Abstract
In this thesis, vertical organic transistors with alternate organic materials are investigated. Pentacene-based planar- and vertical-type organic thin film transistors (OTFTs) are investigated. High operation voltages are observed for the planar-type OTFTs with top source/drain electrodes, which results from the limitations of channel length and low material mobility. With a reduced channel length, a LiF hole-injection enhancement layer and a thin metal gate, the vertical-type pentacene OTFTs exhibit a low voltage operation of less than 5 V and a compatible on/off ratio of larger than 102. The smaller current gain observed from the device under current modulation is attributed to the increase of base recombination current under the common-emitter mode. Although pentacene-based vertical organic transistors with high current gain are successfully fabricated, the reliability of the devices under atmospheric ambient condition is still an issue. Based on the understandings, the development of CuPc-based triodes with similar device structures would not only prove the device structures suitable for alternate organic materials but also improve device reliability. Therefore, CuPc-based vertical organic triodes with thin metal base operation under saturation region are investigated. With an optimized LiF thickness of 0.4 nm, a common-emitter current gain of 3.63 is obtained for the device at JB = -2.5 mA/cm2 and VCE = -5 V. With a close-to-unity transport factor, the observed smaller current gain is attributed to the decrease of emitter current resulted from the non-uniform current flow under the common-emitter mode. Compared with pentacene-based vertical triodes, the CuPc-based devices exhibit a smaller common-emitter current gain resulted from its lower hole mobility. To futher investigate the influence of Al film thickness on the device performances, the characteristics of vertical organic triodes fabricated by using two copper phthalocyanine (CuPc) back-to-back Schottky diodes with different metal base thicknesses are fabricated. The vertical organic triodes exhibit pronounced saturation regions in the output current-voltage characteristics. The common-emitter current gain reduces with increasing the Al base thickness due to the increase of recombination current at the base end resulted from the reduction of opening voids in the Al metal film. The common-emitter current gain of the device with 4.5 nm thick Al base reaches 1.9 at VCE = –6 V and JB =2.5 mA/cm2.