GaAs dual-gate Schottky-barrier FET's for microwave frequencies

The benefits inherent in the tetrode structure and the potential of GaAs are combined to realized a dual-gate FET with low noise and a wide dynamic range at microwave frequencies. A design theory of the dual-gate FET is constructed on the basis of the Lehovec-Zuleeg model for single-gate FET's. The theory has led to a new device structure having a second gate with a deeper pinchoff voltage than the first which shows improved gain and noise performance. Also derived is the importance of minimizing parasitic feedthrough due, for example, to packages. Samples were fabricated using n-type epitaxial GaAs. The first and second gates were Schottky barriers, 1.2 and 2.5 µm long. The improved channel structure was accomplished by reducing the thickness of the epitaxial layer under the first gate. Samples were mounted and characterized in specially designed small-size ceramic packages with a feedthrough capacitance of only 0.004 pF. The possibility of gain control by means of second gate bias over a wide bandwidth is demonstrated.