Thermal noise in field-effect devices

Thermal-noise calculations for both junction-gate and m.o.s. field-effect transistors are performed using a straightforward circuit-analysis technique based on the equivalent circuit. Expressions are obtained for īd2, īg2 and the correlation coefficient of ig and id at moderately high frequencies. A comparison with van der Ziel's results for the bulk f.e.t. shows that the expression for the gate noise can be simplified considerably and that the expression for the correlation coefficient c is in error. The correct expression for c is given in terms of van der Ziel's equations and is also presented in simplified form. Approximate expressions for the noise factor of both bulk and m.o.s.f.e.t. amplifiers are presented in a form convenient for comparison with experimental measurements. It is found that, for the model under consideration, at higher frequencies the thermal-noise performance of the m.o.s.f.e.t. is similar to or better than that of the bulk f.e.t., for comparable gate capacitances and transconductances.