Chapter 5Bipolar Junction Transistor (BJT)2025/11/27
1 2025/11/27 Bipolar Junction Transistor (BJT) Chapter 5
2Content5.0lntroduction5.1DeviceStructureandPhvsicalOperation5.2Current-VoltageCharacteristics5.3 The BJT as an Amplifier and as a Switch5.4BJTCircuits at DC5.5BiasinginBJTAmplifierCircuits5.6Small-SignalOperationandModels5.7Single-StageBJTAmplifiers5.8TheBJTInternalCapacitancesandHigh-FrequencyModel5.9FrequencyResponseoftheCommon-EmitterAmplifier5.10TheBasicBJTDigitalLogicInverter5.11TheSPICEBJTModelandSimulationExamplesSummary2025/11/27
Content 2025/11/27 2 5.0 Introduction 5.1 Device Structure and Physical Operation 5.2 Current-Voltage Characteristics 5.3 The BJT as an Amplifier and as a Switch 5.4 BJT Circuits at DC 5.5 Biasing in BJT Amplifier Circuits 5.6 Small-Signal Operation and Models 5.7 Single-Stage BJT Amplifiers 5.8 The BJT Internal Capacitances and High-Frequency Model 5.9 Frequency Response of the Common-Emitter Amplifier 5.10 The Basic BJT Digital Logic Inverter 5.11 The SPICE BJT Model and Simulation Examples Summary
IntroductionPhysicalStructureCircuitSymbolsforBJTsModesof OperationBasicCharacteristic2025/11/27
Introduction ⚫ Physical Structure ⚫ Circuit Symbols for BJTs ⚫ Modes of Operation ⚫ Basic Characteristic 2025/11/27 3
45.1TransistorTransistorisa three-terminal device,madeofsilicon(Germaniumwasearlyused)Therearetwotypes:NPNandPNPMetalMetalcontactcontactn-typep-typen-typeCollectorEmitterBaseEooCBaseCollectorEmitterEmitterCollectorregionregionregionregionregionregion(E)(C)Emitter-baseCollector-basejunctionjunctionOBBase(EBJ)(CBJ)(B)b)pnptransistor.a)npntransistorSimplifiedstructure2025/11/27
5.1 Transistor ⚫ Transistor is a three-terminal device, made of silicon (Germanium was early used) ⚫ There are two types: NPN and PNP 2025/11/27 4 a) npn transistor b) pnp transistor. Simplified structure
55.1Circuit symbol of the transistorOECBoBoocOEppnpn(b)(a)The emitter is distinguished by the arrowhead2025/11/27
5.1 Circuit symbol of the transistor 2025/11/27 5 The emitter is distinguished by the arrowhead
65.1PhysicalStructureemitterregion(nThreetype)collectorregions:baseregion(ptype)ragierponacollectorCBJregionThreebaseb-baseregionbase (B)Dterminalsemittercollector (C)regionEEJeemitter-base junction (EEJ)Twopnemitterjunctions:collector-base junction (CBJ)2025/11/27
2025/11/27 5.1 Physical Structure emitter region (n type) base region (p type) collector region (n type) emitter (E) base (B) collector (C) emitter-base junction (EEJ) collector-base junction (CBJ). Three regions: Three terminals Two pn junctions: EEJ CBJ emitter region base region collector region c collector base b emitter N P N+ e 6
5.1Structure of Actual Transistorstheemitterregion:>highly dopedEBC..the base region:>very thin>lightly doped7thecollectorregiorCross-section of an npn BJT>largearea2025/11/27
5.1 Structure of Actual Transistors ⚫ the emitter region: ➢ highly doped ⚫ the base region: ➢ very thin ➢ lightly doped ⚫ the collector region: ➢ large area 2025/11/27 7 Cross-section of an npn BJT +
85.1OperationModesDependingonthebiascondition(forwardorreverse)ofeachofthetwojunctions,thereare differentmodesofoperationoftheBJTEBJCBJModesApplicationSwitchingCutoffReverseReverseapplication inForwardForwardSaturationdigital circuitsForwardReverseAmplifierActiveReversePerformanceReverseForwardactivedegradationInthis course,we are mainly interested intheamplificationmode.2025/11/27
2025/11/27 8 5.1 Operation Modes ⚫ Depending on the bias condition (forward or reverse) of each of the two junctions, there are different modes of operation of the BJT. ⚫ In this course, we are mainly interested in the amplification mode. Modes EBJ CBJ Application Cutoff Reverse Reverse Switching application in Saturation Forward Forward digital circuits Active Forward Reverse Amplifier Reverse active Reverse Forward Performance degradation
95.1Transistor and Its FunctionalityAtransistorcanbeusedtorealizeacontrolled current source.In the extreme,the current in the thirdterminal can be changed from zero to a largevalueswitchapplication.BiasinAmplificationMode>EBJisforwardlybiased>CBJisreverselybiased2025/11/27
2025/11/27 9 5.1 Transistor and Its Functionality ⚫ A transistor can be used to realize a controlled current source. ⚫ In the extreme, the current in the third terminal can be changed from zero to a large value ⎯ switch application. ⚫ Bias in Amplification Mode ➢ EBJ is forwardly biased ➢ CBJ is reversely biased
105.1CurrentFlow ina NPNTransistor.EBJ:majoritydiffussionR-lEN-lEP.CBJ:minoritydrift-IcBOIcNEc 二·CBJcollectstheelectronsfromtheemmiterandformsIcBOIcNb·BasecurrentIBNMaj.-Recombination current IBNRbIEPJENEB?2025/11/27
2025/11/27 10 Rb Rc EB EC e c b IEN IEP Maj. ICN ICBO IBN 5.1 Current Flow in a NPN Transistor • EBJ：majority diffussion −IEN −IEP • CBJ：minority drift −ICBO CBJ collects the electrons from the emmiter and forms ICN Base current −Recombination current IBN