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 expression
V^{n} \propto I
at 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.