SiGe, GaAs, and InP Heterojunction Bipolar Transistors

CHAPTER THREE

DC Performance

3.1   GENERAL STRUCTURES AND STEADY-STATE BEHAVIOR

3.1.1   Electron and Hole Currents

The central design principle of heterostructure devices utilizes energy gap variations in addition to electric fields as forces acting on electrons and holes to control their distribution and flow [1]. By a careful combination of energy gap variations and electric fields, it becomes possible, within wide limits, to control the forces acting on electrons and holes, separately. A design freedom not achievable in homostructures is obtained. The resulting greater design freedom permits a reoptimization of the doping levels and geometries of the HBT, leading to higher-speed devices.

In HBT a wide energy bandgap emitter is used. The basic theory behind a wide-gap emitter is explained as follows. Consider the energy band structure of an N-p-n bipolar transistor (the capital N represents the wide bandgap emitter), as shown in Fig. 3.1. The emitter–base band edge is sufficiently graded to eliminate any band-edge discontinuities in the conduction band. The dc currents flowing in such a transistor include a current I_n of electrons injected from the emitter into the base, a current I_p of holes injected from the base into the emitter, and a current I_s due to electron–hole recombination within the emitter–base space-charge layer. In addition, a small part I_r of the electron injection current I_n is lost due to bulk recombination. Neglecting recombination effects, the maximum current ...