X-Band GaN Power HEMTs with Power Density of 2.23W/mm Grown on Sapphire by MOCVD

The growth,fabrication,and characterization of 0.2μm gate-length AlGaN/GaN HEMTs,with a high mobility GaN thin layer as a channel,grown on (0001) sapphire substrates by MOCVD,are described.The unintentionally doped 2.5μm thick GaN epilayers grown with the same conditions as the GaN channel have a room temperature electron mobility of 741cm2/(V·s) at an electron concentration of 1.52×1016 cm-3.The resistivity of the thick GaN buffer layer is greater than 108Ω·cm at room temperature.The 50mm HEMT wafers grown on sapphire substrates show an average sheet resistance of 440.9Ω/□ with uniformity better than 96%.Devices of 0.2μm×40μm gate periphery exhibit a maximum extrinsic transconductance of 250mS/mm and a current gain cutoff frequency of77GHz.The AlGaN/GaN HEMTs with 0.8mm gate width display a total output power of 1.78W (2.23W/mm) and a linear gain of 13.3dB at 8GHz.The power devices also show a saturated current density as high as 1.07A/mm at a gate bias of 0.5V.

基于遥感、地理信息系统和人工神经网络的呼中林区森林蓄积量估测

利用遥感图像光谱信息良好的综合性和现势性以及地理信息系统(GIS)强大的空间分析功能,结合人工神经网络(ANN)可优化求解非线性复杂系统的功能,对呼中林区森林蓄积量进行了估测.结果表明:中红外波段与森林蓄积量间存在明显的负相关关系,说明中红外波段对估测森林蓄积量具有一定潜力;可见光波段和光谱变换第一主成分与森林蓄积量间也存在负相关关系;地形因子中海拔对研究区森林蓄积量的影响最大,坡度和坡向对蓄积量的影响较小.基于最佳的ANN网络参数、适当的GIS提取信息和遥感波段,呼中林区森林蓄积量的预测值和实测值的相关系数达0.973,经主成分变换后,数据量被有效降低,而预测精度只有少量下降(R2=0.934).