Influence of pseudomorphic base-emitter spacer layers on current-induced degradation of beryllium-doped InGaAs/InAlAs heterojunction bipolar transistors

We present in this paper a basis for the design of high performance heterojunction bipolar transistors in which base dopant diffusion can be drastically reduced, or even eliminated. From previous theoretical and experimental studies we have established that the motion of point defects in a semiconductor can be impeded or enhanced by a thin (30-100 Å) pseudomorphic layer. The path preferred by the defect will depend on the local strain exerted by it in the lattice and the strain tensor of the host layer. Based on this, we have studied the outdiffusion of Be dopant atoms from the base region of n-p-n In_0.53Ga_0.47As/In_0.52Al_0.48 As heterojunction bipolar transistors (HBT´s) during short-term high-current (t=18-24 h, J_c=7×10⁴ A/cm² , T=80°C) stress tests. The microwave transistors grown by molecular beam epitaxy have 150-200 Å of lattice-matched, compressively strained, or tensilely strained spacer layers incorporated between the base and emitter layers. Changes are observed in the dc and microwave characteristics of the transistors with lattice-matched and compressively strained spacers, while no changes are recorded in the devices with tensilely strained spacer layer after the current stress test. As expected, the tensiley strained spacer layer is very effective in controlling the outdiffusion of Be dopant atoms, which exert a local strain in the lattice, from the base to the emitter region