Flexible megahertz organic transistors and the critical role of the device geometry on their dynamic performance

Abstract

The development of organic thin-film transistors (TFTs) for high-frequency applications requires a detailed understanding of the intrinsic and extrinsic factors that influence their dynamic performance. This includes a wide range of properties, such as the device architecture, the contact resistance, parasitic capacitances, and intentional or unintentional asymmetries of the gate-to-contact overlaps. Here, we present a comprehensive analysis of the dynamic characteristics of the highest-performing flexible organic TFTs reported to date. For this purpose, we have developed the first compact model that provides a complete and accurate closed-form description of the frequency-dependent small-signal gain of organic field-effect transistors. The model properly accounts for all relevant secondary effects, such as the contact resistance, fringe capacitances, the subthreshold regime, charge traps, and non-quasistatic effects. We have analyzed the frequency behavior of low-voltage organic transistors fabricated in both coplanar and staggered device architectures on flexible plastic substrates. We show through S-parameter measurements that coplanar transistors yield more ideal small-signal characteristics with only a weak dependence on the overlap asymmetry. In contrast, the high-frequency behavior of staggered transistors suffers from a more pronounced dependence on the asymmetry. Using our advanced compact model, we elucidate the factors influencing the frequency-dependent small-signal gain and find that even though coplanar transistors have larger capacitances than staggered transistors, they benefit from substantially larger transconductances, which is the main reason for their superior dynamic performance.

High‐Resolution Lithography for High‐Frequency Organic Thin‐Film Transistors

“…Note that Equation (1) was derived under the assumption that the gate-to-source and the gate-to-drain overlap lengths are identical (L ov = L ov,GS = L ov,GD ) and that they have the same influence on the transit frequency, while in reality, L ov,GS and L ov,GD have slightly different impacts on the transit frequency, at least when the TFTs are biased in saturation. [52,54,83] Author Ref.…”

“…A comprehensive analysis of the dynamic characteristics of these TFTs was provided by Jakob Leise et al., using a compact model based on a closed‐form description of the frequency‐dependent small‐signal gain accurately accounting for all relevant secondary effects, such as contact resistance, charge traps, fringe capacitances, subthreshold regime and non‐quasistatic effects. [ 54 ]…”

“…A comprehensive analysis of the dynamic characteristics of these TFTs was provided by Jakob Leise et al, using a compact model based on a closed-form description of the frequency-dependent small-signal gain accurately accounting for all relevant secondary effects, such as contact resistance, charge traps, fringe capacitances, subthreshold regime and non-quasistatic effects. [54] According to a detailed analysis by Oscar Vazquez-Mena et al of the blurring that occurs when a material is deposited onto a surface through openings in a stencil mask, the resolution limit of stencil lithography is about 0.2 μm. [55] This value therefore represents approximately the smallest channel length that can realistically be expected when using stencil lithography.…”

High‐Frequency fT and fmax in Organic Transistors: Performance and Perspective

“…[13] A comprehensive analysis of the dynamic performance and high-frequency behavior of OTFTs has been conducted based on a compact model. [14] Vertical organic transistors, particularly vertical organic permeable-base transistors (OPBTs), are an appealing alternative that can reach a high transit frequency and gain, making them particularly attractive. [15][16][17] OPBTs exhibit promising potential as candidates for high DC and AC performance, [18][19][20] with the ability to approach the GHz high-frequency.…”

“…[ 13 ] A comprehensive analysis of the dynamic performance and high‐frequency behavior of OTFTs has been conducted based on a compact model. [ 14 ]…”