For the GaAs- and InP-based devices interface charges are carefully controlled and reduced during MBE growth. For the AlGaAs/SiN and InAlAs/SiN interfaces Fermi level pinning prevails at the interface. Variations of the surface potential due to surface roughness, changed surface stoichiometry in the AlGaAs or SiN, or due to traps are therefore reduced.
For AlGaN/GaN HEMTs the control of the overall bulk charge concentration is necessary as due to the polarization effects this quantity is not completely determined by doping alone in advance. In [109] the impact of the surface etching is investigated for the AlGaN/GaN materials system, which shows that the surface treatment by etching significantly interferes with the available carrier concentration. SiN passivation of the device [109] is a means to control the surface and to allow for a less arbitrary transistor design. For AlGaN/GaN, however, the semiconductor/insulator interfaces require further analysis in order to obtain reliable control of effects such as sudden power and drain current reduction due to changes of the amount of available carriers [81].