A 5.2~7.8GHz Cryo-CMOS LNA with 4-K Noise Temperature with Cascode gm-Boosting and Current Reuse for Noise Reduction

Fidelity of quantum state discrimination is limited by the noise temperature of cryogenic low noise amplifier (LNA), due to the temp.-independent shot noise, thermal noise due to device self-heating, and noise-impedance mismatch. This work presents a 5.2~7.8 GHz cryo-CMOS LNA, exploring the noise boundary of CMOS technology at 4.2 K. For the 1st-stage cascode device, the size (240 µm/60 nm) and current density (~44 µA/µm) are optimized for low Fano factor (F) of shot noise and high transconductance gm. Since high transconductance efficiency g²m/ID leads to low noise, a transformer-based gm-boosting circuit enhances the gm by 3×. In addition, the current of the 1st-stage cascode device and the 2nd-stage common source MOSFET is reused, reducing the device self-heating. Furthermore, a high-Q, off-chip inductor is used to address the noise mismatch issue. Measured at 4.2 K, the LNA attains a peak RF gain of 38 dB with a 3 dB bandwidth of 5.2~7.8 GHz. A minimum noise temperature Te of 4 K is recorded at 6.25 GHz with a DC power PDC of 10.6 mW. With a core area of 0.23 mm², a figure-of-the-merit (FoM₁) of 101.3 is achieved.