Cap-layer and charge sheet effect in InP based pnp δ-doped heterojunction bipolar transistor

The effect of varying the cap layer and the δ doped charge sheet has been investigated for an InP based heterojunction bipolar transistor (HBT) using the SILVACO TCAD tool. At the outset the simulation method is validated by comparing with experimental reported results. The InGaAs cap layer is then replaced with InGaAsP. The device figures- of-merit are observed to be remarkable improved with the quaternary cap layer. Some of the noteworthy results include increase in DC current gain from 50 to 126 and the Early voltage is improved from 7.96 V to 19.23 V. The minimum noise figure is reduced from 7.3 to 3 dB. The unity gain cut-off frequency (f_T) and the maximum frequency of oscillation (f_max) are improved from 7.3 MHz to 5.4 GHz and 1.59–12.33 GHz respectively. Now in the presence of InGaAsP cap layer the δ-doped charge sheet is varied from 2 × 10¹¹ cm⁻² to 2 × 10¹² cm⁻², and it is observed that it has little effect on these parameters except that the V_CE,offset voltage decreases with increase in the sheet density.