Surface transport and DC current gain in InGaAs/InP DHBTs for THz applications

InGaAs/InP double heterojunction bipolar transistors (DHBTs) are highly suitable for applications in GHz mixed-signal ICs, >100 GHz digital logic, and millimeter-wave communications and imaging because of their high breakdown voltage and high cutoff frequencies (f_τ/f_max~0.5/1.0 THz)[1,2]. To extend HBT bandwidth, device dimensions must be reduced and the doping concentration in the InGaAs base must be increased. As a result, surface recombination increases, as does lateral electron transport from the emitter to the base contact, both on the exposed base surface and within the bulk base semiconductor. The DC current gain (β) thus decreases. Experimentally measured β are ~10-25 in THz DHBTs [2]. Because it limits the useful range of circuit applications, it is important to understand the mechanisms causing decreased β in scaled DHBTs. Using TCAD simulation, we had earlier found that lateral carrier diffusion within the bulk of the base contributes significantly to the observed high base currents in THz HBTs [3]. Here we model the surface conduction between the emitter and base contacts resulting from Fermi level pinning at the exposed base semiconductor surface, comparing simulations with experimental data. At bias conditions corresponding to peak f_τ/f_max, we find that ~50% of the total base current arises from surface conduction. This finding suggests the need for improved base surface passivation in THz HBTs.