Tetrode transistor

A tetrode transistor is any transistor having four active terminals.

Early tetrode transistors

There were two types of tetrode transistor developed in the early 1950s as an improvement over the point-contact transistor and the later grown-junction transistor and alloy-junction transistor. Both offered much higher speed than earlier transistors.

Point-contact transistor having two emitters. It became obsolete in the mid-1950s.
Modified grown-junction transistor or alloy-junction transistor having two connections at opposite ends of the base.^[1] It achieved its high speed by reducing the input to output capacitance. It became obsolete in the early 1960s with the development of the diffusion transistor.

Dual-emitter transistor, used in two-input transistor–transistor logic gates
Dual-collector transistor, used in two-output integrated injection logic gates
Diffused planar silicon bipolar junction transistor,^[2] used in some integrated circuits. This transistor, apart from the three electrodes (emitter, base, and collector), has a fourth electrode or grid made of conducting material placed near the emitter-base junction from which it is insulated by a silica layer.
Field-effect tetrode

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^ U.S. patent 4,143,421 - Tetrode transistor memory logic cell, March 6, 1979. Filed September 6, 1977.