High performance HBTs with built-in base fields: exponentially-graded doping vs. graded composition

A comparison of exponentially graded doping and graded composition as a means of producing built-in drift fields in the base of npn GaAs-AlGaAs heterojunction bipolar transistors (HBTs) is discussed. The reduction in base transit time (τ_B) associated with the built-in fields is substantial. Compared to uniformly doped devices with the same base width, τ_B is reduced by ~70% for both the graded doping and graded composition profiles. The field associated with exponentially graded base doping is derived, accounting both for bandgap narrowing effects and for degeneracy. It is found that the built-in field due to doping gradients is substantially larger than that expected when computed using only nondegenerate Boltzmann statistics. The increase in device performance when exponentially graded doping is used is demonstrated using a distributed HBT amplifier with 24-GHz bandwidth