1. The resistivity is usually high.
2. The temperature co-efficient of resistance is always negative.
3. The contact between a semiconductor and a metal forms a layer which has a higher resistance in one direction than the other.
4. When some suitable metallic impurity (e.g., Arsenic, Gallium etc.) is added to a semi-conductor, its conducting properties change appreciably.
5. They exhibit a rise in conductivity with the increasing temperature, with the decreasing temperatures their conductivity falls off, and at low temperatures semiconductors become dielectrics.
6. They are usually metallic in appearance but (unlike metals) are generally hard and brittle.

Both the resistivity and the contact effect are as a rule very sensitive to small changes in physical conditions, and the great importance of semiconductors for a wide range of uses apart from rectification depend on the sensitiveness
 
Types of Semiconductor: Semiconductor can be classified into two types:
1. Intrinsic Semiconductors or Pure of semiconductors
2. Extrinsic Semiconductor or Impure of Semiconductors

Instrinsic semiconductors: Two normal (pure) silicon and Germanium are intrinsic semiconductor.They posses  all essential conducting characteristics of a semiconductor. The number of electrons present in the outermost orbit of intrinsic semiconductor is four,hence they are termed as tetra valent . So,intrinsic semiconductor are tetra valent in nature

Extrinsic Semiconductors: The process of adding impurities to an intrinsic semiconductor is known as Doping. With respect to the type of  Impurities  added, extrinsic semiconductor are classified into two types.

1. N-type semiconductors
2. P-type semiconductors

DOPING PROCESS

The pure semiconductor mentioned earlier is basically neutral. It contains no free electrons in its conduction bands. Even with the application of thermal energy, only a few covalent bonds are broken, yielding a relatively small current flow. A much more efficient method of increasing current flow in semiconductors is by adding very small amounts of selected additives to them, generally no more than a few parts per million. These additives are called impurities and the process of adding them to crystals is referred to as DOPING. The purpose of semiconductor doping is to increase the number of free charges that can be moved by an external applied voltage. When an impurity increases the number of free electrons, the doped semiconductor is NEGATIVE or N TYPE, and the impurity that is added is known as an N-type impurity. However, an impurity that reduces the number of free electrons, causing more holes, creates a POSITIVE or P-TYPE semiconductor, and the impurity that was added to it is known as a P-type impurity. Semiconductors which are doped in this manner – either with N- or P-type impurities – are referred to as EXTRINSIC semiconductors.

N-Type Semiconductor :

The N-type impurity loses its extra valence electron easily when added to a semiconductor material, and in so doing, increases the conductivity of the material by contributing a free electron. This type of impurity has 5 valence electrons and is called a PENTAVALENT impurity. Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these materials give or donate one electron to the doped material, they are also called DONOR impurities.

An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is, by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers.

P-Type Semiconductor

The second type of impurity, when added to a semiconductor material, tends to compensate for its deficiency of 1 valence electron by acquiring an electron from its neighbor. Impurities of this type have only 3 valence electrons and are called TRIVALENT impurities. Aluminum, indium, gallium, and boron are trivalent impurities. Because these materials accept 1 electron from the doped material, they are also called ACCEPTOR impurities.

A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order to increase the number of free (in this case positive) charge carriers.

Electricity is made up of atom matter.  To understand that you have to understand the basics of atomic structure.
Matter is commonly made up of mass which occupies space.  This mass can become and take into form several different states:
solid   liquid   gas   plasma
Matter

All matter is comprised of molecules. A molecule is the smallest part of matter which can exist by itself. It contains one or more atoms, which are comprised of protons, neutrons and electrons.
The light in your room requires energy to glow. The energy must find a path through the light switch and through the copper wire. This movement is called electron flow.
The word matter includes copper, wood, water, air….everything. If we take a piece of matter such as a drop of water, divide it into two, and keep dividing it by two, finally we will find that it could be divided no more.  We have a molecule of water.
An atom is divisible. The component of interest to us is the electron.   Electrons are the smallest and lightest parts of the atom and are said to be negatively charged. Another part, protons, are about 1800 times the mass of electrons and are positively charged. 

Electrons repel electrons and protons repel protons. Electrons and protons attract one another.

Like forces repel, unlike forces attract.

When an electron and a proton are brought to close proximity, the electron moves because the proton is much heavier. The electron is small, its field is strong negative, and is equal to the positive field of the proton.
When electrons move, the result is an electron flow – electricity. To move an electron,  a negatively charged field will “push it”, a positively charged field will “pull it”. Or there can be combined efforts! 

Ionisation        
                                                                                                     
When an atom loses an electron, it lacks a negative charge. It is then a called a positive ion. In most metals the atoms are constantly losing and gaining free electrons. In this condition the metal is a good conductor. When gas is ionised under certain conditions, this too becomes a good conductor. Examples are lightning, neon lights and fluorescent lights.

Conductors and insulators

Materials with atoms or molecules with many free electrons will allow an easy interchange of their electrons.  

There is another one, but this shouldn’t really concern you right now.  It is called Superconductivity.  Superconductivity is the phenomenon where the electrons move through the material at no opposition at all, no friction, no resistance, no opposition.  But it is hard to make it work under normal everyday human like environments.  -452 F.  So there is still a lot of research that has to be done there and you shouldn’t concern your self with it.

Examples of good conductors are: Silver; Copper; Aluminium; Gold. (Metals) 
If the free electrons are numerous and loosely held the element is a good conductor. 
If there are few free electrons the element is a poor conductor. 
If there are virtually no free electrons, the element is a good insulator.

Examples of good insulators are: Glass; Mica; Rubber, Plastics.

Semiconductors
Semiconductors exhibit conductivity somewhere between that of good conductors and good insulators. Examples are silicon and germanium.  
Electromotive Force      
To produce a drift of electrons or electric current along a wire, there must be a difference in “pressure” or potential between the two ends of the wire. This potential difference can be produced by connecting a source of electrical potential to the ends of the wire.
For example, and simply put, there is an excess of electrons at the negative terminal of a battery and a deficiency of electrons at the positive terminal. This is due to chemical action within the battery.
A potential difference is the result of the difference in the number of electrons between the terminals. The force or pressure due to a potential difference is termed e.m.f. – electromotive force.
An emf also exists between two objects whenever there is a difference in the number of free electrons per unit volume of the object. If the two objects are both negative, and they are connected together, current will flow from the more negatively charged to the less negatively charged . There will also be an electron flow from a less positively charged object to a more positively charged object.
The emf  is expressed in a unit called the volt. A volt can be defined as the pressure required to force a current of one ampere through a resistance of one ohm.
Consider the following example: Consider the water pressure (volts) required to pass water (current) through a copper pipe of a certain small diameter (resistance).
Try to visualise water going through other pipes of varying diameters. The water pressure required will vary and the volume delivered will vary, or both.

This is Ohm’s law, where E = Volts; I = current in amperes and R = resistance in ohms.   

Electromotive force can be generated in many different ways. 
Some examples:

Chemical (batteries) e.g. dry cell 1.5V, wet cell storage about 2.1V 
Electromagnetic (generators)
Thermal (heating junctions of dis-similar metals)
Piezoelectric (mechanical vibration of certain crystals)
Photoelectric (light sensitive cells)
 

POWER

Energy is the ability to do work and Power is the rate of doing work. 

It is given by: 

P = W/t
P = Power, W = Work in foot-pounds, t = time

In electronics, we know by now that VOLTAGE is the force and that AMPS is the movement of electricity per second or current. 

One AMPERE = 1 COULOMB per second moving through the wire.

Equation:     P = I x E

Power is measured in WATTS.  Watt is named after James Watt, the inventor of the steam engine.  To convert mechanical power to electrical power this is the equation that we use.

1 HP = 746 watts.   HP stands for HORSEPOWER, and in mechanics this is used to measure the rate of work.

This same above Equation can be written in three different ways.  P = I x E, E = P/I or I = P/E.

Cells                                                                                          

Primary cells 

A common method for producing emf is the chemical action in a cell. Two or more cells form a battery. A flashlight cell is in common use in many small appliances. It’s likely to consists of a zinc can (the negative terminal), a carbon centre rod with a copper cap (positive terminal), and a black, damp, paste-like substance called an electrolyte between them. 

These materials were selected from substances so that electrons are pulled from the outer orbits of the molecules or atoms of the positive carbon terminal chemically by the electrolyte and deposited on the zinc can. The massing of these electrons on the zinc produces a backward pressure of electrons, or an electric strain, equal to the chemical energy in the cell. The cell remains static at 1.5V until it is connected to some load.

Once connected, the electrons flowing through the circuit start to fill up the deficient outer orbits of molecules of the positive rod in a continuous stream. It is important to understand that this motion produces the same current throughout the circuit at the same time. 

Alkaline cells 1.2V, have more energy capacity.  The mercury cell 1.34V is long working. The nickel-cadmium or Nicad 1.25V is rechargeable. 

Secondary cells 

The lead-acid storage battery is in near universal use as a vehicle battery. The cell delivers about 2.1V and is rechargeable. This particular battery is made of coated lead plates immersed in a solution of sulphuric acid and water. The acid content of the dielectric varies with the state of the charge. This may be determined by measuring the specific gravity of the electrolyte. A reading of about 1.27 indicates a full charge while a reading of 1.15 or below indicates the cell needs recharging. 

A 12V battery of these cells may be fast charged PROVIDED that care is taken to let escaping gases free themselves and PROVIDED the electrolyte temperature is below 50oC or 125oF.

The automotive battery was specifically designed for rapid charge and rapid discharge. For example, starting a car can cause currents well in excess of 500 amperes to flow – this is why jumper leads use thick wires. This battery was not designed for continuous use – such as running a radio or headlights in a stationary position for an extended period of time. 

Similar types of cells are ’sealed lead acid’ which may be used for emergency stand-by power.

The connection of cells 

When two or more cells are connected together in series, they form a battery. The voltages add together. Some flashlights or portable radios comprise four cells to make up 6V (4 x 1.5V). A car battery has 6 cells in series – so we get 6 x 2.1V = 12.6V. In  actual practice we get 13.8V for a fully charged vehicle battery.

If we put two batteries or cells in parallel, we get the same voltage, but twice the capacity. Twice the energy is available to us.