Effect of Device Layout on the Stability of RF MOSFETs

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In this paper, the stability of RF MOSFETs is investigated in terms of the stability-factor ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -factor) for various layout schemes and device dimensions based on two different RFCMOS technologies. To systematically analyze the effect of small-signal device model parameters on RF MOSFET stability, the expression for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -factor is derived as a function of the small-signal model parameters of RF MOSFETs. Based on the expression, the effect of small-signal model parameters on the stability of RF MOSFETs is explored along with its bias dependence. In addition, the effect of wiring schemes, number of gate fingers, gate finger pitch, and gate length is examined based on various device structures. It is shown that the transconductance and capacitances are the dominant device parameters to determine the stability of RF MOSFETs. The result also indicates that the stability of RF MOSFETs is strongly affected by the details of layout scheme and lateral dimension. Additionally, it was found that there is a tradeoff between device stability and speed. This study is expected to serve a guideline for the device design and optimization for stable operation of RF MOSFETs and circuits based on them.