Bipolar Junction TransistorsEE314

Chapter 13: Bipolar Junction TransistorsHistory of BJTFirst BJTBasic symbols and features A little bit of physicsCurrents in BJTBasic configurationsCharacteristics

Current Flow in BJTpnp BJTInjected h+ current from E to B2. e- injected across the forward-biased EB junction (current from B to E)3. e- supplied by the B contact for recombination with h+ (recombination current)4. h+ reaching the reverse-biased C junction5,6.Thermally generated e- & h+ making up the reverse saturation current of the C junctioniE-iB-iC-VCE

Now, you can trynpn BJT

BJTs Basic configurations

npn BJTs Operation Modes

Forward & reverse polarized pn junctions

Different operation modes:

npn BJTs Operation Modes When there is no IB current almost no IC flowsWhen IB current flows, IC can flowThe device is then a current controlled current deviceOperational modes can be defined based on VBE and VBC

BJT-Basic operationnpn BJTpnp BJT(n+), (p+) heavy doped regions; Doping in E>B>C

Operation mode: vBE is forward & vBC is reverseThe Shockley equationIESsaturation I (10-12-10-16A); VT=kT/q -thermal V (26meV) D diffusion coefficient [cm2/s] m carrier mobility [cm2/Vs]The Kirchhoffs lawsIt is true regardless of the bias conditions of the junctionUseful parameterthe common-emitter current gainfor ideal BJT b is infiniteBJTs Current & Voltage RelationshipsEinstein relation

Useful parameterthe common-base current gainfor typical BJT a is ~0.99The Shockley equationonce moreIf we define the scale current A little bit of math search for iBFinallyBJTs Current & Voltage Relationships

BJTs CharacteristicsSchematicCommon-EmitterInputOutputVBCVBE

If VCE

BJTs Load line analysisCommon-Emitter AmplifierInput loopif iB=0if vBE=0smaller vin(t)

BJTs Load line analysisOutput loopCommon-Emitter AmplifierExample 13.2

Circuit with BJTsOur approach: Operating point - dc operating pointAnalysis of the signals - the signals to be amplifiedCircuit is divided into: model for large-signal dc analysis of BJT circuitbias circuits for BJT amplifiersmall-signal models used to analyze circuits for signals being amplified Remember !