As is known in the art, second generation GaN HEMI devices (30-300 GHz) must have higher sheet charge in the channel, thinner and higher Al mole fraction AlGaN, InAlN, or InGaAlN Schottky contact layer thicknesses, and lower parasitic ohmic contact resistance (<0.2 ohm mm) than present the present devices.
One method used to form ohmic contacts for the first generation devices includes forming a Ti/Al/Barrier/Au with rapid thermal annealing at 850-900 C which generally result in devices having high ohmic contact resistance (>0.2 ohm mm), and lower yields for source/drain contact spacing of <=2 um.
One method suggested to produce devices having lower ohmic contact resistance is shown in FIGS. 1A-1C. Here, a substrate of, for example, silicon carbide (SiC) or silicon Si, has a gallium nitride (GaN) layer epitaxial formed on the substrate. A semiconductor layer (i.e., a Schottky contact layer) having a higher band gap that the GaN, (e.g., a layer of AlGaN, InAlN, or InGaAlN) is formed on the GaN layer resulting in two dimensional electron gas (2DEG) layer being produced at the interface between the GaN layer and the higher band gap Schottky contact layer. Next, a mask is formed on the Schottky contact layer and the exposed portions of the Schottky and GaN are etched in the source and drain contact regions as shown using any suitable dry etch. The resulting structure is a mesa shaped structure as shown in FIG. 1B. An ohmic contact layer of n+ doped GaN is deposited over the etched structure as shown in FIG. 1C. It is noted that the ends (i.e., edges) of the two dimensional electron gas (2DEG) layer now are in direct contact with the ohmic contact layer of n+ doped GaN. This method suffers from two issues: Firstly, etching and exposure of the two dimensional electron gas (2DEG) on the edges might compromise the carrier concentration and mobility of the electrons in the vicinity of the exposed etched surface. Secondly, electron injection at the source and collection of electrons at the drain is only through a thin (˜50 Angstrom) contact between the two dimensional electron gas and the ohmic contact layer of n+ doped GaN.