BJT structurenote: this is a current of electrons (npn case) and so theconventional current flows from collector to emitter.heavily doped ~ 10^15provides the carrierslightly doped ~ 10^8lightly doped ~ 10^6

BJT characteristics

BJT characteristics

BJT modes of operation

BJT modes of operationCutoff: In cutoff, both junctions reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch.

Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, f in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations.

Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the f in inverted mode is several times smaller. This transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.

Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.

BJT structure (active)current of electrons for npn transistor

conventional current flows from collector to emitter.

BJT equations (active) = Common-base current gain (0.9-0.999; typical 0.99)

BJT equations (active) = Common-emitter current gain (10-1000; typical 50-200)

BJT equations (active) = Common-emitter current gain (10-1000; typical 50-200) = Common-base current gain (0.9-0.999; typical 0.99)

BJT large signal models (forward active)

BJT large signal models (reverse)Common-base current gain (0.1-0.5) BJT transistor is not a symmetrical device

BJT Ebers-Moll (EM) model

BJT structureThe npn transistor has beta=100 and exhibits an Ic=1mA at VBE=0.7V. Design the circuit so that a current of 2mA flows through collector and a voltage of +5V appears at the collector.

BJT equationsThe voltage at the emitter was measured and found to be -0.7V. If beta=50, find IE, IB, IC and VC.

BJT equationsA given npn transistor has beta=100. Determine the region of operation if:

IB=50uA and IC=3mA

IB=50uA and VCE=5V

VBE=-2V and VCE=-1V

BJT equationsRB=200k, RC=1k, VCC=15V, beta=100. Solve for IC and VCE

BJT equationsRB=200k, RC=1k, VCC=15V, beta=100. Solve for IC and VCE

Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current.Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so theelectron proceed to the collector where they are again the majority carrier.

Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current.Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so theelectron proceed to the collector where they are again the majority carrier.

Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current.Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so theelectron proceed to the collector where they are again the majority carrier.

Remember that the base region is deliberately made very thin and lightly doped, while the emitter is made much more heavily doped. Because of that, applying a forward bias to the emitter-base junction causes vast majority carriers to be injected into the base, and straight into the reverse-biased collector-base junction. Those carriers are actually minority carriers in the base region, because that region is of opposite semiconductor type to the emitter. To those majority-turned-minority carriers, the collector-base junction depletion region is not a barrier at all but an inviting, accelerating filed; so as soon as they reach the depletion layer, they are immediately swept into the collector region. Forward biasing the emitter-base junction causes two things to happen that might seem surprising at first: Only a relatively small current actually flows between the emitter and the base. much smaller than would flow in a normal PN diode despite the forward bias applied to the junction between them. A much larger current instead flows directly between the emitter and the collector regions, in this case, despite the fact that the collector-base junction is reversed biased.

From a practical point of view, the behavior of bipolar transistors means that, unlike the simple PN-junction diode, it is capable of amplification. In effect, a small input current made to flow between the emitter and collector. Only a small voltage--around 0.6 volts for a typical silicon transistor--is needed to produce the small input current required.