Evaluation of base transit time in ultra-thin carbon-doped base InP/lnGaAs 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 W_B were varied from 50 to 1300 Å. The estimated base transit time τ_B shows linear dependence on W_B when W_B is smaller than 150 Å, whereas it has quadratic dependence on W_B above this value. The linear behaviour of τ_B on W _B is evidence of dominant ballistic electron transport in the ultra-thin base layer of InP/InGaAs HBTs