(Invited) AlGaN Higher Power Devices

The maximum power that GaN and SiC devices can routinely handle with their present designs is ~10⁵ watts. This is determined by the fact that for a given breakdown voltage, V_B, the maximum doping level, n_max (for electrons) and therefore the current density, is limited primarily by the critical filed, ξ_C, and access to the carrier as defined by the donor, E_D, or acceptor, E_A, depth. To a lesser extent, it is also limited by the carrier – usually the electron – mobility, μ_n. GaN, and the III-V semiconductors in general, have an advantage over group IV semiconductors in that they usually form continuous alloys so that ξ_C can be increased incrementally by increasing the fraction of the larger band gap material in the ternary; for the group IV it is Si, SiC, or diamond, and nothing else in between. To a first approximation the energy gap, E_G, increases linearly from 3.39 for GaN to 6.2 eV for AlN and ξ_C from 3.5 to 11.7 V/cm. They actually increase sublinearly with the concentration, and the effects of this will be briefly explored. The carrier of interest is the electron because the depth of the Mg acceptor, which is deep to begin with, increases quite rapidly with the Al concentration. The dopant of interest is the Si donor, which behaves like a hydrogenic atom up to ~80% Al; above this it is thought to form a DX center. E_D increases in depth from 32 to ~75 meV up to this Al%, and although this is ~3kT at room temperature, the large density of states in the conduction band enables most of the donors to be ionized. μ_n does decrease with the Al%, and at 80% Al it is about half the value it is for GaN. However, this deficit is more than made up for by the increased ξ_C, which has more than doubled. The conductivity also has a higher order dependence on ξ_C than it does on μ_n; in the ideal vertical diode it is proportional to ξ_C³ and it is only linearly proportional to μ_n. Utilizing AlGaN creates other issues, especially for the vertical device, which will have to be the structure used because very high power lateral devices would occupy a larger surface area and be more prone to surface breakdown. The higher source and drain contact resistance for AlGaN will be a problem for both lateral and vertical devices, as will be alloy scattering in the 2DEG. It will be difficult to create a vertical structure that can be easily turned on and off be it with a current aperture or a JFET structure. The AlGaN drift region will also contain mismatch dislocations due to its lattice mismatch substrate, and it will be more difficult to confine the majority of dislocations to the interface, as it is done in SiGe devices, because Si and Ge are cubic and have active primary slip systems exposed to a substantial amount of shear. AlN and GaN have the hexagonal wurtzite structure, and neither the primary basal slip system nor the secondary prismatic slip systems have any shear operating on the slip planes so slip, if it occurs, occurs on pyramidal planes which have large critical shear stresses.