Light-emitting field-effect transistor: simple model and underlying functional mechanisms

Abstract

We report on light emission on organic thin film transistors of tetracene and polyfluorene (Poly(9,fluorene) (PF2/6)). The utilized transistor structure is a bottom gate configuration with interdigitated source and drain electrodes on a Si/SiO 2 substrate with a channel length of 5 µm. Light emission occurs above a source drain voltage of 30V even if the gate voltage is higher than the drain voltage. The light output can be controlled by the gate voltage. The light emission occurs close to the drain electrode as observed by light microscope images of operating transistors. In order to understand the functional principle of a light emitting transistor a resistor capacitor equivalent circuit model has been utilized to describe charge carrier transport, carrier distribution and the electrical potential distribution in such a device. The model extends the common thin film transistor theory for unipolar charge transport to ambipolar charge transport. Analytical expressions for output and transfer characteristics as well as for the potential and charge carrier distributions are obtained. Further, the effect of contact resistors on the output and transfer characteristics are simulated. The model is used to explain the underlying mechanisms of the present devices. Imperfections on the contact electrodes, most probably due to under-etching of the electrodes are seen as the main reason for the electron injection.

Light-emitting organic field-effect transistor using an organic heterostructure within the transistor channel

“…4 Ever since, other research groups have published results on tetracene-based LEOFETs, dealing with experimental 5,6 as well as theoretical aspects. 7,8 This same basic type of LEOFETs, based on one unipolar organic semiconductor, has also been realized using polymers 9, 10 and doped small molecules 11 as the organic semiconductor. Electroluminescence in these LEOFETs occurs very close to the electron injecting metal electrode.…”

New concepts for light-emitting transistors

“…It was not possible to operate the transistor in a pure electron accumulation mode. The S-shape at low-voltages in the output characteristics originates from an injection barrier for holes at the source electrode [8]. In the transfer characteristics (not displayed) an unusual saturation is observed, which can be attributed to contact resistors as detected by computer simulation [8].…”

“…In a more recent study light emission was also demonstrated in non-underetched OFET structures, but the transport was also found to be unipolar and the emission originated at the drain contact only. In a theoretical study by Schmechel et al [8] In the present study the still open question of electron injection at the gold tetracene contact is investigated by XPS and UPS. Based on the obtained values for the injection barriers a new technique of interface doping with calcium is introduced in order to increase the electron mobility in the investigated materials.…”

Asymmetric Electrical Properties of Half Corbino Hydrogenated Amorphous Silicon Thin-Film Transistor and Its Applications to Flat Panel Displays

“…Interdigitated electrodes were first adapted to achieve a high device width (W) to length (L) ratios in a limited layout space in metal oxide semiconductor field-effect transistors (MOSFET). 1,2) Organic thin-film transistors (OTFTs) also employed these structures to increase ON-current characteristics, 3,4) while bi-polar power transistors have generally used comb-like contact electrodes to minimize current crowing. 5) Recently, the comb-shaped electrodes have been also used in field-effect hydrogenated amorphous silicon (a-Si:H) solar cells to enhance the output power.…”