Sensing Few Electrons Floating on Helium with High-Electron-Mobility Transistors

We report on low-frequency measurements of few electrons floating on superfluid helium using a bespoke cryogenic cascode amplifier circuit built with off-the-shelf GaAs High-Electron-Mobility Transistors (HEMTs). We integrate this circuit with a Charge-Coupled Device (CCD) to transport the electrons on helium and characterize its performance. We show that this circuit has a Signal-to-Noise ratio (SNR) of $\thicksim$ 2$\frac{e}{\sqrt{Hz}}$ at 102 kHz, an order of magnitude improvement from previous implementations and provides a compelling alternative to few electron sensing with high frequency resonators.