The post-alloy diffused transistor (PADT), or post-alloy diffused-base transistor, was developed by Philips (but GE and RCA filed for patent and Jacques Pankove of RCA received patent for it) as an improvement to the germanium alloy-junction transistor, it offered even higher speed. It is a type of diffused-base transistor.
The Philco micro-alloy diffused transistor had a mechanical weakness that ultimately limited their speed; the thin diffused base layer would break if made too thin, but to get high speed it needed to be as thin as possible. Also it was very hard to control alloying on both sides of such a thin layer.
The post-alloy diffused transistor solved this problem by making the bulk semiconductor crystal the collector (instead of the base), which could be as thick as necessary for mechanical strength. The diffused base layer was created on top of this. Then two alloy beads, one P-type and one N-type were fused on top of the diffused base layer. The bead having the same type as the base dopant then became part of the base and the bead having the opposite type from the base dopant became the emitter.
A doping-engineered electric field was created in the diffused base layer to reduce the charge carrier base transit time (similar to the drift-field transistor).
Read more about this topic: Alloy-junction Transistor
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