Contact Effects in Organic Transistors

Organic thin-film transistors (TFTs) are of interest for applications that require electronic functionality with low or medium complexity distributed over large areas on unconventional substrates, such as flexible polymeric films. Generally these are applications in which the use of silicon devices and circuits is technically or economically not feasible. Examples include flexible displays and large-area sensor arrays. The static and dynamic performance of state-of-the-art organic p-channel transistors is already sufficient for applications in which the transistors operate with frequencies of a few tens of kilohertz. Strategies to improve the performance of organic TFTs include improvements in the field-effect mobility of the charge carriers in the organic semiconductor layer and more aggressive scaling of the transistor dimensions. Additional advances are also required in the environmental and operational stability of organic TFTs and in the application of controlled contact doping for the reduction of the contact resistance of organic TFTs. In this thesis, high-resolution silicon stencil masks are introduced to pattern top-contact organic TFTs with lateral dimensions as small as 0.8 µm. Taking advantage of the recently synthesized high-mobility air-stable organic semiconductor dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]thiophene (DNTT), polycrystalline organic p-channel TFTs with excellent air-stability and an intrinsic mobility of 3.4 cm2/Vs were fabricated. The transistors employ an ultra-thin gate dielectric based on a plasma-grown oxide layer in combination with an organic self-assembled monolayer (gate capacitance ∼1 µF/cm2), allowing the TFTs to operate with low voltages of about 3 V. Transistors with dimensions of less than 100 nm were manufactured using polymer resist shadow masks that have been fabricated by electron-beam lithography. Thanks to the 5 nm thick gate dielectric the TFTs show no short-channel effects. The subthreshold swing, the on/off current ratio, and the threshold voltage are comparable to long-channel TFTs with lateral dimensions of ∼100 µm. However, organic transistors with reduced dimensions are often limited by the energy barrier at the interface between the semiconductor and the source/drain contacts. One approach to reduce this energy barrier is the introduction of a strong electron acceptor into the contact area that acts as a dopant. With contact doping, the contact resistance is reduced and the effective mobility of short-channel TFTs increases by more than 200%. Transistors (without doping) with lateral dimensions of ∼1 µm have a transconductance of about 1 S/m, so that the TFTs operate at frequencies of several megahertz. The influence of strongly reduced lateral dimensions on the contact resistance and the maximum operation frequency is investigated and scaling limitations are discussed.