered which would increase the maximum interruptible anode current I_ATO and blocking voltage while decreasing the switching time and power dissipation. The most difficult design problem is to determine the dominant factors that affect I_ATO. From experimental and computational results, it is found that I_ATO is increased by a reduced p-base sheet resistance, a thicker n-base layer and an increased gate-cathode breakdown voltage. The turn-off performance is also improved by introducing several modified device structures, such as an anode-shorted emitter construction, an asymmetric n⁺-doped based structure, a buried gate, and a cathode emitter heterojunction GTO thyristor. Typical characteristics are given for a 5000-V 3000-A unit. GTO applications are discussed, including variable-voltage variable-frequency inverter-controlled AC induction motor drive systems and PWM converter systems     

Measurement of the transient junction temperature in MOSFET devices under operating conditions

The capabilities of three techniques to measure the transient average junction temperature in power MOS devices based on the electrical thermo-sensitive parameters are assessed experimentally and by compact device simulation. The first two methods make use of the dependency of dI_ds/dt on the temperature, while the third one exploits the temperature dependency of the turn ON delay of the device.     

A numerical approach based on transient thermal analysis toestimate the safe operating frequencies of thyristors

The spatio-temporal distribution of temperatures in high-power SCRs used for switching high di/dt current pulses were simulated using the finite element method (FEM). Two types of SCRs, with amplifying gate (unshorted device), and without amplifying gate (shorted device) structures, were analyzed. The details of the numerical simulation, such as the meshing strategy, the heat source model and the boundary conditions are discussed. Based on the analysis, the failure temperature of the unshorted device was computed to be 1100°C. The peak temperature in the shorted device was, however, found to be 335°C. The instantaneous cooling cycles of the devices and their cooling time constants, as obtained from the simulations, are presented. Based on these parameters, the safe operating frequencies of these devices were estimated     

Deep level transient spectroscopy under conditions of current-carrier exchange between two allowed bands

Some characteristic features of deep level transient spectroscopy (DLTS) spectra of deep levels which effectively exchange current carriers with both allowed bands are studied. It is shown that the maxima of the peaks shift very little on the temperature scale, but analysis of the spectra by the conventional methods leads to errors in the determination of the parameters of the deep levels (ionization energies, capture cross sections). Several methods for determining these parameters more accurately are proposed and a numerical example of such an analysis of DLTS spectra is presented. Fiz. Tekh. Poluprovodn. 31, 437–440 (April 1997)     

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.     

Quasi-three-dimensional spice-based simulation of the transient behavior, including plasma spread, of thyristors and over-voltage protectors

This paper describes a fast and reliable circuit-based simulation method of two-dimensional electrical transient characteristics of thyristors and over-voltage protectors (TOVP). Either device is divided into four-layered square prisms, and a one-dimensional PNP–NPN transistor pair model associated to each of them. The transistors are represented with the Gummel–Poon model, complemented with breakdown and quasi-saturation sub-models. The cells in turn are coupled through the base planes, with current-modulated resistors. Plasma-spreading velocities obtained through simulation are comparable to experimentally obtained ones reported elsewhere. Spatial current density instabilities induced by device asymmetries are also presented. The required spatial resolution was attained by representing these four-layer devices with up to 40,000 Spice circuit elements, including 3000 bipolar junction transistors. The associated Spice lists were assembled effortlessly with a Mathematica program. Simulations took from 1 to 3 h in a 650 MHz Pentium III computer, with no symptoms of convergence problems whatsoever. The authors believe this is the first time that Spice based high-resolution transient behavior simulations of TOVPs and thyristors are presented in the literature.     

Gate-assisted turnoff thyristors

A study of the turnoff physics in gate-assisted turnoff thyristors (GATT's) leads to a proposed mechanism involving the gate bias acting to prevent a forward voltage from appearing on the cathode rather than, as was previously thought, to sweep out excess carriers. It is shown that cathode shunting can be used in GATT's to virtually eliminate an important failure mode and to decrease the gate voltage needed to produce the desired improvement in turnoff time. Implications for designing GATT's are given, one being that a change in the lateral resistance of the p-base will have opposite effects depending on whether the cathode is shunted or not.     

Skip turn-on of thyristors

A lateral current in an emitter layer of a thyristor (SCR) is shown to vary the lateral field in the base layers and also to change the distribution of the current density injected from the emitter to the base. A method of using lateral emitter layer currents in an SCR to increase the spreading velocity of the on-region and, at higher currents, to turn on quickly areas of the SCR remote from the gate contact is demonstrated experimentally.     

A simple linear trigger circuit for thyristors working under variable-frequency anode supply

A simple linear firing circuit using IC chips for an AC voltage controller working under variable-frequency anode supply is described. Its performance for an inductive load supports the theoretical conclusions. Features of the proposed scheme include symmetrical firing for both halves of the supply voltage, adjustable pulse width to the thyristor gates and linear variation in the firing angle with the control voltage which is independent of the anode-supply frequency over a wide dynamic range 1-120 Hz. The scheme may easily be extended to three-phase circuits     

Investigations of transient thermal properties of conductively cooled diode laser arrays operating under quasicontinuous-wave conditions

Thermal properties of diode laser arrays not only affect electrical and optical characteristics, they can also become determining factors for long-term reliability of the devices. Here, we report on the investigations of transient thermal effects in high-power diode arrays operating under quasicontinuous-wave (QCW) conditions. A novel measurement setup for the transient junction temperature determination is proposed. We show the measurement results of conductively cooled diode laser arrays mounted on the conventional copper heat sink as well as on the thermal expansion matched CuW submount. The presented results allow evaluation of the thermal cycling experienced by the device during pulsed operation. In the following part, time-resolved wavelength and temperature variations within an array are presented. The guidance for heat sink material choice for high-power diode laser arrays designed for QCW operation is given.     

Electron irradiation of field-controlled thyristors

The influence of 3-MeV electron irradiation upon the characteristics of asymmetrical field-controlled thyristors has been examined for fluences of up to 16 Mrad. In addition to the lifetime reduction due to the radiation damage, carrier removal effects have also been observed in the very lightly doped n-base region of these devices. The leakage current, even after radiation at the highest fluenee, is not significantly increased and the blocking characteristies of these devices are not degraded. In fact, a small improvement in the blocking gain has been observed at low gate voltages. The electron irradiation has been found to increase the forward voltage drop during current conduction and to reduce the forced gate turn-off time. Gate turn-off times of less than 500 ns have been achieved by irradiation with a fluence of 16 Mrad. However, this is accompanied by a large increase in the forward voltage drop. Tradeoff curves between the forward voltage drop and the gate turn-off time have been obtained. From these curves, it has been determined that gate turnoff times of 1 µs can be obtained without a significant increase in the forward voltage drop for devices capable of blocking up to 600 V.     

The forward characteristics of thyristors

A theory on the forward V-I characteristics of P⁺-P-N-P-N⁺thyristors is proposed. Taking the minority carrier lifetime in the base region into account, the effects of the device structures on the forward characteristics are discussed on the following three cases: 1) low-level operation, 2) middle-level operation, and 3) high-level operation. At middle-level operation, the term that is independent of current and, at high-level operation, the √I dependency, appears in the forward characteristics of the thyristors. The general theory is illustrated by reference to experimental results on silicon-controlled rectifiers.     

High-energy pulse-switching characteristics of thyristors

Experiments were conducted to study the high energy, high di/dt pulse-switching characteristics of silicon controlled rectifiers (SCRs) with and without the amplifying gate. High di/dt, high-energy single-shot experiments were first done. Devices without the amplifying gate performed much better than the devices with the amplifying gate. A physical model is presented to describe the role of the amplifying gate in the turn-on process, thereby explaining the differences in the switching characteristics. The turn-on area for the failure of the devices was theoretically estimated and correlated with observations. This allowed calculation of the current density required for failure. Since the failure of these devices under high di/dt conditions was thermal in nature, a simulation using a finite-element method was performed to estimate the temperature rise in the devices. The results from this simulation showed that the temperature rise was significantly higher in the devices with the amplifying gate than in the devices without the amplifying gate. From these results, the safe operating frequencies for all the devices under high di/dt conditions was estimated. These estimates were confirmed by experimentally stressing the devices under high di/dt repetitive operation     

Analysis of n-channel MOS-controlled thyristors

The turn-off of the n-channel MOS-controlled thyristor (NMCT) is analyzed using two-dimensional simulation. A lateral NMOS-controlled thyristor structure, LNMCT, suitable for HVIC application is also proposed. It is found that the operation of a parasitic lateral n-p-n transistor in NMCT-type structures degrades the forward voltage drop and the turn-off capability and hence should be suppressed. The maximum controllable current in the NMCT is not only a function of internal parameters, but also depends on external supply voltage. This indicates that snubberless operation of an MCT-type device is not feasible. The advantages and disadvantages of the NMCT are compared with those of conventional MCT structures. The LNMCT turn-off speed is limited by the large amount of holes existing in the substrate, resulting in a turn-off waveform similar to that of an LIGBT     

Avalanche injection in high-speed thyristors

The aim of the present paper is to show the possibility of designing high-power semiconductor switches the turn-on time of which is as short as that of hydrogen thyratrons. These devices switch pulses of 10⁵W with turn-on times in the nanosecond range. In the OFF state all the voltage applied to a semiconductor switch is concentrated on the space-charge region (SCR) where there are assumed to be no free carriers. The process of switching into the ON state in a conventional thyristor switching mechanism is to fill the SCR with free electrons and holes injected from emitters by diffusion through base regions. The generation of carriers due to impact ionization in the SCR during the whole transition process accelerates switching. The avalanche injection (AI) suggested by Gunn [1] in diodes is the process providing impact ionization despite the voltage decrease in the device during its switching. At first we consider simplified AI processes and their potentialities in three-layer structures. Then the results are extended to more complex four- and five-layer structures by including the gate current. At the end the experimental data are given.     

Ultrahigh-voltage high-current gate turn-off thyristors

High-power GTO's with ratings of 2500 V . 2000 A have been developed, and a 4500 V . 2000 A GTO was trial fabricated and performance tested, for use in traction motor control equipment. Their low ON-state voltage was attained by applying a unique anode emitter shorting structure which does not require doping of a lifetime killer such as gold to obtain suitable GTO characteristics. Their high interrupt current was obtained by introducing a ring-shaped gate structure which has uniform operation between many segments in the devices during turn-off process.