Symmetric-gain, zero-offset, self-aligned, and refractory-contact double HBT's

We report double-heterostructure bipolar transistors using refractory emitter and base contacts (WSi x /InAs for n-type, and W with rapid diffusion of Zn for p-type) and self-aligned implantation. These devices have near-symmetric gain (∼ 100) across a fairly large current range (∼ four decades), negligible offset voltage, and a weak current gain dependence on device size. This results from suppression of surface recombination obtained using a P-GaAlAs electron barrier at the extrinsic-base surface. We also demonstrate use of a technique for deconvolving various recombination mechanisms. The major gain-limiting mechanisms in these devices are shown to be (a) residual damage from ion implantation and its effects on base transport factor, and (b) Shockley-Read-Hall recombination in the depletion regions and its effect on injection efficiency. Devices similar to the above showed gains approaching theoretical values of ∼ 1000 at high currents, and greater than 10 at picoampere currents when the processing was designed to obtain high lifetime in the implanted region.