DC, RF, and Microwave Noise Performance of AlGaN–GaN Field Effect Transistors Dependence of Aluminum Concentration

AlGaN–GaN heterostructures with different Al concentrations (20, 27, and 35%) were grown by metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates. Ti–Al–Ti–Au ohmic contact optimization was investigated at different temperatures and annealing time. Heterojunction field effect transistors (HFET) with a gate length of 0.25 m were fabricated. Low contact resistances of 0.2, 0.26, and 0.33 mm were obtained for HFET device samples with Al concentrations of 20, 27, and 35%, respectively. For Al concentration of 20, 27, and 35%, the AlGaN–GaN devices exhibited extrinsic transconductances of 143, 152, and 210 mS/mm, of 398, 566, and 784 mA/mm, unity cutoff frequencies ( ) of 24.9, 34.6, and 50 GHz and maximum oscillation frequencies ( MAX) of 54.9, 61.8, and 100.9 GHz and minimum noise figures ( min) of 2.01, 1.47, and 1.02 dB at 12 GHz, respectively. The calculated minimum noise figures have a good agreement with the measured values for all the devices with different Al mole fractions. The noise analysis shows that intrinsic noise of these AlGaN-GaN FETs plays a prominent part in the noise behavior because of improved device parasitics.