A numerical analysis to study the effects of process related variations in the extrinsic base design on dc current gain of InAlAs/InGaAs/InP DHBTs

The 2-D simulation program DESSIS was used to simulate InAlAs/InGaAs/InP DHBTs and compare the calculated Gummel plots with measurements from experimental devices employing the same structure. The simulations show that a combination of bulk recombination in the intrinsic device and lateral diffusion of minority carriers to the base contact limit the peak gain of these DHBTs; the lateral diffusion of minority carriers is strongly related to both the lateral and vertical placement of the base contact. For an emitter-edge to base-contact distance, W_bl, as low as 200 nm the lateral diffusion component of the base contact is essentially negligible and peak gain is limited largely by bulk recombination. Conversely, for short W_bl of 20 nm, which would be typical of self-aligned devices, lateral diffusion is the dominant part of base current and hence, the major factor limiting peak gain. The presence of a residual spacer layer in the extrinsic device is shown to accentuate the gain degradation through enhanced space charge recombination and lateral electron diffusion