Stability analysis and layout design of an internally stabilized multi-finger FET for high-power base station amplifiers

A high-power, discrete, and internally-matched FET, such as for use in base station amplifiers, consists of lots of gate fingers to realize a very large periphery. It is well-known that many active devices combined in parallel likely form many closed loops and cause odd mode oscillation. However, stability analysis among FET fingers is usually complex, because of the existence of lots of active nodes. In this paper, novel internally stabilized multi-finger FET layout methodology with a branched gate feed structure is proposed, to stabilize among gate fingers without increasing the occupied layout area of the FET. The feature of this layout is that the branched gate feed structure, which can be fabricated without any extra processing step, functions as a resistor to isolate each FET cell. Stability analysis and layout design were achieved by using the NDF (Normalized Determinant Function) evaluation technique, which can deal with lots of active nodes. In an experiment for a GaAs FET of 134 mm gate width with 168 gate fingers, this stability analysis precisely predicted an oscillation frequency of the FET having multiple closed loops. The new approach presented here on the gate feed structure effectively suppressed odd mode oscillation.