The IC industry for many years has been employing bipolar junction transistor using the basic-junction-isolated bipolar process. Many engineers call it the standard buried collector (SBC) process. In this SBC process, adjoining devices are separated by back-to-back pn junctions diodes which must be reverse-biased to ensure isolation. Today most digital circuit employ self-aligned processes but SBC is still useful.
 
Bipolar Junction Transistor

 The bjt performance is determined by doping geometry. The doping profile is responsible for:

1. common-emitter forward current transfer ratio B F = IC/IB
2. junction breakdown (maximum collector voltage)
3. intrinsic cutoff frequency
4. sheet resistance of the base diffusion The geometry determines 1. current rating
2. actual frequency response The current gain is determined by:

B -1 = GB/GE + WB2/h LB2

where WB is the width of the electrical neutral base region of the transistor and where h is determined by the shape of the impurity in the base in a range from 2 to 20. LB is the diffusion length for minority carriers and are the Gummel numbers on the base and the emitter respectively.

The Gummel numbers are defined by:

where DB and DE are the minority carrier constants in the base and emitter
Note: GE is typically 1013 to 1015 because of the heavy doping in the base.

In modern high-frequency transistors, the basewidth WB is typically much less than the diffusion length LB, therefore the GB/GE term dominates the B .
 
Junction Breakdown

Emitter-base breakdown voltage is determined by the doping concentration and radius of curvature of the junction. Collector-base breakdown voltage is determined by the doping in the collector and junction radius. Most c-b junctions breakdown by avalanche breakdown. The second breakdown mechanism is punch-through of the base region.

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