The Effect of Base Doping Profile on Horizontal Current Bipolar Transistor’s (HCBT) Beta Recovery at Cryogenic Temperatures

It has been recently demonstrated that horizontal current bipolar transistor (HCBT) shows a recovery of DC current gain $(\beta)$ at cryogenic temperatures (CT) below $\sim 90 \mathrm{~K}$, a unique property among the silicon bipolar transistors. This effect can be explained by the interplay between incomplete ionization (II) of the acceptors in the base and bandgap narrowing (BGN) in the emitter in a specific range of HCBT doping profiles. To better understand the critical role of base doping parameters on $\beta$ recovery at CT, the HCBT with a lower-doped-collector (HCBT 2) was compared to the previously characterized HCBT with an nwell collector (HCBT 1). HCBT 2 also exhibits a $\beta$ recovery at CT, although less prominent than in the case of HCBT 1. HCBT 2 reaches a minimum peak $\beta$ at $\sim 70 \mathrm{~K}$ of 25, whereas HCBT 1’s minimum $\beta$ of 31 is at a higher temperature of $\sim \mathbf{9 0} \mathrm{K}$. Moreover, the $\beta$ recovery at CT of HCBT 2 reaches 37, which is lower than HCBT 1’s $\beta$ of 64 at 20 K. Calibrated TCAD simulations of equivalent BJT structures show that small relative differences in the base doping profile near Mott transition limit can have a significant impact on the cryogenic performance of the device. The demonstrated properties of both devices can enable the implementation of cryogenic circuits in HCBT BiCMOS technology.