Field-effect carrier doping in thin semiconductor layers with small density of states

To provide a guideline for understanding the field-effect doping phenomenon in organic thin film transistors (OTFTs), a simple metal–insulator–semiconductor–metal four-layer system has been discussed because this phenomenon in OTFTs may be different from that in the conventional inorganic devices when the density of states of the semiconductor material used is very small. On the basis of analytical considerations that are allowed by the simplification of the device structure, it has been found that the thickness of the region where the doped charges mostly distribute decreases gradually as the thickness of the semiconductor layer, ds, is decreased and then is always much smaller than ds. However, it has also been shown that the doping phenomenon in extremely small ds cases is nonlinear with respect to the gate voltage. This nonlinearity is enhanced by disorders through an increase in the mobility edge. The enhancement of the nonlinearity is less remarkable in the cases with intermediate ds values than in those with extremely small or large ds values.