78 A Brief Review of Technology4.3 b Current DependenceFigure 4.5 shows the dependence of b on the collector current. b drops off at high currents because the electron concentration in the base-collector depletion region becomes comparable to the background dopant ion concentration, leading to a dra-matic increase in the effective width of the base. This is called the Kirk effect or base pushout. As a result, the base resistance is current dependent. Another effect is emitter crowding, which comes about because of the distributed nature of parasitic resistance in the base region, causing the junction forward bias voltage to be higher at the emit-ter periphery closest to the base contact. This results in the highest current density at the edge of the emitter. In the other extreme, at low currents, b may be reduced due to the excess current resulting from recombination in the E-B depletion region.4.4 Small-Signal ModelOnce the bias voltages and currents are determined for the transistor, it is neces-sary to determine how it will respond to alternating current (ac) signals exciting it. Thus, an ac small-signal model of the transistor is now presented. Figure 4.6 shows a fairly complete small-signal model for the bipolar transistor. The values of the small-signal elements shown, rp, Cp, Cm, gm, and ro will depend on the dc bias of the transistor. The intrinsic transistor (shown directly under the emitter region in Fig-Figure 4.5 Current dependency of b.Figure 4.6 Small-signal model for bipolar transistor.