Date of Award
Doctor of Engineering Science in Electrical Engineering
Raj Pratap Misra
W. H. Warren Ball
Research and Development was done on gate turn-off devices. The aim was i) to improve turn-off capability without compromise of turn-on, on-state, and useful temperature range, ii) to implement "safe" turn-off.
A) The dependence of the electrical device characteristics on cathode width for an optimized vertical structure was determined. Fall times of 100 to 200 nsec and risetimes of <400 nsec were simultaneously obtained for average current densities of 53 A/cm2 @ Tj = 125°C. From spreading resistance analysis the fast tr and tf response is shown to be a consequence of proper Au-distribution in the active device volume.
From transient analysis it is concluded that for safe turn-off d2iA/dt2 should never become positive during the fall phase, and that the differential turn-off gain should remain less than unity during the tail period. These requirements are realized by means of a voltage source with a series inductance as gate input for turn-off. As a result a switching capability of 1.56kW (resistive load) @ 75°C case temperature at 50kHz and with 97% device efficiency is obtained for a chip of 0.15cm2.
B) Anode shorts for improvement of turn-off without the necessity of excessive lifetime reduction were also investigated. These shorts substantially reduce the turn-on sensitivity.
A Schottky barrier in series with the non-regenerative region, which parallels the thyristor section, restored the turn-on sensitivity at low temperatures while retaining the turn-off capability at high temperatures.
For 30A (J = 200 A/cm2) turn-off @ +125°C the gate trigger current was q,10mA and 60mA @ -40°C for devices with Schottky barriers and without Schottky barriers, respectively.
C. The incidence of catastrophic failures due to filamentary burn-out was drastically reduced through introduction of a dynamic ballasting (defocusing) concept. This approach features a resistively ballasted cathode ("7Ω/square) with an insulated center, i.e., contacted only at the periphery.
Therefore, the formation of small area, high current density filaments is largely inhibited. Using this principle, device operation was extended from -20°C to +125°C for conventional devices, to a range from -60°C for sensitive turn-on (Igt = 300μA) to +150°C for safe turn-off @ IT off(max) = 8.5A (J = 55A/cm2) for the ballasted devices.
Becke, Hans Werner, "Investigations of gate turn-off structures" (1981). Dissertations. 1257.