Bipolar Transistors Two PN junctions joined together Two types available – NPN and PNP The regions (from top to bottom) are called the collector (C), the base (B), and the emitter (E) Base Collector Emitter

Operation Begin by reverse biasing the CB junction – Here we are showing an NPN transistor as an example Now we apply a small forward bias on the emitter-base junction – Electrons are pushed into the base, which then quickly flow to the collector – The result is a large emitter-collector electron current (conventional current is C- E) which is maintained by a small E-B voltage Some of the electrons pushed into the base by the forward bias E-B voltage end up depleting holes in that junction – This would eventually destroy the junction if we didn’t replenish the holes – The electrons that might do this are drawn off as a base current

Currents

Conventional View

Origin of the names the Emitter 'emits' the electrons which pass through the device the Collector 'collects' them again once they've passed through the Base...and the Base?...

Original Manufacture

Amplification Properties The C-B voltage junction operates near breakdown. – This ensures that a small E-B voltage causes avalanche – Large current through the device

Common Base NPN

Common Emitter NPN

Common Collector NPN How does I C vary with V CE for various I B ? Note that both dc sources are variable Set V BB to establish a certain I B

Collector Characteristic Curve If V CC = 0, then I C = 0 and V CE = 0 As V CC ↑ both V CE and I C ↑ When V CE  0.7 V, base-collector becomes reverse-biased and I C reaches full value (I C =  I B ) I C ~ constant as V CE ↑. There is a slight increase of I C due to the widening of the depletion zone (BC) giving fewer holes for recombinations with e¯ in base. Since I C =  I B, different base currents produce different I C plateaus.

NPN Characteristic Curves

PNP Characteristic Curves

Load Line For a constant load, stepping I B gives different currents (I C ) predicted by where the load line crosses the characteristic curve. I C =  I B works so long as the load line intersects on the plateau region of the curve. Slope of the load line is 1/R L

Saturation and Cut-off Note that the load line intersects the 75 mA curve below the plateau region. This is saturation and I C =  I B doesn’t work in this region. Cut-off

Example We adjust the base current to 200  A and note that this transistor has a  = 100 – Then I C =  I B = 100(200 X A) = 20 mA Notice that we can use Kirchhoff’s voltage law around the right side of the circuit – V CE = V CC – I C R C = 10 V – (20 mA)(220  ) = 10 V – 4.4 V = 5.6 V

Example Now adjust I B to 300  A – Now we get I C = 30 mA – And V CE = 10 V – (30 mA)(220  ) = 3.4 V Finally, adjust I B = 400  A – I B = 40 mA and V CE = 1.2 V

Plot the load line V CE ICIC 5.6 V20 mA 3.4 V30 mA 1.2 V40 mA

Gain as a function of I C As temperature increases, the gain increases for all current values.

Operating Limits There will be a limit on the dissipated power – P D(max) = V CE I C – V CE and I C were the parameters plotted on the characteristic curve. If there is a voltage limit (V CE(max) ), then you can compute the I C that results If there is a current limit (I C(max) ), then you can compute the V CE that results

Transistors as Switches