Semiconductors and its types

A Semiconductor is a substance which has resistivity in between conductor and insulator .Copper is a good conductor which is resistivity is 1.7*10^-8.Germanium is a semiconductor which is reistivity is 0.6 ohm meter.A Glass is a good insulator which is resistivity is 9*10^ ohm meter.A Nichrome is a resistance material which is resistivity is 10^-4 ohm meter.

Comparing the resistivity of above materials it is apparent that resistivity of germanium is quite high as compared to copper but is quite low when compared with glass.This is shows that resistivity of a semiconductor lies in between conductors and insulators.

Types of semiconductors :

The semiconductors are classifieds as followed

(i) intrinsic semiconductors

(ii)Extrinsic semiconductors

In every elements,the atoms are tied together by the bonding action of valence electrons.Si and Ge atoms contains only 4 valence electrons.These electrons have tendency to fill the outer most orbit.In this way the electrons placed to in the last orbit of an atom share the electrons with their neighbor atoms. Similarly all the electrons are tied together with their neighboring atoms.

(i)Intrinsic Semiconductors :

A pure semiconductor is called intrinsic semiconductor.The silicon and Ge atoms contain only four electrons in the outermost orbit.So they are called tetravalent bonds.In an intrinsic semiconductors even at room temperature hole pair are created.

When electric field is applied across an intrinsic semiconductor ,the current conduction takes same place by two process which is free electrons and holes.Now no electrons get away from the co-valent bonds.So current flows is zero.

Metal oxide Semiconductor FET

Introduction :

A field effect transistor that can be operated in the enhancement -mode is called a MOSFET. A MOSFET is a important semiconductor device and can be used in any of the circuits covered for JFET.

The main draw back of a JFET is that gate must be reverse biased for proper operation of the device which means that negative gate operation for n-channel and positive gate operation for p-channel.This means that we can only decrease width of the channel.This type of operation is called depletion mode operation.There fore JFET can only operate in the depletion mode.

Symbols for MOSFET :

Types of MOSFET :

there are two basic types available in MOSFET 

1.D-MOSFET or Depletion type MOSFET .This type of MOSFET can be operated in both depletion mode and enhancement mode.

2.E-MOSFET or Enhancement type MOSFET.This type of MOSFET can be operated only in enhancement -mode.

1.D-MOSFET : 

The above figure shows that n-channel MOSFET is a piece of n-type material with p-type region on the right and insulated gate on the left side then free electrons are moving from source to drain must passes through the narrow channel between the gate and the n-type region.

It is usual practice to connect the substrate to the source internally so that a MOSFET has three terminals which are source,drain,gate.

gate insulated from the channel we can apply either positive or negative voltage to the gate.therefore D-MOSFET can be operated in both modes then JFET can operated only one mode which means that depletion mode.

The figure given in below the n-channel D-MOSFET 

2.E-MOSFET :

The E-MOSFET has no channel between source and drain unlike the D-MOSFET. The E-MOSFET requires a proper gate voltage to form a channel .It is reminded that E-MOSFET can operate only enhancement mode only.The constructional details of a n-channel E-MOSFET figure given in below.

Lead acid battery

Battery :

Battery is an electrochemical  device,which delivers electric energy by chemical reaction.An emf is produced by chemical means,whenever two dissimilar solid conductors known as electrodes are placed in conducting liquid known as electrolyte current will flow through the external circuit connected across the two electrodes.

classification of cells :
The cells are classified into two types 
1.primary cell
2.secondary cell
1.primary cell :

A cell which cannot be recharged is called Primary cell.The Primary cells convert chemical energy into electrical energy.

Example : Dry cell,Volatile cell,Daniel cell.

In a primary cell ,the chemical action is irreversible.The electrodes and electrolytes undergo changes during discharge which cannot be reversed.

2.Secondary cell :

The cell which can be recharged and brought back to the original state is called a Secondary cell.

Example : Lead acid cell,Alkaline cells.

The secondary cell converts the electrical energy into chemical energy and it is stored into the cells during charges.When the cells discharges the stored chemical energy is converted in to electrical energy.These are known as storage cells.

What is Battery ?
When number of cells are grouped together it is called Battery.
Types of Battery :
1.primary cell :
a) Dry cell b) Volatile cell c) Daniel cell d) Weston cell
2.secondary cell 
a) Lead acid battery 
b) Nickel-cadmium battery 
c) Nickel-iron battery
d) nickel-zinc battery

Care and Maintenance of Lead Acid Battery :

• Install the Battery firmly.
• Tighten all the connection periodically.
• Keep the battery dry and clean to avoid discharging through moisture.
• Keep the battery surface free of any electrolyte.
• When cleaning or washing high voltage batteries,connection may be opened at several points to avoid shock.
•  Apply petroleum jelly to terminals to prevent corrosion.
• The level of electrolyte must be kept above the top of the plates.
• The level of the electrolyte should be checked at regular intervals.
• The cell  should be recharged as soon as possible after discharge.
• Do not leave the battery in a discharged condition for a long period.
• Do not charge the battery at high rate.
• If battery being used as a stand by source to give emergence lighting etc.It should be given trickle charge.
• The temperature of the cell should not exceed above 40 degree.
• The battery terminals should not be short circuited.
• Carefully follow the instructions of the manufactures.
• During charging the positive terminals of the battery is to be connected to the positive terminal of the supply and negative to the negative terminals of the supply.