Horizontal Current Bipolar Transistor f T and f max Recovery at Cryogenic Temperatures

RF characterization of the Horizontal Current Bipolar Transistor (HCBT) is performed in a cryostat over the temperature range from 300 K down to 20 K. Following the expected decrease of small-signal current gain ( $h$${}_{\mathbf {{21}}}$ ), cut-off frequency ( $f$${}_{\mathbf {T}}$ ) and maximum frequency of oscillations ( $f$${}_{\mathbf {\textit {max}}}$ ) from 300 K to 80 K, a recovery of all parameters is observed below 80 K, in contrast to any other published Si BJT technology. Compared with a commercial bipolar junction transistor (BJT) having a similar $f$${}_{\mathbf {T}}$ and $f$${}_{\mathbf {\textit {max}}}$ at 300 K, the HCBT demonstrates a 3.2 times higher $f$${}_{\mathbf {T}}$ and 2.5 times higher $f$${}_{\mathbf {\textit {max}}}$ at 20 K. The $f$${}_{\mathbf {T}}$ recovery is attributed to the mobility improvement in the freeze-out regime, whereas $f$${}_{\mathbf {\textit {max}}}$ increases even more due to the reduction of the effective collector-base capacitance( $C$${}_{\mathbf {\textit {BC}}}$ ) below 80 K. These characteristics could facilitate the use of Si BJTs in cryogenic applications such as quantum computing.