Current-driven instabilities and high-injection phenomena in heterojunction bipolar transistors

A comprehensive numerical investigation of the carrier transport in bipolar transistor collectors at high levels of injection is undertaken with the primary purpose of designing a novel GaAs-based heterojunction bipolar transistor (HBT) capable of intrinsic transferred-electron induced current oscillations. The behavior of III-V HBTs at high collector currents is found to depend on the intricate interplay between the transferred-electron (Gunn-Hilsum) and base widening (Kirk) phenomena arising at similar current levels. While all the conditions for the emergence of Gunn oscillations can be readily met in GaAs transistor collectors, the HBT intrinsic stability under the DC conditions is ensured by the base widening creating the collector field profile detrimental to the nucleation of Gunn charge domains. Under transient conditions, we determine that the onset of base widening upon bias switching is associated with a time delay caused by the slow diffusive influx of carriers in the forming current-induced base region. The ways to utilize the Kirk effect transient phenomenon to enhance silicon transistors high-frequency characteristics are discussed. We develop the improved large-signal models, for both silicon and GaAs transistors, compatible with the standard CAD tools and taking into account the base widening onset delay. During this delay, propagating charge domains are formed in the III-V HBT collectors. The impact of the transferred-electron effect in III-V HBTs on the Kirk effect threshold conditions and the device transient behavior is discussed. We then demonstrate that proper engineering of the collector field profile greatly increases the base widening onset delay and leads to the intrinsic collector current instabilities, similar to those in Gunn diodes. Further modification of the collector doping profile and judicious choice of bias conditions may result in the appearance of the sustainable microwave or millimeter wave current oscillations in III-V transistor structures. The principles of design, operation, and industrial application, as well as technological advantages, of the proposed device are discussed and demonstrated by numerical simulations. The novel three-terminal bipolar transferred electron oscillator (BTEO) can combine signal generation and modulation stages in a single device.