I. Introduction
Bipolar junction transistors (BJTs) with high current gain () have served as a local signal amplifier to significantly improve the signal-to-noise ratio (SNR) of biosensors [1]. An emerging interesting application for such SNR enhancement strategy is the spin readout for semiconductor qubits in quantum computing [2]–[6]. The detection of spin-dependent charge transfer is commonly accomplished by a single electron transistor (SET) capacitively coupled to the qubit at deep cryogenic temperatures [7], [8]. The SET signal in the magnitude of tens to hundreds of pico-ampere is then sent to room-temperature electronics for processing via long cables [4], [9]. Immediate signal amplification for the SET at cryogenic stage will not only improve detection bandwidth (BW) and readout fidelity, but can also enhance the SNR by suppressing the interference from the triboelectric noise present in a dilution refrigerator [9]. Early study has shown that a factor of as much as 100 times increase in SNR could be achieved by pre-amplification of the SET output current using a discrete, commercial silicon-germanium heterojunction bipolar transistor (SiGe HBT) whose base is connected to the source of the SET with a bond wire [4], [5].