In this paper, we present results on the electrical characteristics of both thick and thin film polymer metal–insulator–semiconductor field-effect transistors (MISFETs) based on poly(3-hexylthiophene) semiconducting substrate. These MISFETs were fabricated in eight steps using conventional semiconductor processing technology. The MISFETs had excellent current–voltage characteristics and showed little degradation with time. Electrical parameters of threshold voltage, mobility, and transconductance were − 1 V, 10−4–10−5 cm2 V−1 s−1, and 10−9 S, respectively. These values are typical of π-conjugated polymer-based MISFETs. Electrical transport in MISFETs were by both bulk (or resistive) and FET-like mechanisms in the thick film transistors. The resistive current component and the electrical characteristics were found to be tunable by photolytic degradation of the semiconducting polymer. In this process, the semiconducting π-conjugated regions were spatially converted to insulating regions by novel photo-oxidative chemistry. We were also successful in modelling the MISFETs as an intrinsic FET with parasitic source and drain resistances in series with it and a varistor (for the bulk current component) in parallel with the intrinsic FET. Using this circuit model, we obtained very good agreements between SPICE simulations and the experimental results.