Evaluation of base transit time in ultra-thin carbon-dopedbase InP/InGaAs heterojunction bipolar transistors

Electron transit times in the carbon-doped base of InP/InGaAs heterojunction bipolar transistors (HBTs) are investigated from the current gains. The base layers were heavily doped to 2.5 × 1019 cm-3 and the thicknesses WB were varied from 50 to 1300 Å. The estimated base transit time τB shows linear dependence on WB when WB is smaller than 150 Å, whereas it has quadratic dependence on WB above this value. The linear behaviour of τB on WB is evidence of dominant ballistic electron transport in the ultra-thin base layer of InP/InGaAs HBTs.