The present invention relates generally to lateral bipolar transistors and more specifically to an improved double diffused PNP lateral bipolar transistor.
Conventional lateral PNP bipolar transistors are generally fabricated by simultaneously diffusing P type collector and emitter regions side by side in an N epitaxial base region during the P base diffusion cycle of the vertical NPN transistors. This structure is illustrated in FIG. 1. The main problems in achieving high performance lateral PNP transistors using this scheme are that the distance between the emitter and collector regions cannot be reduced anymore than the limits of the photolithographic process. Thus, there is a technological limitation to the minimum base which is achievable by this method. This limitation correctly influences the performance characters of the device. Another difficulty is that the N.sup.- region which is common to the PNP and NPN devices cannot be independently optimized. Further the uniformity of the N.sup.- epitaxial base region does not contribute to enhancing carrier transport across the base region.
To improve the charcateristics of the lateral PNP transistor, a double-diffused technique was developed. This technique basically involved diffusing N type impurities through an aperture into a P epitaxial region to form the base region and subsequently diffusing P type impurities through the same aperture to form the emitter region. Thus, the base width would be the difference between the P emitter diffusion and the N base diffusion. The resulting structure is specifically shown in FIG. 2. This technique is specifically described in U.S. Pat. No. 3,766,446 for an NPN lateral transistor. A doubly-diffused lateral PNP transistor not using this same aperture but achieving the same results is illustrated in U.S. Pat. No. 3,873,989.
The prior art structure of FIG. 2 although reducing the base width, results in a structure which is incompatible with the formation of other elements in an integrated circuit. This results because of the use of the P.sup.- epitaxial region which prevents readable formation of vertical NPN devices. Similarly, it should be noted that the P.sup.- epitaxial region which forms the bulk collector between the P.sup.+ collector contact region and the N base region contributes to the series collector resistance which in turn influences both the frequency response and saturation voltage for the bipolar transistor.
Thus, there exists a need for a method of fabricating a lateral PNP transistor which is compatible with the formation of other devices in an integrated circuit as well as providing improved frequency response and voltage saturation.