No abstract

MgO has proven effective in the past as a surface passivation layer to minimize current collapse in AlGaN∕GaN high electron mobility transistors (HEMTs). However, MgO is not environmentally stable and more stable oxides need to be developed. MgCaO can be produced that is lattice matched to the GaN. Three samples were grown with 0%, 50% and 75% of Ca, which had respective lattice mismatches of −6.5% for MgO, −1% for Mg0.50Ca0.50O and +4% for Mg0.25Ca0.75O. Drain saturation current in HENTs had increases of 4.5% and 1%, respectively, for Mg0.5Ca0.5O and Mg0.25Ca0.75O passivated devices. However, there was a 10% decrease for the device passivated with pure MgO. This was due to strain applied on the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride HEMT by the oxide, which is consistent with the piezoelectric effect in the nitride form the lattice mismatch between AlGaN and GaN. From pulsed measurements, HEMTs passivated with Mg0.5Ca0.5O and Mg0.25Ca0.75O showed higher passivation effectiveness (90% of dc current) then the MgO passivated HEMTs (83% dc current). This is due to the closer lattice matching of these calcium containing oxides and the reduction in interface traps associated with lattice mismatch.

MgO is a promising gate dielectric and surface passivation film for GaN transistors but little is known of the band offsets in the MgO∕GaN system. X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEv) of MgO∕GaN heterostructures in which the MgO was grown by rf plasma-assisted molecular beam epitaxy on top of thick GaN templates on sapphire substrates. A value of ΔEv=1.06±0.15eV was obtained by using the Ga 3d energy level as a reference. Given the experimental band gap of 7.8eV for the MgO, this would indicate a conduction band offset ΔEC of 3.30eV in this system.

The Sc2O3∕GaN interface shows low trap densities and has been used both to demonstrate inversion in gated metal-oxide-semiconductor diodes and to mitigate current collapse in AlGaN∕GaN heterostructure transistors but little is known of the band offsets at this interface. We measured the energy discontinuity in the valence band (ΔEv) of Sc2O3∕GaN heterostructures using x-ray photoelectron spectroscopy. A value of ΔEv=0.42±0.07eV was obtained using the Ga 3d energy level as a reference. With the experimental band gap of 6.0eV for the Sc2O3 grown by this method, this implies that the conduction band offset ΔEC is 2.14eV in this system.

We report on a comparison of different gate oxides for AlGaN/GaN highelectron-mobility transistor (HEMT) pH sensors. The HEMTs show a linear increase in drain-source current as the pH of the electrolyte solutions introduced to the gate region is decreased. Three different gate oxides were examined, namely the native oxide on the AlGaN surface, a UV-ozone-induced oxide and an Sc 2 O 3 gate deposited by molecular beam epitaxy. The Sc 2 O 3 produced superior results in terms of resolution in measuring small changes in pH. The devices with Sc 2 O 3 in the gate region exhibited a linear change in current between pH 3 and 10 of 37 lA/pH with a resolution of <0.1 pH over the entire pH range. In contrast, the native oxide devices showed a larger change in current, $70 lA/pH, but with a degraded resolution of $0.4 pH. Results for the UV-ozone oxide were intermediate in resolution, 0.2 pH. These HEMTs have promise for detecting pH changes in biological samples and can be readily integrated into a standard package for wireless data transmission.