Cryogenic GaAs high-electron-mobility-transistor amplifier for current noise measurements Available to Purchase

The saturation of I_ch near 20 mA for the W = 3 mm HEMT is due to the parasitic resistance of about 25 Ω, which is the sum of the ohmic contact resistance and the wiring resistance. We do not observe such saturation for the W = 1 mm HEMT over the measured V_g range because of its smaller contact resistance and the resultant smaller total parasitic resistance (about 20 Ω).

The commercial HEMT becomes unstable above V_DD = 0.4 V.

We restricted the measurement up to P ≅ 1 mW to avoid the temperature rise of the 4 K stage.

The S^V_HEMT peak near f = 900 kHz is an artifact originating from the noise in the power supply.

At higher R_L, the operating point becomes close to the pinch-off, leading to lower g_m. This increases S^V_HEMT for a given S^I_ch because S^V_HEMT ≅ S^I_ch/g_m².

In this simulation, we first examined fits to the data for the GaAs line with several sets of |A(f₁)| and $Te$ values to obtain r, $SHEMTI$⁠, and $SHEMTV$ as fit parameters, while C_in = 238 pF and L_in = 33 μH were fixed to give f₁ = 1.794 MHz. Then, we examined fits to the data for the ATF line with fixed values of T_e, r, and S^I_HEMT obtained from the former fit. Using this procedure, we found that only the parameters presented in Table I and r = 14.7 Ω explain our experimental data consistently within the error of ∼4%.

The slight difference between the experimental data and the theoretical curve at G/G₀ = 0.65 might be caused by nonlinear bias dependence of the QPC conductance.