Study On Vertical Organic Field-effect Transistors

With the high-speed development of informationization, today, large scale integrated circuits technology and terminal display technology become the two most important support techniques in information-based society. Traditional Si-based semiconductor techniques have such disadvantages as high investing cost, complicated preparation engineering, not suit for flexible substrate and large area production. With the advantages of novelty, low cost and flexibility, organic field-effect transistor had developed rapidly, which provides the best solution schemes to achieve large area and fexible flat displays array and integrated circuits, and it has become an important leading topic in organic electronics.In this dissertation, we fabricated the vertical organic field effect transistor (VOFET) in which the channel length was reduced to nanometers. The performance of our devices was charactered and factors to affect their characteristics were analysized in detail.Firstly, the electrical properties of a series of insulation films were characterized. By using impedance spectroscopy technique and C-V measurements, we studied the dynamic electrical characteristics of device ITO/LiF(120nm)/Al, its unit capacitance of 15 nF was figured out and the equivalent circuit model was established. By using similar methods, the unit capacitance of SiO₂, PMMA, SiN_x and TiO_x films was measured, respectively.Secondly, we have successfully fabricated VOFETs. By analyzing the contact of organic semiconductor/metal interface and the capacitance characteristics of gate insulating layer, the operation mechanism of the devices was discussed.(1) Pentacene-based VOFETs were fabricated, and the impact of different insulating layers on device performance was investigated. The on/off current ratio of 20 was obtained with LiF, with a relative dielectric constant of 9.036, as gate insulating layer and it was improved to 10³, with a turn-on voltage less than 3V, by both oxidizing the source electrode and by inserting 1nm LiF layer before Al electrode.The unit capacitance of capacitor cell plays a significant role on the performance of our VOFETs. With a thick insulating layer with low dielectric constant as the capacitor cell, the drain current seldomly almost unchanges with increasing gate voltages. However, for a thin insulating layer with high dielectric constant as capacitor cell, the device leak current is large and the phenomena of zero-point shifting of current occurs because of the drain leak current and the charge carriers tunneling Schottky barrier due to the mass collective charges at the interfaces.(2) CuPc-based VOFETs were fabricated and we found that their on/off current ratio is lower than that of pentacene-based VOFETs. In our opinion, the reason is that electron injection barrier at the CuPc/Al interface is relatively lower than that at Pentacene/Al interface, sequentially the drain current from CuPc-based VOFETs is higher than that from pentacene-based VOFETs.