Evaluation of the Beyond- $f_T$ Operation of an IGZO TFT-Based RF Self-Mixing Circuit

The objective of this letter is to show the potential of amorphous indium gallium zinc oxide (IGZO) thin-film transistors (TFT) for designing radio frequency (RF) communication systems operating in the gigahertz regime. For that purpose, the self-mixing operation of in-house fabricated IGZO-TFTs beyond their transit frequency (f
_T
) is reported. The bottom-gate, top-contact TFTs have an f
_T
of 40 MHz including the device interconnects. A differential RF self-mixing circuit was fabricated to act as an RF detector and frequency doubler. The detector shows a peak voltage responsivity (RV) and a minimum noise equivalent power of 445 V/W and 0.1 nW/√(Hz) at 50 MHz and 2 V/W and 30 nW/√(Hz) at 1 GHz, respectively, at a chopping frequency of 28 kHz. As a frequency doubler, the circuit can generate a second-harmonic output voltage up to -36 dBV for an 8-dBV RF input voltage at 100 MHz.