Experimental evidence of surface conduction contributing to transconductance dispersion in GaAs MESFETs

Low-frequency transconductance dispersion in GaAs metal semiconductor field effect transistors (MESFET´s) is commonly attributed to the presence of a high density of surface states under the passivated source-gate and gate-drain separation regions. In this paper, we have found that they also contribute to a temperature dependent surface reverse leakage current with a thermal activation energy of 0.43 eV and a surface electron concentration (2.5×10¹² cm² ). Conductance deep-level transient spectroscopy (DLTS) spectra of these MESFETs have shown apparent “hole-like” peaks with emission energy of 0.48 eV and capture activation energy of 0.05 eV. These data were used for a model-based simulation and the results were compared with those obtained from experimental transconductance versus temperature measurements performed at various frequencies. A close agreement between these provides conclusive proof that the surface conduction at the GaAs-passivant interface is the major cause of the low-frequency transconductance dispersion observed. Finally, a possible explanation for the characteristic activation energy of 0.43 eV for Si ₃N₄ passivant film on GaAs has been presented