Deep level transient spectroscopy has been used to characterise the deep levels in Al0.25Ga0.75As/In0.20Ga0.80As/GaAs pseudomorphic high electron mobility transistors (pHEMTs) grown by solid source molecular beam epitaxy (SSMBE). Only one electron trap was detected in the Si-doped Al0.25Ga0.75As Schottky layer of the pHEMT with doping concentration of 5 x 10(16) and 1 x 10(17) cm(-3), respectively. The activation energy of the electron trap is 0.45 eV for silicon doping concentration of 5 x 10(16) cm(-3) and 0.46 eV for silicon doping concentration of 1 x 10(17) cm(-3) in the Al0.25Ga0.75As Schottky layer. The trap concentrations for these devices are 2.0 x 10(15) and 3.2 x 10(16) cm(-3), respectively. The current-voltage (I-V) characteristics of the devices were measured at 300, 77 and 30 K. Drain current collapse at temperature below 77 K at low drain bias and persistent photoconductivity effect was evident in both the Al0.25Ga0.75As/In0.20Ga0.80As/GaAs pHEMTs, indicating the presence of DX centres in the Al0.25Ga0.75As Schottky layer. The drain saturation current of the devices becomes smaller due to the carriers being captured by the defects, and the transconductance becomes higher due to an increase in carrier mobility in the channel as the temperature was lowered from 300 to 30 K. Microwave measurements of f(T) and f(max) values show no significant effect from the deep levels at room temperature. On the other hand, noise measurements show that higher trap concentration in the device gives rise to higher noise figure at room temperature.