THz Device Design for SiGe HBT under Sub-room Temperature to Cryogenic Conditions

BiCMOS technology can be a possible replacement for FDSOI and FinFET technology due to their higher transconductance, which allows them to operate at in THz range i.e. radio frequencies (RF) in addition to their higher voltage handling ability. The most advanced SiGe heterojunction bipolar transistor (HBT) technology (55-nm BiCMOS) demonstrates room temperature cut-off frequency ( $f_{\mathrm{t}}$ ) and maximum oscillation frequency ( $f_{\max}$ ) of 320 GHz and 370 GHz respectively. In this paper, we performed TCAD analysis to investigate the performance metrics, $f_{\mathrm{t}}$ and $f_{\max}$ of the SiGe HBT at different cryogenic temperatures. The calibrated Gummel characteristics reveals that a record DC current gain of $1.2\times 10^{4}$ is obtained at 77 K for $\mathrm{V}_{\text{BE}}=\mathrm{V}_{\text{CE}}=1.2\ \mathrm{V}$ . The HBT device employs bandgap engineering by linearly varying the Ge concentration in the base region, which enhances the device performance. Both the bandgap engineering with linearly graded Germanium (Ge) profile (induces intrinsic drift field in the base) and the cryogenic operation of the HBT device results in enhancement of $f_{\mathrm{t}}$ and $f_{\max}$ . Our simulations predict that the value of peak $f_{\mathrm{t}}$ decreases below 100 K due to increase in the emitter junction capacitance and the peak $f_{\max}$ increase is due to decrease in collector junction capacitance and base resistance. The aggregate metric $f_{\mathrm{t}}+f_{\max} > 1.2\ \text{THz}$ is achieved under cryogenic condition without scaling the device, this advantage can be utilized in the THz device applications.