| GaN-based heterostructure field-effect transistors(HFETs)are a promising new generation of semiconductor devices.Offering superior advantages such as high electron mobility and a high critical breakdown electric field,they possess broad application prospects in high-frequency and high-power fields,and are the core electronic components that support the next wave of technologies in wireless communications,aerospace,new energy vehicles and other high-tech industries.GaN-based HFETs can also assist a country in meeting major strategic needs,as they are conducive to the innovation,development,transformation and progress of national industries,and have important research value.After nearly 30 years of development efforts,the physical mechanisms of GaN-based HFETs are gradually being understood,and long-term explorations have been conducted on GaN-based HFETs material options,structures and preparation processes.Device performance has also greatly improved,and the commercialization of GaN-based HFETs is progressing,with commercial chip products already available in some areas.However,despite these advancements,GaN-based HFETs still possess certain drawbacks that restrict further commercialization efforts,such as strong polarization effects,surface state problems and current collapse,all of which have a strong impact on device reliability and need to be urgently addressed.At the present time,even enhanced-mode GaN-based HFET technology is not mature enough for commercial applications.Other aspects of GaN-based HFETs also require further research,such as the relationship between polarization Coulomb field(PCF)theory and GaN-based HFET performance,which includes the relationship between device barrier layer optimization,enhanced device performance,PCF scattering and so on.Such studies would be of great significance for clarifying the physical mechanisms of GaN-based HFETs and further enhancing their performance capabilities.A strong polarization effect is an important feature that distinguishes GaN materials from other semiconductor materials,and it has a strong impact on the performance of GaN-based HFETs.Without doping,high-density(?1×1013cm-2)two-dimensional electron gas(2DEG)can be generated at the interface of GaN-based HFET heterostructures.PCF scattering is a scattering mechanism closely related to the strong polarization effect in GaN-based HFETs.The scattering of electrons is caused by the perturbation scattering potential,which is produced by the non-uniform strain distribution in the barrier layer.After the mechanism was first proposed,numerous studies have been conducted on how PCF scattering is influenced by device process and device size,which have shown that it has an important impact on device performance and reliability.However,the theoretical model of PCF scattering still needs further improvement.The optimization of a device's barrier layer is an important way to enhance performance.The strong polarization effect in the AlGaN barrier layer of AlGaN/GaN HFETs is affected by the barrier layer' s Al composition and thickness,which in turn will affect the performance of AlGaN/GaN HFET devices.However,the relationship between these influencing factors and PCF scattering is not well understood and urgently needs further clarification.When developing GaN-based HFETs for ultra-high frequency operation,it is necessary to scale the device size.To maintain a larger gate length to barrier layer thickness ratio and avoid short channel effects,AlN/GaN HFETs with an ultra-thin AlN barrier layer have been developed.Although such devices are becoming one of the most promising GaN-based HFET structures for ultra-high frequency applications,the relationship between the strong polarization effect and ultra-thin thickness of AlN materials and PCF scattering is not known and requires further investigation.Conventional AlGaN/GaN HFETs are depletion-mode(D-mode)devices due to the high density 2DEG in AlGaN/GaN heterostructures.In power conversion applications,when the power converter is turned on,a negative gate bias must be applied to the gate of the power device to keep the device channel off;otherwise,the device will short-circuit.Therefore,to fulfill the requirements of simple structure,low power consumption and low cost,it is necessary to utilize enhanced-mode(E-mode)devices,and among such devices,enhanced P-GaN/AlGaN/GaN HFETs have demonstrated the greatest potential for commercial application use.As the effect of PCF scattering on the performance of P-GaN/AlGaN/GaN HFETs has never been reported,this dissertation aims to deeply explore this issue by conducting a combined investigation on the PCF scattering and current problems of GaN-Based HFETs.First,PCF scattering with the electron system was examined via comparative analysis,and the PCF scattering theoretical model was further developed and improved upon.Next,based on the model,the relationship between the Al composition and thickness of the AlGaN barrier layer and the PCF scattering in AlGaN/GaN HFETs was determined,providing a new idea for barrier-layer optimization.Then,the relationship between the extremely strong polarization effect and ultra-thin thickness of the AlN barrier layer and PCF scattering in the AlN/GaN HFETs was analyzed.Finally,the influence mechanism of PCF scattering on the electrical performance of P-GaN/AlGaN/GaN HFETs was thoroughly investigated.Our study is the first to discover the gate bias dependency of Rs for E-model P-GaN/AlGaN/GaN HFETs due to PCF scattering.Throughout our research,we tried to enhance and develop the PCF scattering theory,deeply understand the interaction mechanism between the strong polarization effect and performance of GaN-Based HFETs,and provide a new theoretical basis for improving device reliability and optimizing device performance.The content of this dissertation is as follows:1.Polarization Coulomb field scattering with the electron systems in AlGaN/GaN heterostructure field effect transistorsAlGaN/GaN HFETs with varying gate lengths of200nm,100nm and 30nm,a gate-source distance of 1?m,and gate width of 40×2?m were fabricated.Since the PCF scattering Hamiltonian selection principle has not been clarified yet,two methods were used to analyze and calculate the PCF scattering in AlGaN/GaN HFETs:considering the 2DEG of the gate-source,gate-drain,and gate regions as three independent electron systems,and considering the 2DEG of the drain-source channel as a unified electron system.Combined with current-voltage(I-V)data and based on PCF scattering theory,the electron mobility and additional polarization charges were calculated through an iterative process by analyzing which of the two methods was more reasonable from the perspective of additional polarized charges.Since the 2DEG density underneath the gate region will decrease with decreasing gate bias(VGS),regarding the 2DEG of the drain-source channel as a unified electron system cannot consider variations in 2DEG density under the gate with changing VGS.Furthermore,the 2DEG density when VGS is 0V is used for the ground state wave function,which overestimates the 2DEG density and affects the accuracy of the calculations,and the larger the gate length and the smaller the gate bias,the greater the impact it has on the results.For the other method,the drain-source channel 2DEG was divided into three electronic systems:the gate-source electron system,the under-gate electron system and the gate-drain electron system.When calculating the PCF scattering of each electronic system,its polarization charge was used as the benchmark,and the difference between the polarization charges of the other two systems and that of the benchmark was used as the additional scattering potential to produce the PCF scattering of the electrons in each system.This method,which can fully consider variations in 2DEG density under the gate with changing VGS,can provide a more accurate reflection of the PCF scattering mechanism and generate more precise calculations,and the results obtained from this method perfected the PCF scattering theory.2.The relationship between the Al composition of the AlGaN barrier layer and polarization Coulomb field scattering in AlGaN/GaN heterostructure field-effect transistorsBased on PCF scattering theory,the relationship between the Al composition of the AlGaN barrier layer and PCF scattering in AlGaN/GaN HFETs was determined.AlGaN/GaN HFETs of the same size(gate length of 0.6?m,gate-source[gate-drain]distance of 1.5?m and gate width of 40×2?m)were fabricated on AlGaN/GaN heterostructure materials with differing Al compositions of 0.17 and 0.26 in the AlGaN barrier layer.Using the current-voltage(I-V)and capacitance-voltage(C-V)data obtained from experimental measurements,the electron mobility and additional polarization charge under the gate were calculated based on the PCF scattering theoretical model.Through a comprehensive analysis of the additional polarization charge,electron mobility and 2DEG density,it was found that increasing the Al composition of the AlGaN barrier layer increased the additional polarization charge and 2DEG density,indicating that both of these factors have an important impact on the intensity of PCF scattering.Since adding additional polarization charges produces a stronger PCF scattering potential,therefore,this factor enhances PCF scattering;however,as PCF scattering is a Coulomb scattering,which is highly sensitive to 2DEG density,when the 2DEG density is higher,the triangle quantum well of AlGaN/GaN heterostructure is deeper,the kinetic energy of 2DEG electron is larger,the scattering effect of PCF scattering potential on 2DEG electrons is weaker.And when the 2DEG density is high,the stronger Coulomb shielding effect will also weaken the PCF scattering intensity.It can be inferred that increasing the additional polarization charge and 2DEG density have opposing effects on PCF scattering.In this study,the increase in 2DEG density had a greater impact on PCF scattering than the increase in additional polarization charges,and therefore,it can be deduced that the intensity of PCF scattering increases or decreases with increasing Al composition,depending on which influencing factor is more dominant(additional polarization charges or 2DEG density).The relationship between the Al composition of the AlGaN barrier layer and PCF scattering in AlGaN/GaN HFETs was also determined,presenting a new idea for optimizing the Al composition of the AlGaN barrier layer and adjusting the PCF scattering intensity according to different needs.These findings provide guiding value for the optimization of AlGaN/GaN HFET materials and device structures.3.The relationship between AlGaN barrier layer thickness and polarization Coulomb field scattering in AlGaN/GaN heterostructure field-effect transistorsBased on PCF scattering theory,the relationship between AlGaN barrier layer thickness and PCF scattering in AlGaN/GaN HFETs was established.AlGaN/GaN HFETs of the same size(gate length of 4 ?m,gate-source[gate-drain]distance of 10?m and gate width of 70 ?m)were fabricated on AlGaN/GaN heterostructure materials with different AlGaN barrier layer thicknesses of 15.5 nm,19.3 nm and 24.7 nm.Using the I-V and C-V data obtained from experimental measurements,the electron mobility and additional polarization charge under the gate were calculated based on the PCF scattering theoretical model.Through an in-depth analysis of the additional polarization charge,electron mobility and 2DEG density,the correlation between AlGaN barrier layer thickness and PCF scattering in AlGaN/GaN HFETs was determined.For samples with a thicker AlGaN barrier layer,when the same gate bias voltage was applied to the gate,the electric field intensity under the gate was smaller and the inverse piezoelectric effect of the barrier layer under the gate was weaker,which resulted in fewer additional polarized charges being generated under the gate,indicating that fewer additional polarization charges correspond to weaker PCF scattering potential.Moreover,the sample with a thicker AlGaN barrier layer possessed a higher 2DEG density.Since PCF scattering is a scattering mechanism that is very highly sensitive to 2DEG density,a higher 2DEG density will weaken the PCF scattering intensity;and as a result,the increase in AlGaN barrier layer thickness reduces the additional polarization charge under the gate and increases the 2DEG density,both of which will weaken the PCF scattering intensity.This implies that the thicker the AlGaN barrier layer,the weaker the PCF scattering(without strain relaxation);and therefore,the PCF scattering intensity can be reduced by increasing the AlGaN barrier layer thickness.This study clarified the correlation between the AlGaN barrier layer thickness and PCF scattering in AlGaN/GaN HFETs,providing a new idea for optimizing AlGaN barrier layer thickness and suppressing the PCF scattering intensity.The results of this study are also beneficial toward the optimization of AlGaN/GaN HFETs materials and device structures.4.The relationship between the AlN barrier layer and polarization Coulomb field scattering in AlN/GaN heterostructure field effect transistorsIn this section,the relationship between the extremely strong and ultra-thin AlN barrier layer and PCF scattering in AlN/GaN HFETs was determined.Sub-micron T-gate AlN/GaN HFETs with varying gate lengths of 0.5 ?m and 0.6 ?m,a gate-source(gate-drain)distance of 1.7 ?m,and gate width of 40×2 ?m were fabricated on AlN/GaN heterostructure materials with ultra-thin AlN barrier layers.Based on PCF scattering theory,the additional polarization charge and electron mobility were calculated.On the one hand,as the AlN barrier layer of AlN/GaN HFETs is ultra-thin,after the gate bias was applied to the gate,it was found that the electric field strength in the AlN barrier layer was very strong and the piezoelectric coefficient of the AlN material was large,indicating that the inverse piezoelectricity has a very strong effect.The large amount of additional polarization charges produced under the gate will generate a larger PCF scattering potential,thereby enhancing the PCF scattering.However,the inverse piezoelectric effect produced by the gate bias cannot cause the infinite strain in the AlN barrier layer,and as gate bias decreases,the additional polarization charge will saturate.On the other hand,the 2DEG density under the gate was quickly depleted due to the thinness of the AlN barrier layer,demonstrating that as gate bias is reduced,the 2DEG density under the gate rapidly decreases,and the decreasing 2DEG density will increase the effect of the additional scattering potential on the 2DEG,therefore increasing the PCF scattering intensity.By determining the relationship between the strong polarization effect and ultra-thin thickness of the AIN barrier layer,and the PCF scattering in AlN/GaN HFETs,a new theoretical basis is provided for the optimization of AlN/GaN HFET materials and device structures.5.The influence of polarization Coulomb field scattering on the parasitic source resistance of E-model P-GaN/AlGaN/GaN heterostructure field effect transistorsEnhanced-mode(E-mode)P-GaN/AlGaN/GaN HFETs with different device sizes were fabricated on Si(111)substrate,and the influence mechanism of PCF scattering on the parasitic source resistance(Rs)of the devices was explored.The Rs corresponding to the different gate biases of each sample was obtained by the gate-probe method.It was found that Rs varied greatly with changing gate bias,and the degree of Rs change with the gate bias of different-sized device samples also varied,indicating that the change of Rs is determined by the electron mobility in the gate-source region.The additional polarization charge generated by the device process and the gate bias produces an additional perturbation potential that has a scattering effect on the electrons in the gate-source region.It was also found that device size and gate bias affect the intensity of PCF scattering,which will lead to varying electron mobility in the gate-source regions of different-sized devices and the gate-source region of same-sized devices under different gate biases.Our study is the first to discover the gate bias dependency of Rs for E-model P-GaN/AlGaN/GaN HFETs due to PCF scattering,providing a new idea for further in-depth studies on the Rs of E-model P-GaN/AlGaN/GaN HFETs and device performance optimization. |
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