Transistor Bipolar

ELG 2135
ELECTRONICS I

FOURTH CHAPTER :
BIPOLAR JUNCTION TRANSISTORS

Session WINTER 2003
Dr M. YAGOUB

Fourth Chapter: Bipolar Junction Transistors

IV - 2

_________________________________________________________________________________________________________________________________________________________

Having studied the junction diode, which is the most basictwo-terminal semiconductor device, we
now turn our attention to three-terminal semiconductor devices. Three-terminal devices are far more
useful than two-terminal ones because they can be used in a multitude of applications.
There are two major types of three-terminal semiconductor devices: the bipolar junction transistor
or BJT, which is the subject of this chapter, and the field-effect transistor orFET, which we shall
study in next chapter.
The bipolar junction transistor consists of two PN junctions constructed in a special way and
connected in series, back to back. Current is conducted by both electrons and holes, hence the name
bipolar.
The simplified structure of a BJT consists of three semiconductors regions:
→

Emitter (E)

→

Base (B) which always refers to the centerregion

→

Collector (C)

We should distinguish between two possible structures of a BJT:
•

A region of P type between two regions of N type (Figure IV-1) :
→

Such a transistor is called a NPN transistor.

Figure IV-1

•

A region of N type between two regions of P type (Figure IV-2) :
→

Such a transistor is called a PNP transistor.________________________________________________________________________________________________________________________________________________________

Fourth Chapter: Bipolar Junction Transistors

IV - 3

_________________________________________________________________________________________________________________________________________________________

Figure IV-2

The transistor consists of two PN junctions:
•

TheEmitter-Base junction (E-B)

•

The Collector-Base junction (C-B)

Depending on the bias condition (forward or reverse) of each of these junctions, different modes of
operation of the BJT are obtained:
Mode

E-B Junction

C-B Junction

Cutoff

Reverse

Reverse

Active

Forward

Reverse

Saturation

Forward

Forward

The active mode is the one used if the transistoris to operate as an amplifier. Switching applications
utilize both the cutoff and the saturation modes.

A – NPN TRANSISTOR
I – NPN transistor in the active mode
The physical operation of a NPN BJT in the active mode is shown in Figure IV-3. Two external
voltage sources (batteries) are used to establish the required bias conditions for active-mode operation:
•

The base-emitter voltage VBEcauses the p-type base to be higher in potential than the n-type
emitter, thus forward-biasing the emitter-base junction.

•

The collector-base voltage VCB causes the n-type collector to be higher in potential than the ptype base, thus reverse-biasing the collector-base junction.________________________________________________________________________________________________________________________________________________________

Fourth Chapter: Bipolar Junction Transistors

IV - 4

_________________________________________________________________________________________________________________________________________________________

Figure IV-3

II – Current flow
As indicated in Figure IV-3, the forward bias of the emitter-base junction will cause two
componentsof the current to flow across this junction: electrons injected from the emitter into the base,
and holes injected from the base into the emitter. As it is highly desirable to have the first component
(electrons) at a much higher level than the second component (holes) the device is designed to have a
high density of electrons in the emitter and a low density of holes in the base. The sum of...