Explain shot noise and transit time noise.
1 Answer

Shot noise :

It is an internal noise that is generated in most of the semi conductor devices due to

1] Random behavior of charge carriers (and holes) in active devices.

2] Random diffusion of minority carriers.

3] Random generation and recombination of electron hole pairs.

4] Random variations in the arrival of or holes at the o/p terminal. In semi conductor diode, the diode current is given by,

$i_d = \frac{in^2}{2eB}$


$i_d$ $\rightarrow$ RMS diode current.

e $\rightarrow$ charge of an e $(1.6 \times 10^{-19C})$

B $\rightarrow$ system bnd width in Hz.

5] In semiconductor diode (or X er) the o/p current 1(t) is sum of mean current Io and the ft noise current in(t) which fluctuates around Io, i.e.

$i(f) = I_o + i_n \ (t)$

$\therefore$ Shot noise appears as a randomly varying noise current on the o/p currents.

Transit time noise :

It is an internal noise that occurs in many $\mu$ wave devices which produce more noise at frequencies approaching their cut off frequencies.

1] This high frequency noise occurs when the time taken by charge carrier to cross junction is comparable to the time period of i/p s/g waveform.

2] A fluctuating current that constitutes transit time noise is produced because some of the charge carriers may diffuse back across the junction.

3] This carrier diffusion gives rise to an admittance whose conductance component increases with frequency, As a result transit time noise occurs.

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