1 Abstract-We present Nyquist stability criterion together with step time responses for small-signal equivalent circuit model of common source configured carbon nanotube field effect transistors (CNTFETs) made of 100 CNTs at 10 nm pitch as well as single-CNTs, for the first time. In this analysis considering tube to tube variations in CNTs’ diameters, dependence of the degree of relative stability on CNTs dimensions is acquired. Simulations show the step responses for both types of CNTFETs, unlike those for CNT/GNR based local interconnects, pose multi-harmonic oscillations. Amplification of such harmonics by CNTFET can cause instability. Index Terms-Carbon nanotube field effect transistors (CNTFETs), Nyquist Stability, Time responses. I. BACKGROUND High mobility, low-defect structure, direct band-gap, symmetric bands and intrinsic nanometer scale of carbon nanotubes (CNTs) have led to intense research efforts into the viability of utilizing CNTFETs as a promising alternative for traditional silicon MOSFETs [1,2]. Performance of CNTFETs with pitched tubes is vital to the analog radio frequency integrated circuits (RFICs). In other words, possible high overshoots/undershoots in the time domain responses of CNTFETs biased in common source (CS) configurations used in an RF-IC can deteriorate the chip performance. In order to analyze the performance of CS-CNTFETs designed for on-chip applications, we need to evaluate their relative stabilities and time domain responses. Relative stability is a tool by which one can evaluate the delay and the maximum amplitude of the overshoot/undershoot. On the other hand, the Nyquist diagram as a complex plane of coordinates can be a powerful tool for investigating the system relative stability [3]. Point (−1, 0), in this complex plane, is the critical point for stability. When inside the diagram, for the system to be more stable, the point must move outward, as the system parameters are varied. When outside, the farther it moves from the diagram, the more stable the system. Relative stability and time domain response analysis for multi-layer graphene nanoribbons (MLGNR) as well as single-wall and multi-wall CNTs (SWCNTs and MWCNTs) interconnects have been recently reported [4-8]. Carbon nanotubes used in CNTFETs, contrary to those employed in interconnects are semiconducting and expected to pose different relative stabilities and step responses. The importance of such analyses for CS-CNTFETs lies with the fact that they can amplify the amplitudes of overshoot/undershoot coming from interconnects. Figure 1 illustrates (a) small signal circuit model and (b) cross section of a CNTFET adopted from [9], and (c) its top view when the channel is made of many CNTs at pitch size of d. The model shown in Fig. 1(a) is a physics-based model that includes non-idealities such as the quantum confinement effects in both circumferential and longitudinal directions of the channel. It also includes capacitances and resistances of the doped SWCNT source/drain regions, and also the series resistances due to Schottky barriers that might appear at the source/drain contacts. The model and geometrical parameters are defined in Table I. There are few other physical based CNTFET models [10-15] that are less comprehensive. Gate to source/drain trans-capacitances and drain to source resistance are given by [16]: Authors are with Advanced Devices Simulation Lab, Faculty of Electrical and Computer Engineering, Tarbiat Modares, Tehran 1411713116, Iran. Corresponding author: farshi_k@modares.ac.ir or moravvej@ieee.org Stability Analysis in CNTFETs Saeed Haji-Nasiri and Mohammad Kazem Moravvej-Farshi Senior Member, IEEE Fig. 1. (a) Small-signal CNTFET circuit model [9], (b) cross sectional and (c) top views of a CNTFET with pitched tubes at pitch size d.