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In the common-emitter transistor switching application, there are occasions in which the collector supply voltage exceeds the transistor sustain voltage. Consequently, the load line could intersect the negative resistance characteristic of the device in the I /sub C/ -V /sub CE/ plane, resulting in a possible unstable or latch-up condition. The purpose of this paper is to study analytically the avalanche region characteristics and their implications for transistor switching applications. The first part is concerned with the derivation of the direct current-voltage relation that, when viewed from the output terminal, represents a negative resistance. The characteristic of this negative resistance depends on the base-emitter circuit condition. The ac terminal behavior is treated in the second part where consideration of the frequency dependence of alpha leads to an equivalent circuit consisting of an inductance in series with a negative resistance. Both elements are nonlinear as well as frequency-dependent. With an external load connected to this nonlinear circuit, a technique of nonlinear analysis is employed to investigate the circuit stability. From this analysis, latch-up and oscillation phenomena in the transistor switching circuit can be predicted. Since the second breakdown involves additional mechanisms besides avalanche multiplication, it is not discussed in this paper.