Quantum Computing Hardware
Which correctly describes a change in MOSFET characteristics at low temperature that must be considered when designing cryo-CMOS circuits that run qubit control electronics at about 4 K?
- AThe threshold voltage rises and the subthreshold swing decreases (the current turns on more steeply), so transistor models made for room temperature cannot be used as they are
- BCarrier mobility falls, and subthreshold leakage becomes larger than at room temperature
- CSilicon becomes superconducting, so the channel resistance drops to zero
Show answer and explanation
Answer: A. The threshold voltage rises and the subthreshold swing decreases (the current turns on more steeply), so transistor models made for room temperature cannot be used as they are
At low temperature, scattering by lattice vibrations (phonons) decreases, so carrier mobility rises, and leakage current also drops sharply. Meanwhile, the threshold voltage rises and the subthreshold swing decreases, but it saturates at a value much larger than the theoretical (k_B T/q) ln 10 (about 0.8 mV/dec at 4 K). There are other effects as well, such as carrier freeze-out and increased device variability, so circuits are designed with models rebuilt from characteristics measured at low temperature. The power consumption must also fit within the cooling power of the 4 K stage (roughly 1 W).
- A. Correct. Models must be rebuilt to match characteristics measured at low temperature.
- B. These changes are the opposite of what happens: at low temperature mobility rises and leakage falls.
- C. The channel of a silicon MOSFET does not become superconducting even at low temperature.