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 middle 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.

Modern tetrode transistors

• 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