Bipolar junction transistors (BJTs)

The BJT is a transistor with three regions and two pn junctions. The regions are named

as the emitter, base, and collector and each is connected to a lead.

There are two types of BJTs – npn and pnp.

1. Arrow on the emitter terminal.

2. Arrow from p-type to the n-type material.

( direction of hole current flow)

3. Arrow along the direction of emitter current.

npn transistor

• Under normal circumstances, IE has the greatest value of

the three, a little bit larger than IC .

• IB normally has the smallest value than the others

• IE and IC are controlled by IB

• forward current gain, β ~ a factor by which current

increases from the base of a transistor to its collector.

where ~ 100

11

B C E

E B C

C B

C E

I I I

I I I

I I

I I

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Terminology

Operation of npn Transistor

B-E junction B-C junction Mode Voltages

Forward Reverse Forward-active (활성) VE < VB < VC

Forward Forward Saturation (포화) VE < VB > VC

Reverse Reverse Cut-off (차단) VE > VB < VC

Reverse Forward Reverse-active VE > VB > VC

There is an avalanche breakdown mode for very high VCE

Active (Forward, Reverse)

When VBE is sufficient to overcome the barrier potential of the junction, current is generated in the emitter and base region (IBE)

The BJT operates between saturation (maximum IC) and cutoff (minimum IC) regions

6.3.1 Transistor Currents

current controlled (current driving) device

: a small change in IB results in larger change in the other terminal current

6.3.3 DC Beta

The ratio of dc collector current to dc base current

The most common transistor circuits have an input signal applied to the base and an output signal taken from the collector

6.3.4 DC Alpha

The ratio of dc collector current to dc emitter current.

Also referred to as collector current efficiency.

6.3.5 The Relationship Between Alpha and Beta

EXAMPLE 6.5

Determine the value of alpha for the transistor shown in Figure 6.16. Then, determine the value of IC using both the alpha and the beta ratings of the transistor.

6.4 Transistor Characteristic Curves

6.4.1 Collector Curves

Kirchhoff’s law

Assuming VBE = 0.75 V

Saturation Mode !!

6.4.1 Collector Curves

400 W

IC is determined by IB and .

Active Mode !!

Changing the value of RC has not affected the value of IC. But, VCE changed from 0.3 to 6 V

IC is not controlled by VCE when a transistor is operated in the active region.

6.4.2 Base Curves