Lateral current flow in thyristors with partially shorted cathodes—II. Three-terminal operation, theory of turn-on area, plasma spreading velocity and the effect of gold doping
The lateral current flow IG( x) in the p-gate region of a thyristor under three-terminal operation in the high-impedance region is investigated, the cathode except where stated, being partially short-circuited. For the simple model employed it appears that a large fraction of the gate current drive flows out of the shorting dot (SD). In the region of high cathode bias, close to the gate contact, IG( x) follows a very different—though admittedly estimated—course compared to the two-terminal case of paper (I). It falls in value with increasing distance from the gate contact until VK the cathode junction voltage bias has fallen to such a low value that IG( x) starts rising again similarly to the two-terminal case. Estimates are also made of VK( x) and JK( x) the cathode current density and a theory of the turn-on area is put forward based upon the assumptions of (a) a much faster n- p- n transistor response compared to the p- n- p section and (b) a critical cathode current density derived from the two-terminal (no SD) breakover condition. The influence of gold doping is surmised in keeping with the measured differences between gold-doped and nominally gold-free devices. Finally, as in paper (I) an expression for effective current gain is sought for three-terminal operation proving successful for an n- p- n transistor but more elusive for a thyristor structure. Nevertheless the equations graphically illustrate the influence of the SD in reducing the applied gate drive to an effective gate drive. Approximate analytical solutions are obtained for IG( x), JK( x) and turn-on area, the latter two also as a function of gate drive. This solution also yields important constants of the theory. Initial computer calculations confirm this.