Different types of noise are as follows:

1.Thermal noise:

• Thermal noise also known as Johnson noise.

• The free electrons within a conductor are always in random motion.

• This random motion is due to the thermal energy received by them.

• distribution of these free electrons within a conductor at a given instant of time is not uniform.

• It is possible that an excess number of electrons may appear at one end or the other of the conductor.

• The average voltage resulting from this non-uniform distribution is zero but the average The power is not zero.

• As this power results from the thermal energy, it is called as the “thermal noise power”.

• The average thermal noise power is given by,

$P_n$= kTB watts

Where k= Boltzmann’s constant =1.38*10-23 Joules/Kelvin

B= Bandwidth of the noise.

T= Temperature of the conductor, “Kelvin”.

2.Shot noise:

• The shot noise is produced due to shot effect.

• Due to the shot effect, shot noise is produced in all the amplifying devices rather in all the active devices.

• The shot noise is produced due to the random variations in the arrival of electrons (holes) at the output electrode of an amplifying device.

• The exact formula for the shot noise can be obtained only for diodes. The mean square shot noise current for a diode is given as

  

Where I = direct current across the junction

$ I_0$= reverse saturation current

q = electron charge

B = effective noise bandwidth in Hz.

3.Partition noise:

Partition noise is generated when the current gets divided between two or more paths.

It is generated due to the random fluctuations in the divisions

Therefore the partition noise in a transistor will be higher than in a diode.

4.Low frequency or flicker noise:

The flicker noise will appear at frequencies below a few kilohertz.

It is sometimes called as “1/f”noise.

In the semiconductor devices, the flicker noise is generated due to the fluctuations in the carrier density.

5.High frequency or transit time noise:

• If the time taken by an electron to travel from the emitter to the collector of a transistor becomes comparable to the period of the signal which is being amplified then the transit time effects takes place.

• This effect is observed at very high frequencies.

• Due to transit time effect some of the carriers may diffuse back to the emitter.

• Once this noise appears, it goes on increasing with frequency at a rate of 6 dB per octave.