Deep level transient spectroscopy has been used to characterise the deep levels in Al0.24Ga0.76As/In0.20Ga0.80As/GaAs and In0.48Ga0.52P/In0.20Ga0.80As/GaAs pseudomorphic high electron mobility transistors (pHEMTs) grown by solid source molecular beam epitaxy. Only one electron trap was detected in the Al(0.24)Ga(0.76)AS and In0.48Ga0.52P layers of the Al0.24Ga0.76As/In0.20Ga0.80As/GaAs and In0.48Ga0.52P/In0.20Ga0.80As/GaAs pHEMT, respectively. The activation energy of the electron trap is 0.49 eV for the Al0.24Ga0.76As/In(0.20)Ga(0.80)AS/GaAS pHEMT and 0.40 eV for the In0.48Ga0.52P/In0.20Ga0.80As/GaAs pHEMT. The trap concentration is 1.72 x 10(20) and 2.38 x 10(20) cm(-3) for these devices, respectively. The current-voltage 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 the (Al0.24Ga0.7As)-As-6/In0.20Ga0.80As/GaAs pHEMT. Such effects were not observed in the In0.48Ga0.52P/In0.20Ga0.80As/GaAs devices, indicating that the DX centres are only present in the Al(0.24)Ga(0.76)AS layers. 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.