Thermal/ White noise:
- MOS transistors also exhibit thermal or white noise. The significant source is generated in the channel.
For MOS devices operating in saturation region the channel noise can be modeled by a current source connected between the drain and source terminal and expressed as
K - Boltzman Constant = 1.38 x 10-23 J/K;
T - Temperature;
γ = 2/3 - for long channel devices;
γ = 2 - for sub-micron devices;
$g_m$ - Transconductance
The dependence of thermal noise upon temperature suggests that low temperature operations can decrease the noise in analog circuits.
- The ohmic sections of a MOSFET also contribute thermal noise. The gate, source and drain materials exhibit finite resistivity, thereby introducing noise. For a relatively wide transistor, the source and drain resistance is typically negligible whereas the gate distributed resistance may become noticeable.
- The interface between the gate oxide and the silicon substrate in a MOSFET entails an interesting phenomenon. Since the silicon crystal reaches an end at this interface, many “dangling” bonds appear, giving rise to extra energy states.
- As charge carriers move at the interface, some are randomly trapped and later released by such energy states, introducing “flicker” noise in drain current.
In addition to trapping, several other mechanisms are believed to generate flicker noise.
The flicker noise is more easily modeled as a voltage source in series with the gate and is given by
Where K is the process dependent constant and W and L are width and length of MOSFET.
The inverse dependence on WL suggests that to decrease 1/f noise, the device area must be increased. Also PMOS devices exhibit less 1/f noise than transistors because the former carry the holes in a ‘buried channel’ i.e at some distance from the oxide silicon interface.
Another observation shows that flicker noise decreases with increase in frequency and at appoint it starts falling much below the thermal noise. The frequency at which flicker is equal to the thermal noise is called corner frequency. (fc)
The fc of the output current is determined as
Therefore, fc generally depends on device dimensions and bias current.