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Lecture 2: Semiconductor Diodes

The pn Junction Diode

R ectifiers

Power Supplies

Diode Clippers

Diode Clampers

Diode Ther mometer

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Diodes

Diodes are semiconductor devices which might be

descr ibed as passing current in one direction only. Diodes

however are far more versatile devices than that.

Diodes may be used as voltage regulators, tuning devices

in RF tuned cir cuits, f requency multiplying devices in RF

cir cuits, mixing devices in RF cir cuits, switching

applications or can be used to mak e logic decisions in

digital cir cuits.

There are also diodes which emit ³light´, of course theseare known as light-emitting-diodes or LED¶s.

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Figure

9.5

A pn Junction

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Figure

9.7

Forward- and Reverse-biased pn Junctions

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Figure 9.8,

9.9

Semiconductor Diode i-v

Characteristic

Semiconductor Diode Circuit

Symbol

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Common Diode The first diode in the previous Figure is a semiconductor

diode which could be a small signal diode. Notice the

str aight bar end has the letter ³k´, this denotes the

³cathode´ while the ³a´ denotes anode.

Current only flows f r om anode to cathode and not in the

reverse direction, hence the ³arr ow´ appear ance. This is

one ver y impor tant pr oper ty of diodes.

The anode ter minal must be positive and the cathode

ter minal negative for a current to flow.

In an ideal diode, there is no voltage dr op acr oss a diode

when current is flowing.

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Figure

9.10

The i-v Characteristic of the Semiconductor Diode

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Figure 9.11

Large-Signal on/off Diode Model

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Physical Str ucture of the Junction Diode

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For war d Biasing the Junction

Electr ons in the p-type mater ial, near

the positive ter minal of the supply,

break their electr on pair bonds and

enter the supply, there by pr oducing

new holes. Also electr ons f r om thenegative ter minal of the supply enter

the n-type mater ial and migr ate towar ds

the junction.

Free electr ons f r om the n-type then

flow acr oss the junction and move into

the holes which have migr ated f r om the

positive ter minal. This current flow will

continue as long as the exter nal supply

is connected and is called for war d

current flow.

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R everse Biasing the Junction

When the polar ity of the supply isreversed, the potential within thedevice is eff ectively reinfor ced andthe de pletion layer becomes wider .

This is because the f ree electr ons inthe n-type are attr acted towar ds the

positive ter minal, away f r om the junction, while the electr ons f r omthe negative ter minal of the supplyenter the p-type and migr atetowar ds the junction. Current flow

is extremely small and is calledreverse current. Note that thiscurrent is pr oduced by minor itycarr iers and the device is said to bereverse biased.

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Figure

9.12,

9.13,

9.14

Circuit containing ideal diode Assuming that the ideal diode conducts

Assuming that the ideal diode does not

conduct

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Zener Diode

The second of the diodes is a zener diode which are fair ly

popular for the voltage regulation of low current power

supplies. While it is possible to obtain high current zener diodes,

most regulation today is done electr onically with the use of

dedicated integr ated cir cuits and pass tr ansistors.

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Varactor Diode

This is actually two var actor diodes mounted back

to back with the DC contr ol voltage applied at the

common junction of the cathodes. These cathodes

have the double bar appear ance of capacitors toindicate a var actor diode.

When a DC contr ol voltage is applied t o the

common junction of the cathodes, the capacitance

exhibited by the diodes (all diodes and tr ansistors

exhibit some degree of capacitance) var ies in

accor dance with the applied voltage.

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Vacuum Tube

The next diode is the simplest for m of vacuum

tube or valve. It simply has the old cathode and

anode.

These ter ms were passed on to moder n solid state

devices. Vacuum tube diodes are mainly only of

interest to restorers and tube enthusiasts.

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LED

The last diode de picted is of course a light emitting diode

or LED.

A LED actually does not emit as much light as it first

appears, a single LED has a plastic lens installed over it

and this concentr ates the amount of light.

Seven LED¶s can be arr anged in a bar fashion called a

seven segment LED dis play and when decoded pr oper ly

can dis play the numbers 0 - 9 as well as the letters A to F.

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Diode CircuitsFr om Neamen, Electr onic Cir cuit Analysis and Design, McGr aw Hill

SignalInput

SignalOutput Recti ied

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Figure

9.20,

9.21

deal Diode Rectifier Input and Output Voltages

T T

[

rms P

T T T

T

P

i I

dt dt t I T

dt t iT

i

2

0sin1

)(1

0

2/

0 2/

!!

¼½

»¬-

«!! ´ ´ ´

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Half-Wave Rectifier(Sedr a/Smith Micr oelectr onic Cir cuits, Oxfor d)

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Figure 9.39

Full-Wave Rectifier

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Full-Wave Rectifier

(Sedr a/Smith Micr oelectr onic Cir cuits, Oxfor d)

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Figure9.42

Operation of Bridge Rectifier

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The Bridge Rectifier

(Sedr a/Smith Micr oelectr onic Cir cuits, Oxfor d)

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Figure 9.45

DC Power Supply

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Figure 9.49

(a)A Zener diode voltage regulator; (b) simplified circuit for Zener

regulator

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Figure 9.54,

9.55

Two-sided diode clipper Circuit model for the diode clipper

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Figure9.58, 9.60

Two-sided (ideal diode) clipper

input and output voltages

Voltages for the diode clipper

(piecewise linear diode model)

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The Filter(Sedr a/Smith Micr oelectr onic Cir cuits, Oxfor d)

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