InAlAs/InGaAs heterojunction bipolar transistors with an n-doped InGaAs spacer

Static and high frequency characteristics of InAlAs/InGaAs heterojunction bipolar transistors (HBTs) with a 200-A-thick n-doped InGaAs spacer between the emitter and base are investigated. The presence of the deep notch (ΔEn) at the emitter-base hetero-interface is confirmed from the low temperature I-V data, in which the specific bumps corresponding to electrons tunneling to the quantized electron levels in the deep notch are found in the base current for the first time. Electron injection energy (Ei) into the base for the proposed HBTs is reduced by ΔEn as compared with that for the HBTs without the n-doped spacer. Although the spacer is as thick as 200 A, recombination current is low because the Ei still remains sufficiently high. A current gain of as high as 200 is obtained despite the higher base doping of 2 × 1019 cm-3. By employing the 200 A spacer, Ei is optimized successfully for minimizing the sum of the base and collector transit times.