Noise Parameter Characterisation of Graphene Field Effect Transistors in the 2-8 GHz Frequency Range

Abstract

Graphene is promising for being used as a channel material in high frequency and low noise field effect transistors (FETs). This is facilitated by its superior near-room temperature mobility (10^5 cm^2/V-s) for both type of carriers and predicted high value of saturation velocity (4*10^7 cm/s). This thesis presents noise parameter characterisation of graphene field effect transistors (GFETs) using source-pull measurement technique in the 2 to 8 GHz frequency range. Commencing from cleanroom fabrication, all stages of the work including measurement and modelling for device characterisation are dealt with in this study. In the first part of the thesis, a procedure for GFET fabrication utilising CVD graphene is developed. The device properties such as gate leakage, contact resistance, and annealing condition are optimised. The obtained contact resistance is 135 ohm-µm which is state-of-the-art. GFETs fabricated using CVD graphene on silicon dioxide (300 nm)/silicon substrate with 1 µm long and 230 µm wide graphene channels are characterised to obtain the noise performance at device level. The cut-off frequency and the maximum frequency of oscillation of the GFETs are on the order of 10.5 GHz and 13 GHz, respectively. The measured minimum noise figure was 2.4 to 4.9 dB for the extrinsic device with a corresponding associated gain of 10.6 to 2 dB in this frequency range. The intrinsic device has minimum noise figure of 0.8 to 4.3 dB after de-embedding the parasitic noise contribution using noise correlation matrices. Subsequent application of Pospieszalski two-temperature noise model provided a drain noise temperature of 1950 K and a gate noise temperature of 700 K.