Implications of hole vs electron transport properties for high speed Pnp heterojunction bipolar transistors

Abstract

This paper examines the implications of differences in hole and electron transport properties on the microwave performance potential of InP-based Pnp HBTs. In particular, the carrier mobility, diffusion length, saturation velocity and ballistic transport properties for holes vs electrons are compared and their implications for Pnp device performance, such as the current gain and high frequency performance. The effects of the doping level on the mobility and minority carrier lifetime are included using empirical models fit to the available experimental data. The results show that, while the hole's transport properties are inferior to those for the electron, the minority carrier lifetime is larger for holes than electrons so that the hole diffusion length in the base is smaller but comparable to the electron's. As a result, Pnp HBTs with adequate current gain can be achieved with only a modest reduction in the base width compared with the Npn. For the device's microwave performance, the hole's characteristics determine the base transit time and the collector delay time, which limit the Pnp HBT's high frequency performance.