Scalable Fabrication of Ambipolar Transistors and Radio‐Frequency Circuits Using Aligned Carbon Nanotube Arrays

Electronic devices based on carbon nanotubes (CNTs) have attracted signifi cant attention for potential radio frequency (RF) applications. [ 1–3 ] It has been shown that intrinsic current-gain and power-gain cutoff frequencies ( f T and f max ) above 1 THz should be possible, but experimental demonstration using fi eldeffect transistors (FETs) based on individual CNTs has suffered from excessive parasitic effects and impedance mismatch problems. [ 2,3 ] In order to overcome these limitations, great efforts have been concentrated on FETs made from aligned arrays of CNTs. [ 4,5 ] Since all of the published A-CNT RF circuits were designed to work in the linear region, it is often stated that it is necessary to use dense arrays of all-semiconducting nanotubes to achieve high performance. [ 2 ] While impressive progress has been made in achieving high-density arrays of CNTs, [ 6–12 ] for example in several cases the required density of tens of nanotubes per micrometer has been realized, it is still challenging to eliminate all metallic CNTs without damaging semiconducting CNTs and thus severely degrading the performance of the CNTarray FETs. [ 11–16 ] Therefore most of the published RF integrated circuits based on CNT array on quartz only can operated at a relative low frequency far below 1 GHz owing to the low cutoff frequency of FETs. Here we argue that rather than trying to avoid the requirement of pure semiconducting CNTs, we may indeed make a good use of natural growing CNTs for RF applications. We demonstrate that perfect ambipolar modulation may be achieved using FETs based on as-grown CNTs arrays on quartz near the minimum current point (MCP). Using the ambipolar region rather than the linear region of these FETs, RF circuits including frequency multipliers and mixers are batchfabricated and shown to retain their function up to 40 GHz, outperforming all previously reported carbon-based RF circuits. The core device in our RF circuits is a FET fabricated on a horizontally aligned large-diameter (about 2.4 nm on average) SWCNT arrays. The carrier mobility in the CNT is known to increase rapidly with increasing tube diameter, making the largediameter CNTs more suitable for RF applications. [ 17 ] The geometry of the RF FET is illustrated in Figure 1 a, where the source (S) electrodes are connected to the common ground and the double parallel gate (G) fi ngers and drain (D) electrodes are used for the input and output ports, respectively. Figure 1 b is an optical image showing arrays of fabricated FETs, and Figures 1 c and d are, respectively, low-magnifi cation optical microscope and high-magnifi cation scanning electron microscope (SEM) images showing the detailed electrode structure of the FETs. The transfer characteristics of the fabricated FET shows ambipolar behavior (Figure 1 e) in the sense that the branch at positive gate voltage (n-branch) and that at negative gate (p-branch) are symmetric about the MCP. Owing to the participation of metallic CNTs, as well as the large diameter of the CNTs, [ 10 ] the transfer characteristics shown in Figure 1 e have a current on/off ratio of less than 1.5, which is comparable to that of graphene top-gated FETs. [ 18,19 ] Although low current on/ off ratio would preclude applications in digital logic, it is acceptable in analogous RF systems where the device operates in a narrow range of voltage around a fi xed point. It should be noted that Ti contacts and a high-effi ciency top gate were used here to improve the symmetry between nand p-branches in transfer characteristics owing to the suitable work function of Ti and high gate effi ciency of Y 2 O 3 insulator. [ 20,21 ] When V ds is fi xed at 1 V, as in Figure 1 e, the drain current I ds is modulated by gate voltage V gs and varies between 1.31 and 0.94 mA, which corresponds to a channel resistance ranged from 780 to 1060 Ω . The transfer characteristics shown in Figure 1 e are very similar to those measured from a graphene FET, and the corresponding output characteristics (Figure 1 f) show a saturation tendency at large bias, which is diffi cult to realize in graphene FETs. [ 22,23 ]