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Accounting for Dynamic Behavior in FET Device Models Ad Reprint

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Today’s engineer faces a range of challenges, including inadequate nonlinear device models that contribute heavily to inaccurate large-signal high frequency simulation. Because of this, designers need to know if a model they are given is suitable for their application before they commit a design to manufacture. Unfortunately, the hurdle for pre-qualifying a nonlinear model is often perceived as high, so few engineers will even ask the question: “Is my model good enough?” Failure to accurately take into account the dynamic behavior of a device in its electrical model can have a detrimental effect on an application. Therefore, modeling such behavior is essential to accurate nonlinear simulation. A powerful simulation-based qualification method now offers designers a viable means of testing field-effect transistor (FET) device models for critical large-signal, high frequency behavioral characteristics at the operating point required by an application. This early insight is critical in allowing designers to avoid costly and time consuming design iterations and their consequences. Understanding dynamic Behavior To better understand why taking a device’s dynamic behavior into account is so critical, it is first important to understand exactly what this behavior is. Dynamic behavior occurs when an object gives measured results that are a function of when a change started (implying memory of past conditions) and/or for how long the change was applied (process/mechanism time constants). It may be exhibited by all semiconductor devices in response to appropriate changes in electrical stimuli, but is particularly prevalent in applications where large-signal, high frequency signals are present (that is RF power amplifiers, mixers, oscillators and high speed digital circuits). Device technology and size do impact a device’s dynamic behavior, but it is unreasonable to assume that an application employing the latest deep sub-micron silicon process will be any less likely to exhibit dynamic behavior than a 50 W GaN device. Essentially, when a device in a quiescent (steady) state experiences a change in electrical conditions, that change does not instantly result in a new steady-state condition. Rather, physical processes, each with differing time constants, begin to respond. The observed result is a time variant change in the electrical conditions observed at the device terminals.

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