The initial region of the characteristics of a transistor in the common-emitter connection

Abstract

The emitter-base forward characteristic with the collector at base potential is given as the physical boundary line of the initial region of the collector and emitter characteristics, respectively, in the common-emitter circuit. Using two examples, the formation of the initial region of the emitter and collector characteristics, respectively, of constant input voltage is examined and explained. The emitter-current/collector-voltage curve at constant input voltage is discussed. The rise of emitter current at constant input voltage is explained by a decrease in input resistance, caused by the collector-base forward current. The decrease of emitter current in the region of small collector voltages arises from the increase in the charge carrier concentrations at the collector junction, by which the gradient of the charge carriers between emitter and collector is smoothed and thus the current is decreased. To explain the collector characteristics a current distribution factor Ax is defined in the initial region, the value of which is in approximate agreement with the current gain A beyond the initial region. With the current distribution factor so defined and using the collector-base forward characteristic, we can construct, with the aid of the emitter characteristics, a family of collector characteristics which, within the range of the accuracy of measurement, corresponds to the measured values. The dependence of the envelopes limiting the collector characteristics to small collector voltages and of the collector voltage, whose collector current is zero, upon the geometry of the transistor is explained. A simple construction of the envelope is shown. Steep envelopes and small collector voltages for IC = 0 may be obtained by means of a high current gain, a small distance between base and emitter and as little overlapping of the projections of base and collector as possible.