Write short note modeling source in MOSFETs.

Write short note modeling source in MOSFETs.

Ans. In MOSFET, flicker noise and thermal noise are the primary contributors to the presence of noise. In the Si-SiO2 interface region, flicker noise connected with the trapping and releasing of electrons. In the channel, thermal noise connected with the carriers. In  either the small signal or large signal device model can be modeled as a current source between the source and drain because these noises sources contribute to the total drain current, as shown in fig. 2.17 (a).

The thermal noise current is white noise, which has zero mean. It is simply characterized by its spectral density as given below –

SIw   = M           (ohmic region)

Where,  RFET  = Equivalent FET resistance

T  = Temperature in 0K

K  = Boltzmann’s constant

Gm= Small signal transconductance.

In the ohmic region and saturation region, flicker noise current characterized by the spectral density –



Where,    IDQ = Quiescent current

Kf  = Flicker noise coefficient

F   = Frequency

K’, Cox,  L   = MOSFET model parameters,

The spectral current density of noise current source IN can be determined by adding SIf and SIW as given below –

SN = SIW + SIf

   If the frequency band [f1, f2], RMS flicker noise and white noise current source can be determine from  the spectral density and is given by –

INB = M2

It is noted that, if we define RMS flicker noise and white noise currents by the square root  of the integral of the spectral densities –

IWB =     M3

IFB  =    m4

Hence,    INB  =      m5

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