| AlGaN/GaN heterostructure field-effect transistors (HFETs) are very representative electron devices for the third generation semiconductor. The wide band gap and the stable chemical properties of GaN material induce high breakdown voltage, high radioresistance and high resistant to corrosion for AlGaN/GaN electron devices. What’s more, due to the spontaneous polarization and the piezoelectric property, two-dimensional electron gas (2DEG) is induced at the AlGaN/GaN heterostructure interface without any doping. The 2DEG is highly quantum confined at the interface. And the sheet density and carrier mobility are very high. So, AlGaN/GaN HFETs are with great development potential and application prospect in microwave power and power electronics fields. With the development for more than 20 years, AlGaN/GaN electron devices have developed much. However, there are still some challenges. The further improvement for high frequency and high power and the reliability are some of the most important issues.The polarization is a very important characteristic for GaN based electron devices. The irregularly distributed barrier layer strain will induce an irregular distribution of polarization charges, and will induce polarization Coulomb field (PCF) scattering. This scattering is specific for GaN based electron devices. Some studies have shown that this scattering is critical for the low field carrier mobility and device reliability. But due to the thickness of barrier layer, the strain and polarization cannot be measured directly and experimentally. Moreover, due to converse piezoelectric effect, different bias conditions correspond to different distributions for strain and polarization, and thus different PCF scattering potential for the electrons in the gate-source channel, and thus different source access resistance Rs and extrinsic transconductance gm. However, until now, PCF scattering has not been taken into account to analyze Rs and gm. In addition, due to the lattice and thermal misfits between the substrate and the epilayers, there is residual strain in the GaN layer for the AlGaN/GaN heterostructures, which is one of the factors that affect the reliability of AlGaN/GaN HFETs. The trap in GaN buffer layer is one of the reasons for the OFF-state leakage current, which is quite unfavourable for switching application. This dissertation focuses on these issues above, striving to promote the development of AlGaN/GaN HFETs. The content of this dissertation is as follows:1. Determination of the additional polarization charge distribution at the heterostructure interface and analysis of barrier layer strain energy in AlGaN/GaN heterostructure field-effect transistorsAlGaN/GaN devices with micron and submicron gate length were fabricated. With the measured capacitance-voltage (CV) and current-voltage (IV) curves, the low-field electron mobility is obtained. With the scaling down of device size, PCF scattering plays a more and more important role in the carrier transport, comparing with the devices with larger sizes in the previous study. For the devices with a same drain-source distance, PCF scattering gets stronger with the decrease of gate length.With the relationship between PCF scattering and polarization charge distribution at the AlGaN/GaN interface, using the obtained low-field mobility and the theoretical model for PCF scattering, a self-consistent iterative method is proposed to determine the polarization charge distribution at the AlGaN/GaN interface and the strain distribution of AlGaN barrier layer through the whole area of drain-source space. The relationship between polarization/strain, voltage drop through the barrier layer under the gate and device size is figured out, and is demonstrated by the low-field mobility of other devices with different sizes.The strain energy of the whole AlGaN barrier layer is also analyzed, based on the obtained releationship. In the-2 V to 0 V range, gate bias induces a decrease of AlGaN barrier layer tensile strain and strain energy under the gate, but also induces an increase of barrier layer tensile strain in the gate-source and gate-drain region. So, the strain energy for the whole barrier layer keeps relatively constant.2. Influence of polarization Coulomb field scattering on source access resistance and extrinsic transconductance in AlGaN/GaN heterostructure field-effect transistorsThree AlGaN/GaN HFET devices with 20μm drain-source distance were fabricated. The gate length is 12 p.m,4μm and 4μm, respectively, and the gate-source distance is 4μm,4μm and 8μm, respectively. Under different forward gate-source current IGS, using traditional gate-probe method, the source access resistance Rs for the three devices is measured. All the Rs values decrease with the increase of IGS.-This phenomenon is attributed to the increase of the positive additional polarization charges under the gate with the increase of forward IGS.A greater degree of an offset is induced for the negative additional polarization charges near the drain and source Ohmic contacts, inducing a weaker PCF scattering and a decrease of Rs. A longer gate length or shorter gate-source distance is accompanied by a stronger influence of the additional polarization charges under the gate and a larger degree of the Rs decrease. Different extrinsic transconductance gm behaviors for the three devices also demonstrate the influence of PCF scattering on Rs and gm.Then, another AlGaN/GaN device is chosen for a further study. This device is with a mainstream size for microwave power application. The drain-source distance is 2 p.m and the gate length is 100 nm. It will be accompanied by a more remarkable hot carrier and hot phonon effect. Based on the traditional gate-probe method, an improved method is proposed to determine the Rs value corresponding to each different direct current quiescent point (DCQP) in the saturation region of the current-voltage output characteristic. The Rs values obtained from this method correspond to the additional polarization charge distributions under the DCQPs, and the results show a nonlinear increase of Rs with the increase of IDS.Using the obtained Rs values, the additional polarization charge distribution corresponding to each DCQP at the AlGaN/GaN heterostructure interface is obtained. The additional polarization charges influence the carrier transport for the electrons in the gate-source channel, and thus influence Rs. The expressions for this mechanism are presented to reflect the influence of PCF scattering on Rs.With the expressions and the obtained Rs values, taking other scattering mechanisms into consideration, including polar optical phonon (POP) scattering, piezoelectric (PE) scattering and interface roughness (IFR) scattering, the contributions of these scatterings to Rs are obtained. It is found that PE and IFR scatterings have little influence on Rs, and hardly change with IDS.It is mainly the POP and PCF scatterings that influence Rs. With the increase of IDS, carrier temperature increases and thus POP temperature increases, due to so strong interaction between electrons and POP. So the intensity of POP scattering quickly gets stronger, and thus the influence of POP scattering on Rs quickly gets stronger. The Rs proportion for POP scattering increase quickly with IDS. However, with the increase of IDS, the absolute value of the additional polarization charges at the AlGaN/GaN heterostructure interface under the gate decreases, inducing a weaker PCF scattering to the electrons in the gate-source channel. So the Rs proportion for PCF scattering decreases. Comparing with PE and IFR scatterings, the share of PCF scattering is much larger, and the biggest share is more than 50%, indicating that PCF scattering plays a very important role in the Rs values.Comparing with varying other device size parameters, such as drain-source distance and gate-source distance, varying the gate width can exclude the influence from other factors besides PCF scattering. So this will offer a more clear view for PCF scattering. Three AlGaN/GaN HFET devices were fabricated. The drain-source distance is 2μm and gate length is 100 nm. The gate is symmetrically placed in the middle between the source and the drain. The gate widths are 20μm,40μm and 75 μm, respectively. From the measured results, it is found that, with the increase of gate width, the gm peak value decreases. This is because that a wider gate width corresponds to stronger influence of the additional polarization charges under the gate, which induces a larger Rs value. Meanwhile, with the increase of gate width, the gm curve drops slower. The reason is as follows. With the increase of gate bias, the absolute value of the additional polarization charges under the gate decreases, and PCF scattering gets weaker. The Rs proportion for PCF scattering decreases, which offsets or partially offsets the increase of Rs proportion for POP scattering. So Rs varies slower. A wider gate width corresponds to a stronger PCF scattering and a larger offset. So the gm curve for the wider gate width drops slower.Based on the analysis above, an approach to improve the AlGaN/GaN device performance by taking advantage of PCF scattering is proposed. Enhancing the influence of PCF scattering can modulate the Rs variation and thus flat the gm curve to improve the performance of the large signal linearity. From the perspective of heterostructure material, thinning the AlGaN barrier layer can enhance the electric field under a same gate bias, and increasing Al-component can change the elastic coefficient and piezoelectric coefficient and thus increase the additional polarization charges under the gate, both enhancing PCF scattering. From the perspective of device structure, a longer gate length or a wider gate width corresponds to a stronger influence of the additional polarization charges at the AlGaN/GaN heterostructure interface. And a shorter gate-source distance corresponds to a shorter distance between the additional polarization charges at the AlGaN/GaN heterostructure interface under the gate and the carriers in the gate-source channel. So the PCF scattering gets stronger. Meanwhile, it should be noted that, enhancing PCF scattering makes the total scattering stronger, inducing a larger Rs value and a smaller gm peak value. As a result, there is a tradeoff between a flatter gm curve and higher gm peak value, and different designs can be made for different needs.3. Influence of substrate thickness and substrate bias on the characteristics of AlGaN/GaN heterostructure field-effect transistorsAn AlGaN/GaN heterostructure field-effect transistor with 5μm drain-source distance,1μm gate length and 1μm gate-source distance was fabricated on a sapphire substrate with an original thickness of 420μm. The substrate thickness is thinned and the characteristics under different substrate thickness were measured to investigate the influence of sapphire substrate thickness on the device performance. It is found that, when the substrate thickness is more than 170μm, the electrical properties of the device hardly change. When the substrate thickness is less than 170 μm, the electrical properties change:the drain-source current decreases, the 2DEG sheet density decreases and the threshold voltage increases. Raman and photoluminescence (PL) measurements were performed to investigate the strain of the GaN buffer layer, indicating an increase of the compressive strain after the thinning of substrate. By self-consistently solving Schrodinger’s and Poisson’s equations, the polarization charge sheet density at the AlGaN/GaN heterostructure interface is obtained. It also indicates that, when substrate thickness is more than 170 μm, the polarization charge sheet density keeps almost constant, and when substrate thickness is less than 170μm, the density decreases. Atomic force microscopy measurement for the AlGaN surface morphology was performed on a 2x2μm2 area between the gate and the drain. After substrate thinning, the AlGaN surface roughness increases, indirectly indicating the increase of dislocation density and dislocation scattering strength. The carrier mobility under 70μm substrate thickness is larger than that under 120μm substrate thickness. It suggests a reduction of PCF scattering, and it is consistent with the variation of polarization charges.Besides the strain of GaN buffer layer, the traps in the GaN buffer layer can also influence the device performance. Based on this, a method is proposed to improve the switching characteristics of the AlGaN/GaN electron device by applying voltage bias on substrate. It is found that, positive substrate bias can effectively reduce the OFF-state leakage current, and keep the ON-state drain-source current constant. So the switching characteristics are improved. The reason for this phenomenon is attributed to the interaction between the electric field induced by the substrate bias and the traps in the GaN buffer layer. The positive substrate bias can induce the ionization of acceptor traps and (or) the deionization of donor traps, reducing the conductivity of the GaN buffer layer and the leakage current. Moreover, the positive substrate bias will drive the electrons in the GaN buffer layer to the bottom of GaN buffer layer or even AlN nucleation layer, and the holes to top. It will further reduce the conductivity of the GaN buffer layer. After a thinning of substrate thickness, a same substrate bias will induce a larger electric field, and the interaction with the traps in the GaN buffer layer will be stronger. So the improvement for the switching characteristics is more remarkable.
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