Alloy-junction transistor

The germanium alloy-junction transistor, or alloy transistor, was an early type of bipolar junction transistor, developed at General Electric and RCA in 1951 as an improvement over the earlier grown-junction transistor.

The usual construction of an alloy-junction transistor is a germanium crystal forming the base, with emitter and collector alloy beads fused on opposite sides. There were several types of improved alloy-junction transistors developed over the years that they were manufactured.

All types of alloy-junction transistors became obsolete in the early 1960s, with the introduction of the planar transistor which could be mass-produced easily while alloy-junction transistors had to be made individually. The first germanium planar transistors had much worse characteristics than alloy-junction germanium transistors of the period, but they cost much less, and the characteristics of planar transistors improved very rapidly, quickly exceeding those of all earlier germanium transistors.

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.

The micro-alloy diffused transistor (MADT), or micro-alloy diffused-base transistor, was developed by Philco as an improved type of micro-alloy transistor; it offered even higher speed. It is a type of diffused-base transistor.

Before using electrochemical techniques and etching depression wells into the base semiconductor crystal material, a heated diffused phosphorus gaseous layer is created over the entire intrinsic semiconductor base crystal, creating a N-type graded base semiconductor material. 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 diffused base layer and through most of the intrinsic base semiconductor region, forming an extremely thin base region. 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).

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