The micro-alloy transistor (MAT) was developed by Philco as an improved type of alloy-junction transistor, it offered much higher speed.
It is constructed of a semiconductor crystal forming the base, into which a pair of wells are etched (similar to Philco's earlier surface-barrier transistor) on opposite sides then fusing emitter and collector alloy beads into the wells.
Read more about this topic: Alloy-junction Transistor