A three-terminal device whose output current, voltage, and/or power are controlled by its

input current (Bipolar Junction Transistor, BJT) or voltage (Field-Effect Transistor, FET).

Function of transistor

1. Amplification (BJT)

a circuit used to increase the strength of an ac signal.

2. Switching (FET)

switch “on” and “off” at a rate of several billions of times per second (~ GHz)

Type of transistor

Bipolar junction transistor (BJT)

Field effect transistor (FET)

Transistors

This picture shows the workbench of John Bardeen and Walter Brattain

at Bell Laboratories. They were supposed to be doing fundamental

research about crystal surfaces. The experimental results hadn't been

very good, though, and there's a rumor that their boss, William

Shockley, came near to cancel the project.

But in 1947, they switched to pure materials. And it dawned on them

that they could build the circuit in the picture. It was an amplifier!

The three shared a Nobel Prize. Bardeen and Brattain continued in

research (and Bardeen later won another Nobel). Shockley quit to start

a semiconductor company in Palo Alto. It folded, but its staff went on to

invent the integrated circuit (the "chip") and to found Intel Corporation.

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

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

Zero Biasing at room temperature with no potentials applied

The depletion layer extend farther into the base region than either of the other two because the base region has a lower doping level

++

+

Cut-off (Reverse, Reverse)

Both pn junction in a transistor are reverse biased.

IC is nearly zero even with the most high VCE

B-E junction B-C junction Mode Voltages

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

Saturation (Forward, Forward)

IC reaches its maximum value of CCC

C E

VI

R R

n-type collector is lightly doped than the emitter region,

for the built-in potential to be +0.4 V between the base

and the collector.

No more increase of IC even with more larger IB

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

Transistor operating 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

EXAMPLE 6.1

Determine the value of collector current for the values of base current shown in the figure.

6.3.2 The Relationship Among IE, IC, and IB

From Kirchhoff’s current law,

IE = IB + IC for npn transistor

Since IB is normally much less than IC,

IE ≈ IC for npn transistor

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

EXAMPLE 6.3

Determine the values of IC and IE for the circuit shown in the figure.

EXAMPLE 6.4

Determine the values of IB and IE for the circuit shown in Figure 6.14.

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.

6.3.6 Maximum Current Ratings

Most transistor spec sheets list maximum collector current ratings (IC(max))

The maximum allowable base current for a given IC(max) :

EXAMPLE 6.6

The transistor shown in Figure 6.17 has the following ratings: IC(max) = 500 mA and βmax = 300. Determine the maximum allowable value of IB for the device.

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.

A composite of collector characteristic curves for a BJT

6.4.2 Base Curves

6.6 Transistor Testing