The dielectric identity of organic semiconductors: Bulk dielectric relaxation time as a universal figure of merit

Abstract

Organic field-effect transistors have been analyzed for four decades within a semiconductor framework that their active materials do not support. In undoped organic semiconductors, the Debye screening length exceeds the film thickness by approximately two orders of magnitude, prohibiting band bending and depletion, while the dielectric relaxation time is three to six orders of magnitude shorter than the free-carrier lifetime—placing these materials unambiguously among relaxation-type dielectrics rather than lifetime-type semiconductors. This Perspective argues that organic semiconductors are correctly treated as relaxation-type dielectrics governed by Maxwell–Wagner physics, with the dielectric relaxation time τbulk=ε0εr/σbulk as the fundamental figure of merit for contact physics. Within this framework, the gate-voltage-independent contact-resistance floor is identified as the Maxwell–Wagner resistance of the organic semiconductor volume in the contact region, predicted from bulk dark conductivity without free parameters. A cross-material benchmark constructed from published data across independent groups confirms that τbulk ordering predicts the contact-resistance floor across ten orders of magnitude—from undoped conjugated polymers to electrolyte-gated devices—unifying three experimentally distinct pathways to sub-100 Ω cm contact resistance as different engineering levers on the same dielectric relaxation parameter. A structural consequence of the framework is that the critical channel length for contact-dominated operation depends on the dark carrier density but not on carrier mobility, which cancels exactly: the organic-transistor scaling roadmap is, therefore, written in τbulk, not in mobility.