The presence of traps in GaInP/GaAs and AlGaAs/GaAs HEMT's was investigated by means of low frequency noise and frequency dispersion measurements. Low frequency noise measurements showed two deep traps (E(alpha1) = 0.58 eV, E(alpha2) = 0.27 eV) in AlGaAs/GaAs HEMT's. One of them (E(alpha2)) is responsible for the channel current collapse at low temperature. A deep trap (E(alpha1') = 0.52 eV) was observed in GaInP/GaAs HEMT's only at a much higher temperature (approximately 350 K). These devices showed a transconductance dispersion of approximately 16% at 300 K which reduced to only approximately 2% at 200 K. The dispersion characteristics of AlGaAs/GaAs HEMT's were very similar at 300 K (approximately 12%) but degraded at 200 K (approximately 20%). The low frequency noise and the transconductance dispersion are enhanced at certain temperatures corresponding to trap level crossing by the Fermi-level. The transition frequency of 1/f noise is estimated at approximately 180 MHz for GaInP/GaAs HEMT's and resembles that of AlGaAs/GaAs devices.