Studies On Vertical Field-effect Transistors Based On Pentacene

The field-effect transistor (FET) is an important member of the electronic family. Traditional inorganic silicon FETs have some disadvantages such as the high cost, the complicated preparation process, and it is not suit for flexible substrate. Comparing with inorganic counterpart, organic FETs (OFETs) are developing rapidly and becoming an important part of organic electronic devices, being the advantages of novelty, low cost, compatibility on flexible substrates.In this thesis, we fabricated the vertical field-effect transistors (VFETs) based on pentacene and analyzed the influence of the dielectric layer, the middle source electrode and the modified layer on the device performance, also, bipolar transistors were fabricated and characterized. Basically, the following conclusions can be drawn from our experiments:Firstly, the vertical OFETs (VOFETs) with different thickness of dielectric LiF layer (ITO/LiF (x nm)/Al (25 nm)/Pentacene(80 nm)/Al) were fabricated, the performance of these devices was characterized and the influence of dielectric layer on the control capability of gate voltage was investigated. Experiments showed that the best performance of the device was obtained with the LiF thickness of 150 nm.Secondly, the middle source electrode was studied and the influence of the source electrode on the control capability of gate voltage was analyzed:(1) when the thickness of Al electrode was 20 nm, the device showed the best performance with the "on/off' current ratio of 85; (2) the effect of ozone treatment with different time on the middle Al electrode (20 nm) was investigated, and we found that the "on/off" current ratio of the device after ozone treatment of 12 minutes could reach 103, also, the effect of surface morphology and composition of the Al middle electrode on the device was studied; (3) the device performance with different metals (Al,Au,Ag) as the middle electrode was investigated, it showed n-type characteristics when Al and Ag used as source electrode but p-type characteristics when Au as source electrode.Meanwhile, we fabricated VOFETs with a modified layer (LiF, Cs2CO3) on the middle Al electrode, and we found that the "on/off"current ratio of the device increased by 5 times and 10 times, respectively, and the device showed n-type characteristics. By increasing of the thickness of Cs2CO3 modification layer, we found that the increase of of the device performance declined; But if we replaced the middle Ag electrode by Cs2CO3, the device performance was not improved significantly; In addition, we prepared a good performance device with Au as the source and Cs2CO3 as the drain electrode modification layer, and the "on/off"current ratio of the device reached to 6×102.Finally, we investigated the polarity of the VOFETs, and found that the device showed ambipolar characteristics (n-type and p-type) by using two different types of metals (Al and Au) together as source and drain electrodes. With the change in ratio of Al and Au, the device showed the relationship of n-type characteristics "stronger than", "equal to" and "weaker than" p-type characteristics.