Al composition dependence of breakdown voltage in AlxGa1−xN Schottky rectifiers

Planar geometry, lateral Schottky rectifiers were fabricated on high resistivity ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}\text{\hspace{0.05em}(x=0–0.25)}$ epitaxial layers grown on sapphire substrates. The reverse breakdown voltages of unpassivated devices increased with Al composition, varying from 2.3 kV for GaN to 4.3 kV for ${\mathrm{Al}}_{\mathrm{0.25}}{\mathrm{Ga}}_{\mathrm{0.75}}\mathrm{N}.$ The reverse current–voltage $\mathrm{(I–V)}$ characteristics showed classical Shockley–Read–Hall recombination as the dominant mechanism, with ${\mathrm{I\propto V}}^{\text{0.5}}.$ The reverse current density in all diodes was in the range ${\text{5–10×10}}^{\text{−6}} {\mathrm{A cm}}^{\mathrm{-2}}$ at 2 kV. The use of $p^{+}$ guard rings was effective in preventing premature edge breakdown and with optimum ring width increased $V_B$ from 2.3 to 3.1 kV in GaN diodes.