ABSTRACT
Figure 8.2a shows the schematic of the five-stage of ring oscillator which was fabricated by Chen et al. [2]. Figure 8.2b shows the simulation result of the ring oscillator output waveform at 0.92 V supply voltage using Verilog-AMS codes. Figure 8.2c shows the oscillation frequency with different supply voltages. The observed derivation of experimental results from the modeled characteristic can be attributed to non-optimized fabrication process, such as the CNT is not perfectly straight. Our model equations and experiments show that the oscillating frequency of this ring oscillator is only about 70-80 MHz at 1.04 V supply voltage. This is due to the CNT-FETs in the ring oscillator, which are 600 nm long, and there are parasitic capacitances associated with the metal wire in the ring oscillator. Shorter length of CNT-FETs will increase the oscillating frequency, as shown in Fig. 8.3. This is due to shorter channel length CNT-FETs, which will give high current and less parasitic capacitances.
