Increasing sophistication, improved performance, and application of new material systems ranging from wide band gap oxides to van der Waals materials make testing Thin Film Transistor (TFT devices and circuits a growing challenge. We recently developed a new testing technique using the sub-terahertz (sub-THz) and THz radiation for testing electronic components, including transistors, Microwave Monolithic Integrated Circuits (MMIC) and Very Large Scale Integrated (VLSI) circuits. The THz radiation impinging on the transistors couples via contacts or interconnects and excites overdamped waves of the electron density – plasma waves - that are rectified by the transistor nonlinearities. This rectification results in the induced DC signal between the transistor contacts. The impinging THz modulation is modulated at a low frequency and a lock-in amplifier amplifies the resulting slowly modulated DC response. The resolution of this technique is better than the device feature size and scanning the beam allows to record the response images. The transistor response is sensitive to the applied bias voltage. The comparison of the responses between good and defective devices or circuits allows for developing the defect signatures. To analyze the application of this technique to TFTs, we introduce and analyze the TFT current dispersion length that determines the penetration of the radiation induced signals into the device channel and calculate the responsivity as a function of frequency for different TFT feature sizes using TFT Unified Charge Control Model. We also analyze the effect of the contact resistances and fringing parasitic capacitances on the response. Our results demonstrate the applicability of this new testing technique for the TFT testing.