MODULE 4.1

Semiconductors - Diode

Syllabus for DiodeDiode symbols Diode characteristics and properties Diodes in series and parallelMain characteristics and use of

silicon controlled rectifiers (thyristors), light emitting diode, photo conductive diode, varistor, rectifier diodes;

Functional testing of diodes

Diode and One Way ValveDiodes are similar to check

valves.They allow fluid to flow in one

direction but not the reversed.

How Diode Works

The PN junction consists of P and N material joint together.

The area where P material meets the N material is called the junction.

Current in N-Type Material

The flow is made up of electron, similar to the current in copper.

The electrons leave the crystal and flow towards the positive terminals of the battery.

Current Flow in P-Type Material

Holes flow within the semiconductor from +ve terminal to -ve.

The flow external to the semiconductor is always electron.

Junction Barrier

Both N and P types of semiconductors are electrically neutral.

The number of electrons and protons is balanced.

We expected the electrons will neutralize and balance the holes, but that do not happen

What happen, the electrons from N material move to P material, thus creating ++ ions on N material.The hole transferred to N material.This create a field barriers, ++ at N material and -- at P material.

Barrier JunctionVideo

Forward Biased PN Junction

Reverse Biased PN junction

Reverse biased◦No current due to

majority carrier.◦ small current due to

minority carrier.Resistance

characteristic.◦Small resistance in

forward bias◦High resistance in in

reversed bias. Refer to the graph.

PN Junction Characteristic

The resistance is given by ohm’s law.

Example, forward bias, at 1 v point, R=250 ohms.In reverse bias – R= 800k.

Diode Identification…

As there are many types of diode, a system is required to identify it.

The semi conductor identification is used to distinguish one from the other.

…Diode IdentificationAs there are many types of diode,

a system is required to identify it.The semi conductor identification

is used to distinguish one from the other.

• The first digit XN – X =is the number of junction. N = a semiconductor.

• YYY – Identification Number.

Diode MarkingsThere is the

needs to identify anode/cathode

The cathode is marked as “k”, “+” or color code or band.

Diodes Color CodeExample

◦ Brown, orange and white bands Brown = 1 Orange = 3 White 9

◦ So the diode is 1N139

Diode Maintenance

Diode Maintenance…Diodes are rugged and design to

last a long time.Damages can be due to:

◦Current overloads can damage the junction

◦Excess voltage◦High temperature. Heat increases

the current flow which in turn produce more heat which will cause higher current.

… diode maintenanceThe followings are the

precautions when working with diodes:◦ Power off when installing and removing

diodes◦ Do not use force to pull the diode from the

PCB.◦ Soldering diode need care to avoid

excessive heat.◦ Replace of diode – must be direct

replacement, right direction. ◦ Avoid touching the exposed wires to avoid

static electricity.

Checking Diodes - analogue

Checking Diode – Positive lead is black

Why?+ –

+ –

Checking Diodes digital

Fwd bias – about 1 K

Reverse bias – open circuit.

Different Types of Diodes

… Different Types of Diodes

◦Point Contact Diode◦PN Junction◦Schottky diodes◦Schottky Barrier Diode ◦Varicap or varactor diode◦Zener diode

Different Types of Diodes …

Different types of diodes are manufactured for different purposes:◦Laser diode – the diode

produces laser light. The laser generation function is cheaper than the alternative, however the diode is more expensive than other types of diodes.

LED – Light Emitting Diode

PhotodiodeWhen light strikes, it produces

electrons or holes causing current flow.

Used to detect light.Typically photo-diodes are

operated under reverse bias conditions where even small amounts of current flow resulting from the light can be easily detected.

Diode – PIN Diode

The pin diode acts as an ordinary diode at frequencies up to about 100 megahertz, but above this frequency the operational characteristics change.

The large intrinsic region increases the transit time of electrons crossing the region. Above 100 megahertz, electrons begin to accumulate in the intrinsic region. The carrier storage in the intrinsic region causes the diode to stop acting as a rectifier and begin acting as a variable resistance.

Point Contact Diode

A point-contact diode works the same as the junction diodes, but their construction is simpler.

A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor.

Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact.

The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics.

Schottky Diodes

Lower forward voltage drop about 0.15 – 0.4 v

Different Construction – metal to semiconductor contact

Mainly used for clamping circuit

Schottky Barrier Diodes

Use for rectification Shorter reverse time thus suited

for high frequency switching.

Diode Switching CharacteristicsWhen  diode  is  switched  from  forward 

biased  to reverse biased  state  or vice versa, it takes finite time to attain a steady state.

This  time  consists  of  a  transient  and  an  interval  of  time before  the  diode attain the steady state is attained.

The  behavior  of  the  diode  during  this  time  is  called switching characteristics of the diode.

In  the forward-bias state, there are a large number of electrons from the n side diffusing to the p side.

This  diffusion  process  establishes  a  large  number  of  minority carriers in each material

Zener Diode

RESOURCES

Title Author ISBN

Aircraft Electricity and Electronics

Eisman 0-02-801859-1

Art of Electronics Horowitz /Hill

Horowitz /Hill

0-521-37095-7

Elements of Electronics

Hickey/ Villines

0070286957

Modern Aviation Electronics

Helfrich 0-13-118803-8

Micro Electronics in Aircraft systems

E Pallet 0-273-08612-X

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