International Business Machines Corporation

David E De Bar

Geoffrey B Stephens

Francisco H De La Moneda

Raymond W Hamaker

A vertical PNP bipolar transistor structure with Schottky Barrier diode emitter is disclosed which simplifies the structure and process steps for combining a complementary PNP in an NPN integrated circuit and improves the speed and density of the vertical PNP. The PNP emitter is formed with a Schottky contact such that only the PNP base region is contained in the NPN emitter junction structure. The structure uses a separately masked ion/implant for the NPN intrinsic base implant which also forms the PNP collector region so that the PNP base doping profile can intercept the PNP collector profile at a lower concentration resulting in lower collector/base capacitance, lower series collector resistance and higher collector/base breakdown voltage for the PNP. Since the base doping concentration is lower in the structure and the emitter has no sidewall capacitance, the PNP emitter-base capacitance is greatly reduced. These features result in an improved frequency response for the PNP structure.

Fabrication method for vertical PNP structure with Schottky barrier

Variations of current gain from element to element in a phototransistor array are eliminated by covering the array with an opaque mask and etching openings in the mask over each phototransistor based upon an area reduction factor (ARF). The area reduction factor for an opening is equal to (I.sub.m /I.sub.x).sup.1-n where n is a constant definitive of the change in beta of a phototransistor in the array over a given range of collector currents; I.sub.m is the minimum collector current measured for the array and I.sub.x is the collector current for the phototransistor beneath the opening. Based upon the ARF's, the openings etched in the mask or cover initiate uniform current from each phototransistor element when uniform light flux is directed on the array. The process of fabricating the array comprises measuring the collector current for each phototransistor element at a given uniform light flux; determining the element with minimum collector current in the array; calculating the ARF for each phototransistor to achieve a uniform current response from the array; coating the array with an opaque cover, and etching the cover at each phototransistor based upon the ARF.

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Manassas, VA, United States of America

David E De Bar