ON BISTABLE BEHAVIOR AND OPEN-BASE BREAKDOWN OF BIPOLAR-TRANSISTORS IN THE AVALANCHE REGIME - MODELING AND APPLICATIONS

In the presence of carrier multiplication in the base collector diode the I(c)(I(b), V(ce)) characteristics of bipolar transistors show a bistable region of operation. This phenomenon is investigated experimentally and theoretically with the help of the extended Gummel/Poon model. Analysis of the output characteristics yields two breakdown voltages which mark the boundaries of the bistable regime on the V(ce) axis. A generalization of Miller's formula is used to model the decrease of the multiplication factor with increasing transfer current, required for proper description of the input characteristics in bistable regime. The input characteristics in the bistable regime are used for a discussion of a special BiCMOS SRAM cell that only employs two devices. Formulas for LO and HI level, as well as the switching point are derived and used for a discussion of power dissipation, area requirements, and noise immunity of the cell, as well as the effects of temperature variation, and device degradation. Bistable transistor operation can also be exploited for detection and switching of signals-this possibility is briefly discussed, focussing on threshold control and the realization of fast optically controlled switches.