Modeling And Design Of Microwave Devices Based On Carbon Nanotubes

Since the invention of the first transistor,the semiconductor industry has experienced a rapid evolution that has profoundly changed the way human societies produce and live.However,an undeniable truth is emerging,semiconductor technologies,typified by silicon,are inexorably approaching the thresholds of physical and technical limitations,making it increasingly difficult to maintain the expected progress envisaged by Moore’s Law.Against this backdrop,novel semiconductor materials such as carbon nanotubes have begun to emerge,attracting widespread attention and scientific research.Due to their unique quasionedimensional structure,carbon nanotubes exhibit unique properties such as enhanced electron mobility and ultrathin dimensions.Because of these properties,carbon nanotubes are considered to be the ideal materials for continuing Moore’s Law.Over the past two decades,carbon nanotube electronics has made important breakthroughs in a variety of areas,including material preparation,device fabrication and circuit applications.These remarkable advances have made the path to the industrialization of carbon nanotubes much clearer,and it is expected that they will be the mainstream semiconductor technology of the future.The Carbon Nanotube Field-Effect Transistor（CNTFET）uses carbon nanotubes as a conductive channel.Benefiting from the distinctive properties of carbon nanotubes,such as high electron mobility,high saturation velocity and low intrinsic capacitance,the CNTFET has remarkable potential in the field of radio frequency and microwave applications.However,current manufacturing processes have not fully utilized its inherent benefits.Several factors limit the performance of CNTFETs,such as excessive contact resistance and less than ideal alignment of the carbon nanotubes within the channel.In light of these issues,this dissertation undertakes a comprehensive investigation of carbon nanotube devices,with a particular focus on CNTFET devices intended for microwave circuit applications.This research includes the analysis of their operating mechanisms,the development of device models,the characterization of critical parameters and the study of circuit design.The main research includes:1.Research on DC modeling of CNTFETs.Existing CNTFET DC models are too idealized to accurately describe the DC characteristics of carbon nanotube array transistors.To address this problem,this dissertation proposes a physicsbased DC model that integrates various nonideal influences in real CNTFET devices.The model is based on the Landauer formula and includes the modeling of Schottky barrier effects,short channel effects,metallic carbon nanotube effects,trap effects,and,for the first time,the influence of carbon nanotube diameter dispersion on CNTFET current.Through mathematical approximations and the careful introduction of empirical models,the formulation of this CNTFET DC model remains simple and compatible with SPICE circuit simulation software.Experimental validation shows that this model closely matches actual measurement data and provides reliable current prediction at high bias voltages.2.Research on contact resistance and extraction methods for CNTFETs.At present,the contact resistance of practical CNTFET devices is generally high,and there is a lack of suitable extraction methods.To address this issue,the applicability of traditional contact resistance extraction methods（transmission length method and Yfunction method）to CNTFETs is investigated in detail,and a method based on transfer curve extrapolation is proposed to extract the contact resistance of CNTFETs.Through theoretical analysis and simulation studies,it is shown that conventional methods are not fully suitable for CNTFET contact resistance extraction.In contrast,the novel approach proposed in this dissertation enables accurate and efficient extraction of biasindependent contact resistance,making it particularly suitable for statistical measurement and analysis of large device batches.329 CNTFET devices on the same wafer were statistically analysed using the method,and an average contact resistance of 1133.65 kΩ/CNT was extracted,and the corresponding histograms of the distribution were plotted.3.Research on characterization and modeling of CNTFETs based on Sparameters.Currently,the research on microwave CNTFET devices is still limited.In particular,the studies using highfrequency measurements techniques for CNTFET characterization are rare.To address this problem,this dissertation applies the Sparameter measurement technique to characterize CNTFETs.A method for extracting CNTFET mobility using Sparameters is proposed for the first time.Under the special bias condition of V_DS=0 V,the smallsignal equivalent circuit model is used to extract information such as contact resistance,channel resistance,and channel capacitance from the Sparameters.In addition,the overall device mobility,carbon nanotube intrinsic mobility and semiconductortype carbon nanotube mobility is calculated based on this information.This approach contributes to a deeper understanding of the internal characteristics of CNTFETs.At last a comprehensive smallsignal equivalent circuit model for CNTFETs is established and a corresponding parameter extraction procedure is developed.4.Microwave performance analysis and circuit design of CNTFETs.To date,research on CNTFETs in the field of highfrequency and microwave circuits are at an early stage,and their application in microwave circuits still presents a number of challenges.To address this issue,this dissertation investigates the performance metrics of CNTFETs in the microwave domain using experimentally measured Sparameters.Using the smallsignal equivalent circuit model established in previous sections,the impact of various device parameters on microwave performance is quantitatively analyzed and validated by electrostatic doping experiments.The results show that electrostatic doping significantly improves the gain of the device and increases the maximum operating frequency by 44%.To determine the feasibility of microwave circuits based on carbon nanotubes,two microwave amplifier chips are designed and simulated using measured Sparameters.Finally,using existing devices,a microwave amplifier module based on CNTFETs is successfully fabricated and tested,achieving a breakthrough gain of 10.9 d B at a high frequency of 9 GHz.