Analysis and Simulation of Simple Transistor Structures Exhibiting Negative Differential Resistance

We investigate analytically and via simulations one-port circuits consisting of two bipolar junction transistors (BJT’s) and a few linear resistors connected in a feedback structure. These circuits possess topologies and parameter values such that their terminal one-port i v characteristics exhibit a negative differential resistance (NDR) region. These structures have been used in the past to model silicon controlled rectifier (SCR) devices and the latch-up phenomena in CMOS integrated circuits. We show analytically that the large voltages across transistor pn junctions predicted by SPICE are not numerical artifacts of this simulator, but are intrinsic properties of the circuits. Our analysis explicitly accounts for the influence of the Early voltage effects on the calculations of the break-over voltages and currents, and suggests that this sometimes neglected effect may indeed play a dominant role in these circuits’ behavior.