| The gate turn-off thyristor (GTO) is the only power semiconductor, with self turn-off capability, that can be used for switching voltages above 2 kV, or currents above 1500 A. In spite of being in existence for the past three decades, it remains a very poorly understood device, mainly due to the complexity of the turn-off process, and its potential largely unexploited. As an important step toward better design of these devices, it is essential to understand the dependence of the various modes of operation on the physical parameters and dimensions. In our physical modeling work, we have tried to raise the general level of understanding by developing new physical analytical models for the on-state and the turn-off mechanisms from first principles, and design considerations to link the blocking capability with steady state dissipation. No fitting parameter was used in any of the analyses.; Our turn-off model, originating in the current continuity equations, characterized the squeezing of minority carrier plasma, addressed the physical basis for a minimum "on" region dimension (typically 25 {dollar}mu{dollar}m), and predicted, for the first time, the dependence of storage time on the anode and gate currents, device dimensions and physical parameters. The predictions are in good agreement with our experiments at G.E., Malvern, and are also corroborated by measurements of other workers that were unexplained before our work.; A new physical model was also developed to analyze the temperature dependent steady on-state mode of operation. It includes Auger and SRH recombination, the dependence of the carrier mobilities on temperature and injection level, and the effect of device geometry. Experimental current-voltage characteristics for anode currents upto 3000 A, obtained from G.E. devices at 25{dollar}spcirc{dollar}C and 115{dollar}spcirc{dollar}C, corresponded closely with the theoretical predictions.; Also, design procedures were developed, for the first time, to demonstrate the overall superiority of the GTO with a two layered n base over the conventional pnpn structure (A blocking capability of 8000 V requires 1100 {dollar}mu{dollar}m of n base for the latter while only about 780 {dollar}mu{dollar}m for the former). Our design calculations linked the forward blocking and on-state characteristics of each device in terms of their dimensions, doping densities and basic physical parameters. Best blocking voltage versus on-state voltage characteristics were predicted for a doping density of 5 {dollar}times{dollar} 10{dollar}sp{lcub}12{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar} in the high resistivity n base. These calculations can be directly used by device designers to select the set of parameters most suited to a given application. |
