High Speed Characterization of Organic Thin Film Transistors

Abstract

We have developed an electronic method to characterize the drift velocity and mobility of charge carriers in organic thin film transistors. The measurement is based on the movement of a packet of carriers injected into the channel. Drift mobilities obtained are greater than 1 cm2/Vs. These results can lead to the development of a class of circuits that can be operated in the megahertz range. This technique can also be used to explore trap states and therefore obtain a comprehensive understanding of charge transport in these materials. This experiment is the first of its kind to be performed on an organic transistor. Organic semiconductors are endowed with qualities such as easy processibility and variety of materials. However organic circuit applications are hindered by low speeds of operation. This is due to the fact that majority of the carriers are trapped in localized states resulting in low field-effect mobilities. However, existence of fast carriers in the channel of a pentacene transistor has been demonstrated by Dunn et. al. by exciting the transistor with a step voltage[ 1]. Drift mobilities of 0.18 cm2/Vs have been obtained for devices with field effect mobilities of 0.07 cm2/Vs. The impulse voltage method is an extension of the step voltage technique which can be used to measure the drift velocity under varying bias conditions. The schematic of the experiment is shown in figure 1. To study transport using the impulse method, the transistor is switched on and allowed to settle in a conducting state. This is followed by the superposition of a small perturbation, a 5 V impulse with a width (FWHM) of 15 nsec, at the source of the transistor, which is otherwise at 0 V (figure 2). Consequently a packet of charge carrier is injected into the channel. Simulations results shown in figure 3 indicate that the drift component of the injected carrier current exceeds its displacement counterpart. The diffusion component can also be neglected for these small timescales of operations. By restricting the drain bias to low voltages it can be safely assumed that the electric field is uniform inside the channel. The transit time can, therefore, be related to the drift mobility as t = L2/_ VDS. Once the carrier flux arrives at the drain, a bias-t is used to divert the transient current into a 500 Q resistor. The voltage across the resistor is therefore a measure of the current generated by the pulse. The shape of the current pulse for Vg = -90 at varying drain voltages is shown in figure 2. By subtracting the parasitic RC delays in the experimental setup, the true transit-time of current pulse can be extracted. This yields a mobility of 1.5 (+0.45, -0.2) cm2/Vs, which is -10 times higher than the FET mobility. Poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT) was used for this study because of its high field effect mobilities [2]. Linear region mobility of 0. 12 cm2/Vs was obtained at VGS of -100 V. The experimental and simulated I-V characteristics are displayed in Figure 4. The electric fields (E) used in the transient experiments were 104 V/cm. The average drift velocity (v) of the carriers at these fields was greater than 4x 04 cm/s. These results would indicate the presence of carriers in the channel of a transistor that can respond to input frequencies (f) in the megahertz range. (f= v/271L) One of the major significance of these fast response times is the opportunity to construct circuits that can surpass the performance barrier posed by low FET mobility. The maximum operating frequency for digital circuits is governed by the capacitive delays in forming the channel. However, even with a partially formed channel, switching operations can be performed by the faster carriers capable of responding to the input frequency. Therefore a class of circuits can be constructed that doesn't require the completion of channel formation. An example of such a circuit is a rectifier. A rectifier is an integral component of an RFID tag, which converts the inductively coupled ac input power into dc voltages used to operate the rest of the circuit. Such circuits can be operated at frequencies much higher than that predicted by the steady state mobility. In summary, we have made the first measurements of drift velocity and mobility in a polymer transistor. The drift mobilities are 10 times higher than the field-effect mobilities. These high mobilities have been used to explain high frequency operation of organic transistor circuits. This method can be also used to explore trap states and achieve a more complete understanding of charge transport.