Operation and properties of ambipolar organic heterostructure field-effect transistors

In a recent article, we reported detailed numerical simulations for an ambipolar organic single-layer field-effect transistor (FET) with source and drain bottom contacts aimed to clarify basic ambipolar effects including both the actual contact properties and the recombination process [Paasch et al., J. Appl. Phys. 98, 084505 (2005)]. Here we present two-dimensional simulations of an ambipolar organic heterostructure (or bilayer) FET with a bottom Au contact and a Mg top contact, a structure for which current-voltage characteristics have been determined experimentally [Rost et al., J. Appl. Phys. 95, 5782 (2004)]. The difference between the single-layer bottom contact structure studied in the previous article and the heterostructure is demonstrated. Further, the influence of different electron and hole mobilities in both layers, and of the band offset at the interface of the two layers on the device characteristics is clarified. In particular, the formation of an dominating electron channel at the interface between the two organic layers, and separated from the hole channel formed at the interface of the first layer with the gate oxide, can occur only if the electron mobility of the upper layer is orders of magnitude larger than the hole mobility in the first layer. However, due to the different control of the distant electron channel by the gate voltage, one obtains current-voltage characteristics that are qualitatively different from the observed ones. With more realistic material parameters, the simulated current-voltage characteristics of the heterostructure feature all specific characteristics arising from the ambipolar operation. (c) 2007 American Institute of Physics.