This invention relates to semiconductor devices such as field-effect transistors (FETs), Schottky-barrier diodes (SBDs), and pn-junction diodes. More particularly, the invention deals with the surface stabilization of such semiconductor devices.
Crystalline semiconductor regions of semiconductor devices have unique interface states at their surfaces. The semiconductor surface will become electrically unstable, and current leakage will occur between the electrodes, if charge carriers (e.g., electrons) are captured there. Current leakage should be reduced to a minimum by any means. It is customary in the semiconductor industry to assess the voltage-withstanding capability of a semiconductor device in terms of current leakage. The more prone a semiconductor device is to current leakage, the lower will be the assessment of its voltage-withstanding capability.
Conventional approaches to surface stabilization of crystalline semiconductor regions, as far as the applicant is aware, are: (a) surface cleaning; (b) treatment of what are known as “dangling bonds,” unsaturated (broken) interatomic bonds at the semiconductor surface, which disrupt the flow of electrons; and (c) surface passivation. All these conventional methods have their own strengths, but weaknesses do appear particularly when they are applied to nitride or like compound semiconductors because, unlike silicon semiconductors, compound semiconductors have many crystal defects and interface states.
Another problem that manifests itself in relation to the electrification of the semiconductor surface is the so-called current collapse which takes place in a high electron mobility transistor (HEMT). The HEMT by itself is of the familiar make comprising an electron transit layer of, say, undoped gallium nitride and an electron supply layer of, say, either n-type or undoped aluminum gallium nitride. Overlying the electron supply layer are source and drain electrodes with a Schottky gate electrode therebetween. The resistance of the electron supply layer is so low in its thickness direction, and so high in its transverse direction, that the current flows between the drain and source electrodes through the two-dimensional electron gas layer in the electron transit layer. The two-dimensional electron gas layer appears because of piezo and/or spontaneous depolarization of the heterojunction between electron transit layer and electron supply layer.
In use of the HEMT in alternating current circuitry, negative charge carriers (electrons) are arrested at the interface state of the surface of the electron supply layer, as described in Japanese Unexamined Patent Publication No. 2004-214471. The electron density of the two-dimensional electron gas layer lessens due to the arrested negative charge carriers, resulting in current collapse in which the maximum drain current during alternating current operation grows less than that during direct current operation. The lessening of the maximum drain current is ascribed to the decrease of electron density in the two-dimensional electron gas layer by reason of the collection of negative charge carriers in the interface state surface of the electron supply layer.
The unexamined Japanese patent application cited above proposes a silicon nitride overlay on the electron supply layer for prevention of current collapse. This solution proved unsatisfactory as it reduced the gate-drain voltage strength of the HEMT. Moreover, the silicon nitride overlay was not nearly so effective as might be desired for prevention of current collapse as it served only for reduction of the carriers captured by the interface state, not for annihilation of the captured carriers.
The same unexamined Japanese patent application also suggests a silicon dioxide overlay on the silicon nitride overlay, and a field control electrode on the silicon dioxide overlay, in order to enhance the voltage strength. A field plate, instead of the field control electrode, is provided on the silicon nitride overlay via the silicon dioxide overlay for the same purpose according to Japanese Unexamined Patent Publication No. 2004-200248. The field control electrode and field plate are both objectionable because they incur additional costs for the manufacture of the HEMTs.