Chapter 4 Bipolar Junction Transistors

Chapter 4Chapter 4Bipolar Junction Bipolar Junction

TransistorsTransistors

ObjectivesObjectives

Describe the basic structure of the bipolar junction transistor (BJT) Explain and analyze basic transistor bias and operation

Discuss how a transistor can be used as an amplifier or a switch

Troubleshoot various failures typical of transistor circuits

Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits

IntroductionIntroduction

A transistor is a device that can be used to either switch flow on/off or “control flow”

device.

Let’s first consider its operation in a simpler view as a flow switching device.

+

- +

-II I

On/Off Flow

1 - open

2 - closed

mittermitter

ollector

ollector

IntroductionIntroduction

Let’s first consider its operation in a simpler view as a current controlling

device.

+

- +

-II I

I

Variable Flow

Flow

ollector

ollector

mitter mitter

Transistor StructureTransistor Structure

With diodes there is one p-n junction. With bipolar junction transistors (BJT), there are three layers forming two p-n junctions. Transistors can be either pnp or npn type.

BJT ConstructionBJT Construction

Physical construction of a BJT is deposited layers of “doped “ N and P semiconductor materials.

Note: Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seed

crystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate. This is

different from other thin-film deposition methods which deposit polycrystalline or amorphous films, even on

single-crystal substrates. If a film is deposited on a substrate of the same composition, the process is called

homoepitaxy; otherwise it is called heteroepitaxy.

http://en.wikipedia.org/wiki/Seed_crystal

Basic Transistor OperationBasic Transistor OperationLook at this one circuit as two separate circuits:• The base-emitter (left side) circuit and • The collector-emitter (right side) circuit. Note that the emitter leg serves as a conductor for both circuits. • The amount of current flow in the base-emitter circuit controls the amount of current that flows in the collector circuit. • Small changes in the transistor base-emitter current yields a large change in collector-current.

IE = IC + IB

http://www.bcae1.com/transres.htm

http://www.bcae1.com/transres.htm

Basic Transistor OperationBasic Transistor OperationBelow, is the proper bias configuration for the npn and the pnp transistor. Note that base-emitter junction is forward-biased, while the base-collector junction is reverse-biased (known as forward-reverse bias). For the npn, the forward-bias BE, narrows the BE junction depletion zone, while the reverse-bias BC depletion zone grows larger. Let’s examine this more closely (next slide).

IC = ß IB

Forward-Reverse bias of a bipolar transistor.

Basic Transistor Operation - Conventional Basic Transistor Operation - Conventional FlowFlow

Looking at a forward-reverse bias NPN

1. Forward-bias BE narrows “Depletion Zone” (see forward-biased diode).

2. Reverse-bias BC widens “Depletion Zone”.

3. Emitter (doped) region is “rich” with free (conduction-band) electrons.

4. These “conduction-band” electrons easily migrate across the BE junction to become “minority carriers” in the base.

5. Since there are few “holes, only a few recombine to form a small “valence electron current” thru RB to BE supply.

6. The “free electrons” are attracted across the BC junction into the Collector and thru to the +C supply as Ic (collector current).

7. The PNP transistor follows a similar logic.

1

2

3

456

“valence electron” current

NPNTransist

or

Collector

Base

Emitter

Transistor Currents (Conventional Flow)Transistor Currents (Conventional Flow)

IE = IC + IB Note relative current

sizes.

Regions of operationRegions of operation

• Applied voltages Mode

• E < B < C Forward active

• E < B > C Saturation

• E > B < C Cut-off

• Forward active: base higher than emitter, collector higher than base (in this mode the collector current is proportional to base current by βF).

• Saturation: base higher than emitter, but collector is not higher than base.

• Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor

• is not letting conventional current to go through collector to emitter.

• In terms of junction biasing: ('reverse biased base–collector junction' means Vbc < 0 for NPN, opposite for PNP)

• Forward-active (or simply, active): The base–emitter 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, βF, 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.

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

• Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch.

• There are basically three possible ways to connect a Bipolar Transistor within an electronic

• circuit with each method of connection responding differently to its input signal as the static characteristics of the transistor vary with each circuit arrangement.

• 1. Common Base Configuration - has Voltage Gain but no Current Gain.

• 2. Common Emitter Configuration - has both Current and Voltage Gain.

• 3. Common Collector Configuration - has Current Gain but no Voltage Gain

Transistors Characteristics and Transistors Characteristics and ParametersParameters

For proper operation, the base-emitter junction is forward-biased by VBB and conducts just like a diode. IB is set.

The collector-base junction is reverse biased by VCC and blocks current flow through it’s junction just like a diode.Current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter. IC + IB results in IE

BC Reverse-biased

BE Forward-biased

Transistor Characteristics and ParametersTransistor Characteristics and ParametersAnalysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law and the ß for the transistor.

Determine the base BIAS current. Using Kirchhoff’s voltage law, subtract the 0.7VBE from VBB 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 (ohm’s

law)The collector DC current is determined by multiplying the base current by beta. ICdc = IBdc

or DC = IC/IBNote:0.7 VBE will be used in most analysis examples.

Transistor Characteristics and ParametersTransistor Characteristics and Parameters

What we ultimately determine by use of Kirchhoff’s voltage law for series circuits is that; In the base circuit, VBB is distributed across the base-emitter junction and RB in the base circuit.

In the collector circuit we determine that VCC is distributed proportionally across RC and the transistor (VCE).

Transistor Characteristics and ParametersTransistor Characteristics and Parameters• Small ΔIDCbase-emitter yields Large ΔIDCcollector-emitter• This relationship of current change (collector to base)

(ΔIDCcollector-base) is called beta. ( )

DC = IC /IB (Current Gain)

Data sheets will refer to hybrid (h)

parameters hFE = DC (typical hFE values 20 – 200)

∝

See Ex. 4-1

• The relationship of current change (collector to

emitter)

(ΔIDCcollector-emitter) is called

alpha. (∝∝) ∝DC = IC / IE is always >1. (Typical ∝ values 0.95 – 0.99+)


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

Relationships of Key Parameters:VBE = 0.7 V.

IB = (VBB – VBE)/RB

VCE = VCC – ICRC

VCB = VCE - VBE

See Ex 4-2

Collector Characteristic CurvesCollector characteristic curves give a graphical illustration of the relationship of: Ic (collector current), and VCE (Voltage-Collector to Emitter) with specified amounts of IB (base current).

With greater increases of VCC , VCE continues to increase until it reaches breakdown, but the current remains about the same in the linear region from .7V to the breakdown voltage.

IBaseIC

Cutoff

regionActive Region

IB

VCE

Ic

Vcc

Linear Region

VCE

Ic

VCE

See Ex. 4-2

Transistor Characteristics Curves - ReviewTransistor Characteristics Curves - Review

1

22

34

1. Vcc = 0 IB is thru the base & emitter.

Transistor is in Saturation

2. VCC , VCE and Ic with increase Vcc

3. VCE exceeds 0.7V

Base/Collector junction is

reverse-biased

Transistor operates

in Linear region

4. “breakdown” or

Avalanche region.

……. Bad!!

See Ex. 4-3


With no IB this transistor is in the cutoff region and just as the name implies there is practically no current flow in the collector part of the circuit. With the transistor in a cutoff state the full VCC

can be measured across the collector and emitter (VCE). Rc≈0V.

Transistor Characteristics and ParametersTransistor Characteristics and ParametersCurrent flow in the collector part of the circuit is, determined by IB multiplied by .

However, there is a limit to how much current can flow in the collector circuit regardless of additional increases in IB.

Once this maximum is reached, the transistor is said to be in saturation. Note that saturation can be determined by application of Ohm’s law. IC(sat) =VCC/RC

The measured voltage across the now “shorted” collector and emitter is VCE = 0V.


The dc load line graphically illustrates IC(sat) and cutoff for a transistor.

See Ex.4-4

NOTE:

This will be the area

of operation for all

out transistor

applications.

Transistor Characteristics and ParametersTransistor Characteristics and ParametersMore about More about ββ

The beta for a transistor is not always constant. Temperature and collector current both affect ββ. Variances in the manufacture of the transistor may also result in variable ββ’s.’s.

There are also maximum power ratings to consider. The data sheet below, provides information on these characteristics.

See End of Chap. Q.4-20 & 21

Note: ΔβDC for various temperatures.

Note: ΔβDC for ΔIC

Transistor Characteristics and ParametersTransistor Characteristics and ParametersMaximum Power RatingsMaximum Power Ratings

• Maximun ratings are available from datesheets and on-line.

• Typical max. ratings address: VCE max, VCB max, VEB max, IC max & Power.

• A primary limitation is; Ic = PD(max) / VCE rearranged: VCE = PD(max)/IC

Assuming PD(MAX) is 500mW:

VCEmax is 20V, IC(MAX) is 50mA.

The graph shows the transistor can’t be operated in the “shaded“ areas.

IcMAX

PDMAX

VCE(MAX)

• Power: P= EI, P= I2R, P=E2/R

See Ex. 4-5 & 4-6

Transistor Characteristics and ParametersTransistor Characteristics and ParametersPower Derating/Data SheetsPower Derating/Data Sheets

Power Derating – PDmax

3 factors decide Power Deratings: see the previous slide.

ICmax , VCEmax & PDmax -------- IC = PD(max) / VCE

See the Datasheet for DeratingsSee Ex. 4-7

Transistor Datasheets

Provide:

• Physical details

• Absolute Maximum ratings:

Voltage, Current, Temperature & Power

• Thermal characteristics

• Electrical Characteristics – hfe, Cutoff, Breakdown voltage, Gain, Capacitance, Switching speeds, etc.

See Ex. 4-8

BJT (Transistor) AmplifierBJT (Transistor) Amplifier

Amplification is the process of linearly increasing the amplitude of an electrical signal.

Some designation standards and conventions:

DC Currents: IC IE IB

DC Voltages: VBE VCB VCE VB VC VE

External DC Resistance: RE RC RB

AC Currents: Ic Ie Ib

AC Voltages: Vbe Vcb Vce Vb Vc Ve

External AC Resistances: Re Rc Rb

Internal Transistor Resistances: r ’ r ’e

Transistor SwitchTransistor Switch

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.

VCE(cutoff) = VCC IC(sat) = VCC – VCE(sat)

βDC

IB(min) = Ic (sat)

βDCSee Ex 4-10

Transistor SwitchTransistor Switch

Application: Transistor Switch

Base of transistor is “pulsed” with a “squarewave”.

“Off” periods provide no base biasing….transistor remains OFF.

“On” periods provide DC bias, switching the transistor “ON”

IB is max. Ic is max.

LED lights!!

See Ex 4-11

Transistor Types & PackagingTransistor Types & Packaging

2N39042N3906

Same transistor type in metal packaging

Transistor Types & PackagingTransistor Types & Packaging

High-density, multi-transistor packaging

Power transistors

RF Transistors

TroubleshootingTroubleshooting

Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm’s law and Kirchhoff’s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.


Internal opens within the transistor itself could also cause transistor operation to cease.

Erroneous voltage measurements that are typically low are a result of point that is not “solidly connected”. This called a floating point. This is typically indicative of an open.

More in-depth discussion of typical failures are discussed within the textbook.

Opens in the external resistors or connections of the base or the circuit collector circuit would cause current to cease in the collector and the voltage measurements would indicate this.


Testing a transistor can be viewed more simply if you view it as testing two diode junctions. Forward bias having low resistance and reverse bias having infinite resistance.


The diode test function of a multimeter is more reliable than using an ohmmeter. Make sure to note whether it is an npn or pnp and polarize the test leads accordingly.

Troubleshooting – Bias TestingTroubleshooting – Bias Testing

SummarySummary

The bipolar junction transistor (BJT) is constructed of three regions: base, collector, and emitter. The BJT has two pn junctions, the base-emitter junction and the base-collector junction.

The two types of transistors are pnp and npn.

For the BJT to operate as an amplifier, the base-emitter junction is forward-biased and the collector-base junction is reverse-biased.

Of the three currents IB is very small in comparison to IE and IC. Beta is the current gain of a transistor. This the ratio of IC/IB.

Summary

A transistor can be operated as an electronics switch. When the transistor is off it is in cutoff condition (no current). When the transistor is on, it is in saturation condition (maximum current).

Beta can vary with temperature and also varies from transistor to transistor.

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Chapter 4 Bipolar Junction Transistors