Threshold voltage shift in organic field effect transistors by dipole monolayers on the gate insulator

Abstract

We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self assembled monolayer (SAM). The observed shift of the transfer characteristics range from -2 to 50 V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4 × 10 12 /cm 2 . In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold region is broadened, indicating that the SAM creates additional trap states, whose density is estimated to be of order 1 × 10 12 /cm 2 .

Threshold voltage control in organic thin film transistors with dielectric layer modified by a genetically engineered polypeptide

“…Shifts in V T similar to the shift caused by QBP under neutral assembling conditions have also been reported by using SAMs. 4,5 However, the ability to vary assembling conditions of the peptide offers an efficient method for controlling V T .…”

“…3,4 It can be shifted by modifying the dielectric with self assembled monolayers ͑SAMs͒ to shift V T by tens of volts. 5,6 A less studied route in the pursuit of improved device performance involves integrating functional biological materials into OTFTs. 7 Singh et al demonstrated using deoxyribonucleic acid as a gate dielectric to produce memory effect.…”

Organic field-effect transistors

“…V T voltage additionally has been found to depend strongly on the preparation of the surface on which the organic material is deposited. Threshold voltage shifts in thin film as well as single−crystal OFETs have been found to be induced by di− pole monolayers on the gate insulator [41]. Thickness of the semiconductor layer influences the values of the threshold voltage and, to a lesser extent, the saturation current.…”

Continuous tuning of the threshold voltage of organic thin-film transistors by a chemically reactive interfacial layer

“…This has been achieved by modifying a parylene gate dielectric with oxygen plasma [9] or a UV-ozone [10] treatment, or by inserting a layer of metal oxide in the transistor structure [11]. Also, thin siloxane layers have been found to significantly modify the threshold voltage [12,13].…”