Generalized transport-band field-effect mobility in disordered organic and inorganic semiconductors

Field-effect mobility mu(FE) and its activation energy in disordered inorganic and organic semiconductor thin-film transistors is strongly dependent on bias conditions. This implies a nonlinear dependence of conductivity on carrier concentration, which stems from the high density of trapped carriers while only a few contribute to conduction. When mu(FE) is extracted from measurement data, the nonlinear conductivity-concentration dependence is averaged over the semiconducting film. Consequently, mu(FE) becomes mingled with device attributes such as gate capacitance in addition to terminal bias, which undermines the physical interpretation of mu(FE) and subsequent comparison of measured values for different devices and different semiconductors. This paper presents an effective mobility mu(eff) description at a reference carrier concentration, which separates the physical conductivity-concentration dependence from the device and bias attributes, enabling comparison of carrier transport in disordered semiconductors. In particular, by using the generalized concept of mobility edge and exponential band tails we show that mu(eff) can be applied to a wide range of inorganic and organic semiconductors. Indeed, three parameters, viz., mu(eff), exponential band tail slope T(t), and bias-independent activation energy E(a0) of mu(eff), can describe carrier transport in the transistor together with its bias and temperature dependence.