Optimal noise figure of microwave GaAs MESFET's

The optimal value of the minimum noise figure (F sub 0) of GaAs MESFET's is expressed in terms of either representative equivalent circuit elements or geometrical and material parameters in simple analytical forms. These expressions are derived on a semiempirical basis. The predicted values of F sub 0 for sample GaAs MESFET's using these expressions are in good agreement with the measured values at microwave frequencies. The expressions are then applied to show design optimization for low-noise devices. This exercise indicates that shortening the gate length and minimizing the parasitic gate and source resistances are essential to lower F sub 0. Moreover, a simple shortening of the gate length may not bring an improved F sub 0 unless the unit gate width is accordingly narrowed. The maximum value of the unit gate width is defined as the width above which the gate metallization resistance becomes greater than the source series resistance. Short-gate GaAs MESFET's with optimized designs promise a superior noise performance at microwave frequencies through K band. The predicted values of F sub 0 at 20 GHz, for example, for a half-micrometer gate device and a quarter-micrometer gate device are 3 and 2 dB, respectively. These devices could be fabricated with the current technology.