High-gain n-p-n and p-n-p InGaAs/InAlAs double-heterojunction bipolar transistors with InAs cap layers by molecular-beam epitaxy

The vertical and lateral diffusion of the contact metal during thermal processing become increasingly critical as device dimensions shrink. To alleviate these problems while achieving low contact resistances, InAs contact layers have been used to facilitate the formation of nonalloyed ohmic contacts in n-p-n and p-n-p double-heterojunction bipolar transistors (DHBTs) grown by MBE on (100) InP at 500°C substrate temperature. Si was used for n-type and Be for p-type dopants. Standard photolithography and conventional wet chemical etching were used to fabricate self-aligned HBTs with emitter sizes varying from 1×5 to 50×50 μm². Metal contacts for n-type and p-type layers were formed by evaporating AuGe/Ni/Au and AuBe, respectively. Contact resistances smaller than 1.5×10^-7 Ω-cm² were obtained for nonalloyed n-type contacts and 1×10^-5 Ω-cm² for nonalloyed p-type contacts. Not only are these nonalloyed contact resistances very small, but the InAs has also made the resistance data more uniform and reproducible as compared to direct contact between metal and InGaAs. In devices with a 50×50 μm² emitter area, common emitter current gains of 1500 and 40 were measured for nonalloyed n-p-n and p-n-p structures, respectively. These values increased to 2000 and 70 for alloyed n-p-n and p-n-p devices, respectively. The collector current densities for maximum gains were 3-4×10³ A-cm^-2 and 1-1.5×10³ A-cm^-2, and the ideality factors were 1.3 and 1.4 respectively