18-05-2012, 02:52 PM
Bipolar Junction Transistors
BJTs - Copy.ppt (Size: 378 KB / Downloads: 33)
Physical Structure
The BJT consists of 3 alternating layers of n- and p-type semiconductor called emitter (E), base (B) and collector ©.
The majority of current enters collector, crosses the base region and exits through the emitter. A small current also enters the base terminal, crosses the base-emitter junction and exits through the emitter.
Carrier transport in the active base region directly beneath the heavily doped (n+) emitter dominates the i-v characteristics of the BJT.
Transport Model for the npn Transistor
The narrow width of the base region causes a coupling between the two back to back pn junctions.
The emitter injects electrons into base region; almost all of them travel across narrow base and are removed by collector.
Base-emitter voltage vBE and base-collector voltage vBC determine the currents in the transistor and are said to be positive when they forward-bias their respective pn junctions.
The terminal currents are the collector current(iC ), the base current (iB) and the emitter current (iE).
The primary difference between the BJT and the FET is that iB is significant, while iG = 0.