Effect of uniaxial strain on the subthreshold swing of ballistic carbon nanotube FETs R. Yousefi Department of Electrical Engineering, Nour Branch, Islamic Azad University, Nour, Iran article info Article history: Received 9 May 2011 Accepted 28 June 2011 Available online 5 July 2011 abstract In this paper, using the non-equilibrium Green’s function formalism (NEGF), i have investigated the effect of uniaxial strain on the subthreshold swing of the Tunneling and Schottky barrier carbon nanotube FETs. By a qualitative description of the quantum capacitance of the structures, i have shown that the uniaxial strain has strong effect on the subthreshold swing. The minimum voltage swing to obtain a given I ON /I OFF ratio has also been investigated. The results show that, a smaller voltage swing and subthreshold swing can be obtained using a higher value of the uniaxial strain. Although a simultaneous decrease in the ON current and increase in the delay time also occurs. Hence, whenever the power consumption is the main purpose of the design, uniaxial strain can be used to achieve a very low voltage swing value. & 2011 Elsevier B.V. All rights reserved. 1. Introduction In carbon nanotube field-effect transistors (CNTFETs), the strain engineering is a method for simultaneously increasing the ON current and decreasing the leakage current. The effects of uniaxial strain on the performance metrics such as, power-delay product, delay, ON and OFF currents of the Schottky Barrier CNTFET (SB-CNTFET) [1], MOS-like CNTFET (MOSCNT) [2] and Tunneling CNTFET (T-CNTFET) [3] were investigated [4–6]. It is basically understood that uniaxial strain can influence on the bandgap, band-structure-limited velocity [4] and density of state (DOS) [5] of the CNTs. An important figure of merit in a field-effect transistor is the minimum voltage swing needed to turn a transistor from OFF to ON. The subthreshold swing is defined as the gate voltage needed to change the drain current by one order of magnitude [7]. The minimum power supply and power dissipation of a technology are a decreasing function of the subthreshold swing [7]. The (2.3 k B T/q) limit for subthreshold swing (60 mV/dec at 300 K) is applicable to MOSFETs such as the MOSCNT in which the thermionic emission is the dominant current flow mechanism [8]. This limit can also be applied to the SB-CNTFETs [9] and increases by the transistor scaling. It has been shown that the subthreshold swing of the tunneling transistors can be reduced below the 60 mV/dec limit [10,11]. In this paper, i have studied the effect of uniaxial strain on the subthreshold swing of the SB-CNTFETs and T-CNTFETs. In Section 2, i present modeling and simulation method, results and discussions is presented in Section 3 and we conclude this paper in Section 4. 2. Modeling and simulation method Schematic structures of a T-CNTFET and SB-CNTFET are shown in Fig. 1. A coaxial structure is considered since it was shown that the subthreshold swing could be lowered further by increasing the gate control on the channel [8]. I have solved the Schr ¨ odinger equation self-consistently with the Poisson equation in the NEGF formalism [13,14], using a mode space approach [15]. I have taken into account only the first subband that is sufficient within the range of the uniaxial strain considered here. The Poisson equation has discretized based on a non-linear finite difference method and then solved by the Newton– Ralphson method [16]. The tight-binding method with only a p z orbital coupling [17] has been used to calculate the Hamiltonian of the carbon nanotube. For the SB-CNTFET, the self energy of metal contact was calculated using Ref. [14]. A method introduced in Ref. [4] was used to include the uniaxial strain in the NEGF formalism. New hopping parameters were cal- culated by the method of Harrison [18] as follows: t i ¼ t 0 i d 0 i d i ! p i ¼ 1, 2 and 3 ð1Þ where t 0 i ¼ 3 eV and t i is hopping parameter before and after defor- mation, respectively, i is the three nearest neighborhoods, d 0 i and d are the bond lengths before and after deformation, respectively. I have used p ¼ 2 as considered by Harrison himself. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2011.06.034 E-mail address: Yousefi@novinpardazesh.ir Physica E 43 (2011) 1896–1901