Lateral current flow in thyristors with partially shorted cathodes—II. Threeterminal operation, theory of turnon area, plasma spreading velocity and the effect of gold doping
Abstract
The lateral current flow I_{G}( x) in the pgate region of a thyristor under threeterminal operation in the highimpedance region is investigated, the cathode except where stated, being partially shortcircuited. 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, I_{G}( x) follows a very different—though admittedly estimated—course compared to the twoterminal case of paper (I). It falls in value with increasing distance from the gate contact until V_{K} the cathode junction voltage bias has fallen to such a low value that I_{G}( x) starts rising again similarly to the twoterminal case. Estimates are also made of V_{K}( x) and J_{K}( x) the cathode current density and a theory of the turnon 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 twoterminal (no SD) breakover condition. The influence of gold doping is surmised in keeping with the measured differences between golddoped and nominally goldfree devices. Finally, as in paper (I) an expression for effective current gain is sought for threeterminal 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 I_{G}( x), J_{K}( x) and turnon 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.
 Publication:

Solid State Electronics
 Pub Date:
 February 1978
 DOI:
 10.1016/00381101(78)902678
 Bibcode:
 1978SSEle..21..373F