THEORY OF SOLIDS(SEMICONDUCTUORS ,COMDUCTORS &INSULATORS)

PREPARED BY GROUP

C.U. SHAH POLYTECHIC

Lakhatariya sanket s.

CONCEPT

Electron band

SEMICONDUCTER

INSULATOR

CONDUCTOR

CONDUCTIVITY

PN JUNCTION

RECTIFIER

I.ENERGY BAND

VALANCE BAND

CONDUCTION BAND

FORBIDDIN GAPE

Valance band

The band of energies occupied by the valance electron is called valance band.The electron in the outermost orbit of an atom are known as valance electrons.in Normal atom and based posses the electron of higher energy.This band may be completely of partial filled. electrons can be moved from one valance band by the application of external energy.

Conduction band

The band of energies occupied by conduction electrons is known as conduction band. This is the uppermost band, all electronsIn the conduction band are free electrons. The conduction band is empty for insulator and partially filled for conductors

Forbidden energy gap

The gape between the valance band and the conduction band on energy level diagram Known as forbidden energy gap. Electrons are never found in the gape. Electrons may jumpFrom back and forth from the bottom valance band to the top conduction band.

II.SEMICONDUCTORS

In semiconductors, electron is loosely bound to the nucleus hence requires less energy for separating them from the nucleus. Semiconductors are materials whose electrical resistivity lies between insulator and conductor e.g. germanium and silicon.

The resistivity of semiconductors lie between 10 ohm to 1000 ohm meter at room temperature.

The forbidden gape is very small equal to 1 eV, the energy band diagram of a semiconductors .

The conductivity increase with temperature. As the temperature is increased, some of the valance electron acquire thermal greater than forbidden energy gape and hence moves into the conduction band.

III.INSULATOR

The valance band is full but the conduction band is totally empty .so free electrons from conduction band is not available. In an insulator , the energy gape between valance band and conduction band is very large and approximately equal to 5 eV or more. Hence electron cannot jump from valance band to conduction band. So, a very high energy is required to push the electrons to the conduction band.

The resistivity of insulator lie between 10000 to n10(17) ohm meter at a room temperature .

An insulator does not conduct at room temperature because there are no conduction electrons in it, an insulator may conduct if its temperature is very high or if a high voltage is applied across it. This is known as breakdown of the insulator.

V. CONDUCTOR

Conductors are characteristised by high electrical conductivity .these are the solids. In which plenty of free electrons are available for electrical conduction . Ex. Silver , copper, iron.

In general electrical resistivity of conductor is very low and is of the order of 10 (-6) ohm cm.

There is no forbidden gap and the conduction band and valance band are overlapping each other ,so a slight potential difference the conductor causes the free electrons to constitute electric current.

Due to the absense of forbidden gape ,there is no structure to elablish holes. The total current in conductor is simply a flow of electrons for conductor , the energy gape is of the order of 0.01 eV .

VI. PN Junction diode

p-n JUNCTION:

p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ;

“diffusion” = movement due to difference in concentration, from higher to lower concentration;

in absence of electric field across the junction, holes “diffuse” towards and across boundary into n-type and capture electrons;

electrons diffuse across boundary, fall into holes (“recombination of majority carriers”);

formation of a “depletion region” (= region without free charge carriers)

around the boundary;

charged ions are left behind (cannot move):

negative ions left on p-side net negative charge on p-side of the junction;

positive ions left on n-side net positive charge on n-side of the junction

electric field across junction which prevents further diffusion.

What is a PN Junction?

A PN junction is a device formed by joining p-type ( doped with B, Al) with n-type (doped with P, As, Sb) semiconductors and separated by a thin junction is called PN Junction diode or junction diode.

Electronic Symbol …..the triangle shows indicated the direction of current

Depletion layer forms an insulator between the 2 sides

P type

N type

Forward biased PN junction

It forces the majority charge carriers to move across the junction ….decreasing the width of the depletion layer.

Once the junction is crossed, a number of electrons and the holes will recombine .

For each hole in the P section that combines with an electron from the N section, a covalent bond breaks and an electron is liberated which enters the positive terminal

Thus creating an electron hole pair.

Current in the N region is carried by ….electrons

Current in the P region is carried by …. Holes.

Reverse biased PN junction

If the + of the battery is connected to the n-type and the – terminal to the p-type,

the free electrons and free holes are attracted back towards the battery, hence back from the depletion layer, hence the depletion layer grows.

Thus a reverse biased PN junction does not conduct current

Diode

diode = “biased p-n junction”, i.e. p-n junction with voltage applied across it

“forward biased”: p-side more positive than n-side;

“reverse biased”: n-side more positive than p-side;

forward biased diode:

the direction of the electric field is from p-side towards n-side

p-type charge carriers (positive holes) in p-side are pushed towards and across the p-n boundary,

n-type carriers (negative electrons) in n-side are pushed towards and across n-p boundary

current flows across p-n boundary

PN junction can basically work in two modes, (A battery is connected to the diode )

forward bias mode ( positive terminal connected to p-region and negative terminal connected to n region)

reverse bias mode negative terminal connected to p-region and positive terminal connected to n

region

VOLTAGE –CURRENT (V-I) CHARACTERISTICS OF PN JUNCTION DIODE

The curve drawn between voltage across the junction along X axis and current through the circuits along the Y axis.

They describe the d.c behavior of the diode.

When it is in forward bias, no current flows until the barrier voltage (0.3 v for Ge) is overcome.

Then the curve has a linear rise and the current increases, with the increase in forward voltage like an ordinary conductor.

Above 3 v , the majority carriers passing the junction gain sufficient energy to knock out the valence electrons and raise them to the conduction band.

Therefore , the forward current increases sharply .

With reverse bias

potential barrier at the junction increased. …junction resistance increase…and prevents current flow.

However , the minority carriers are accelerated by the reverse voltage resulting a very small current (REVERSE CURRENT)….in the order of micro amperes.

When reverse voltage is increased beyond a value ,called breakdown voltage,the reverse current increases sharply and the diode shows almost zero resistance .It is known as avalanche breakdown.

Reverse voltage above 25 v destroys the junction permanentaly.

Thus the P N junction diode allows the electrons flow only when P is positive .

This property is used for the conversion of AC into DC ,Which is called rectification

Automatic switch

When the diode is forward bias ,the switch is CLOSED.

When it is reverse biased , it is OPEN

APPLICATIONS

….as rectifiers to convert AC into DC.

As an switch in computer circuits.

As detectors in radios to detect audio signals

As LED to emit different colours.

When the diode is forward bias ,the switch is CLOSED.

When it is reverse biased , it is OPEN

Automatic switch

Working of a PN junction

Forward Bias

Reverse Bias

Zener or Avalanche Breakdown

Voltage

Current

I-V characteristic of a PN junction diode.

•PN junction diode acts as a rectifier as seen in the IV characteristic.

•Certain current flows in forward bias mode.

•Negligible current flows in reverse bias mode until zener or avalanche breakdown happens.

VII. Rectifier

introduction

Half wave rectifier

Full wave rectifier

Bridge rectifier

COMPONENTS

1.What is rectifier?

One of most widely used electronic circuit to convert AC voltage to DC voltage.

Since the rectifier circuit uses diodes to convert ac voltage to dc, its also called a converter circuit.

All power that supply to a modern factory is alternating current, so it is important to have circuit that can convert the ac power to dc power since most solid-state device require a source of dc power to operate.

2.Single phase half wave rectifier

The transformer feeding a resistor as its load with a rectifier inserted in the circuit.

The rectifier will conduct each time its anode is positive with respect to its cathode.

So when the end of the secondary winding shown + is positive, the diode acts as a short-circuit and the + appears across the load.

Current flows around the secondary circuit for the time that the diode is conducting.

The waveform appearing across the load is shown in red on the graph.

3.Two diode full wave rectifier

This is two half-wave rectifiers combined - it uses a center-tapped secondary winding and one additional diode. 

Each side of the centre-tap has the same number of turns as our previous example - and each "works" for half the cycle as our half-wave rectifier did.

The "top half" of the secondary works with one diode like the half-wave circuit we have just considered. 

When the polarity of the secondary changes, the upper diode shuts off and the lower diode conducts. 

4. Four diode Bridge Rectifiers

This uses one single winding as the secondary and four diodes - two are conducting at any one time.

The operation is simple: Parallel-side diodes conduct at the same time. Note that the two + points are connected by a diode - same as in the two previous cases. The other end of the load returns to the transformer via the other parallel diode. When the polarity changes, the other two diodes conduct. 

The output waveform is the same as the full-wave rectifier example shown before. 

Another use of semiconductor technology is in the fabrication of transistors, devices that amplify voltages or currents in many kinds of circuits. The first transistor was developed in 1948 by John Bardeen, William Shockley, and Walter Brattain (Nobel Prize, 1956). As an example we consider an npn-junction transistor, which consists of a thin layer of p-type semiconductor sandwiched between two n-type semiconductors. The three terminals (one on each semiconducting material) are known as the collector, emitter, and base. A good way of thinking of the operation of the npn-junction transistor is to think of two pn-junction diodes back to back.

Transistors

Figure 11.22: (a) In the npn transistor, the base is a p-type material, and the emitter and collector are n-type. (b) The two-diode model of the npn transistor. (c) The npn transistor symbol used in circuit diagrams. (d) The pnp transistor symbol used in circuit diagrams.

Consider now the npn junction in the circuit shown in Figure 11.23a. If the emitter is more heavily doped than the base, then there is a heavy flow of electrons from left to right into the base. The base is made thin enough so that virtually all of those electrons can pass through the collector and into the output portion of the circuit. As a result the output current is a very high fraction of the input current. The key now is to look at the input and output voltages. Because the base-collector combination is essentially a diode connected in reverse bias, the voltage on the output side can be made higher than the voltage on the input side. Recall that the output and input currents are comparable, so the resulting output power (current × voltage) is much higher than the input power.

Figure 11.23: (a) The npn transistor in a voltage amplifier circuit. (b) The circuit has been modified to put the input between base and ground, thus making a current amplifier. (c) The same circuit as in (b) using the transistor circuit symbol.

Transistor

Field Effect Transistors (FET)

The three terminals of the FET are known as the drain, source, and gate, and these correspond to the collector, emitter, and base, respectively, of a bipolar transistor.

Figure 11.25: (a) A schematic of a FET. The two gate regions are connected internally. (b) The circuit symbol for the FET, assuming the source-to-drain channel is of n-type material and the gate is p-type. If the channel is p-type and the gate n-type, then the arrow is reversed. (c) An amplifier circuit containing a FET.