SiGe, GaAs, and InP Heterojunction Bipolar Transistors

CHAPTER SEVEN

Breakdown and Thermal Instability

7.1   AVALANCHE BREAKDOWN

For heterojunction bipolar transistors the collector doping concentration is on the order of 10¹⁷cm⁻³. As a result of higher collector doping, the electric field in the base–collector space-charge region increases accordingly. The resulting peak electric field can be as high as 10⁵ V/cm and impact ionization-induced current can become prominent even under normal operating conditions. Understanding the avalanche breakdown mechanism will help device designers and circuit designers make reliable, high-performance transistors and integrated circuits.

The breakdown phenomenon occurs in the collector–base depletion region, where the electric field is highest. A high electric field accelerates the carriers and imparts to them a large kinetic energy. Indeed, this kinetic energy can become sufficiently large to cause an electron, or even more than one electron under the right conditions, to be removed from the valence band and transferred to the conduction-band. Take the case of the GaAs energy-band diagram shown in Fig. 7.1. The electron- and hole-initiated transition, involving a single electron–hole pair generation, consists of a transfer of energy from the hot carrier to an electron in the valence band, which jumps into the conduction-band leaving a hole behind [1]. In Fig. 7.1 the ionization process initiated by a hot electron is shown by solid lines. The ionization process initiated by a hot hole from the split-off ...