1.  Field of the Invention
The present invention generally relates to methods for fabricating semiconductor devices and more particularly to those methods utilized for the fabrication of microwave transistors. 
2.  Prior Art
The methods for fabricating semiconductors as disclosed by the prior art indicate a number of procedures wherein silicon nitride films are utilized.  In one of the patents disclosed by the prior art, a semiconductor element utilizes a surface coating of silicon nitride and silicon dioxide, the individual films covering different surface portions of the semiconductor substrate.  The utilization of different surface materials enables selective diffusion of impurities such as gallium and antimony.  In a semiconductor device formed by this method, the surface coating acts as a satisfactory surface protective film against the external atmosphere.  In addition, the process is used to terminate the PN junction beneath the silicon nitride film. 
In another method disclosed by the prior art, silicon nitride is used as an etching stencil so as to better locate contact windows relative to one another in a silicon dioxide layer.  This method uses additional photoresist layers after etching through the silicon nitride layer to more accurately control positioning of the region openings.  As with the other pieces of prior art, the use of silicon nitride layers has no relationship to the present invention. 
In the construction of transistors pursuant to the prior art methods, a basic problem is encountered.  Where an NPN transistor is to be fabricated, a semiconductor wafer of N-type conductivity has a base region formed therein through the use of a conventional P-type base dopant such as boron.  The transistor is completed by the disposition of an N-type emitter region within the base region, the typical emitter dopants being phosphorus or arsenic.  In the fabrication of transistors, it is clearly necessary that there be two consecutive diffusion steps.  In such a case, the diffusion of the emitter region uses a surface layer having a high surface concentration.  As is described in the publication Physics and Technology of Semiconductor Devices by A. S. Grove, John Wiley & Sons, Inc., 1967 at Page 64, an emmitter-push effect is created.  Where boron is used for a base dopant and phosphorus is utilized for the emitter dopant, the boron distribution in the silicon is both under a high concentration phosphorus-diffused region and elsewhere in the sample.  The emitter-push effect arises where the phosphorus diffusion "pushes" the boron distribution ahead of it.  This brings about an irregular junction boundary. 
Where the emitter-push effect is created, a number of problems result.  A lower limit on the value of base width is imposed since further penetration of the emitter diffusion front into the wafer can cause the emitter region to punch through the PN junction at the irregular boundary caused by the emitter-push effect.  Restricting the value of the base width will degrade the frequency response and performance of a transistor since the transmit time will be degraded. 
Another problem which exists in the prior art is based upon the necessity to provide for P+ regions within the base region for the purpose of improving base resistance characteristics.  The present invention solves this problem by totally eliminating the need for the deposition and diffusion of P+ regions.  The present invention method provides for a base region which has a higher concentration outside of the emitter region.  As a result, the high concentration diffusion front of the base region is directly adjacent the emitter diffusion front thereby reducing base resistance without the need for performing the additional process steps.