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The effect of ambient illumination is investigated for differently processed GaN/AlGaN/GaN heterostructure materials. For samples of the same material with different passivation, the difference in sheet resistance of illuminated and non-illuminated material can be as large as 130% (for annealed heterostructure without passivation) and as small as 3% (for heterostructure passivated with low pressure chemical vapor deposition (LPCVD) silicon nitride). The time constant for the decay of the persistent photoconductance (PPC) is also very different for the differently processed samples. The majority of the effect on the conductance is from photons with energies between 3.1 and 3.7 eV. The investigation indicates that delayed recombination of electrons emitted from surface states and from deep level states in the AlGaN layer dominates the PPC. A theory is formulated by which the difference in illumination sensitivity for the differently passivated materials can be explained by different distributions of electrons between the channel two dimensional electron gas and an accumulation layer formed in the cap layer. For practical heterostructure field effect transistor (HFET) measurements, the illumination sensitivity is generally lower than that of the Hall measurements. Furthermore, HFETs fabricated with the LPCVD silicon nitride passivation are practically illumination invariant.

journal of applied physics

Illumination effects on electrical characteristics of GaN/AlGaN/GaN heterostructures and heterostructure field effect transistors and their elimination by proper surface passivation