Nonlinear Modeling of FETs for Microwave Switches and Amplifiers
Abstract: The exponential growth in wireless systems require rapid prototyping of radio frequency circuits (RF) using computer-aided design (CAD) enabled models. Most of the RF circuits (e.g. switches, ampliﬁers, mixers, etc.) use transistors as their active component for a variety of key functions. This thesis deals with application-oriented empirical modeling of high electron mobility transistors (HEMTs) for RF switches and ampliﬁers.Transistors used in switching and resistive mixer circuits are typically intrinsically symmetrical around the gate. Therefore, a symmetrical small-signal model is proposed, which mirrors the transistor behavior as its source and drain terminals are interchanged. The proposed model allows a signiﬁcant reduction in the number of measurements required to extract the model parameters with a minimum compromise in accuracy. The proposed small-signal equivalent circuit is extended to create a symmetrical nonlinear transistor model. It is shown that only one current and one charge expression is suﬃcient to model the overall nonlinear characteristics of a symmetrical transistor. The method is demonstrated ﬁrst with a GaAs transistor and then extended to a GaN device, where a new symmetrical nonlinear current model is proposed. Transistors also show trapping eﬀects caused by the capture of electrons (and holes) in energy levels within the bandgap. This deviates the high frequency operation of a transistor from its dc-IV characteristics. Therefore, a new model based on Shockley-Read-Hall (SRH) theory is presented to correctly model the trapping eﬀects. The proposed model diﬀerentiates the trap potential and how the trapped electrons modulate the current in a transistor. Furthermore, high power transistors often have ﬁeld-plates to relax peak electric-ﬁelds, which inﬂuence both the number and distribution of the trapped electrons. Therefore, the proposed model is also used to investigate the eﬀect of ﬁeld-plates on the trapping, showing an interesting trade-oﬀ between the trap potential and modulation of the current by the trapped electrons. The investigation also opens a scope to build a trap model scalable with respect to the ﬁeld-plate dimensions in GaN HEMTs. In this work, the modeling procedures, although exempliﬁed using GaN and GaAs HEMTs for switches and ampliﬁers, can be applied equally well for other FET technologies e.g., Si, SiC, GaAs, InP, and other application areas e.g. mixers, oscillators. The work has shown that by incorporating physical information in the modeling, simpler models with improved accuracy can be developed which can reduce time-to-market for new products. HEMT, model, symmetry, trap, GaAs, GaN, nonlinear model, small-signal model, symmetrical model, trap model, ﬁeld-plate.
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