Organic thin-film transistors (TFTs) have potential as pixel drivers in flexible active-matrix organic light-emitting diode displays . Hence it is essential to analyze the charge-carrier injection and extraction dynamics of organic TFTs to gain a better understanding of the trapping and detrapping at the TFT interfaces. From the current-voltage characteristics of the TFTs, many important parameters can be extracted, such as carrier mobility, threshold voltage, on/off ratio, subthreshold slope and transconductance. But to quantitatively evaluate the trapping and detrapping dynamics, displacement current measurements on two-terminal long-channel capacitors (LCCs) are far more useful [2, 3]. The cross-section and the layout of an LCC are schematically shown in Fig. 1. Unlike a TFT, an LCC has only one contact, so that carriers are injected into and extracted from the semiconductor through the same contact. To increase the signal-to-noise ratio, a very large channel length (up to 6 cm) is employed. While Liang et al. have performed displacement current measurements on pentacene-based LCCs [2,3], we report here on displacement current measurements on LCCs based on four different organic semiconductors: pentacene, dinaptho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), 2,9-didecyl-DNTT (C 10-DNTT) and diphenyl-DNTT (DPh-DNTT). In TFTs, these semiconductors show hole mobilities ranging from 1 to 7 cm2/Vs. The goal of the displacement current measurements reported here is to study how the choice of the semiconductor affects the trapping and detrapping dynamics in organic TFTs.