Introduction To Semicon (bjt)

Transistors Bipolar Junction Transistors

Engr. Katrina B. Acapulco

Introduction • Basically, it consists of two back-to-back PN junctions manufactured in a single semiconductor crystal. • These two junctions give rise to three regions called emitter, base and collector.

Introduction Emitter – more heavily doped than any of the other regions because its main function is to supply majority charge carriers to the base. Base – forms the middle section of the transistor. It is very thin as either the emitter or collector and it is very lightly-doped. Collector – its main function is to collect majority charge carriers coming from the emitter and passing through the base. – this region is made physically larger than the emitter region because it has to dissipate much greater power.

Transistor Biasing • For proper working of a transistor, it is essential to apply voltages of correct polarity across its two junctions. 1. Emitter-base junction is always forward biased. 2. Collector-base junction is always reverse-biased.

Important Biasing Rule • For a PNP transistor, both collector and base are negative with respect to the emitter. Collector is more negative than the base. • For NPN transistor, both collector and base are positive with respect to the emitter. Collector is more positive than the base.

Transistor Currents • The three primary currents which flow in a properlybiased transistor are IE, IB and IC. • IE = IB + IC • A small part (about 1-2%) of emitter current goes to supply base current and the remaining major part (9899%) goes to supply collector current. • Moreover, IE flows into the transistor whereas IB and IC flow out of it.

Transistor Currents

Summing Up • The four basic guideposts about all transistor circuits are: 1. Conventional current flows along the arrow whereas electrons flow against it. 2. E/B junction is always forward-biased. 3. C/B junction is always reverse-biased. 4. IE = IB + IC

Transistor Circuit Configurations • Basically, there are three types of circuit connections for operating a transistor. 1. CB 2. CE 3. CC - the term “common” is used to denote the electrode that is common to the input and output circuits.

CB Configuration • In this configuration, emitter current IE is the input current and collector current IC is the output current. • The input signal is applied between the emitter and base whereas output is taken out from the collector and base.

CB Configuration • The ratio of the collector current to the emitter current is called dc alpha (αdc) of a transistor. αdc = -IC / IE •

The negative sign is due to the fact that current IE flows into the transistor whereas IC flows out of it. IC = - αdc IE

• If we write αdc simply as α, then α = IE/IC • It is also called forward current transfer ratio(-hFB).

CB Configuration • The α of a transistor is a measure of the quality of the transistor: the higher the value the better the transistor in the sense that collector current more closely equals the emitter current. • Its value ranges from 0.95 to 0.999. • It applies only to CB configuration of a transistor. IC = α I E IB = IE - α IE = (1- α) IE

CB Configuration • Incidentally, there is also an ac α for a transistor. It refers to the ratio of change in the collector current to the change in emitter current. αac = - ∆IC / ∆IE • It is also known as short-circuit gain of a transistor and it is written as –hfb. • For all practical purposes: αdc = αac = α

CE Configuration • Here, input signal is applied between the base and emitter and output signal is taken out from the collector and emitter circuit. • IB is the input current and IC is the output current.

CE Configuration • The ratio of the collector current to the dc base current is called dc beta (βdc) or just β of a transistor. β = - IC / - IB = IC / IB or IC = βIB • It is also called common-emitter dc forward transfer ratio and it is written as hFE. • It is possible for β to have a value as high as 500. • For ac analysis: βac = ∆IC / ∆IB • It is also written as hfe. • IE = IB + IC = IB + βIB = (1 + β)ΙΒ

Relation Between α and β ∀ β = IC / IB and α = IC / IE • Therefore : β / α = ΙΕ / ΙΒ • Now : IB = IE – IC • Therefore: β = IC / IE – IC = (Ι

/IE) / (IE/IE) – (IC/IE)

C

= α / 1 −α • Cross-multiplying the previous equation: β(1−α) = α or β = α (1+ β) or α = β / (1+ β)

CC Configuration • In this case, input signal is applied between base and collector and output signal is taken out from emittercollector current. • Conventionally speaking, here IB is the input current and IE is the output current.

CC Configuration • IE/IB = IE/IC x IC/IB = β / α = β / β(1+ β) = (1 + β) • IE = IB + IC = IB + βΙΒ = (1 + β)ΙΒ • Therefore: output current = (1+ β) x input current

Relations Between Transistor Currents While deriving various equations, following definitions should be kept in mind: α = IC / IE , β = IC / IB , α = β / (1+ β) and β = α / 1−α •IC = βΙΒ = β / (1+ β) x IE ∀ΙΒ = ΙC/ β = IE / (1+ β) = (1−α)ΙΕ 3. IE = IC/ α = (1+ β) / β x IC = (1+ β) ΙΒ =Ι

Β

/ (1−α)

Relations Between Transistor Currents 4. The three transistor dc currents always bear the following ratio: IE: IB : IC 1 : (1−α) : α Incidentally, it may be noted that for ac currents, small letters ie, ib and ic are used.