AC Response of a Field Effect Transistor Composed of Electrons on Helium*

We report on the AC response of a unique field effect transistor composed of electrons on helium in the frequency range between 10 kHz and 100 kHz. The impedance of the device can be modeled as a two-dimensional transmission line formed between the source and drain electrodes. At low temperature the coupling between the source and drain is predominantly capacitive when the channel above them is fully populated. In this regime the current that flows from source to drain has a phase angle φ = 90° relative to the excitation voltage. However, at sufficiently high temperature φ decreases and can become negative in the vicinity of depletion. In this case the imaginary component of the source-drain current reverses sign. We have analyzed this behavior and find that it can be understood by considering the propagation of strongly damped voltage oscillations along the transmission line formed by the system of two-dimensional electrons above, and between, the source and drain electrodes.