Investigation Of Ⅲ-Nitrides Polarization Doped Field Effect Transistors

As one of the most salient features of group III nitrides, the polarization effect plays an important role in the fields of microelectronics and photonics.Especially in microelectronics applications, the two-dimensional electron gas in the abrupt AlGaN/GaN heterostructures caused by the polarization discontinuity is the work foundation of the GaN-based heterostructure field effect transistors (HFETs). However, in the graded AlGaN/GaN heterostructures, a three dimensional electron slab would be formed across the graded AlGaN/GaN heterostructure region. The geometry extension of the carriers provides the basis of application space expansion, such as Polarization-doped FETs (PolFETs). Compared with the traditional nitrides devices, the PolFET exhibit some novel features, such as high linearity and high reliability, etc.In this dissertation, the design, fabrication and characterizations of AlGaN/GaN graded heterostructure and field effect transistors were presented.The growth process optimization of GaN on SiC was investigated, including the optimizations of buffer, modulations of growth parameters and the stress control of GaN layer. For The optimized GaN epilayer on SiC substrate, the FWHMs of (002) and (102) were 185arcsec and 261arcsec, respectively. The stress evolution model for GaN epilayer on SiC substrate was proposed.The graded AlGaN/GaN heterostructure was grown on 2 inch sapphire substrate by MOCVD. The electron mobility of 732cm2/V-s at a bulk density of 1.2E+18 cm-3 is obtained. The monotonic grading profiles of Al and Ga composition across the graded AlGaN layer were obtained by the secondary ion mass spectrometry (SIMS) measurements.The Al fraction was ranged from about 35% on the top to 0 at the bottom of the graded layer. The graded Al fraction and the subsequent polarization gradient are responsible for the quasi-bulk three-dimensional electron slab. Frequency-dependent conductance-voltage measurements were employed to characterize the trap behaviors in AlGaN/GaN based PolFET material structure. No obvious effective trap density peak was found in the graded AlGaN/GaN epilayers of the PolFETs, due to the absence of the abrupt heterostructure as in HFETs. The exactly exponential dependence of the trap state time constants across the graded layers on the bias voltage similar with those in HFETs was observed, indicating multi-interface characteristics.The high linearity AlGaN/GaN PolFET was obtained. The DC transfer and frequency characteristics of PolFETs exhibit a bias-insensitive feature throughout the low-and high-voltage operating conditions, demonstrating the potential for high-linearity applications, which is suitable for the high-linearity power amplifiers of wireless communication systems, such as WiMAX and LTE-Advanced. In the current collapse measurements, small current collapses were observed in the unpassivated PolFET device. It was found that, the upper part of the channel in PolFET has a large number of mobile electrons and effectively screens the lower channel part from potential fluctuations on device surface. Consequently, surface trapping/detrapping has little chance to reduce the current or delay the frequency response of the lower part of the channel.The temperature dependent DC performance of AlGaN/GaN PolFET was studied. The rough decrement of drain current and transconductance with the operate temperature was obtained. Compared with the conventional HFETs, the drain current drop of PolFET was smaller. The transconductance drop of PolFETs at different gate bias showed different temperature dependence. At VGS=-2V, the temperature index of transconductance drop was-0.5, while the temperature index was-1.5 at FGS=-5V. The quasi-multi-channel model was proposed to investigate the physics behind from the aspect of the unique carriers’ behaviors of AlGaN/GaN PolFET.