Electrical characterization of Cu-doped CdS p-type thin film transistors

In this work, the growth of CdS thin films by chemical bath deposition, and doping of the active layer by ion exchange method to fabricate a pseudo-metal–oxide–semiconductor field-effect transistor (Ψ-MOSFET) is reported. The introduction of copper ions as dopants in CdS films was employed to control the semiconductor bandgap and to produce transistors with adjustable channel currents. A cubic crystalline structure was corroborated by the preferential cubic (111) plane located at 2θ = 26.75°, detected on both CdS as-grown and Cu-doped CdS samples. The optical absorption edges were located near 515 and 505 nm, for as undoped and doped samples, respectively. In addition, the calculated bandgap presented a slight variation from 2.40 to 2.46 eV, after copper doping. To compare the performance between undoped and Cu-doped CdS films as active layer in thin film transistors, pseudo-MOSFET devices were manufactured. The surface morphology of the CdS films was studied by atomic force microscopy and electron microscopy. The SEM cross-section micrograph of the device showed a 110-nm-thick CdS layer deposited on SiO₂/Si–n substrates. Electrical characterization showed that the addition of Cu ions into CdS films produced improvements on the electrical behavior of the thin film transistors (TFTs). It was found that after Cu doping, the channel was modified from n to p-type. In addition, the resistivity increased along the transistor channel, while the calculated mobilities values of 0.02 and 6.6 cm²V⁻¹S⁻¹ were obtained for p-type and n-type TFTs, respectively. The threshold voltage value was − 6.4 eV and 5.64 eV corresponding to the Cu-doped and undoped devices, respectively. These results represent a promising contribution to the development of thin film transistors with the possibility of selecting the n-type or p-type behavior by the introduction of Cu ions and modifying its electrical properties.