A tetrode transistor is any transistor having four active terminals.
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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.
Modern tetrode transistors
References
Tetrode transistor Wikipedia(Text) CC BY-SA