# Ku-Band Mixers Based on Random-Oriented Carbon Nanotube Films
## 3 Results and Discussion  
### Electrical Characterization of RF Transistors  
Top-gated RF transistors were designed based on randomly oriented carbon nanotube films (\(-\)40-60 CNTs/\(\upmu\)m, see Figure 1a) with the ground-signal-ground (GSG) test architecture for RF measurements and fabricated using the lift-off process described in the Materials and Methods section. Usually, metal stack Palladium and Gold (Pd/Au) of 20/10 nm were deposited onto CNT films to form the p-type contact. Atomic-layer deposition-grown 5 nm HfO\({}_{2}\) is the high-k gate dielectric layer for efficient gate control. The wire connection was formed by the deposition metal stack Titanium/Gold (Ti/Au) with a thickness of 5/90 nm. In the overlapping region, a bridge of PMMA resist was overexposed and sandwiched between two metal layers to reduce the parasitic capacitance compared with high-k dielectric HfO\({}_{2}\). First, we fabricated two-finger-configuration RF transistors to verify the manufacturing process and measurement method. The channel length (Lch) of the two-finger gate transistor was about 130 nm, the gate length (L\({}_{\text{g}}\)) was 50 nm, and the total gate width was 20 \(\upmu\)m with one finger width of 10 \(\upmu\)m. A high-magnification SEM image of the active region shows the two-finger configuration as the schematic shown in Figure S2a. Our two-finger-configuration RF transistors exhibited a current density of nearly 0.4 mA \(\upmu\)m\({}^{-1}\) and peak transconductance (g\({}_{\text{m}}\)) of over 0.4 mS \(\upmu\)m\({}^{-1}\) at a bias (\(V_{\text{ds}}\)) of \(-\)1.4 V (Figure S2b in the Supplementary Materials). To achieve high-performance RF transistors, we needed to design a large gate width [12; 31] to ensure adequate drive capacity and, more importantly, to reduce the power loss caused by the resistance mismatch between the device and the 50 \(\Omega\) impedance. It is worth noting that enlarging gate width for improving device current relies on material uniformity, which is more difficult for aligned CNT films, although they have much better normalized currents (namely, current density). So, we designed randomly oriented CNTfilm transistors with the twenty-finger configuration of 100 \(\upmu\)m gate width (\(W_{\text{ch}}\)) and 50 nm gate length (\(\text{L}_{\text{g}}\)), as shown in Figure 2a. The total driving current (\(I_{\text{on}}\)) reached 30 mA and peak transconductance (\(g_{\text{m}}\)) reached 34 mS at a bias (\(V_{\text{ds}}\)) of \(-1.2\) V, presenting practical performance in randomly oriented CNT-based FETs (Figure 2b,c). The DC performance of carbon nanotube-based FETs can help us estimate the RF performance of transistors and also help us determine the DC bias conditions required for transistor RF measurements. As shown in Figure 2b, the total driving current of the CNT-based FET reached 30 mA, which met the requirements of RF measurement. Based on the DC performance of the fabricated 20-finger-configuration RF transistor, we characterized its cut-off frequency performance which is the basis for RF application. The RF performance of a transistor could be characterized by the current-gain cut-off frequency (\(f_{T}\)), defined as the frequency at which the current gain becomes unity, and the power-gain cut-off frequency (\(f_{max}\)), defined as the frequency at which the power gain becomes unity. As shown in Figure 2d, the current gain (\(H_{21}\)) and power gain (MSG/MAG) versus frequency curves demonstrated that the cut-off frequency (\(f_{T}\)) and maximum oscillation frequency (\(f_{max}\)) of the device reached 78 GHz and 60 GHz, respectively, both with the extraction slope of \(-20\) dB/dec which met the minimum requirements for CNT-based RF transistors building RF devices operating in the Ku band (the working frequency is at least one-third of \(f_{max}\)) [32].