  Optical illumination of quantum-dot qubit devices at cryogenic temperatures,
while not well studied, is often used to recover operating conditions after
undesired shocking events or charge injection. Here, we demonstrate systematic
threshold voltage shifts in a dopant-free, Si/SiGe field effect transistor
using a near infrared (780 nm) laser diode. We find that illumination under an
applied gate voltage can be used to set a specific, stable, and reproducible
threshold voltage that, over a wide range in gate bias, is equal to that gate
bias. Outside this range, the threshold voltage can still be tuned, although
the resulting threshold voltage is no longer equal to the applied gate bias
during illumination. We present a simple and intuitive model that provides a
mechanism for the tunability in gate bias. The model presented also explains
why cryogenic illumination is successful at resetting quantum dot qubit devices
after undesired charging events.
