easily realized in the top-contact geometry, the limitation in minimum separation of source and drain electrodes prevents further advancement in on-off switching speed; it is essential to realize OFETs with a high-mobility semiconductor and a short channel length L , because the maximum circuit operating frequency is proportional to the transconductance g m and inversely proportional to L 2 . Moreover, the technique using shadow masks cannot be applied to mass-producible processes to fabricate integrated OFETs in an identical substrate with high yield.Photolithography is the well-established technique to fabricate fi ne patterns, though it is rarely employed in the top-contact OFETs because conventional organic semiconductor layers are seriously damaged by solutions used for common wet processes in the method. A method using orthogonal fl uorinated photoresist was proposed, [ 10,11 ] which enables to make fi ne patterns without dissolution of organic semiconductors. Though the method enables the fabrication of micro-and nano-scale OFETs, it has not matured yet to be applied for circuitries with short-channel and high-mobility semiconductor devices mainly because of the very large contact resistances. [ 11 ] In OFETs with bottom-contact geometry, photolithography is more often employed because the electrode patterning is fi nished before the semiconductor deposition. However, a major concern for the bottom-contact OFETs with a polycrystalline semiconductor fi lm relates to the molecular disorder occurring in the vicinity of the metal electrodes. [ 12 ] To overcome this problem, electrode surface modifi cations on gold fi lms by thiol-based selfassembled monolayers [ 13,14 ] or UV/O 3 treatment [ 15 ] are effective to reduce the contact resistance, however, these devices suffer from irreproducibility and poor long-term reliability due to chemical instability of most Au-thiolate SAMs. [ 13 ] In this communication, we report for the fi rst time a micropatterning process based on photolithography with simple wetetching of gold electrodes deposited directly on pristine organic fi lms without any adverse effect of residual photoresist. Since the wet-etching of gold is governed by iodide/iodine redox reaction, it is essential to use organic semiconductor materials that are robust to the reaction. Employing a newly developed highmobility organic semiconductor materials with tuned ionization potential, these processes are indeed suited to fabricate fi ne-pitch electrodes with low contact resistance, so that the highest cut-off frequency of 19 MHz is demonstrated for p-type transistors.In order to examine the robustness of organic semiconductors to the gold wet-etching process, we have carefully Based on the recent development of high-mobility and organic semiconductors, [1][2][3][4] organic fi eld-effect transistors (OFETs) offer promising prospects to realize such attractive applications as high-speed fl exible displays, fl exible radio frequency identifi cation (RF-ID) tags, and light-weight wearable smartsensing logics....
the other hand, it was recently reported by our group that inchsize single-crystalline fi lms with unprecedentedly high carrier mobility can be fabricated from solution using a simple "edgecasting" method. [11][12][13] In this study, we aim to link the highmobility of solution-processed organic crystalline fi lms to a high dynamic response in organic transistors for both p-type and n-type operation by micropatterning the crystalline semiconductors and source/drain electrodes. The cut-off frequency f c of a transistor in the linear regime is described aswhere V D is the applied drain voltage, and L and W are the channel length and width, respectively. µ eff is the effective carrier mobility of an organic semiconductor, including the effects of contact resistance, C para is the parasitic gate capacitance, and c i WL represents the channel capacitance. In the saturation regime, V D is replaced by the gate voltage V G . From Equation ( 1) , it is clear that short-channel high-mobility transistors are strongly important to raise up the maximum operational speed of organic transistors. To realize a high fi eld-effect mobility in a short-channel device, it is crucial to reduce the contact resistance between the organic materials and the contact electrodes, which has been a challenging issue in organic transistors. In this Communication, top-contact organic confi gurations were adopted to realize extremely low contact resistances of 123 Ω cm for p-type transistors and 1.2 kΩ cm for n-type transistors, in which the contact electrodes were fabricated using photolithography process on solution-processed organic semiconductors. Complementary ring oscillators consisting of p-type and n-type transistors were demonstrated in ambient conditions to examine the operational speed of the organic circuits. Moreover, organic rectifi ers based on high-speed p-type transistors in which the drain and gate electrodes were diodeconnected were examined to determine their dynamic response speed in rectifying AC signals to DC output voltages at frequencies above 22 MHz. In the design of logic circuits, complementary circuits have the advantage of low power consumption, so the mainstream advancement of silicon technology has been based on complementary metal-oxide-semiconductor (CMOS) circuits. In organic transistors, stable n-type operation in ambient conditions has been a crucial issue because of the unstable Organic complementary circuits based on organic semiconductors have been proposed to enable attractive devices such as fl exible, or wearable organic devices. [1][2][3][4][5] Radio-frequency identifi cation (RFID) tags are one prospective application because organic devices are low-cost, light-weight, and fl exible, which are attractive features for this application. [6][7][8] One of the most important features of organic semiconductor materials is the strong self-aggregation of molecules, which enables fi ne crystalline fi lms to be easily formed, even at room temperature. This strong self-aggregation of organic materials also enables ...
Herein, the influence of postannealing on the properties of ZnO-SnO2 (ZTO) thin-film transistors (TFTs) was investigated. Postannealing in ambient air induced recovery of the electrical properties of wet-etch-damaged TFTs and a decrease in the subthreshold swing. Also, the field effect mobility increased with increasing postannealing temperature. Further improvement of the electrical properties of ZTO TFTs was not obtained with a further increase in annealing time. After postannealing at 200 °C for 60 min in the air, the ZTO TFTs exhibited a field effect mobility of about 10 cm2/V s.
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