| With the widely application of the microwave technology, especially the rapid development of wireless communications, aerospace and modern defense electronics, the power performance, frequency performance, high temperature resistant characteristics as well as the radiation resistant characteristics should be further improved. As a new third generation of semiconductor materials, GaN has many excellent physical properties, such as wide band gap, high breakdown electric field, high electron saturated velocity, high thermal conductivity, small permittivity, steady chemical properties and strong radiation resistant capacity, so it is the preferred material of high temperature, high frequency, high power microwave electronics field. The main application in high frequency and high power field is high electron mobility transistors (HEMTs), or called heterostructure field-effect transistors (HFETs)In GaN HFETs, the2DEG electron mobility is an important parameter, it directly decides the frequency characteristics and power characteristics of the devices, so the study of the2DEG electron mobility is of great importance. Polarization Coulomb field scattering was first put forward by Jianzhi Zhao in2007, and it has been pointed out that the polarization Coulomb field scattering is an important scattering mechanism in the AlGaN/GaN HFETs, AlGaN/AlN/GaN HFETs, InAlN/AlN/GaN HFETs and AlN/GaN HFETs. However, those researches were all made with the low channel electric field (corresponding to0.1V drain-source bias, Fo=0.1V). In fact, different biases will be applied at the source and drain contacts when devices are working, to our knowledge, the relationship between the polarization Coulomb field scattering and the channel electric field distribution has not been studied. As a new scattering mechanism of the GaN electronic devices, the interaction mechanism of the polarization Coulomb field with the carrier should be further studied. The main work in this dissertation is to study the influence of the channel electric field and device size on the2DEG electron mobility for different GaN HFETs, especially the influence on the polarization Coulomb field scattering, so as to establish the theory model of the polarization Coulomb scattering mechanism. The main conclusions of the dissertation are listed below:1. The2DEG electron mobility in the linear region of the AlGaN/AlN/GaN HFETs. We prepared four AlGaN/AlN/GaN HFETs samples, the drain-to-source distance of the samples are all100μm and the gate lengths are different, then the C-V characteristics and Ⅰ-Ⅴ characteristics are measured. Based on the test data, the low field mobility at the drain end of the channel for different drain-source voltage was calculated, and then the fitting mobility was obtained by nonlinear fitting. Based on the fitting mobility, a mobility model related to the electric field was given. We simulated the Ⅰ-Ⅴ characteristics of the AlGaN/AlN/GaN HFETs using the Quasi-2-D model, and obtained the2DEG sheet density, electron mobility, channel electric field and potential at any position of the channel. By analyzing the variation of the electron mobility with the drain-source voltage and electric field in the AlGaN/AlN/GaN HFETs, we found that the polarization Coulomb field scattering still plays an important role in the linear region of the AlGaN/AlN/GaN HFETs. Gate-source voltage and drain-source voltage can change the2DEG sheet density and polarization charge distribution, and then affect polarization Coulomb field scattering.2. The influence of the channel electric field distribution on the electron mobility in the AlGaN/AlN/GaN HFETs.(a) Based on the test data, Ⅰ-Ⅴ characteristics of different gate length AlGaN/AlN/GaN HFETs were calculated using the Quasi-2-D model. It is found that when the2DEG sheet density was modulated by drain-source voltage, with the different samples, there are the electron mobility peaks which are corresponding to the different2DEG sheet density. The radio of the gate length to drain-to-source distance is smaller, the2DEG sheet density at the peak points is higher. With the increase of the2DEG sheet density, the electron mobility difference between the curves when the2DEG sheet density was modulated by drain-source voltage and the curves when the2DEG sheet density was modulated by gate-source voltage gradually decreases, and finally tends to the same value for the electron mobility. The electron mobility difference at the peak points for the four AlGaN/AlN/GaN HFETs can reach up to1829.9cm2/V·s (sample a),1226cm2/V·s (sample b),1594.3cm2/V·s (sample c) and1033.1cm2/V·s (sample d), respectively. We analyzed the channel electric field distribution at the peak points, and calculated the piezoelectric polarization of the AlGaN barrier at different channel position. It is found that there are significant differences in the channel electric field distribution and the piezoelectric polarization of the AlGaN barrier in both cases. Since the elastic scattering potential of the polarization Coulomb field is proportional to the absolute value of the negative polarization charge density difference at the AlGaN/AIN interface, and all the negative polarization charges along the channel scatter the2DEG electrons. While the total amount of negative polarization charge density difference when the2DEG sheet density was modulated by gate-source voltage is much more than that when the2DEG sheet density was modulated by drain-source voltage, so the polarization Coulomb field scattering is stronger, and the electron mobility is lower. So it can be concluded that the electron mobility difference in both cases is caused by polarization Coulomb field scattering, and the channel electric field distribution has an important influence on the polarization Coulomb field scattering.(b) By Analyzing the2DEG electron mobility of the AlGaN/AlN/GaN HFETs with different drain-to-source distance (100μm and20μm), it was found that when the ratio of the gate length to the drain-to-source distance is larger (more than1/2), the polarization Coulomb field scattering is weaker, the LO phonon scattering and interface roughness scattering play an important role at higher2DEG sheet density, so the electron mobility increases with the decrease of the2DEG sheet density (increase of the channel electric field). When the2DEG sheet density reduces to a certain value, the polarization Coulomb field scattering begins to dominate, which results in the decrease of the electron mobility with the decrease of the2DEG sheet density, so the electron mobility curve appears a peak. But as to the device with smaller ratio of the gate length to drain-to-source (less than1/2), the polarization Coulomb field scattering is stronger, so the electron mobility increase with the increase of2DEG sheet density. Conclusions can be made that with the increase of the channel electric field, the scattering mechanism which influences the electron mobility is related to the ratio of the gate length to drain-to-source distance, the ratio is larger, the polarization Coulomb field scattering is weaker, and just the reverse for the smaller ratio devices. For the devices with drain-to-source distance as60μm, when the2DEG sheet density was modulated by drain-source voltage, although the2DEG electron mobility of all the samples appear peaks with the variation of2DEG sheet density, but it can be seen that the ratio of the gate length to drain-to-source distance is smaller, the2DEG sheet density at the peak is higher, which also shows clearly that the ratio of the gate length to drain-to-source distance is smaller, the polarization Coulomb field scattering is stronger.3. The study of the electron mobility in the InAlN/AlN/GaN HFETs and the AIN/GaN HFETs. (a) We simulated the I-V characteristics of different gate length InAlN/AlN/GaN HFETs, and the results show that the simulation method and mobility model used in the AlGaN/AlN/GaN HFETs are also applicable to the InAlN/AlN/GaN HFETs. By analyzing the electron mobility of the InAlN/AlN/GaN HFETs, we found that similar conclusions as the AlGaN/AlN/GaN HFETs can be made. When the2DEG sheet density is modulated by drain-source voltage, the electron mobility appears peak with the2DEG sheet density. The ratio of the gate length to drain-to-source is smaller, the2DEG sheet density is higher at the peak. With the increase of the2DEG sheet density, the electron mobility difference between the curves when the2DEG sheet density was modulated by the drain-source voltage and the curves when the2DEG sheet density was modulated by the gate-source voltage gradually reduces and finally tends to the same value for the electron mobility. The electron mobility differences at the peaks can reach up to1522.9cm2/V·s(sample1),1241.1cm2/V·s(sample2) and1273.2cm2/V·s(sample3), respectively. Although in the In0.18Al0.82N/AlN/GaN heterostructure, the InAIN barrier layer is lattice-matched to GaN, and there are not strain and piezoelectric polarization for the InAIN barrier layer, however, there is a thin AlN interlayer between the InAIN and GaN, the AlN interlayer is strained due to the lattice-unmatched to GaN. Therefore, due to the converse piezoelectric effect, the change of the channel electric field will induce the strain variation of the AlN interlayer in the In0.18Al0.82N/AlN/GaN heterostructures, then, the In0.18Al0.82N barrier layer will follow the strain variation of the AlN interlayer. As a result, the polarization charge density underneath gate metals will be changed by both the drain-source biases and the gate-source biases. With the above reason, the polarization charges at the In0.18Al0.82N/AlN interface are distributed irregularly. The difference between the nonuniform polarization with applied electrical field and the uniform polarization which is corresponding to zero bias constitutes the elastic scattering potential and sets up a polarization Coulomb field that scatters2DEG electrons. So conclusion can be made that in the InAlN/AlN/GaN HFETs, the polarization Coulomb field scattering is the main scattering mechanism and the channel electric field distribution has an important influence on the polarization Coulomb field scattering. With the different channel electric field distribution, the electron mobility difference appears between the curves when the2DEG sheet density was modulated by the drain-source voltage and the curves when the2DEG sheet density was modulated by the gate-source voltage. In addition, the ratio of the gate length to drain-to-source distance is smaller, the polarization Coulomb field scattering is stronger,(b) We simulated the I-V characteristics of four different gate length AIN/GaN HFETs and analyzed the variation of the electron mobility in AIN/GaN HFETs. With the sample a, b, and c, the ratio of the gate length to drain-to-source distance is larger, when the2DEG sheet density was modulated by drain-source voltage, the polarization Coulomb field scattering is screened at higher2DEG sheet density, the LO photon scattering and interface roughness scattering play the dominate role, so the electron mobility increases with the decrease of the2DEG sheet density. With the decrease of the2DEG sheet density, the shielding effect is weakening, and the polarization Coulomb field scattering will play a leading role at certain2DEG sheet density, so the electron mobility will reduces with the decrease of the2DEG sheet density, which result in the peak. As to the sample d, the ratio of the gate length to drain-to-source distance is smaller, the polarization Coulomb field scattering is stronger, so the electron mobility increase with the increase of2DEG sheet density. Similar to the AlGaN/AlN/GaN HFETs and InAlN/AlN/GaN HFETs, with the increase of the2DEG sheet density, the electron mobility difference between the curves when the2DEG sheet density was modulated by the drain-source voltage and the curves when the2DEG sheet density was modulated by the gate-source voltage gradually reduces and finally tends to the same value for the electron mobility. We calculated the channel electric field and the piezoelectric polarization of the A1N barrier layer, and found that they have obvious difference in both cases, based on the analysis, we can conclude that the electron mobility difference was caused by the polarization Coulomb field scattering, it is the same with the conclusions obtained in the AlGaN/AlN/GaN HFETs and InAlN/AlN/GaN HFETs. |
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