Thickness Engineering of the Top Organic Semiconductor in OPBTs: A Barrier-Dependent Study

Abstract

Organic permeable-base transistors (OPBTs) are vertical organic transistors that achieve high current densities at low operating voltages via charge transport through nano-pinholes in a permeable base1. In OPBTs, carriers are injected from the emitter, traverse the top organic semiconductor and the nano-pinholes, and then flow through the bottom organic semiconductor to the collector. To optimize performance, we investigate how the top organic semiconductor thickness impacts injection under controlled barrier conditions.

Using the two-dimensional finite element device simulator (ATLAS, Silvaco), we fixed the bottom organic semiconductor thickness at 100 nm and swept the top organic semiconductor thickness from 0 to 100 nm in 20 nm steps under three emitter injection barriers (ΦB.E = 0, 0.2, 0.4 eV). For all injection barriers, thinning the top organic semiconductor tends to increase current density because the emitter transport contact resistance decreases as top organic semiconductor thickness reduces. Under ohmic-like injection (ΦB.E = 0 eV), thinning the top organic layer monotonically increases current density and the on-current density peaks at 0 nm. In contrast, for higher injection barriers, excessively reducing the thickness of top organic semiconductor to 0 nm reduces current density. This reversal occurs because higher barriers require a finite transfer length (LT) to support lateral current spreading prior to injection2. With an extremely thin top organic semiconductor thickness of 0 nm, organic material is insufficient to sustain LT formation, thereby lowering the current density. Therefore, at higher emitter barriers, OPBT performance cannot be improved by unconditional thinning of top organic semiconductor. Instead, a finite top organic semiconductor thickness is required to establish LT and sustain efficient injection.