Organic CMOS Technology Based on Interface Doped Pentacene
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I4.10.1
Organic CMOS Technology based on interface doped pentacene Marcus Ahles, Roland Schmechel, Heinz von Seggern Institute of Materials Science, Darmstadt University of Technology, Petersenstraße 23, 64287 Darmstadt, Germany ABSTRACT An organic complementary-metal-oxide-semiconductor (CMOS) inverter based on pentacene acting as both n- and p-type organic semiconductor is presented. The circuit consists of two spatially separated transistors which are realized within one continuous pentacene layer. Both transistors act exclusively in unipolar mode with electron and hole mobilities of 0.11 cm2V-1s-1 and 0.10 cm2V-1s-1, respectively. In the domain of the n-channel, electron accumulation in the pentacene is enabled by deposition of traces of calcium acting as electron donator. The CMOS inverter works reliably within the range of the supply voltage (60 V) with a gain in between 17 and 24 which is among the highest values observed in organic systems. Nevertheless, the circuit shows hysteresis, which is explained by a gate voltage depending trap occupation in the nchannel. INTRODUCTION Organic field-effect transistors (OFETs) are currently attracting great attention for their possible use in flexible low-cost electronic circuits [1,2]. For logic circuit design, complementary-metaloxide-semiconductor (CMOS) technology is preferred, due to its very low-power consumption and compensating effect for device tolerances. This technology requires the integration of both n- and p-type OFETs on the same substrate. However, up to now, all organic semiconducting materials which are known to exhibit sufficiently high charge carrier mobilities are suitable either for n- or for p-type OFETs [3]. Hence, the organic CMOS inverter circuits, which have been realized up to now, require two spatially separated semi-conducting materials [4,5] or two ambipolar transistors consisting of interpenetrating networks of both n- and p-type materials [6,7]. The main disadvantage of the utilization of two ambipolar transistors in a CMOS inverter is the problem of power consumption, i.e. that in both logic states a significant current passes through the device. Recently, it has been demonstrated that pentacene, well known for its good hole transport properties in OFETs [8], allows for an operation in electron accumulation mode with field effect mobilities up to 0.19 cm2V-1s-1 [9,10]. Generally, electron traps at the insulator interface, e.g. SiO2, prohibit the accumulation of mobile electrons in pentacene. This problem has been solved by utilizing a suitable polymer as insulator [10] or by interface doping of a SiO2 surface with a low work function metal acting as electron donator [9]. Based on the latter work a CMOS inverter utilizing pentacene applied as one continuous layer is realized. Both n- and p-channels in this circuit act exclusively in unipolar mode.
I4.10.2
EXPERIMENTAL The schematic configuration of the organic CMOS inverter is illustrated in figure 1 (a) and (b) in top view and cross section, respectively. The device is fabricated on
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