Quasi-Ballistic Transitions in AlGaN/SiC Heterojunction Bipolar Transistors

The effect of base scaling on the performance of wurtzite n-p-n AlGaN/SiC HBTs is investigated using analytical expressions. The transition to the quasi-ballistic base transport regime is identified within the free path formalism. The effect of spontaneous and piezoelectric polarization on the HBT characteristics is studied. Getting current gains of 200 or more becomes extremely difficult without extreme base width scaling for base carrier concentrations of > 1×10
¹⁹
cm
^-3
, required for obtaining high maximum frequency of oscillation f
_max.
Since the base transit times tend to be large for these HBTs, the tradeoff of frequency versus power is not seen except for devices scaled to mesoscopic base dimensions. The impact of dislocation generation in AlGaN emitters with high lattice mismatch to the GaN base on the transistor characteristics are also studied within a Shockley-Read-Hall type model for recombination at dislocation sites.