BIPOLAR JUNCTION TRANSISTOR (BJT)

EMT116 – ELECTRONIC DEVICESDR. NUR SYAKIMAH ISMAIL

EMT116 - COURSE OUTLINE

Semiconductor Material

Diode Application­ Lab 1­ Lab 2

Special Purpose Diode

Bipolar Junction Transistors (BJTs)­ Lab 3

BJT – Transistor Bias Circuit

Field Effect Transistors (FETs)

CHAPTER OUTLINE

BJT Structure

Basic Operation

BJT Characteristic and Parameters

BJT Circuit Analysis

Collector Characteristic Curve

BJT as an Amplifier

BJT as a Switch

Phototransistor

Troubleshooting

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.

TYPE/SYMBOL

BJT CONFIGURATIONIn normal operation, the base-emitter is forward-biased while the base-collector is reverse-biased.

This condition is called forward-reverse bias.

–

+ –

+

–

+

+

BC reverse-biased

–

BE forward-biased

–

+

+

–

–

BC reverse-biased

+

BE forward-biased

–

+

• 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

• 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.

• most free electrons don’t recombine with holes as the base is very thin→movetoward BC junction.

•Swept across into collector region by attraction of +vesupply.

• free electrons move through collector region into external circuit.

• then return into emitter region along with the base current.

OPERATION NPN TRANSISTOR

Forward-biased junction Reverse-biased junction

OPERATION PNP TRANSISTOR

BJT CURRENTS

!" = !$ + !&

The emitter current is the sum of the collector current and the small base current.

BJT CHARACTERISTICS & PARAMETERSFor both npn and pnp transistors, VBBforward-biases the BE junction and VCCreverse-biases the BC junction.

Look 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 base-emitter current yields a large change in collector-current.

BJT CHARACTERISTICS & PARAMETERSThe 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 (b) is the ratio of collector current to base current.

!"# = %#%&

Typical value of '() range from 20 to 200 or higher. '() is usually equivalent hybrid (h)parameters hFE on transistor datasheets.

ℎ+, = '()-() is ratio of collector current (IC) to the emitter current (IE). Less used parameter than beta in transistor circuits.

."# = %#%/

Typical value of -() range from 0.95 to 0.99 or greater, but -() is always less than 1.

AMPLIFICATION FACTORSRelationship Between Currents:

!" = !$ + !&!$ = '!&!$ = (!"

Relationship between amplification factors b and a:

( = ))*+

' = ,+-,

BJT CHARACTERISTICS & PARAMETERS

§ Unsaturated BJT can be viewed as a device with a current input and a dependent current source in output circuit.

§ Input circuit is a forward-biased diode through which there is base current.

§ The output circuit is a dependent current source with a value dependent on base current.

BJT CHARACTERISTICS & PARAMETERS§The beta for a transistor is not always constant. Temperature and collector currentboth affect beta, not to mention the normal inconsistencies during the manufacture of the transistor.

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

BJT CHARACTERISTICS & PARAMETERS§!"# is not truly constant but varies with collector current and temperature.

§Keeping the junction temperature constant and increasing $# à !"# increases to maximum.

§Further increase in $# beyond this max point à !"# will decrease.

§If $# held constant and temperature is varied, !"# changes directly with temperature.

EXAMPLEWhat is the βDC for the transistor shown?

SOLUTION1. Choose a base current near the center of the range, in this case IB3.

2. Read the corresponding collector current.

3. Calculate the ratio.

!"# =%#%&= 5 mA30 µA = 167

EXAMPLE

Determine the dc current gain, !"# and the emitter current, $% for a transistor where $& = 50 *A and $# = 3.65 mA.

Solution:!"# = 73

$% = 3.70 mA

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

BJT CIRCUIT ANALYSIS

Not in picture:Terminal voltage of transistor:

!"!#!$

For proper operation 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.

When base-emitter is forward-biased, it is like forward-biased diode and has a nominal voltage drop:!"# ≅ %. ' (

Remember current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter.

BJT CIRCUIT ANALYSIS

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 V 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.

)" =!+","

The collector current is determined by multiplying the base current by beta.

-./ =)/)"

BJT CIRCUIT ANALYSIS

Base-Emitter (Forward Bias)

!"# ≅ %. ' ! will be used in most analysis examples

B

BEBBB

BEBBBB

RVVI

VRIV-

=

=-- 0BECECB

BEBBBB

VVVVRIV

-==-- 0

Collector - Emitter

BDCC

CCCCCE

CECCCC

IIRIVVVRIV

b=-=

=-- 0

Collector – Base (Reverse Bias)

BJT CIRCUIT ANALYSIS

BJT CIRCUIT ANALYSIS

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.

BJT CIRCUIT ANALYSIS

Previously explained for npn transistor, what about pnp ???

EXAMPLE

Determine !", !$, !%, &"%, &$% and &$" if the transistor has a '($ = 150.

COLLECTOR CHARACTERISTICS CURVESThe Three Operating Regions Saturation Region Operation • Base–Emitter junction is forward biased• Base–Collector junction is forward biased!" reached max à independent of !#Active or Linear Region Operation • Base–Emitter junction is forward biased• Base–Collector junction is reverse biased!" increases slightly for a given !# as $"%continues to increase due to widening of base-collector depletion region.Cut-off Region Operation • Base–Emitter junction is reverse biased Nonconducting state of transistor à a very small collector leakage current

COLLECTOR CHARACTERISTICS CURVES

The collector characteristic curves show the relationship of the 3 transistor currents.

Curve shown is for a fixed base 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 reverse-biased and transistor goes into active/linear region. ICincreases 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.

Cutoff – condition in which there is no base current, !" = $ which results in only an extremely small leakage current (ICEO) in the collector circuit. For practical work, ICEO is assumed to be 0.

%&'()& + +&' − +&& = 0∴ /01(345677) = /00

In cutoff, neither the BE junction

nor the BC junction are forward-biased.

IB = 0 –

+

–

+ ICEO

RC

VCCVCE ≅VCC

RB

COLLECTOR CHARACTERISTICS CURVES

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 collector-emitter voltage, !"# is minimum (≈0.2 V).

When BE junction become forward-biased, $% is increased and $& also increases ($& = ($%) and )&* decreases à voltage drop across +& ()&* = )&& − $&+&).

In saturation ()&*(./0)), increase of $% has no effect on the $& and the relation $& = (2&$%is no longer valid.

Collector saturation current:

$&(./0) =)&& − )&*(./0)

+&Minimum base current to produce saturation:

$%(345) =$&(./0)(2&

–

+ –

+VCC

VBB

VCE = VCC – ICRC

RB

RC

IB

IC

–

+

– +

COLLECTOR CHARACTERISTICS CURVES

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

Cutoff – condition in which there is no base current, !" = 0; %&' = %&& .

Saturation – condition in which there is maximum IC; collector-emitter voltage is minimum.

The transistor characteristic curves are shown superimposed on the load line. The region between the saturation and cutoff points is called the active region. 0

IC

VCE

IB = 0 Cutoff

VCE(sat) VCC

IC(sat)

Saturation

COLLECTOR CHARACTERISTICS CURVES

1. Calculate the saturation current and the cutoff voltage for the circuit.

2. Determine whether or not the transistor in saturation.

Assume VCE = 0.2 V in saturation.

EXAMPLE

VVV

mAkR

VI

CCCO

C

CCSAT

15

48.43.32.0152.0

==

=-

=-

=

mAAII

Ak

I

BC

B

09.2)45.10(200

45.10220

7.03

===

=-

=

µb

µ

Q1

Q2 – make known if !" is large enough to produce !#(%&')

Since IC < ISAT, the transistor is not saturated.

SOLUTION

EXAMPLE1. Determine whether or not transistor is in saturation. Assume !"#(%&') =0.2 V.

2. Determine whether or not the transistor is saturated for the following values: ./" = 125, !33 = 1.5 !, 43 = 6.8 7Ω, 4" = 180 Ω, !"" = 12 !

MAXIMUM TRANSISTOR RATINGS

§BJT and other electronic devices has limitations on its operation.

§This limitation stated in maximum ratings (datasheet).

§Typically given maximum ratings: !"#, !"%, !#%, &" and power dissipation.

§Product of !"% and &" must not exceed max power dissipation.

§If !"% is maximum:

&" =()(+,-)!"%

§If &" is maximum:

!"% =()(+,-)&"

DATASHEET

Datasheet 2N3904

BJT AMPLIFIERS§Amplification à process of linearly increasing amplitude of an electrical signal.

§BJT transistor amplifies current, !" = $!%. !% is very small compared to !" and !& à !" ≈ !&.

§AC voltage suppy, () is superimposed on DC bias voltage, (%% by capacitive coupling. DC bias voltage ("" is connected to collector through collector resistor, *".

§AC input voltage produce AC base current à results in larger AC collector current.

§AC collector current produces AC voltage across *" àproducing amplified but inverted reproduction of AC input voltage.

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.

BJT SWITCHES

PHOTOTRANSISTOR§Similar to regular BJT.

§Base current is produced and controlled by light instead of voltage source.

§Effectively converts light energy to an electrical signal.

§!" = $%"!&; where !&: light-generated based current.

§Typical phototransistor is designed to offer large area to incident light.

§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 (important).

§With live circuits it is most practical to troubleshoot with voltage measurements.

TROUBLESHOOTING

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.

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

TROUBLESHOOTING

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.

TROUBLESHOOTING

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

SUMMARY

§Bipolar Junction Transistor (BJT) is constructed of three regions: base, collector, and emitter.§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.

§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.