The making of many semiconductor devices such as the Schottky barrier gate field-effect transistors has required precision etching of a moat or the like in a semiconductor body. The need for a critically controlled etching step is often a major source of difficulty in maintaining quality control and high yields in production. Moreover, the etching step severely limits the geometry of the semiconductor device.
The Schottky barrier gate field-effect transistor is a solid state signal amplifying device whose operation depends on the control of current by an electric field. It works on the same principles and similar electrical characteristics as the standard junction field-effect transistor (JFET). It differs from the JFET in that the carrier depletion region and in turn gating electric field is formed in the conduction channel at least in part by a Schottky barrier rather than two PN junctions. This difference gives the Schottky barrier gate field-effect transistor electrical characteristics uniquely suited to certain applications such as high power, micro-wave amplifiers.
With the small geometries required by these devices and particularly those for high frequency applications, major problems are encountered with alignment and resolution during the fabrication process. Such devices require small source -- drain contact spacings (e.g. 4 microns) with accurate alignment of the gate between them. The Schottky barrier contact cannot touch either the source or drain regions or ohmic contacts to those regions. Otherwise a low voltage breakdown or a short circuit will result. Self-alignment of the Schottky barrier contact is accomplished by vapor or sputter deposition of the barrier metal through a window in a metal mask layer corresponding to the ohmic contacts to the source and drain regions of the transistor. The mask layer has a cantilever shaped overhang adjacent the window that shields the surfaces of the channel beneath the overhang and prevents deposition of the metal in contact with those surfaces. The cantilevered metal overhang is formed by etching the semiconductor body through the window and undercutting the metal layer. See Proceedings of the IEEE, Vol. 59, pp. 1244- 45 (Aug., 1971).
The main problem with the conventional method for making self-aligned Schottky barrier gate field-effect transistors is shaping the overhang structure by etching. In the etching step, the depth of the gate opening must be controlled to a fraction of a micron to retain a predetermined thickness of the channel layer corresponding to the desired electrical characteristics for the transistor. Further, the lateral undercut of the metal contact layer must be precisely controlled to provide sufficient overhang for self-alignment but yet avoid weakening and sagging of the metal layer with the resulting misalignment of the gate contact.
The present invention overcomes these difficulties and disadvantages. It involves no critical etching or metal deposition steps. The thickness of channel, source, and drain regions can be accurately predetermined and established. Further the Schottky barrier gate contact and the metal contacts to the source and drain can be formed simultaneously in the same metal deposition step.