Super-Beta or high current gain npn transistors

In certain analog ICs it is necessary to have very high input impedance and very low base currents. For such applications, the typical current gains of an integrated npn transistor are not high enough. It is possible to increase the current gain of an npn transistor significantly by improving the base transport efficiency. In this case the base is very narrow ( a few hundred angstroms or less). The collector to emitter breakdown of a structure like this is relatively low ( 2V - 3V) because the collector base depletion layer can punch through the active base region into the emitter. This is the punch-through or "super-beta" transistor. Current gains of 5000 are obtainable using this technique at currents of 20uA or so with a Vce of around 0.5V. The fabrication of super-beta transistors in a standard process can be done by using one extra masking step and diffusion. After the base diffusion for the normal NPN transistors a special mask is used to open up the emitter diffusion for the super-beta transistors. At this stage the emitter of the super-beta transistor is only partially diffused.This step is then followed by the masking and n+ diffusion of the standard npn. Owing to the extra diffusion step for the super-beta transistor, the emitter of the super-beta transistor is diffused slightly deeper
than the normal npn resulting in a narrow base width.