Thermal Noise in Field-Effect Transistors

The limiting noise mechanism in field-effect transistors is thermal noise of the conducting channel. The noise can be represented by a current generator ¿i2 in parallel to the output. The value of i2 is calculated; for zero drain voltage the noise corresponds to thermal noise of the drain conductance, and for other bias conditions the noise at a given gate voltage depends only slightly upon the drain voltage. Because of modulation effects in the channel, i2 is somewhat larger than the thermal noise of the dc drain conductance, except for zero drain bias and beyond saturation. The noise resistance of the device is approximately equal to gmax/g,m2, where gm is the transconductance of the transistor and gmax its maximum value. The approximation becomes even closer if feedback due to the series resistances of the channel must be taken into account.