Light sensitivity of current DLTS and its implications on the physics of DC-to-RF-dispersion in AlGaAs-GaAs HFETs

The light sensitivity of current deep-level transient spectroscopy (I-DLTS) is analyzed with the aim of gaining insight about the physics of surface-trap related dc-to-RF dispersion effects in AlGaAs-GaAs heterostructure field-effect transistors. I-DLTS experiments under dark reveals three surface-trap levels with activation energies 0.44 eV (h1), 0.59 eV (h2), and 0.85 eV (h3), as well as a bulk trap with activation energy 0.45 eV (e1). While the I-DLTS signal peaks associated with the two shallower surface traps h1 and h2 are suppressed by optical illumination with energy larger than the AlGaAs bandgap, that which is associated with the deepest surface trap h3 is nearly unaffected by light up to the highest intensity adopted. Two-dimensional device simulations assuming that surface traps behave as hole traps provide an interpretation for the observed different light sensitivity of surface traps, explaining it as the result of the temperature dependence of surface hole concentration and negative trap-charge density, making trap-charge modulation at increasing temperature less and less sensitive to excess carriers generated by light.