Direct evidence supporting the premises of a two-dimensional diode model for the parasitic thyristor in CMOS circuits built on thin epi

A key concept of a recent model to describe the holding point of thyristors in epitaxial CMOS is that due to conductivity modulation, the two-dimensional (2-D) thyristor is equivalent to a p-i-n diode of the same geometry. Here, the physical origin of negative resistance near the holding point is traced to resistance modulation in the epi. This positive feedback mechanism, inherent in the model, is conceptually different from that of the conventional model, in which negative resistance arises out of an increase in the feedback gain as the bipolar transistors come out of saturation. The I-V characteristics of thyristors and p-i-n diodes obtained from PISCES are shown to be almost identical. If the conventional mechanism were solely responsible for the negative resistance region, a thyristor would exhibit negative resistance while a 2-D diode would not. Experimental evidence showing that diodes exhibit negative resistance regions is presented to support the feedback mechanism of the model.<>