Effects of buried oxide layer on working speed of SiGe heterojunction photo-transistor**Project supported by the National Natural Science Foundation of China (Grant Nos. 61604106, 61774012, and 61901010), the Beijing Future Chip Technology High Precision Innovation Center Research Fund, China (Grant No. KYJJ2016008), the Beijing Municipal Natural Science Foundation, China (Grant No. 4192014), and the Municipal Natural Science Foundation of Shangdong

Abstract

The effects of buried oxide (BOX) layer on the capacitance of SiGe heterojunction photo-transistor (HPT), including the collector–substrate capacitance, the base–collector capacitance, and the base–emitter capacitance, are studied by using a silicon-on-insulator (SOI) substrate as compared with the devices on native Si substrates. By introducing the BOX layer into Si-based SiGe HPT, the maximum photo-characteristic frequency ft,opt of SOI-based SiGe HPT reaches up to 24.51 GHz, which is 1.5 times higher than the value obtained from Si-based SiGe HPT. In addition, the maximum optical cut-off frequency fβ,opt, namely its 3-dB bandwidth, reaches up to 1.13 GHz, improved by 1.18 times. However, with the increase of optical power or collector current, this improvement on the frequency characteristic from BOX layer becomes less dominant as confirmed by reducing the 3-dB bandwidth of SOI-based SiGe HPT which approaches to the 3-dB bandwidth of Si-based SiGe HPT at higher injection conditions.