Dynamic characteristics measurement of organic thin film transistors (OTFTs) is one of important topics from both standpoints of fundamental physics and device application; however, there have been few works on this subject (T. Miyadera, et.al., Appl. Phys. Lett. 89, (2006) 172117.). Accordingly, we have investigated frequency response and transient phenomena of pentacene TFTs and analyzed quantitatively by means of a CR equivalent circuit model, where the effect of trap density, charge injection, and grain boundaries was discussed.
In this study, AC characteristics were examined by using top contact pentacene TFTs fabricated on Si/SiO2 substrates. Local doping technique was adopted where the charge transfer molecule (CTM) of F4TCNQ was inserted between pentacene and Au electrodes. Charge injection properties were investigated by comparing doped and non-doped device. From the analysis of transmission line model (TLM), suppression of contact resistance was observed. Moreover, metal insulator semiconductor (MIS) capacitance analysis revealed the existence of parasitic impedance at the contact. That is to say, charge injection part can be described by an equivalent circuit of capacitance and resistance. More information can be obtained from the MIS capacitance analysis than that from TLM analysis. Quantitative analysis of charge injection was made for the devices with several film thicknesses. We found that optimal film thickness for charge injection was around 20-40 nm. Around that film thickness, parasitic impedance takes minimum value.
The device operation frequency or cutoff frequency was estimated from frequency and gate voltage dependent capacitance. Maximum cutoff frequency of 100 kHz was obtained in the device with 100 µm channel length with CTM doping. Distributed constant circuit model well agreed with the experimental results and the effect of interfaces can be discussed from the model. These analyses make it possible to discuss the component of contact and channel independently and relationship between DC and AC characteristics is clarified.
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