Stability of pentacene-based top gate thin film transistor with thick parylene as dielectric under humid environment

Abstract

This study presents the influence of humidity effect in top gate transistors using pentacene as active layer. The devices are made on a planarized kapton substrate, and 1 μm thick parylene has been used both as gate dielectric and encapsulating layer. Two series of measurement conditions have been used. First, the humidity rate was varied from 0 to 90% in a cell by setting exposure times of 15 min and device characterized, followed to 15 min vacuum. Secondly, humidity rate is fixed at 90% during 144 min and the electrical characteristics recorded successively. The main electrical parameters such as threshold voltage (VT), mobility (μ), on-current (Ion), depletion current (Ioff) and subthreshold slope (S) have been investigated. The results showed that only the electrical parameters in subthreshold region have been affected at different relative humidity (RH) levels for the first series. Under vacuum, the device exhibited stable electrical performances whereas in RH ranging from 24% to 90% we observe an increase of depletion current, and this Ioff increase was found to be reversible. After exposing the device with 90% of RH (second series), from 0 to 108 min in humid environment, threshold voltage, Ion current, VT and charge carrier mobility remain unchanged while Ioff and S are highly affected. Beyond 108 min, Ioff current keeps increasing with a noticeable Ion current degradation. Two effects have to be taken into account: (i) Ioff current increase without any change in on-state regime is the consequence of cumulative effect of Leakage current increase in the parylene dielectric and electrical conductivity increase in pentacene semiconductor, (ii) Ion current decrease is the consequence of absorbed water molecule diffusing and interacting with pentacene. All these parameters are reversible under vacuum after pumping down.