Cryogenic $\text{In}_{0.8} \text{Ga}_{0.2} \text{As}$ Quantum-Well High-Electron Mobility Transistors from Lowpower Quantum Computing to Tera-Hz Applications

We present cryogenic <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{In}_{0.8} \text{Ga}_{0.2} \text{As}$</tex> QW HEMTs with a gate length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(L_{g}\right)$</tex> of 35 nm, achieving a record combination of low-power and high-frequency performance. A meticulous modeling of source resistance <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(R_{s}\right)$</tex> incorporating ballistic channel resistance (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left.R_{\text {ball }}\right)$</tex> - provides key insights for advancing low-power quantum computing and terahertz (THz) applications. At 4 K, the fabricated device exhibits exceptional performance metrics, including a minimum subthreshold-swing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(S_{\min }\right)$</tex> of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4.41 \text{mV} / \text{dec}$</tex>., a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$g_{m_{-} \max }$</tex> of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.49 \text{mS} / \mu \mathrm{m}$</tex>, and the highest record <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{T}$</tex> of 813 GHz, with an average gain-bandwidth product (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text {avg }}$</tex>) of 810 GHz. These results stem from a tightly controlled gate-recess process, minimizing the side length (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$L_{\text {side }}$</tex>) to below 20 nm. Delay-time analysis indicates further THz performance can be achieved by scaled <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$L_{g}$</tex> below 20 nm and reducing fringing gate capacitance (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$C_{\mathrm{g}\_\text{fringe }}$</tex>) by 20 %. This work demonstrates the potential of cryogenic <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{In}_{0.8} \text{Ga}_{0.2}$</tex> As QW HEMTs to revolutionize quantum computing and THz electronics