The consequence of the reciprocal relation between the temperature and current distributions in heterojunction bipolar transistors (HBTs) has been determined. The dc current voltage (I-V) characteristics, RF small-signal parameters, and temperature distributions of discrete devices with emitter fingers of varying lengths were analyzed empirically and their thermal profiles calculated numerically. The lateral temperature gradient induced in the finger due to power dissipation under normal operating conditions is shown to directly affect the current distribution in the transistor. The negative temperature dependence of the HBT base-emitter junction turn-on voltage results in positive feedback between current and temperature. This current temperature relationship leads to higher localized current densities in the hottest portion of the device, the center of the emitter. The temperature of the hot section rises with increasing power dissipation, continually drawing more current. Ultimately, the current through HBTs is localized to a comparable area at the finger center, independent of the emitter length.
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