DET 115 ELECTRONIC DEVICES CHAPTER 3 BIPOLAR JUNCTION

DET 115 – ELECTRONIC DEVICES CHAPTER 3 BIPOLAR JUNCTION TRANSISTOR (BJT) 1

BJT Structure BJT is constructed with 3 doped semiconductor regions separated by 2 p-n junctions (Base-Collector & Base-Emitter). 3 regions are called emitter, base and collector. Emitter (E) – most heavily doped region. Base (B) – thin and lightly doped region. Collector (C) – largest and moderately doped region. 2

Type/Symbol npn transistor Not pointing in pnp transistor Pointing in 3

BJT Operation In normal operation, the base-emitter is forward-biased while the base-collector is reverse-biased. For npn type, the collector is more positive than the base, which is more positive than the emitter. For pnp type, the voltages are reversed to maintain the forwardreverse bias. 4

Operation pnp transistor Forward-biased junction Reverse-biased junction IE = I C + I B IC = IC majority + ICO (minority) is called leakage current 5

Operation npn transistor • free electrons move through collector region into external circuit. • base has low density of holes (majority carriers). • electrons that have recombined with holes as valance electrons leave the crystalline structure of the base, they become electrons in the metallic base lead and produce the external base current. • then return into emitter region along with the base current. • most free electrons don’t recombine with holes as the base is very thin→move toward BC junction. • Swept across into collector region by attraction of +ve supply. • heavily doped n-type emitter region has a very high density of free electrons. • free electrons easily diffuse through BE junction into lightly doped and thin base region 6

BJT Currents The emitter current is the sum of the collector current and the small base current. IE = I C + I B 7

BJT Configuration 8

Basic Operation For both npn and pnp transistors, VBB forward-biases the BE junction and VCC reverse-biases the BC junction. The amount of current flow in the base-emitter circuit controls the amount of current that flows in the collector circuit. Small changes in base-emitter current yields a large change in collector-current. 9

BJT Characteristics & Parameters The collector characteristic curves illustrate the relationship of the 3 transistor currents. By setting up other values of base current, a family of collector curves is develop. Beta( ) is the ratio of collector current to base current. b. DC= IC/IB DC is usually equivalent hybrid (h) parameters h. FE on transistor datasheets. h. FE = DC DC is ratio of collector current (IC) to the emitter current (IE). Less used parameter than beta in transistor circuits. DC = IC/IE 10

Beta (β) Relationship between amplification factors and Relationship Between Currents 11

BJT Configuration There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting. IB: dc base current IE: dc emitter current IC: dc collector current VBE: dc voltage across base-emitter junction VCB: dc voltage across collector-base junction VCE: dc voltage from collector to emitter 12

BJT Configuration For properation the base-emitter junction is forward biased by VBB and conducts just like a diode. The collector-base junction is reverse biased by VCC and blocks current flow through it’s junction just like a diode. Remember current flow through the baseemitter junction will help establish the path for current flow from the collector to emitter. 13

BJT Configuration Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law and the beta for the transistor. Analysis begins with the base circuit to determine the amount of base current. Using Kirchhoff’s voltage law, subtract the 0. 7 VBE and the remaining voltage is dropped across RB. Determining the current for the base with this information is a matter of applying of Ohm’s law. VRB/RB = IB The collector current is determined by multiplying the base current by beta. DC = IC/IB 14

BJT Configuration Base-Emitter (Forward Bias) Collector – Base (Reverse Bias) Collector - Emitter 0. 7 VBE will be used in most analysis examples. 15

BJT Configuration What we ultimately determine by use of Kirchhoff’s voltage law for series circuits is that: 1. VBB is distributed across the base-emitter junction and RB in the base circuit. 2. VCC is distributed proportionally across RC and the transistor (VCE) in the collector circuit. 16

BJT Characteristics The beta for a transistor is not always constant. Temperature and collector current both affect beta, not to mention the normal inconsistencies during the manufacture of the transistor. There also maximum power ratings to consider. The data sheet provides information on these characteristics. 17

EXAMPLE What is the βDC for the transistor shown? 18

SOLUTION 1. 2. 3. Choose a base current near the center of the range, in this case IB 3. Read the corresponding collector current. Calculate the ratio. 19

BJT Characteristics The collector characteristic curves show the relationship of the 3 transistor currents. Curve shown is for a fixed based current. Saturation region – collector current has reached a maximum and is independent of the base current. Ideally, when VCE exceeds 0. 7 V, the BC junction become reversebiased and transistor goes into active/linear region. IC increases very slightly as VCE increases due to widening of BC depletion region. When VCE reaches a sufficiently high voltage, reverse-biased BC junction goes into breakdown; and IC increases rapidly as point C. A transistor should never be operated in the breakdown region 20

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BJT Characteristics Cutoff – condition in which there is no base current, IB=0 which results in only an extremely small leakage current (ICEO) in the collector circuit. For practical work, ICEO is assumed to be 0. So VCE=VCC. In cutoff, neither the BE junction nor the BC junction are forward-biased. 23

BJT Characteristics Saturation – condition in which there is maximum IC. The saturation current is determined by the external circuit (VCC and RC in this case) because the emittercollector voltage is minimum (≈0. 2 V). In saturation, an increase of base current has no effect on the collector circuit and the relation IC=βDCIB is no longer valid. 24

BJT Characteristics DC load line – represent circuit that is external to the transistor. Drawn by connecting saturation and cutoff point. 25

EXAMPLE What is the saturation current and the cutoff voltage for the circuit? 2. Is the transistor saturated? Assume VCE = 0. 2 V in saturation. 1. 26

SOLUTION Q 1 Q 2 Since IC < ISAT, the transistor is not saturated. 27

BJT Amplifiers BJT amplifies AC signals by converting some of the DC power from the power supplies to AC signal power. An AC signal at the input is superimposed in the dc bias by the capacitive coupling. The output AC signal is inverted and rides on a DC level of VCE. 28

BJT Switches A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0 V in saturation. 29

Datasheet 30

Summary Ø Bipolar Junction Transistor (BJT) is constructed of three regions: base, collector, and emitter. Ø BJT has two pn junctions, the base-emitter junction and the base-collector junction. Ø The two types of transistors are pnp and npn. Ø For the BJT to operate as an amplifier, the base-emitter junction is forward biased and the collector-base junction is reverse biased. Ø Of the three currents IB is very small in comparison to IE and IC. Ø Beta is the current gain of a transistor. This the ratio of IC/IB. 31

Summary Ø A transistor can be operated as an electronics switch. Ø When the transistor is off it is in cutoff condition (no current). Ø When the transistor is on, it is in saturation condition (maximum current). Ø Beta can vary with temperature and also varies from transistor to transistor. 32