[Transistors].

Transistors are three-terminal devices that use a small voltage (or current) applied to one contact to modulate (i.e. control) a large voltage (or current) between the other two contacts. An analogy is the vacuum tube from the 1900s. A small voltage applied to the 'grid' modulates a large current between the anode and cathode. The bipolar junction transistor The BJT (1947) is a minority carrier device Emitter p+ p Collector n Base Four modes (forward active, reverse active, saturation, cutoff) Forward active operation (pnp transistor) The emitter E emits minority carriers into the base B: EB is forward biased Holes survive their journey through the base The collector C collects minority carriers from the base B: BC is reverse biased. A small voltage (or current) at B has a large effect on the current I E