The micro-alloy diffused transistor (MADT), or micro-alloy diffused-base transistor, was developed by Philco (but GE and RCA filed for patent and Jacques Pankove of RCA received patent for it) as an improved type of micro-alloy transistor; it offered even higher speed. It is a type of diffused-base transistor.
Before etching wells into the base crystal, a diffused base layer is created over the entire base crystal. The emitter well is etched very shallow into this diffused base layer.
For high-speed operation, the collector well is etched all the way through the intrinsic base semiconductor to the diffused base layer. 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).
For high-voltage operation, the collector well is not etched all the way to the diffused base layer, leaving some intrinsic base semiconductor.
Read more about this topic: Alloy-junction Transistor
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