Analysis of high current effects on the performance of Pnp InP-based heterojunction bipolar transistors

The limiting effects of high current density operation on the performance of Pnp InP-based heterojunction bipolar transistors have been studied using a one dimensional, analytical model. To realize high gain and high frequency performance, the device must be operated at high current densities (approximately 104 A/cm2), though, for excessively high current densities, a series of effects, such as base pushout and an increase in the collector junction capacitance markedly degrade device performance. In this model we investigate the onset of these effects and describe their individual and collective impact on device performance for use in device design and bias point selection. As a starting point, saturation of the hole velocity in the base-collector space charge region imposes a finite hole concentration in the region and at the collector end of the base, which produces a reduction in the peak electric field, an expansion of the space charge region width and, eventually, the onset of base pushout. At the same time, the mobile holes produce an increase in the collector junction capacitance. The extent of these effects on the device's current gain, transit times and cutoff frequency are calculated as a function of the collector current density and collector junction bias and shown to produce a falloff in the current gain and cutoff frequency at high current densities. These effects are incorporated in a Gummel-Poon model and compared with experimental results and results from a commercial numerical device simulator.