CURRENT-TEMPERATURE FEEDBACK EFFECTS IN III-V HETEROJUNCTION BIPOLAR-TRANSISTORS

The consequence of the reciprocal relation between the temperature and current distributions in heterojunction bipolar transistors (HBT's) h as been determined. The dc current-voltage (I-V) characteristics. RF s mall-signal parameters, and temperature distributions of discrete devi ces with emitter fingers of varying lengths were analyzed empirically and their thermal profiles calculated numerically. The lateral tempera ture gradient induced in the finger due to power dissipation under nor mal operating conditions is shown to directly affect the current distr ibution 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 relationshi p leads to higher localized current densities in the hottest portion o f the device, the center of the emitter. The temperature of the hot se ction rises with increasing power dissipation, continually drawing mor e current. Ultimately, the current through HBT's is localized to a com parable area at the finger center, independent of the emitter length.