Research On Switch-HEMT Modeling

Modern microwave circuit design is highly dependent on computer aided design(CAD),which greatly improves circuit performance and scale,and saves design costs and time.The complex linear and nonlinear characteristics of III-V FETs contribute to the main functions of various microwave active circuits,and transistor is also the most difficult part to model in microwave active circuits,its accurate modeling is the most critical factor to the accuracy of circuit CAD.This study focuses on the HEMT modeling for microwave switching circuit design—switch HEMT modeling.The application of HEMT switch is very important,but there are very few relevant research.HEMT modeling is almost confined to common-source device modeling for amplifier applications.The FET models embedded in commercial microwave simulation software are also designed for common-source amplifiers,not suitable for switching circuits.Switch HEMT modeling has been a well-known problem in the HEMT modeling world.In recent years,many international microwave semiconductor giants(Skyworks solutions,Qorvo,Win semiconductors)are discussing switch HEMT modeling,revealing their special modeling requirements and challenges.With the help of the advanced HEMT production line in China,this study defined and solved the problems in switch HEMT modeling,including:1.High frequency switch HEMT small signal modeling.Traditionally,the test structure of the HEMT with a large gate resistor is directly used for accurate high-frequency small-signal modeling.However,the large resistor significantly complicates the parameter extraction.Optimization method cannot extract the small signal parameters with physical significance,which causes difficulties in hierarchical large signal modeling and process feedback.Meanwhile,accurate HEMT model obtained by traditional modeling methods cannot accurately predict the switch characteristics even at relatively low frequency(Ku band)after connecting an accurate large resistor model to its gate.In this study,a special physical phenomenon of switch HEMT modeling is found.Since the gate of the switch HEMT needs to isolate the RF signal(e.g.with large gate resistance),the accuracy of high-frequency switch model is very sensitive,even at relatively low frequencies(Ku band),to the common capacitance-mix of various capacitance extraction methods.This study also explained the reason why the test structure of the HEMT with a large gate resistor cannot be used to analytically extract parameters.This study proposed a switch HEMT small signal lumped model which contains six parasitic capacitance.Using only a single complete HEMT electromagnetic simulation structure,this study proposed analytical capacitance extraction methods that can accurately divide the capacitance-to-ground and interelectrode-capacitance.This modeling method can achieve an accurate and scalable switch HEMT small signal equivalent circuit model from RF to millimeter-wave band.Moreover,the small signal parameters with physical significance not only contributes to the hierarchical switch HEMT modeling,but also provide feedback to HEMT design,thus expanding the circuit optimization space.The model can be used for the precise design of wide-band microwave switches(such as antenna switches),digital and analog attenuators.2.Switch HEMT large signal modeling.Switch HEMT large signal model can predict the power capacity and harmonic characteristics of the switch.By using the measured data,the nonlinear source of switch HEMT was analyzed in detail.In view of the limitation that the traditional HEMT intrinsic small signal model is only applicable to the positive drain-source bias,this study extends the intrinsic small signal model to make it also applicable to the negative drain-source bias.In view of the traditional model cannot accurately simulate(harmonic error up to 20 dB)the only nonlinear source of off-state switch HEMT—deep subthreshold capacitance,a new switch capacitance model is proposed.The new capacitance model can accurately fit the subthreshold capacitance and accurately predict the harmonic characteristics of the switch.Meanwhile,the initial values of all parameters can be extracted analytically.In view of the importance of deep subthreshold capacitance in switch modeling,this paper uses the capacitance model to directly fit the measured small signal capacitance values of the off state switch HEMT,so that the off state surface trapping effect can be considered into the capacitance model.For the traditional channel current model cannot accurately fit the operation area of switch HEMT,this study improved the voltage dependence of the parameters of the Angelov channel current model,and proposed a simple hyperbolic tangent switching function to isolate the positive and negative channel current modeling.This model can not only accurately predict S parameters,power capacity and harmonic characteristics of HEMT switch,but also shows good convergence and simulation speed in circuit simulation.The simple equations and accurately dividing the capacitance make the scaling rules accurate.3.Dual-gate switch HEMT modeling.Dual-gate HEMT contains two intrinsic cores and more parasitic parameters,so its modeling and parameter extraction face greater challenges.For dual-gate switch HEMT,the capacitance-mix problem still needs to be resolved since the gates of the switch HEMT still need to isolate the RF signal(e.g.with large gate resistance),which further increases the difficulty.Traditional modeling requires the use of the test structure of a dual-gate HEMT with large gate resistors,and the parameter extraction heavily depends on multivariate optimization.This study theoretically revealed that the complete lumped topology of the single-gate switch HEMT contains 6 parasitic capacitance,and the complete lumped topology of the dual-gate switch HEMT contains 10 parasitic capacitance.This study proposed a dual-gate switch HEMT model which contains 10 parasitic capacitance,and extracted the capacitance analytically.Based on the successful single-gate switch HEMT modeling,the extracted intrinsic core of the single-gate switch HEMT was directly embedded into the extracted parasitic shell of dual-gate switch HEMT.The accuracy of the "core transplant" hierarchical modeling technique is highly dependent on the accurate extraction of the parasitic shells of the single-gate and dual-gate switch HEMT.The experimental results show that the complete parasitic capacitance topology and extraction method are highly accurate.This scalable dual-gate switch HEMT model can accurately predict linear and nonlinear characteristics.