Dynamics of charge flow in the channel of a thin-film field-effect transistor

Abstract

The local conductivity in the channel of a thin-film field-effect transistor is proportional to the charge density induced by the local gate voltage. We show how this determines the frequency-and position-dependence of the charge induced in the channel for the case of "zero applied current": zero drain-source voltage with charge induced by a square-wave voltage applied to the gate, assuming constant mobility and negligible contact impedances. An approximate expression for the frequency dependence of the induced charge in the center of the channel can be conveniently used to determine the charge mobility. Fits of electro-optic measurements of the induced charge in organic transistors are used as examples.2

Dynamic Infrared Electro-optic Response of Soluble Organic Semiconductors in Thin Film Transistors

“…To maximize the absorption of infrared light (ν = 900-1000 cm -1 ) in the conducting layers adjacent to the dielectric, the thicknesses of the dielectric layer were ~1 micron, which is ~ ¼ of the wavelength in the dielectric. [8] Spatially resolved measurements, using an infrared microscope for measurement areas 30 x 30 µm, were taken of the change in reflectance while applying a gate voltage (V gs ) to a thin film transistor while holding the drain and source at ground. Infrared light, from a tunable leadsalt laser, is reflected off the gate (passing through the TIPS-pentacene twice), and absorbed by accumulated charges in the conducting layer resulting in a change in reflection ~10 -4 .…”

“…where D is the Einstein diffusion constant, µ is the mobility, and C is the capacitance of the dielectric. Numeric solutions at various positions and frequencies are presented in Reference [8]. This one-dimensional equation assumes that fringing fields, leakage current, and contact impedances are negligible, so that ρ(0,t) = ρ(L,t) = CV gs (t).…”