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. Indium and antimony were commonly used to form the alloy junctions on a bar of N-type germanium. The collector junction pellet would be about 50 mils (thousandths of an inch) in diameter, and the emitter pellet about 20 mils. The base region would be on the order of 1 mil (0.001 inches, 25 μm) thick. 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).