This dissertation consists of three main chapters: Pentacene-based low voltage p-channel organic filed-effect transistors (OFETs) with anodized gate dielectrics; Charge transport in N,N'-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) [PDI-8CN2] based n-channel OFETs; and Dual-channel OFETs. Pentacene-based low voltage p-channel OFETs were realized using three different anodized gate dielectrics: a 470 A SiO2, a 1,700 A Ta2O5, and an 800 A Ta2O 5 formed by anodizing an n-Si wafer, a sputtered Ta thin film, and an e-beam evaporated Ta layer, respectively. Devices with the anodized SiO2 gate dielectric exhibited decent characteristics at VDS ≤ -10 V and VG ≤ -4 V, and the device performance was further improved by an octyltrichlorosilane (OTS) treatment. The two anodized Ta2O5 gate dielectrics were successfully employed to fabricate devices with high mobility at VDS ≤ -5 V and VG ≤ -2.5 V for the 1,700 A Ta2O5 devices, and at VDS ≤ -10 V and VG ≤ -5 V for the 800 A Ta2O5 devices. A hexamethyldisilazane (HMDS) treatment and a mono-docecyl phosphate (MDP) treatment proved to remarkably enhance the characteristics of the two Ta2O5 devices. However, the two treatments had the opposite influence on the threshold voltages of the devices from each other because of the capacitance difference resulting from their molecular length difference. In order to establish the suitable charge transport mechanisms in PDI-8CN2 and related n-channel organic semiconductors, the gate voltage and temperature dependence of electrical behavior and the contact resistance effects were studied in PDI-8CN2 based OFETs. The dependence of electrical behavior such as mobility, field-dependent mobility, trap density, and off current on gate voltage and temperature was derived using the multiple trapping and release (MTR) model. The contact resistance effects were determined by calculating the contact-corrected linear regime mobility and contact resistance by means of a