Low frequency noise was measured in silicon MOSFET and GaN and InGaAs based HFET devices with special emphasis on the RTS noise. The RTS (Random Telegraph Signal) dependence on the biasing conditions and temperature was analyzed in order to obtain new information regarding production technology. From the time dependence of the RTS noise voltage the mean time of charge carriers capture and emission by traps in the gate oxide layer was determined as a function of applied gate and drain voltage or electron concentration and then several important trap parameters, such as activation energy and position in the channel could be estimated.