A high-current-gain low-temperature pseudo-heterojunction bipolar transistor utilizing sidewall base-contact structure (SICOS)

An experimental pseudo-heterojunction bipolar transistor (HBT) is described. The pseudo-HBT is a homojunction bipolar transistor having a moderately doped emitter and a heavily doped base, providing a bandgap profile similar to those of actual HBTs. Analyses including real constraints, such as a heavily doped emitter region for ohmic contact and the profile tail in the base region, show a significant change in the way they affect injection characteristics between 300 and 77 K. Based on these analyses, an impurity profile is carefully designed for upward mode operation. The electron injection into the external base region. which is thought to be unfavorable for high current gain in the upward mode, is avoided by using the sidewall base-contact structure. The fabricated transistor clearly displays a negative temperature dependence on current gain. The current gain is 107 at 77 K, which is 5 times higher than the room-temperature current gain. In addition, current gain excluding the nonideal effects at 77 K is as high as 25,000. These results not only remove gain degradation at low temperatures, but also verify the pseudo-HBT concept, in which an injection efficiency as high as that of an HBT can be obtained using only homojunctions. Based on these results, the small emitter transit time inherent in pseudo-HBTs is analyzed