4 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 8, NO. 1, JANUARY 2009 Threshold Voltage and On–Off Ratio Tuning for Multiple-Tube Carbon Nanotube FETs Albert Lin, Student Member, IEEE, Nishant Patil, Member, IEEE, Koungmin Ryu, Alexander Badmaev, Lewis Gomez De Arco, Chongwu Zhou, Subhasish Mitra, Senior Member, IEEE, and H.-S. Philip Wong, Fellow, IEEE Abstract—In this paper, we demonstrate postprocessing tech- niques to adjust the threshold voltage (V t ) and on–off ratio (I ON /I OFF ) of multiple-tube carbon nanotube field effect transistors (CNFETs). These postprocessing techniques open up an additional degree of freedom to further tune individual CNFETs in addition to various device synthesis and processing techniques. We demon- strate proof-of-concept experiments and fully characterize their design spaces and tradeoffs. The techniques, Threshold Voltage Setting and On–Off Ratio Tuning, were able to adjust the threshold by as much as 2 V and tune the on–off ratio across 5 × 10 3 to 5 × 10 5 . In addition, V t Setting could be used as an analysis tool to infer the V t distribution of grown carbon nanotubes (CNTs). These tuning techniques, combined with processes such as doping, will enable high-performance multiple-nanotube devices. Index Terms—Carbon nanotube field effect transistor (CNFET), ICs, nanotechnology. I. INTRODUCTION C ARBON nanotube field effect transistors (CNFETs) are promising extensions to silicon complementary metal– oxide–semiconductor (Si-CMOS) technology because of their excellent intrinsic delay (CV/I) [1], [2]. Many groups have re- ported the fabrication of CNFETs consisting of only a single carbon nanotube (CNT 1 ) [3]. But single-tube CNFETs, while appropriate for scientific exploration, do not have sufficient cur- rent drive for practical circuit applications [4], so CNFETs with multiple nanotubes per device are required. Such multiple-tube CNFETs can show significant performance advantage over Si- CMOS in both inverter fanout-of-4 (FO4) delay (4.7×) and energy per cycle (2.6×) [5]. Multiple-tube CNFETs also enable new methods for tuning the device characteristics previously impossible with single-tube CNFETs. Here, we demonstrate two such techniques for tuning the threshold voltage and on–off ratio of multiple-tube CNFETs. In FETs, the ability to tune the device threshold voltage (V t ) and on–off ratio (I ON /I OFF ) is extremely important. For example, Manuscript received May 29, 2008. First published August 22, 2008; current version published January 16, 2009. This work was supported in part by the Focus Center Research Program (FCRP) on Functional Engineered Nano Architectonics (FENA), C2S2, and the Gigascale Systems Research Center (GSRC), and in part by the National Science Foundation (NSF). The review of this paper was arranged by Associate Editor J. Rogers. A. Lin, N. Patil, S. Mitra, and H.-S. P. Wong are with Stanford Uni- versity, Stanford, CA 94305 USA (e-mail: mrlin@stanford.edu; hspwong@ stanford.edu). K. Ryu, A. Badmaev, L. G. De-Arco, and C. Zhou are with the University of Southern California, Los Angeles, CA 90089 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2008.2004706 1 The nanotubes used in this paper are all horizontally aligned, single-walled carbon nanotubes. in digital logic, an incorrect V t can lead to digital gates with large short-circuit currents during switching, while a tuned V t can result in optimal performance and reduced power dissipation. Similarly, a poor I ON /I OFF can cause slow output transitions or impractically low output swings, while an appropriate I ON /I OFF can improve speed and minimize leakage. We demonstrate V t Setting and I ON /I OFF Tuning as effective techniques for tuning the device characteristics of multiple-tube CNFETs. The V t Setting and I ON /I OFF Tuning techniques presented here are applied postsynthesis/postprocessing, thus they com- plement and can be used in conjunction with many other tech- niques to achieve greater control. For example, there are ex- isting and on-going efforts to develop techniques to control V t and I ON /I OFF , including preferential growth of CNTs [6], [7], sorting/separating metallic CNTs from semiconducting CNTs [8], [9], plasma etching of metallic CNTs [10], and chemical doping of CNTs [11]. V t Setting and I ON /I OFF Tuning can be applied to CNFETs in addition to the previous processes to at- tain greater device control; however, in this paper, we focus on proof-of-concept demonstrations of V t Setting and I ON /I OFF Tuning as stand-alone techniques since many of the other devel- oping techniques have not yet matured to sufficient robustness. V t Setting and I ON /I OFF Tuning adjust the threshold and on–off ratio of multiple-tube CNFETs by selectively pruning the com- posing CNTs within the device. This concept of device tuning is enabled by multiple-tube CNFETs, and would not otherwise be possible in single-tube CNFETs. A multiple-tube CNFET consists of many CNTs, each with its own threshold voltage and on–off ratio due to natural variations in CNT synthesis. Thus, there is a distribution of thresholds and a distribution of on–off ratios for the CNTs within the CNFET. The overall CNFET threshold voltage and on–off ratio is a function of this distribu- tion, i.e., the aggregation of the individual CNT thresholds and on–off ratios determines the overall observed CNFET device threshold voltage and on–off ratio. Consequently, by selectively pruning the CNTs and shaping the distributions, V t Setting and I ON /I OFF Tuning can adjust the device characteristics; however, this incurs tradeoffs with device drive current. In this study, CNT pruning is implemented using selective electrical burning of CNTs 2 , a method similar to [12], but modified and extended appropriately to allow for the precise control and selectivity re- quired for device tuning. The tradeoffs and design spaces for V t Setting and I ON /I OFF Tuning via selective electrical burning are comprehensively characterized and analyzed. 2 CNT pruning can potentially be implemented using other methods, such as selective plasma etching based on diameter, which is strongly correlated with CNT properties such as threshold and current drive. 1536-125X/$25.00 © 2009 IEEE Authorized licensed use limited to: IEEE Xplore. Downloaded on March 13, 2009 at 23:07 from IEEE Xplore. Restrictions apply.