Floating Drain-Based Measurement of ON-State Voltage of an OTFT for Sensing Applications

Abstract

The threshold voltage of an organic thin-film transistor (OTFT) is sensitive to several variables, which makes it attractive for sensing applications. However, its measurement is not straightforward, especially, when mobility is gate voltage-dependent, and source and drain resistances are significant. The on-state voltage of a TFT does not suffer from these drawbacks, and thus, offers an alternative variable for use as a sensing variable. However, conventional measurement of on-state voltage marking the onset of conduction in the device requires measurement of very low current values, and extrapolation as a result of which it is not commonly employed. This paper demonstrates a simpler alternative method for measurement of on-state voltage that requires measurement of floating drain voltage as source voltage is linearly ramped while maintaining gate voltage constant. The drain voltage tracks the source voltage until the channel is completely removed. 2-D numerical simulation results are presented to validate that the proposed technique for the extraction of on-state voltage yields reliable estimates under different device conditions. Experimental results obtained with pentacene/poly (4-vinyl phenol)-based top contact bottom-gate OTFTs are also presented to illustrate the proposed concept.