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.     

Initial turn-on area of gate-controlled thyristors

The initial turn-on conditions have been examined for gate-controlled thyristors. Thermal measurements by means of an i.r. microscope detecting surface temperature show that hot spots are formed around the gate, confirming previous reported results obtained by recombination radiation measurements. Electrical measurements on a specially prepared device with a cathode divided into a number of sectors prove that these hot spots are not formed due to initial load current pinching effects, but to a current preference in certain crystallographic directions. The hot spot formation is strongly influenced by the blocking voltage level and the gate current level as well as by the mechanical stress conditions in the gate region. Some suggestions are made concerning the qualitative physical understanding of these phenomena.     

Probed determination of turn-on spread of large area thyristors

Thyristors (SCR's) were specially constructed to permit the direct observation of the lateral spread of turn-on within the device. The effects on the spread of turn-on of the load current, basewidths, temperature, anode-cathode voltage, gate control pulse, and a large inhomogeneity were observed. The spreading velocity of the on-state and the load current are related approximately by the expressionV^{n} propto Iat high load currents. The spreading velocity is higher in devices with narrower basewidths and increases with temperature. Neither the anode-cathode voltage before turn-on nor the gate control pulse affect the spreading velocity of the on-state. Measurements on end gate devices and center gate linear devices show that triggering at the center enables an equivalent area of the SCR to turn on in less time than occurs when triggering at the end. A large gap in the emitter layer will delay the spread of the on-state but will not necessarily stop the spreading.     

Investigation of the lateral turn-on process of thyristors with an epitaxial p-base

In this paper, the effect of the p-base doping concentration NAon the spreading velocity vsin power thyristors is examined. Chemical vapor deposition (CVD) techniques are used to produce the p-base layer, in order to change the p-base doping concentration and thickness independently. The results show a large reduction of vswith growing p-base doping concentration. At a doping concentration higher than 5 × 10¹⁶/cm³the spreading velocity follows a power law with an exponent of -0,9. The introduction of a sandwiched low-doped player between the p⁺-base and the n-emitter slows down the plasma propagation. The decrease of vs, in both cases, is attributed to the reduction of the current gain β2of the n-p-n transistor with doping concentration. In order to explain this behavior, a simple expression for the spreading velocity is derived, which relates the spreading velocity to the time constant of current rise trand consequently to the feedback loop gain (β1β2) of the two transistor components. In this derivation, only the lateral drift current in the p-base is taken into account. It was found that vsis given byv_{s} sim 1/t_{r} sim ln beta_{1}beta_{2}, in good agreement with the experiment.     

Role of the amplifying gate in the turn-on process of involutestructure thyristors

The switching characteristics of involute thyristors with and without the amplifying gate structure are discussed. The effects of peak gate currents (10-100 A) on the anode current di/dt, switching delay, and energy loss in both types of devices are presented. The performance of the devices without the amplifying gate was far superior than that of the devices with the amplifying gate. A model is presented to explain this difference. Thyristors without the amplifying gate successfully switched anode currents on the order of 12.6 kA, at a di/dt of 100000 A/μs, from an anode voltage of 2 kV on a single-shot basis     

Computer-aided experimental investigation of the correct turn-on in thyristors with amplifying gate

A 2-dimensional computer model has been developed for the analysis of the amplifying gate thyristors correct turn-on at the auxiliary emitter prior to the main one. The results of investigation were used in the design of devices having essentially the same di/dt high capability in any possible turn-on conditions.     

A quasi-stationary treatment of the turn-on delay phase of one-dimensional thyristors: Part I—Theory

A qualitative discussion of the turn-on delay phase of one-dimensional thyristors is given, taking into account the effect of the charge of the free carriers on the width of the space-charge region. It is shown that in a resistive load circuit the slope of the load current rise goes to infinity for finite values of a critical current density. Along a simple quasi-stationary model the influence of varying voltage, n-base doping, n-base width, and gate current on the load current rise and on the critical current density is discussed quantitatively.     

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     

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.     

Gate current flow in cylindrical thyristors with partially shorted cathodes—two and three terminal operation with inner and outer gate configuration—turn-on area

Following on the work of linear thyristors, the gate current flow in cylindrical thyristors is investigated with partially shorted cathodes. Once more the models employ single shorting “dots” (SD) for the two symmetrical configurations possible for the cylindrical geometry, inner gate—outer SD and inner SD—outer gate for a range of sheet resistance values (?/W) for the gate region.The non-linear differential equation governing gate current flow is given by y″+ξ^?1y′(y?1)?Cy = 0 (y = normalised gate current, ξ = normalised radial co-ordinate) with Non-Cauchy boundary conditions and the latter difficulty is overcome both for two and three terminal operation by similar methods previously employed, when solving for y(ξ) for a variety of device geometries and (?/W) values as well as bias conditions. The theory of turn-on area in terms of a critical cathode current density J_KB is also applied to this cylindrical model and expressions are obtained for the gate drive I_GD required to achieve a given turn-on area. Of particular interest are the plots of gate current flowing into the SD as a function of I_GD for a variety of (?/W) values as well as for the two possible gate/SD configurations.     

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.     

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     

Breakover phenomena in field-controlled thyristors

The occurrence of breakover in the forward-blocking anode current-voltage characteristics of field-controlled thyristors is analyzed. It is demonstrated that this breakover occurs due to gate current flow which causes a debiasing of the gate potential in the presence of any series gate resistance. Based upon this mechanism, analytical expressions have been derived which describe the anode current-voltage breakover characteristics. These theoretical expressions have been found to be in very good agreement with the measured data obtained from asymmetrical field-controlled thyristors fabricated with various device thicknesses. From the analysis presented in this paper, it can be concluded that the occurrence of breakover in the forward-blocking characteristics of field-controlled thyristors can be minimized by ensuring low series gate resistances which biasing the devices and by using gate bias voltages well in excess of the minimum value required to block any desired anode voltage.     

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     

Switching characteristics of electron-irradiated MOS-controlled thyristors

MOS-controlled thyristors (MCTs) were designed and fabricated. The effect of electron irradiation on static and dynamic characteristics has been studied. Electron irradiation was found to substantially reduce the MCT turn-off time. An increase in the controlled current density was observed.