GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> II. GAN DEVICES FOR LATERAL INTEGRATION A. DEVICE PHYSICS AND STRUCTURE <s> Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ... <s> BIB001 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> II. GAN DEVICES FOR LATERAL INTEGRATION A. DEVICE PHYSICS AND STRUCTURE <s> A three-stage W-band GaN monolithic microwave integrated circuit power amplifier (MMIC PA) is reported. Electron-beam lithography has been employed to define a 100-nm T-shaped gate on the AlGaN/GaN HEMT structure with ultrahigh aluminum content. The MMIC PA offers a peak small signal gain of 16.7 dB in the 90–97 GHz bandwidth. Moreover, it achieves a peak 1.66-W (32.2 dBm) output power at 93 GHz in a continuous-wave mode, with an associated power added efficiency of 21% and an associated power gain of 13.7 dB. Most notably, the peak power density is 3.46 W/mm with a 480- $\mu \text{m}$ wide output stage, which exceeds 3 W/mm in AlGaN/GaN HEMT at W-band for the first time. <s> BIB002 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> II. GAN DEVICES FOR LATERAL INTEGRATION A. DEVICE PHYSICS AND STRUCTURE <s> In this paper, an analytical switching loss model for a gallium nitride (GaN) based control switch (CS) and synchronous rectifier (SR) in low-voltage buck converters is proposed. Based on the consideration of parasitic inductances, SR reverse conduction voltage drop, and overshoot current effect caused by output capacitance of SR, the total switching loss is modeled with regard to both CS and SR, which can lead to more accurate prediction of power loss and system efficiency in low-voltage buck converters. The modeling results show that the percentage of SR switching loss in total switching loss is 25.4% in the 12–1.2-V converter, which is far larger than the 6.3% in the 48–5-V converter, and this verifies the necessity of including SR into the model for low-voltage converters. Experiment results exhibit an average increase of 52.3% and 66.1% in the accuracy of predicted system efficiency and converter power loss through the proposed model with SR than conventional models from 400 kHz to 1 MHz. <s> BIB003
The advantages of GaN over other solid state materials are so apparent that the GaN-based electronic devices should be the better choice for high power applications including RF, microwave and power conversion applications. The GaN material has a noncentrosymmetric wurtzite structure. Due to the difference between electronegativity of Ga and N atoms, the centers of the positive and negative charge in electron cloud do not coincide, thus GaN shows spontaneous polarization. In peseudomorphically grown epitaxial AlGaN/GaN structure, there will be strains due to the lattice mismatch and dislocations, which lead to the piezoelectric polarization. For Ga-face AlGaN/GaN heterostructures, the polarization induced charge density values can be calculated BIB001 . Fig. 1 shows the calculated polarization charge and 2DEG density values with various Al mole fraction with an inset of basic AlGaN/GaN heterostructure. The 2DEG channel conduction is the key feature of lateral GaN devices, where the electron density can be around 1 × 10 13 cm −2 . The source, gate and drain electrode configuration in AlGaN/GaN based devices further paves the way of integration in the same manner as Complimentary-Metal-Oxide-Semiconductor (CMOS) or Lateral-Diffusion-MOS (LDMOS) process. Fig. 2 show the SEM images of AlGaN/GaN HEMTs specially designed for MMIC BIB002 and power conversion application  , respectively. BIB002 and (b) 2 µm gate length for power conversion application  . In order to get high power-gain cutoff frequency f max , the gate length can be designed as 100 nm, for example, in Fig. 2 (a). While for GaN HEMTs in power applications, the gate length as well as gate-drain separation are designed to be larger, such as 2 µm and 7.5 µm, respectively, in Fig. 2 (b). These lateral distance values, which are all larger than that of GaN HEMTs for MMIC application, can contribute to higher breakdown voltage for GaN power HEMTs. Despite the difference in in dimensions, the same heterostructure and electrode layout establish the common structure basis of lateral power integration schemes. For a single device, the switching frequency is related with its structure. However when devices are applied with external circuits or ICs, the parasitic inductance in gate drive loop also significantly affects the maximum operation frequency BIB003 . The parasitic resistance and inductance in all the interconnections, through-vias, wires and pads can degenerate the high frequency switching performance of GaN ICs. This is a critical aspect that current GaN integration technology are trying to improve.