Temperature dependent transient velocity and mobility studies in an organic field effect transistor

We have investigated the temperature-dependent behavior of a pentacene organic field effect transistor (OFET) by measuring dc current and voltage characteristics and the transient response of the OFET to a sharp voltage pulse applied at the source contact. Standard transistor equations were used to extract the dc field-effect mobility, mu(FE), and the turn on time of the OFET was used to extract a dynamic mobility, mu(dynamic). Temperature dependent measurements were used to extract both field-effect and dynamic activation energies, E(a,FE) and E(a,dynamic), over a range of effective gate voltages in order to vary the charge carrier density in the channel. We found that at higher temperatures mu(dynamic)>mu(FE), while at lower temperatures mu(dynamic)<mu(FE). We attribute this varying relationship between dynamic and field effect mobilities to the fact that mu(FE) is an average mobility of all the charge carriers in the channel, while mu(dynamic) is a measure of the mobility of the fastest charge carriers in the channel. We also found E(a,dynamic) to be greater than E(a,FE) by approximately 7 meV at the same effective gate voltage. We attribute this to the fact that E(a,dynamic) is extracted from transient measurements made during the process of channel formation, and therefore at lower charge carrier densities than the corresponding values of E(a,FE). We also note that E(a,dynamic) is a measure of the activation energy of the most energetic charge carriers in the channel, while E(a,FE) is a measure of the average activation energies of all charge carriers in the channel. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3415546]