Theory and measurements of thyristors with particular reference to lateral effects

Abstract

The characteristics and theories of four-layer pnpn semiconductor devices have been considered with particular reference to the two transistor analogy of thyristor in order to obtain an expression for the current-voltage characteristics of the device. The Small-signal, low-frequency current gains of the npn and pnp transistor sections of high and low power thyristors were measured by a three terminal method originally employed by W. Fulop. The technique is shown to be applicable to measurements on high power thyristors. The current gains of two transistor component were also measured as a function of temperature at a series of anode currents. From this it can be shown that the temperature dependence of two-terminal latching current can be qualitatively estimated from the plot of current gain measured at a series of temperature for constant anode current. Analysis of instability of devices with temperature showed that-, the saturation current-of isolated centre junction plays a dominant role. Gold doping predictably leads to low current gain of the pnp transistor section. Current gain measurements of thyristors with and without shorting dot showed an. almost similar variation with frequency. For both type of devices αnpn0 was almost equal and even closer at high temperature (1000C). The theory of the influence of shorting dots on current flow in thyristors outlined in a report by W. Fulop, was developed. The value of IL’ hole lateral current in the p-base is found and its dependence upon p-base width and shorting dot area investigated. The voltage distribution in the two central base regions just after the device has switched on but before the plasma has had time to spread is investigated. It was found that junctions J1 and J2 become reverse biased and share the bias reverse voltage. Calculations shows that for very large V OFF' the voltages in these two junctions are equal. In order to have a better understanding of thyristor behaviour, the set of one-dimensional non-linear partial differential equations describing the Poisson's equation and the two current continuity equations are solved numerically under steady-state conditions. The potential distribution and the hole and electron current density distribution within the device plotted one-dimensionally are given.