Modeling and Simulation of n-Type Carbon Nanotube Field Effect Transistors Using Ca as Contact Electrodes
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Modeling and Simulation of n-Type Carbon Nanotube Field Effect Transistors Using Ca as Contact Electrodes Aurangzeb Khan1, A. Q.S Shah2, and Jihua Gou3 1 Electrical and computer engineering, University of South Alabama, 307 University Blvd., ECEB51, Mobile, AL, 36688 2 ECE, University of south Alabama, 307 university Blvd., Mobile, AL, 36688 3 Mechanical Engineering, University of South Alabama, 307 University Blvd., Mobile, AL, 36688 ABSTRACT In this research work, a model has been proposed in view of the recent experimental demonstration using Calcium (Ca) as a contact metal to realize the n-type carbon nanotube field effect transistors (CNTFET). In order to fully optimize this proposed device model, effects of different parameters like the work function, oxide thickness, the oxide capacitance and the source velocity limits were studied. Among all the parameters, the work function of the contact metal plays an important role for controlling the flow of carriers through the carbon nanotube channel and to reduce the threshold voltage. A semi-classical simulation of the proposed n-type CNTFET has been performed. Results show an excellent subthreshold swing value of 62.91 mV/decade, close to the International Technology Roadmap for Semiconductor (ITRS) specifications. INTRODUCTION Recently, carbon nanotubes have emerged as promising candidates for nanoscale field effect transistors. The carbon nanotube field effect transistors (CNTFETs) are excellent candidates for the current microelectronic technology that exhibits improved performance in nanometer-regime. Intense research is in progress to study the transport properties of carbon nanotube based field effect transistors. Due to the ability of ballistic transport, CNTFETs have been studied in recent years as a potential alternative to CMOS devices [1, 2]. It is shown that these CNTFETs can be made with ohmic or Schottky type contacts [3, 4]. The operation of Schottky contact CNTFETs is by modulating the transmission coefficient of the barriers at the contact between the metal and the CNT [5]. It has been observed that ambipolar conduction (both holes and electron conduction) is the matter on which to focus in these type of devices. Due to the ambipolar conduction of Schottky barrier CNTFETs, the Ion/Ioff ratio is limited. Recently, fabrication of an n-type carbon nanotube field effect transistor was reported [6-8] where Nosho et al. used a smaller work function contact metal rather than the large work function contact metal commonly used in carbon nanotube transistors (CNTs). Here, an n-type CNTFET is fabricated by selecting a contact metal. A recent study explains the operation mechanism of the nanotube FETs by a Schottky barriers modulation model in which the gating action is dominated by the Schottky barriers formed at the contact between the nanotube and the source metal. Javey et al. [9] reported that electrode / nanotube contact resistance can be reduced by using ëPdí as the contact metal in p-type nanotube FETs. Since the work function of ëPdí me
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