A new 4H silicon carbide metal semiconductor field-effect transistor (4H-SiC MESFET) structure with a buffer layer between the gate and the channel layer is proposed in this paper for high power microwave applications. The physics-based analytical models for calculating the performance of the proposed device are obtained by solving one- and two-dimensional Poisson's equations. In the models, we take into account not only two regions under the gate but also a third high field region between the gate and the drain which is usually omitted. The direct-current and the alternating-current performances for the proposed 4H-SiC MESFET with a buffer layer of 0.2 μm are calculated. The calculated results are in good agreement with the experimental data. The current is larger than that of the conventional structure. The cutoff frequency (fT) and the maximum oscillation frequency (fmax) are 20.4 GHz and 101.6 GHz, respectively, which are higher than 7.8 GHz and 45.3 GHz of the conventional structure. Therefore, the proposed 4H-SiC MESFET structure has better power and microwave performances than the conventional structure.

The Raman shifts of Si—Si， Ge—Ge and Si—Ge in Si（1-x）Gex alloys are calculated by Keating model. The calculated Raman shifts are 40275，41339 and 38815 cm-1 when the concentrations of Ge are 01， 05 and 09 respectively. These results are consistent with the reported experimental results， which indicates the validity of the Keating model for obtaining the Raman frequency of strained materials by changing the elastic coefficients of stretching and compression and bond-bending interaction. The single-phonon scattering peak at 477029 cm-1 in amorphous silicon is obtained for the first time by Keating model, which is in agreement with the result of 4800 cm-1 from the literature, indicating that the atoms of amorphous silicon as a whole are stretched compared with that of crystalline silicon.

In this paper, experimental results are reported about the new Al0.25Ga0.75N/GaN high electron mobility transistor (HEMT) with a step AlGaN layer. The rule of 2DEG concentration variation with the thickness of AlGaN epitaxial layer has been applied to the new AlGaN/GaN HEMTs: The step AlGaN layer is formed at the gate edge by inductively coupled plasma etching, the 2DEG concentration in the etched region is much lower than the other parts of the device. A new electric field peak appears at the corner of the step AlGaN layer. The high electric field at the gate edge is decreased effectively due to the emergence of the new electric field peak, and this optimizes the surface electric field of the new AlGaN/GaN HEMTs. The new devices have the same threshold voltage and transconductance as the conventional structure, -1.5 V and 150 mS/mm. That means, the step AlGaN layer does not affect the forward characteristics of the AlGaN/GaN HEMTs. As the more uniform surface electric field distribution usually leads to a higher breakdown voltage (BV), with the same gate to drain length LGD=4 m, the BV can be improved by 58% for the proposed Al0.25Ga0.75N/GaN HEMTs as compared with the conventional structure. At VGS=1 V, the saturation currents (Isat) is 230 mA/mm for the conventional Al0.25Ga0.75N/GaN HEMT and 220 mA/mm for the partially etched Al0.25Ga0.75N/GaN HEMT (LEtch=4 m, LGD=4 m). The decrease of Isat is at most 10 mA/mm. However, as the BV has a significant enhancement of almost 40 V, these drawbacks are small enough to be acceptable. During the pulse I-V test, the current collapse quantity of the conventional structure is almost 40% of the maximum IDS(DC), but this quantity in the new devices is only about 10%, thus the current collapse effect in Al0.25Ga0.75N/GaN HEMTs has a significant remission for a step AlGaN layer. And as the high electric field peak at the gate edge is decreased, the effect of the gate electrode on electron injection caused by this electric field peak is also included. The injected electrons may increase the leakage current during the off-state, and these injected electrons would form the surface trapped charge as to decrease the 2DEG density at the gate. As a result, the output current and the transconductance would decrease due to the decreased electron density during the on-state. That means, with the region partially etched, the electron injection effect of the gate electrode would be remissed and the stability of Schottky gate electrode would be improved. In addition, due to the decrease of the high electric field at the gate edge, the degradation of the device, which is caused by the high electric field converse piezoelectric effect, will be restrained. The stability of the partially etched AlGaN/GaN HEMT will become better.

In order to optimize the surface electric field of the traditional AlGaN/GaN high electron mobility transistor and improve the breakdown voltage and reliability, a new AlGaN/GaN high electron mobility transistor is proposed with the partial fixed positive charges in the Si3N4 passivation layer in this paper. The partial fixed positive charges of the Si3N4 passivation layer do not affect the polarization effect of the AlGaN/GaN heterojunction. The surface electric field tends to the uniform distribution due to the new electric field peak formed by the partial fixed positive charges, which modulates the surface electric field by applying the electric field modulation effect. The high electric fields near the gate and drain electrode decrease due to the new electric field peak. The breakdown voltage is improved from the 296V for the traditional structure to the 650V for the new structure proposed. The reliability of the device is improved due to the uniform surface electric field. The effect of the electric field modulation is explained by the horizontal and vertical electric field distribution between the Si3N4 and AlGaN interface, which provides a scientific basis for designing the new structure with the partial fixed positive charges in the Si3N4 layer. Because of the fixed positive charge compensation, the two-dimensional electron gas concentration increases, and the on-resistance decreases. So, the output current of the new structure increases compared with that of the traditional AlGaN/GaN High Electron Mobility Transistor.

In order to alleviate the leakage current of AlGaN/GaN High Electron Mobility Transistors (HEMT) device with the N-type GaN buffer, the new Al0.25Ga0.75N/GaN HEMT with the Fluoride ion implantation is proposed for the first time in this paper. Firstly, the output characteristic has the ohmic characteristic for the AlGaN/GaN HEMT without acceptor-type trap, which explains why Fe and Mg are doped into the GaN buffer layer as reported in the literature in theory and simulation. By using the output characteristics of the Ids-Vds for the AlGaN/GaN HEMTs with and without low density drain, the results are obtained that fluoride ion implantation can capture effectively the electrons emitted from the source to reduce the leakage current of the GaN buffer compared with fluoride ions in the gate and the drain regions. The breakdown voltage goes up to 262 V. The scientific basis is set up for desiging the new AlGaN/GaN HEMT with both the low leakage current and the high breakdown voltage.