A technique for simultaneously improving the product of cutoff frequency-breakdown voltage and thermal stability of SOI SiGe HBT

The product of the cutoff frequency and breakdown voltage (f_T×BV_CEO) is an important figure of merit (FOM) to characterize overall performance of heterojunction bipolar transistor (HBT). In this paper, an approach to introducing a thin N⁺-buried layer into N collector region in silicon-on-insulator (SOI) SiGe HBT to simultaneously improve the FOM of f_T×BV_CEO and thermal stability is presented by using two-dimensional (2D) numerical simulation through SILVACO device simulator. Firstly, in order to show some disadvantages of the introduction of SOI structure, the effects of SOI insulation layer thickness (T_BOX) on f_T, BV_CEO, and the FOM of f_T×BV_CEO are presented. The introduction of SOI structure remarkably reduces the electron concentration in collector region near SOI substrate insulation layer, obviously reduces f_T, slightly increases BV_CEO to some extent, but ultimately degrades the FOM of f_T×BV_CEO. Although the f_T, BV_CEO, and the FOM of f_T×BV_CEO can be improved by increasing SOI insulator SiO₂ layer thickness T_BOX in SOI structure, the device temperature and collector current are increased due to lower thermal conductivity of SiO₂ layer, as a result, the self-heating effect of the device is enhanced, and the thermal stability of the device is degraded. Secondly, in order to alleviate the foregoing problem of low electron concentration in collector region near SOI insulation layer and the thermal stability resulting from thick T_BOX, a thin N⁺-buried layer is introduced into collector region to not only improve the FOM of f_T×BV_CEO, but also weaken the self-heating effect of the device, thus improving the thermal stability of the device. Furthermore, the effect of the location of the thin N⁺-buried layer in collector region is investigated in detail. The result show that the FOM of f_T×BV_CEO is improved and the device temperature decreases as the N⁺-buried layer shifts toward SOI substrate insulation layer. The approach to introducing a thin N⁺-buried layer into collector region provides an effective method to improve SOI SiGe HBT overall performance.