1096 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 4, APRIL 2011 Electrical Characteristics of GaAs Nanowire-Based MESFETs on Flexible Plastics Changjoon Yoon, Gyoujin Cho, and Sangsig Kim Abstract—GaAs nanowire (NW)-based metal–semiconductor field-effect transistors (MESFETs) were constructed on flexible plastic substrates by a conventional top–down approach. The top–down approach utilized in this paper combines photolithog- raphy of high-quality GaAs bulk wafers with anisotropic chemical etching processes for preparation of GaAs NWs and photolitho- graphic processes for formation of metal electrodes. For a repre- sentative GaAs NW-based MESFET, peak transconductance, the I on /I off ratio, and the subthreshold slope are estimated to be ap- proximately 19.7 μS, ∼10 7 , and ∼100 mV/dec, respectively. The electrical characteristics of the GaAs NW-based MESFETs were maintained during 3000 times of bending cycles under maximal tensile strains of 0.77% and 1.02%. These results demonstrate the possibility of using these devices in high-speed and high-perfor- mance flexible electronics. Index Terms—Flexible electronics, GaAs, metal–semiconductor field-effect transistors (MESFET), nanowire (NW). I. I NTRODUCTION S EMICONDUCTOR NANOWIRES (NWs) have attracted a great deal of attention as potential alternatives for re- alizing nanoelectronic devices. Most field-effect transistors (FETs) based on NWs have been researched and developed with device structures of metal–oxide–semiconductor FETs (MOSFETs) [1]–[5]. Recently, a lot of effort has been made worldwide to enhance electrical characteristics of NW-based MOSFETs. NW-based MOSFETs composed of InAs–InP het- erostructures exhibited a significant improvement in their ON-current, transconductance, and electron mobility; in par- ticular, for these MOSFETs, an electron mobility value of Manuscript received September 29, 2010; revised December 3, 2010; accepted January 13, 2011. Date of current version March 23, 2011. This work was supported in part by the Ministry of Knowledge Economy/Korea Evalua- tion Institute of Industrial Technology Information Technology R&D Program under Grant 10030559 (Development of Next Generation High Performance Organic/Nano Materials and Printing Process Technology), by the Ministry of Commerce, Industry and Energy Medium-Term Strategic Technology Devel- opment Program, by the Nano R&D Program under Grant M10703000980- 08M0300-98010, by the Ministry of Education, Science and Technology Korea Science and Engineering Foundation World Class University Project under Grant R32-2008-000-10082-0, and by the Hynix–Korea University Nano- Semiconductor Program. The review of this paper was arranged by Editor A. C. Seabaugh. C. Yoon and S. Kim are with the Department of Electrical Engineering, Korea University, Seoul 136-701, Korea (e-mail: sangsig@korea.ac.kr). G. Cho is with the Department of Printed Electronics Engineering and Chemical Engineering, Sunchon National University, Sunchon 540-742, Korea. 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/TED.2011.2107518 11 500 cm 2 /V · s was reported in [6]. In addition, electrical characteristics of omega-shaped-gated and vertical surround- gated NW-based MOSFETs were remarkably enhanced by modification of the gate geometry [7], [8]. While these struc- tures and technologies of NW-based MOSFETs have been successfully optimized, NW-based metal–semiconductor FETs (MESFETs) have not been researched as much, although they have been recognized as one of the most promising devices having many advantages over NW-based MOSFETs. One of the main advantages of NW-based MESFETs is their immunity to problems related to oxide–semiconductor interfaces encoun- tered in most MOSFETs, such as interface traps and reliability issues arising from hot electron injection and trapping. NW- based MESFETs can be constructed by simpler processes, com- pared with NW-based MOSFETs, since deposition of insulating oxide layers is not necessary. From the viewpoint of device function, MESFETs have more suitable device structures for constructing high-speed electronic devices than MOSFETs. In particular, GaAs and specific semiconductor compound- based electronic devices have been developed on the basis of MESFETs, due to a lack of suitable insulating materials for them [9], [10]. Many routes have been investigated to synthesize GaAs NWs. Most GaAs NWs have been synthesized using vapor– liquid–solid (VLS) growth and laser ablation [11], [12] methods or chemical vapor deposition (CVD) [13]. Nevertheless, as- synthesized GaAs NWs always have very broad distribution of their lengths, widths, doping concentrations, and doping uniformity. In recent years, a top–down approach has been re- searched to overcome these problems. The top–down approach allows high-quality GaAs NWs to be arrayed and their doping concentration, length, and width to be well controlled [14]. Specifically, their length and width are precisely dependent on the size of a photoresist (PR) mask formed by photolithography. In this paper, we constructed high-performance GaAs MESFETs on flexible plastic substrates. The GaAs MESFETs consist of a high-quality GaAs NW channel made by the top–down approach, with AuGe–Ni–Au layers as source/drain electrodes and Ti–Au layers as gate electrodes. We character- ized device performance of the GaAs MESFETs and inves- tigated their reproducibility, as well as their flexibility, under various strain conditions. II. EXPERIMENTAL PROCEDURES The fabrication process of the GaAs NW-based MESFETs under study consists of three major steps. The first step is 0018-9383/$26.00 © 2011 IEEE