Simulation and design of InAlAs/InGaAs pnp heterojunction bipolar transistors

The performance capabilities of InP-based pnp heterojunction bipolar transistors (HBT´s) have been investigated using a drift-diffusion transport model based on a commercial numerical simulator. The low hole mobility in the base is found to limit the current gain and the base transit time, which limits the device´s cutoff frequency. The high electron majority carrier mobility in the n⁺ InGaAs base allows a reduction in the base doping and width while maintaining an adequately low base resistance. As a result, high current gain (>300) and power gain (>40 dB) are found to be possible at microwave frequencies. A cutoff frequency as high as 23 GHz and a maximum frequency of oscillation as high as 34 GHz are found to be possible without base grading. Comparison is made with the available, reported experimental results and good agreement is found. The analysis indicates that high-performance pnp InP-based HBT´s are feasible, but that optimization of the transistor´s multilayer structure is different than for the npn device