Resistors

Ø It is a device which opposes the flow of current through it.
Ø The resistor's function is to reduce the flow of electric current.
Ø This symbol is used to indicate a resistor in a circuit diagram, known as a schematic.
Ø Resistance value is designated in units called the "Ohm."(Ω). A 1000 Ω resistor is typically shown as 1KΩ (kilo Ohm), and 1000 KΩ is written as 1MΩ (Mega ohm).
Ø There are two classes of resistors; fixed resistors and the variable resistors. They are also classified according to the material from which they are made. The typical resistor is made of either carbon film or metal film. There are other types as well, but these are the most common.

SPECIFICATIONS:

Ø The resistance value / Ohmic value: It is the value of the resistor expressed in Ω, KΩ or MΩ.
Ø Tolerance: Tolerance: The tolerance of a resistor denotes the maximum permissible plus or minus deviation from the actual rated resistance value expressed in percentage. For example, a ±5% tolerance would indicate a resistor that is within ±5% of the specified resistance value. So it indicates the accuracy.
Ø Power rating: It is the maximum power in watts that the resistor can safely dissipate. Power is calculated using the square of the current ( I2 ) x the resistance value ( R ) of the resistor. If the maximum rating of the resistor is exceeded, it will become extremely hot, and even burn. Resistors in electronic circuits are typically rated 1/8W, 1/4W, and 1/2W. 1/8W is almost always used in signal circuit applications.

Resistor colour coding:

On the small body of the resistors, it is very difficult to print the values. So a method called colour coding is used in which the resistor values are shown using coloured bands. In this method colour bands are printed on one end of the resistor casing. The colour bands (three or more in colour) which are close together represent the normal value of the resistor and the other band which is away from other bands represents the tolerance.









Most resistors have 4 bands:
The first band gives the first digit.
The second band gives the second digit.
The third band indicates the number of zeros that must be put after the first two digits.
The fourth band is used to shows the tolerance (precision) of the resistor

RESISTOR CLASSIFICATION:

Resistors

Fixed Variable
Resistor Resistor

Wirewound Carbon Metal film Rheostat Potentiometer Trimmer
Resistor Resistor Resistor
Carbon Compound Carbon film


( I ) FIXED RESISTORS:
A fixed resistor is one in which the value of its resistance cannot change. Three types of fixed resistors are Wire wound Resistors, Carbon Resistors and Metal film Resistors.

1. Wirewound resistor:
A wirewound resistor is made of metal resistance wire which is wound over an insulating former such as ceramic. By selecting desired length of wire, they can be manufactured to precise values. Also, high-wattage resistors can be made by using a thick wire material. The resistance wire is usually Nichrome (Nickel, Chromium alloy).
Nichrome is used because of its high tensile strength, high thermal stability and low noise disturbance.
After winding the resistance wire it is covered with vitreous enamel or cement to provide electrical insulation.
Resistance range is 1Ω to 100KΩ.
Tolerance is 5% to 10%.
Power rating is 1W to 200W.
Operating temperature as high as 275°C.
.
* For precision & Ultra wirewound resistors
Resistance range is 0.1Ω to 20MΩ.
Tolerance is ±0.05%.
Power rating is 1/10W to 100W.
Advantages:
* Accurate resistance values
*Can withstand large power dissipation
* Can be used in high temperature situations.
* Capable of carrying large currents
* Resistance value doesn’t change with aging
* Can withstand Mechanical shocks and vibrations.
* Can be used in high voltage circuits.
* Low cost.
Disadvantages:
* Wirewound resistors cannot be used for high-frequency circuits. Since a wirewound resistor is a wire wrapped around an insulator, it is also a coil, in a manner of speaking. Using one could change the behavior of the circuit
* Large size and weight
* Costly
* Wires may break leading to breakdown of circuits.




2. CARBON resistors:

a. CARBON COMPOSITION RESISTOR:
It is the most common type of low wattage resistor. The resistive material is carbon clay ie. A combination of finely powdered carbon or graphite mixed with powdered insulating material or filler such as talc with a synthetic resin as binder. This mixture is made in shape of a rod by compression and molding. The ends of carbon resistance elements are joined to silver plated end caps and these caps are provided with leads made of Tinned copper. The resistance is enclosed in a plastic jacket.
Resistance range is few ohms to 22MΩ.
Tolerance is 5% to 20%.
Power rating is 1/8W to 2W.
Advantages:
* Large range of resistance values
* Good RF performance.

* Nil inductance and low capacitance.
* Small in size.
* Low cost.
Disadvantages:
* No precision.
* High tolerance.
* Easily heated and crackdown on soldering.
* Not suitable for applications involving power levels of 5W or more.
* Resistance value varies with aging.

b. CARBON FILM RESISTORS:
They are made by spraying carbon over an insulating material. These are of two types- Thin film and Thick film resistors.
THIN FILM RESISTORS: It is made by vacuum deposition method. Carbon powder is placed inside an evacuated ceramic container and it is heated. Then the carbon vapours produced come in to contact with the walls of the container and then it is allowed to cool slowly. Thus a thin coating of carbon of thickness less than 5μm is formed inside the container. The carbon film can also be made by pyrolysis of some hydrocarbon gas on the Ceramic core.
Leads
End caps
Grooved Carbon Film
Ceramic core
Resistance range is 1 to 10 MΩ.

Tolerance is ±5%


THICK FILM RESISTORS: The thickness of carbon coating of this type is greater than 5μm. They are of three types – Metal oxide resistors, Bulk property resistor and Cermet (Ceramic + Metal).
Resistance range is 1 to 10 MΩ.
Tolerance is ±1%

Advantages:
* Large range of resistance values
* Good frequency property.
* Small in size.
* Low cost.

Disadvantages:
* Cannot withstand high temperature.
* Vulnerable to atmospheric moisture and humidity.
* Vulnerable to mechanical shocks.
* Chemically reactive and hence are unstable.

3. Metal film resistors:
Metal film resistors are used when a higher tolerance (more accurate value) is needed. They are much more accurate in value than carbon film resistors. They have about ±0.05% tolerance. Metal film resistors are manufactured by an evaporation/deposition process or sputtering process. That is the base metal is vaporized in a vacuum and deposited on a ceramic rod or wafer. Thickness of film will be of the order of 1 micron. Ni-Cr (Nichrome) seems to be used for the material of resistor. The metal film resistor is used for bridge circuits, filter circuits, and low-noise analog signal circuits.









( II ) Variable Resistors:
A variable resistor is one in which the value of resistance can be varied between 0 and a maximum value. They are mainly three types- Rheostat, Potentiometers and Trimmers.

1. Rheostat:
This is the simplest way of using a variable resistor. Two terminals are used: one connected to an end of the track, the other to the moveable wiper. Turning the spindle changes the resistance between the two terminals from zero up to the maximum resistance. Rheostats are often used to vary current, for example to control the brightness of a lamp or the rate at which a capacitor charges.

Rheostat Symbol


2. Potentiometer:
They consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available. The resistance and type of track are marked on the body. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

(Potentiometer Symbol)

Pots are of three types:
a. Linear (LIN) type: A linear pot has a resistive element of constant cross-section, resulting in a device where the resistance between the contact (wiper) and one end terminal is proportional to the distance between. This is the standard arrangement.


*K7 LIN means 4.7 k linear track.
b. Nonlinear or Logarithmic (LOG) pot: LOG track means that the resistance changes slowly at one end of the track and rapidly at the other end, so halfway along the track is not half the total resistance.


1M LOG means 1 M logarithmic track.
c. Volume control type: Usually logarithmic track arrangement is used for volume (loudness) control pots because the human ear has a logarithmic response to loudness so fine control (slow change) is required at low volumes and coarser control (rapid change) at high volumes.





Construction of a wire-wound circular potentiometer is shown above. The resistive element (1) of the shown device is trapezoidal, giving a non-linear relationship between resistance and turn angle. The wiper (3) rotates with the axis (4), providing the changeable resistance between the wiper contact (6) and the fixed contacts (5) and (9). The vertical position of the axis is fixed in the body (2) with the ring (7) (below) and the bolt (8) (above).
In modern usage, a potentiometer is a potential divider, a three terminal resistor where the position of the sliding connection is user adjustable via a knob or slider.
Ordinary potentiometers are rarely used to control anything of significant power (even lighting) directly due to resistive losses, but they are frequently used to adjust the level of analog signals (e.g. volume controls on audio equipment) and as control inputs for electronic circuits (e.g. a typical domestic light dimmer uses a potentiometer to set the point in the cycle at which the triac turns on). Potentiometers used to control high power are normally called rheostats.
Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board.



3. Presets / Trimmers / Trimpots:
These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. For example, to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit. A small screwdriver or similar tool is required to adjust presets. Presets are much cheaper than standard variable resistors so they are sometimes used in projects where a standard variable resistor would normally be used.
(Preset Symbol)
Multiturn presets are used where very precise adjustments must be made. The screw must be turned many times (10+) to move the slider from one end of the track to the other, giving very fine control.

Preset (open style) Presets (closed style) Multiturn preset


SPECIAL RESISTORS:

1. Thermistor (Thermally sensitive resistor):

The resistance value of the thermistor changes according to temperature.


There are mainly three types of thermistor.
1. NTC (Negative Temperature Coefficient Thermistor): With this type, the resistance value decreases continuously as the temperature rises.
2. PTC (Positive Temperature Coefficient Thermistor): With this type, the resistance value increases suddenly when the temperature rises above a specific point.
3. CTR (Critical Temperature Resister Thermistor): With this type, the resistance value decreases suddenly when the temperature rises above a specific point.

LIGHT DEPENDANT RESISTOR:

A LDR is a special type of resistor. The light-sensitive part of the LDR is a wavy track of cadmium sulphide. As more light falls onto the track, the resistance decreases...A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. It can also be referred to as a light-dependent resistor (LDR), photoconductor, or photocell.
An LDR is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
Apllications:
1. Auto flash for cameras 5. Room light control
2. Industrial Control 6. Photo lamp
3. Photoelectric Control 7. Photomusical I.C.
4. Photo switch 8. Electronic toys


Capacitors

The capacitor's function is to store electricity, or electrical energy.
The capacitor also functions as a filter, passing alternating current (AC), and blocking direct current (DC).
This symbol is used to indicate a capacitor in a circuit diagram.
The capacitor is constructed with two electrode plates facing each other, but separated by an insulator.
When DC voltage is applied to the capacitor, an electric charge is stored on each electrode. While the capacitor is charging up, current flows. The current will stop flowing when the capacitor has fully charged.
The value of a capacitor (the capacitance), is designated in units called the Farad (F).

CAPACITOR CODING:
In the case that the value is displayed with the three-digit code, the 1st and 2nd digits from the left show the 1st figure and the 2nd figure, and the 3rd digit is a multiplier which determines how many zeros are to be added to the capacitance. Pico farad (pF) units are written this way.
For example, when the code is [103], it indicates 10 x 103, or 10,000pF = 10 nanofarad (nF} = 0.01microfarad (µF). If the code happened to be [224], it would be 22 x 104 pF =220,000pF = 220nF= 0.22µF. Values under 100pF are displayed with 2 digits only. For example, 47 would be 47pF.

CAPACITOR SPECIFICATIONS:

Capacitance value in Farad (F).
Voltage rating: It is the maximum voltage that can be applied across a capacitor without breakdown of the dielectric.
Breakdown voltage: When using a capacitor, you must pay attention to the maximum voltage which can be used. This is the "breakdown voltage." The breakdown voltage depends on the kind of capacitor being used. You must be especially careful with electrolytic capacitors because the breakdown voltage is comparatively low. The breakdown voltage of electrolytic capacitors is displayed as Working Voltage. The breakdown voltage is the voltage that when exceeded will cause the dielectric (insulator) inside the capacitor to break down and conduct. When this happens, the failure can be catastrophic.
Power rating: Maximum power dissipated by a capacitor by applying a maximum voltage specified by the voltage rating.
Tolerance: It is the maximum permissible % deviation from the actual capacitance value.
Insulation resistance: It is the resistance offered by the dielectric of a capacitor.
Temperature coefficient: The variation of capacitance value per degree rise in temperature.


CAPACITOR CLASSIFICATION:

Capacitors
Fixed Variable
Capacitor Capacitor

Electrostatic Electrolytic Airgang Trimmer
Capacitor Capacitor Capacitor
Paper Mica Ceramic Plastic
Capacitor Capacitor Capacitor Capacitor


« FIXED CAPACITORS:
A fixed capacitor is constructed in such manner that it possesses a fixed value of capacitance which cannot be adjusted. Fixed capacitors are classified as electrostatic capacitors and electrolytic capacitors.

1. Electrostatic capacitors:
They contain two conducting plates separated by an insulating material called dielectric. They are classified according to the type of material used as its dielectric, such as paper, mica, ceramic or plastic.

a) Paper capacitors:
A paper capacitor is made of flat thin strips of metal foil conductors that are separated by paper (paper impregnated with dielectric such as wax, tissue paper etc.). The contacts to foils are made by welding leads to it. Capacitance usually ranges in value from about 0.0005μF to several microfarads. The voltage rating of paper capacitor is 200v to 10000volts. Paper capacitors are sealed with wax to prevent the harmful effects of moisture and to prevent corrosion and leakage.
Advantages:
4 High voltage rating
4 Mechanically very strong
4 Low cost
4 Wide capacitance range
Disadvantages:
4 Poor high frequency characteristics
Applications:
4 Used as RF suppression capacitor
4 As bypass capacitor in amplifier
4 Very good application in HVDC circuits
4 Used with SCRs

b) Mica capacitors:
For this type of capacitor the Silver or Aluminium electrodes are plated directly on to the mica dielectric. Again several layers are used to achieve the required capacitance. Wires for the connections are added and then the whole assembly is encapsulated in a plastic insulating material.
The capacitance range is 50pF to .2μF.
Voltage rating is 500v.


Advantages:
4 Can withstand high voltages
4 Mechanically very strong
4 Suitable for HF applications
4 High insulation resistance(1000MΩ)
4 Negligible voltage coefficient(Variation of capacitance with voltage change)
4 Can be operated in temperature up to 900ºC.
Disadvantages:
4 Mica capacitors do not have high values of capacitance.
4 They are relatively expensive.
4 Silver migration occurs under high DC voltages, high temperature and high humidity.

c) CERAMIC capacitors:

A Ceramic capacitor is so named because it contains a ceramic dielectric.
Disc type ceramic capacitor is manufactured in the shape of a disk. After leads are attached to each side of the capacitor, the capacitor is completely covered with an insulating moisture-proof coating. Ceramic capacitors usually range in value from 1pF to 0.01μF and may be used with voltages as high as 30,000 volts. Some different shapes of ceramic capacitors are shown in figure
Tubular type of ceramic capacitor uses a hollow ceramic cylinder as dielectric. The plates consist of thin films of metal deposited on the ceramic cylinder. Capacitance range is 1μF to 500μF. Voltage rating is 50V.


Outer coating

Ceramic dielectric


Advantages:
4 Can withstand high voltages
4 They are inexpensive
4 Increased reliability in operation
4 Can be used in high frequency applications.
4 Increased reliability in operation.
Disadvantages:
4 High capacitance values are not possible.

d) Plastic film capacitor/Polyester film capacitor:

There are a number of different types of plastic film capacitors according to the dielectric used. PVC, PFC, Polycarbonate, polyester and polystyrene are some of the most common types. Each has its own properties, allowing them to be used in specific applications. Their values may range anywhere from several picofarads to a few microfarads dependent upon the actual type. Normally they are non-polar. Their tolerance is about ±5% to ±10%.





Advantages:
4 Can withstand high voltages(1000v)
4 They are available in very thin rolls.
4 Increased reliability and stability.
4 High dielectric strength and permitivity (ε).
4 Low dissipation factor.
4 Can withstand temperature up to 250ºC.
Disadvantages:
4 Capacitance > 1μF are not possible.
4 Costlier than any other electrolytic capacitors.

2. Electrolytic Capacitors:

In electrolytic capacitors an electrolyte is used as medium to produce high dielectric constant. Two types of electrolytic capacitors are in common use..
ü Aluminium electrolytic capacitors (Aluminium as conducting plate) and
ü Tantalum electrolytic capacitors (Tantalum as conducting plate).
Both are produced in polarized and nonpolarised forms.

Polar type: These capacitors uses two metal plates of which one side of one of the plate is coated with Aluminium oxide. This oxide layer act as the dielectric and the plate on which it is coated is the anode. The electrolyte and the other plate form the cathode. These capacitors are used only in DC circuits.

Metal Plates
Aluminium oxide



Borax
(i) Polar type

A is the aluminum anode B is the aluminum oxide film C is the electrolyte


The lead with more length is the anode and the other one is the anode. Also the –ve plate can be identified by the black stripe marking in the plastic case near the cathode lead.
Reversal of polarity of the applied voltage will cause dielectric breakdown. ie. the dielectric will be removed from anode and large current will flow as oxide is formed on cathode. The gases released from the electrolyte may build up and cause damage to the capacitor, or cause the capacitor to explode. Capacitance range 1 to 10000μF, voltage rating 450V and tolerance 20 to 50%
Non polar type:
In non polar type oxide film is formed on both the plates. The voltage to this can be applied in both directions. It can be used in dc as well as ac circuits.
Aluminium oxide

Borax Metal Plates





(ii) Non polar type
Advantages:
4 High capacitance in small volume.
4 For capacitance exceeding 1μF electrolytic capacitors are more suitable.
Disadvantages:
4 Large leakage resistance due to the porous nature of Aluminium oxide
Applications:
4 As bypass capacitor
4 For interstage coupling b/w two amplifiers.
4 As smoothening capacitors in DC supplies.
4 As phase shift capacitors.

*Tantalum electrolytic Capacitors:
Tantalum Capacitors are electrolytic capacitors that use a material called tantalum for the electrodes. Large values of capacitance similar to aluminum electrolytic capacitors can be obtained. Also, tantalum capacitors are superior to aluminum electrolytic capacitors in temperature and frequency characteristics. When tantalum powder is baked in order to solidify it, a crack forms inside. An electric charge can be stored on this crack.These capacitors have polarity as well. Usually, the "+" symbol is used to show the positive component lead. Do not make a mistake with the polarity on these types. Tantalum capacitors are a little bit more expensive than aluminum electrolytic capacitors. They are highly stable. So they are used for circuits which demand high stability in the capacitance values. Also, the current-spike noise that occurs with aluminum electrolytic capacitors does not appear with tantalum type.












*Electric Double Layer Capacitors (Super Capacitors)
This is a "Super Capacitor," which is quite a wonder. The capacitance can be as high as 0.47 F (470,000 µF). Care must be taken when using a capacitor with such a large capacitance in power supply circuits, etc. The rectifier in the circuit can be destroyed by a huge rush of current when the capacitor is empty. For a brief moment, the capacitor is more like a short circuit. A protection circuit needs to be set up. The size is small in spite of capacitance. Physically, the diameter is 21 mm; the height is 11 mm. Care is necessary, because these devices do have polarity.









« VARIABLE CAPACITORS:
The value of capacitance of variable capacitors can be adjusted manually. Variable capacitors are used for adjustment etc. of frequency mainly. These are of two types, Airgang and Trimmer.

Airgang capacitor:





Variable capacitors are devices that can be made to change capacitance values with the twist of a knob. These devices come in either air variable or trimmer forms. Air variable capacitors consist of 2 sets of aluminum plates (stator and rotor) that mesh together but do not touch. Rotating the rotor plateswith respect to the stator varies the capacitor's effective plate surface area, thus changing the capacitance. Air variable capacitors typically are mounted on panels and are used in frequently adjusted tuning applications (eg. : fine tuning fixed frequency comunications receivers, crystal frequency adjustments, adjusting filter characteristics).


TRIMMER:
Trimmers are small variable units consisting of two metal plates usually separated by dielectric made of thin piece of mica, ceramic or polyester film. The capacitance is varied by varying the distance b/w the plates by means of a screw which is attached to one of the plates as shown in figure. Trimmers are available with value from 3 pF to 30 pF. There are trimmers with more set of plates, which are called Padders. They are available in values from 4 pF to 70 pF.
Plate2
Ceramic base
Plate1
Mica dielectric









INDUCTORS
Inductors store electrical energy in magnetic field. The inductors have the property of opposing any change in current flowing through it. Inductors have a coil of wire wounded over a core. Inductance is represented by ‘L’ and the unit of inductance is Henry (H).
Self inductance:
It is the property of an electric circuit to oppose any change in current flowing through it and induces a voltage called back emf.
Self inductance of the coil
L = μ N2 A / l
Where L= inductance
N= number of turns of the coil
μ= permeability of the core
A= area of crossection of the core.
l = mean length of the core in m.
Mutual inductance:
It is the property by which a change in the current passing through one coil induces an emf in the coil placed near to each other. ie. When two coils are placed in close the flux lines from one coil cut the turns of the other coil and a voltage will be induced in the coil.
Mutual inductance of the coil
M = K√ (L1.L2)
M = μ N1 N2 A / l
Where K = coefficient of coupling.
Specifications of inductors:
4 Inductance value expressed in H, mH, μH or nH.
4 Tolerance: The maximum permissible %deviation from the marked value.
4 Current rating: Maximum current that can be applied continuously through the inductor.
4 DC resistance: It is the resistance offered by the coil to dc currents.
4 Frequency range: It is the upper and lower working frequency limits.
4 Q factor: the ability to store electric charge in magnetic field.

Classification:

*Fixed type:
They have fixed inductance value. They are classified according to the type of dielectric used.
Aircore inductor:
The air-core inductor may be nothing more than a coil of wire, but it is usually a coil formed around a hollow form of some nonmagnetic material such as hollow cylindrical cardboard paper. So air acts as the core. This material serves no purpose other than to hold the shape of the coil. It is used in circuits in which minimum value of inductance of the order of μH is needed. An air-core inductor and its schematic symbol are shown in figure.


Iron core inductor:
It uses insulated iron core over which inductor wire is wound. The iron magnetic core's high permeability has less reluctance to the magnetic flux, resulting in more magnetic lines of force. This increase in the magnetic lines of force increases the number of lines of force cutting each loop of the coil, thus increasing the inductance of the coil. Iron core inductors are used in Audio frequency applications. Low frequency iron core inductors are used to smooth out the ripple voltage in rectifier circuits. The core material used for chokes is Silicon-iron laminations to avoid eddy current loss. The symbol is given below.




Ferrite core inductor:
Ferrite core is made by spraying iron, cobalt and Nickel granules in to an insulating binder. The Ferrite core has a very low eddy current loss. Due to this they are used in HF applications.


*VARIABLE TYPE:
In certain situations we require to vary the inductance (such as tuning circuits, phase shifting circuits, switching of bands in amplifiers etc.), so we go for variable inductors. These are of three types- tap switching type, sliding contact type and movable core type. The symbol of variable inductor is shown below.

Tap switching type:
Here different tapings are taken from the coil as shown in figure.



Sliding contact type:
The sliding contact can be moved over the coil to vary the number of turns used, thus varying the inductance value. This method is used with larger coils.

Movable core type:
Here the inductance is varied by changing the positions of the core. This changes the flux linkage of the coil thereby its inductance. Usually screwed ferrite core is used for construction of variable inductors.











SEMICONDUCTOR AND
SEMICONDUCTOR DEVICES

The electrical properties of a metal largely depends on how tightly outer electronics within the atom of that material are bound to the central cell. On the basis of this, materials can be classified into the following groups: - Conductors, Insulators and Semi conductors.
Conductors:-
Material in which electrons are loosely bound to the central nucleus is called a conductor. So, they are good conductors of electricity. They have very low electrical resistivity (in the order of 1 x 10-6 ohm-metres).
eg :- Copper, Aluminum, Silver, Gold etc.
Insulators:-
Material in which outer electrons are tightly bounded is called insulators. This has no free electrons and so electricity will not pass through it. They have very high electrical resistivity (in the order of 1 x 1013 ohm-metres).
eg:- Rubber, Glass, Wood etc.
Semi conductor:-
Semiconductors are materials that have conductivity in between those of conductors and insulators, they are neither good conductors nor good insulators.
eg:- Silicon, Germanium, Carbon, Selenium etc.

Energy banDs in solid

The range of energy possessed by an electron in a solid is known as energy band. The range of energy possessed by the valance electrons (electrons in the outermost orbit) in a solid is known as valance band. Valance band have the electrons of highest energy.
In a metal, the valance electrons are loosely attached to the nucleus. Even at ordinary temperatures, some of the valance electrons may get detached to become free electrons. These electrons are responsible for conduction of current in a conductor. So, these electrons are called conduction electrons. The range of energy possessed by conduction electrons in a solid is called conduction band.
There is a small energy gap between every two energy bands. The energy gap between the valance band and the conduction band is called forbidden energy gap (Eg). The width of the energy gap is the difference between the bottom of the conduction band and the top of the valance band. The forbidden energy gap is different for different materials.
Conduction band
Valance band
Second energy band
First energy band
Forbidden energy gap
(Energy band diagram)
Conductors:-
In all conductors the valance band overlaps the conduction band. ie. no energy is needed to move electrons from valance band to conduction band.
Valance
band

Conduction band
Insulators:-
In the insulators the energy gap is very large. In general it is more than 5 eV. So a large amount of energy is needed to move valance electrons to conduction band. For Diamond Crystal, Eg = 6 e V.
Eg≥5eV
Valance
band
Conduction band
Semiconductors:-
In case of semiconductors, the Eg is of the order of l eV.
Eg ≈1eV
Valance
band
Conduction band
For Ge, Eg = 0.785 eV &
For Si, Eg = 1.21 eV.
(a) Intrinsic Semiconductors:
A semiconductor in an extremely pure form is called intrinsic semiconductor.
eg:- 14Si28 , 32Ge73 , Grey tin ( 50Sn119).
Every semiconductor has four valancies. Each of the valance electrons is shared by one of its neighbouring atoms, by covalent band. So, all electrons are tightly bounded and thus the intrinsic semiconductors will not allow conduction of electricity.
As the atmospheric temperature increases, some of the valance electrons acquire enough heat energy to break away from the valance band and move to the conduction band. When an electron moves from the valance band and become free a vacancy is left behind in the valance band and it is termed as hole. Thus in a semiconductor electrons and holes are produced in pairs by thermal agitation. These electrons are responsible for the small conductivity of semi conductors.
But at room temperature, relatively few electrons gain enough energy to become free electrons, so the over all conductivity of such materials is low, thereby their name semiconductors.

Si
Si
Si
Si
Si
Si
Si
Si
Si
(b) Extrinsic semiconductors:
To be useful in electronic devices, the conductivity of intrinsic semiconductor is increased by adding small amount of suitable impurity to it. This process of adding impurity to an intrinsic semiconductor is called doping and doped intrinsic semiconductor is called extrinsic semiconductor. The impurity added is very small; say 1 to 2 atoms of impurity for 106 atoms of semiconductor. Depending upon the type of impurity added, the extrinsic semiconductors are of two types- P type & N type.

P-type semiconductor:
When a small amount of trivalent impurity (group 3 element) such as boron, aluminium, gallium or indium, a silicon atom in the lattice may be replaced by a trivalent atom and its three valance electrons will form covalent bonds with neighbouring silicon atoms. However there are not enough electrons to form the fourth covalent bond. This leaves a vacancy of electron ie, a hole in the covalent bond structure and therefore a hole in the valence band of the energy level diagram. Every impurity atom will produce a hole in the valence band. Hole has a tendency to snatch electrons from the neighbouring silicon atom. This tendency is so great that an electron from the neighbouring covalent bond jump to occupy the vacant position and thereby creating a new hole. Thus each trivalent impurity gives one free hole to the crystal by accepting one electron in the outermost orbit of the semiconductor atom. So these impurities are called acceptor impurities.
This material is called P type semiconductor because the majority of charge carriers which contribute to an electrical current are positively charged holes produced by the doping process. There will be some contribution to the current flow from negatively charged electrons due to electron hole pair generation but these electrons are the minority charge carriers in this material.
The P type material itself is not positively charged because the negative charge of the electrons of the donor atoms are balance by the positive charge in the nucleus.

Si
B
Si
Si
Si
Si
Si
Si
Si
-
-
-
-
-
-
-
-
-
-
-
-
-
Trivalent impurity

Silicon

Electron

Hole

Negative ion









N-type semiconductor:
When a small amount of pentavalent element (group 5 element) such as Phosphorous, Antimony or Bismuth is added to an intrinsic semiconductor, a silicon atom in the lattice may be replaced by a pentavalent atom with four of its valence electrons forming the covalent bounds and one extra free electron. This electron is free to wander within the crystal. Such a crystal with excess of free electrons is called an n-type semiconductor whose conductivity is much improved compared to the intrinsic semiconductors. Here electrons are the majority current carriers. There also exists a few numbers of thermally generated holes called minority carriers. Sine each impurity atom donates an electron to the crystal it is called donor impurity.
The N type material itself is not negatively charged. The negative charge of the electrons of the donor atoms is balanced by the positive charge in the nucleus.

Si
P
Si
Si
Si
Si
Si
Si
Si

+
+
+
+
Pentavalent impurity

Silicon

Electron

Hole

Positive ion


+
+
+
+
+
+
+
+
+





DRIFT & DIFFUSION:
The flow of charge carriers through semiconductors takes place due to two phenomenon- drift and diffusion. The net current flow through a semiconductor has two components- drift current and diffusion current.

Drift: It is defined as the flow of electric current due to the motion of charge carriers under the influence of an applied electric field. When electric field is applied to a semiconductor the charge carriers attain certain velocity to move. Thus the electrons move towards the +ve terminal of battery and the holes move towards the –ve terminal of battery. This combined effect constitutes a current known as drift current.
Diffusion: It is the flow of carriers due to the gradient of carrier concentration. ie. Due to the difference of the carrier concentration from one region of the semiconductor to other. In a semiconductor the charge carriers has a tendency to move from higher concentration region to lower concentration region. This movement of carriers is called diffusion and results in a current called diffusion current.

P N JUNCTION DIODE
When a P type semiconductor is suitably joined to an N type semiconductor the contact surface so formed is called p-n junction semiconductor. This is also known as a PN junction diode. All the semiconductor devices contain one or more PN junction. P N junction is fabricated by special techniques namely growing, alloying and diffusion methods. The most common method is alloying. In this method an alloyed junction is formed from an N type slice of semiconductor by melting a pellet of impurity (trivalent) placed on the slice. The trivalent impurity is absorbed in to the n type material and hence a PN junction is formed.



Example: Circuit symbol:


In electronics, a diode is a component that restricts the direction of movement of charge carriers. It allows an electric current to flow in one direction, but essentially blocks it in the opposite direction. The arrow of the circuit symbol shows the direction in which the current can flow. In a p-n diode, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but not in the opposite direction. Circuits that require current flowing in only one direction will typically consist of one or more diodes in the circuit design.

Unbiased PN junction:
The p-type semiconductor is having negative acceptor ions and holes. The n-type semiconductor is having positive donor ions and negatively charged electrons. When the two pieces are joined together and suitably treated they form a p-n junction. The moment they form a p-n junction, some of the conduction electrons from n-type material diffuse over to the p- type material and undergo electron – hole recombination with the holes available in the valence band. Simultaneously holes from p-type material diffuse over to n- type material and undergo hole-electron combination with the electrons available in the conduction band. This process is called diffusion.










When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile. Thus, two charge carriers have vanished. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator. After a few recombinations the immobile ions accumulate at the junction as shown in figure. This region consisting of immobile ions is called depletion region / space charge region / depletion zone / barrier. The physical distance from one side of the depletion region to the other side is called barrier width.
However, the depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field, ie a barrier develops through the depletion zone which acts to slow further movement of charge carriers and then finally stop recombination. The difference of potential from one side of barrier to the other side is referred to as height of the barrier or barrier potential. For silicon diodes, the barrier potential is approximately 0.6 V and for germanium diodes, it is 0.3V.

Biased PN junction:
When a p-n junction is connected across an electric supply, the junction is said to be under biasing. The potential difference across the p- n junction can be applied in two ways, namely- forward biasing and reverse biasing.

Forward biased PN junction:
When the positive terminal of a dc source is connected to p-type, and negative terminal is connected n-type semiconductor of a p-n junction, the junction is said to be in forward biasing. Now, the negative voltage applied to the N-type end pushes or repels electrons towards the junction, while the positive voltage at the P-type end pushes holes towards the junction. As a result the potential barrier is weakened and so the barrier height. As the applied voltage exceeds the barrier potential, the depletion region become very thin and current carriers of both types can cross the junction into the opposite ends of the crystal. Now, electrons in the P-type end are attracted to the positive applied voltage, while holes in the N-type end are attracted to the negative applied voltage. Because of this behavior, an electrical current can flow through the junction in the forward direction, but not in the reverse direction. Thus with forward bias, a low resistance path called forward resistance (Rf) is set up in the p-n junction, and hence current flows through the circuit.













(Forward Characteristics of a PN diode)



Reverse biased PN junction:
When the positive terminal of a dc source is connected to n-type, and negative terminal is connected p-type semiconductor of a p-n junction, the junction is said to be in reverse biasing. With reverse bias, a high resistance path is set up and no current flows through the circuit. This property is best suited for rectification of ac into dc.
In this case, the positive voltage is applied to the N-type material and negative to the P-type material. In response, we see that the positive voltage applied to the N-type material attracts any free electrons towards the end of the crystal and away from the junction, while the negative voltage applied to the P-type end attracts holes away from the junction on this end. The result is that all available current carriers are attracted away from the junction, and the depletion region grows correspondingly larger. There is no current flow through the crystal because all available current carriers are attracted away from the junction, and cannot cross junction due to the high resistance path called reverse resistance (Rr). So no current will flow. However a small minority current (of the order of few nA in Silicon and μA in Germanium) will flow across the junction.






BREAKDOWN IN PN JUNCTION:

A reverse biased PN junction allows a very small current to flow through it. If the reverse voltage is increased to a large value a phenomenon called breakdown takes place. The high current may enough heat to destroy the junction and hence in normal conditions it is avoided. There are two stages that occur as a material begins to breakdown due a large applied voltage. These are zener breakdown and avalanche breakdown.

(a) Zener breakdown:
In Zener breakdown the electrostatic attraction between the negative electrons and a large positive voltage is so great that it pulls electrons out of their covalent bonds and away from their parent atoms. ie Electrons are transferred from the valence to the conduction band. In this situation the current can still be limited by the limited number of free electrons produced by the applied voltage, so it is possible to cause Zener breakdown without damaging the semiconductor.

(b) Avalanche breakdown:
Avalanche breakdown occurs when the applied voltage is so large that the thermally generated minority carriers get accelerated to great velocities. These electrons collide with the silicon atoms and knock off more electrons. These electrons are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons which lead to more collisions etc. The current in the semiconductor rapidly increases and the material can quickly be destroyed.














Zener diode


A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage or "Zener voltage".
A conventional solid-state diode will not let current flow if reverse-biased below its reverse breakdown voltage. By exceeding the breakdown voltage, a conventional diode is destroyed in the breakdown due to excess current which brings about overheating. The process is however reversible, if the device is operated within limitation.
A Zener diode exhibits almost the same properties, except the device is especially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. A reverse-biased Zener diode will exhibit a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage. For example, a 3.2-volt Zener diode will exhibit a voltage drop of 3.2 volts if reverse biased. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.
The breakdown voltage can be controlled quite accurately in the doping process. Tolerances to within 0.05% are available though the most widely used tolerances are 5% and 10%.
Zener effect was discovered by the American physicist Clarence Melvin Zener. Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient. All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of 'zener diode'.

(Symbol)


Reverse Characteristics of ZENER diode:

Light Emitting Diode
In normal PN junction diodes made of Si or Ge, the release of energy during the electron hole recombination is in the form of heat. LEDs are p-n junction devices constructed of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP) in which the energy released is in the form of Infrared or visible light. The junction in an LED is forward biased and when electrons cross the junction from the n- to the p-type material, the electron-hole recombination process produces some photons in the IR or visible region in a process called electroluminescence. An exposed semiconductor surface can then emit light.

Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to violet. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs are monochromatic; 'white' LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.
Gallium Arsenide (GaAs) à invisible IR light
Gallium Arsenide Phosphide (GaAsP) à Red or Yellow light
Gallium Phosphide (GaP) à Red or Green light
Gallium Nitrite à Blue light