A. Field of the Invention
The present invention relates generally to a semiconductor device, and more specifically to a metal-insulator-semiconductor field effect transistor (MISFET) having an oxidized aluminum nitride gate insulator formed on a silicon or gallium nitride or other semiconductor substrate, and a method of making the same.
B. Discussion of the Related Art
Manufacture of a conventional MIS (metal-insulator-semiconductor)-type semiconductor device involves forming a gate insulator on a semiconductor substrate, and subsequently forming a gate electrode on the gate insulator. Typically, the semiconductor substrate consists of silicon (Si) or gallium arsenide (GaAs), and the gate insulator comprises a film of SiO.sub.2, SiN, AlN, or the like, directly deposited on the substrate.
Conventional MISFETs include a silicon dioxide (SiO.sub.2) gate insulator deposited on a silicon substrate. However, it is preferable that MISFETs utilize an insulator made from a material other than SiO.sub.2, and a substrate made from a material other than silicon. For example, substrates made from a gallium nitride-based compound semiconductors such as, gallium nitride (GaN), gallium aluminum nitride (GaAlN), and indium gallium nitride (InGaN), are preferable since such substrates have direct band gaps in the range of 1.95 eV to 6 eV. For this reason, these compound semiconductors are promising as materials for light-emitting devices such as light-emitting diodes and laser diodes, and for high operating temperature circuits.
Unfortunately, when a silicon dioxide gate insulator is deposited on a gallium nitride substrate or a substrate made from a material other than silicon, the electrical properties of the deposited silicon dioxide gate insulator erode the performance of the conventional MISFET. Thus, a MISFET structure on a GaN substrate is not practical and will have poor performance characteristics.
The reason for this is that the best oxides for use as gate insulators in MISFETs are thermally formed by the chemical reaction of oxygen (O.sub.2) gas with the surface of the MISFET substrate. Such native oxides, as they are conventionally called, are not chemically stable for all materials. Further, semiconductor materials that form good FETs and react with oxygen to form a good gate insulator rarely exist. For example, a pure gallium nitride substrate will not react directly with oxygen to form stable oxides. Rather, the gallium nitride reacts with oxygen to form a nitrous oxide (NO) gas and water-soluble Ga.sub.2 O.sub.3. This creates reliability problems because the Ga.sub.2 O.sub.3 will absorb water (H.sub.2 O) from the ambient environment, changing its electrical behavior. The electrical behavior of the transistor will change too, rendering it unstable and unreliable over time. The unstable transistor will render unstable the circuit incorporating the transistor.