Rectangular AlGaN/AlN/GaN heterostructure field-effect transistors （HFETs） were fabricated, and the gate and the source of the HFETs consisted of AlGaN/AlN/CaN Schottky barrier diodes （SBDs）. Based on the measured forward current-voltage and the capacitance-voltage characteristics of the AlGaN/AlN/GaN SBDs, the series resistance under the Schottky contacts （Rs） was calculated using the method of power consumption, which has been proved to be valid. Finally, the method of power consumption for calculating RS was successfully used to study the two-dimensional electron gas electron mobility for a series of circular AlGaN/AlN/GaN SBDs. It is shown that the series resistance under the Schottky contacts cannot be neglected and is important for analysing and characterizing the AIGaN/AIN/GaN SBDs and the AlGaN/AlN/GaN HFETs.
Cao Zhi-FangLin Zhao-JunLŰYuan-JieLuan Chong-BiaoYu Ying-XiaChen HongWang Zhan-Guo
Ni Schottky contacts on A1GaN/GaN heterostructures have been fabricated. The samples are then thermally treated in a furnace with N2 ambient at 600℃ for different times （0.5, 4.5, 10.5, 18, 33, 48 and 72 h）. Current-voltage （I-V） and capacitance-voltage （C-V） relationships are measured, and SchrSdinger＇s and Poisson＇s equations are self- consistently solved to obtain the characteristic parameters related to A1GaN/GaN heterostructure $chottky contacts： the two-dimensional electron gas （2DEG） sheet density, the polarization sheet charge density, the 2DEG distribution in the triangle quantum well and the Schottky barrier height for each thermal stressing time. Most of the above parameters reduce with the increase of stressing time, only the parameter of the average distance of the 2DEG from the A1CaN/GaN interface increases with the increase of thermal stressing time. The changes of the characteristic parameters can be divided into two stages. In the first stage the strain in the A1GaN barrier layer is present. In this stage the characteristic parameters change rapidly compared with those in the second stage in which the AlGaN barrier layer is relaxed and no strain is present.
The influence of annealed ohmic contact metals on the electron mobility of a two dimensional electron gas （2DEG） is investigated on ungated AlGaN/GaN heterostructures and AlGaN/GaN heterostructure field effect transistors （AlGaN/GaN HFETs）. Current-voltage （I-V） characteristics for ungated AlGaN/GaN heterostructures and capacitance-voltage （C-V） characteristics for AlGaN/GaN HFETs are obtained, and the electron mobility for the ungated AlGaN/GaN heterostructure is calculated. It is found that the electron mobility of the 2DEG for the ungated AlGaN/GaN heterostructure is decreased by more than 50% compared with the electron mobility of Hall measurements. We propose that defects are introduced into the AlGaN barrier layer and the strain of the AlGaN barrier layer is changed during the annealing process of the source and drain, causing the decrease in the electron mobility.
Using measured capacitance-voltage curves and current-voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances, the influence of drain bias on the electron mobility is investigated. It is found that below the knee voltage the longitudinal optical （LO） phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance （here 4/5）, and the polarization Coulomb field scattering is dominant for the sample with a small ratio （here 1/5）. However, the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance （here 20 μm） compared with the one with a large distance （here 100 μm）. This is due to the induced strain in the AlGaN layer caused by the drain bias.
Using the measured capacitance voltage curves and the photocurrent spectrum obtained from the Ni Schottky contact on a strained Al0.3Ga0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving SchrSdinger＇s and Poisson＇s equations. It is shown that the calculated values of the relative permittivity are different from those formerly reported, and reverse biasing the Ni Schottky contact has an influence on the value of the relative permittivity. As the reverse bias increases from 0 V to -3 V, the value of the relative permittivity decreases from 7.184 to 7.093.
Ni/Au Schottky contacts with thicknesses of either 50（？）/50（？） or 600（？）/2000（？） were deposited on strained Al_（0.3）Ga_（0.7）N/GaN heterostructures.Using the measured C-V curves and I-V characteristics at room temperature,the calculated density of the two-dimensional electron-gas（2DEG） of the 600（？）/2000（？） thick Ni/Au Schottky contact is about 9.13×10^（12） cm^（-2） and that of the 50（？）/50（？） thick Ni/Au Schottky contact is only about 4.77×10^（12） cm^（-2）.The saturated current increases from 60.88 to 86.34 mA at a bias of 20 V as the thickness of the Ni/Au Schottky contact increases from 50（？）/50（？） to 600 A/2000 A.By self-consistently solving Schrodinger＇s and Poisson＇s equations,the polarization charge sheet density of the two samples was calculated,and the calculated results show that the polarization in the AlGaN barrier layer for the thick Ni/Au Schottky contact is stronger than the thin one.Thus,we attribute the results to the increased biaxial tensile stress in the Al_（0.3）Ga_（0.7）N barrier layer induced by the 600（？）/2000（？） thick Ni/Au Schottky contact.
Ni Schottky contacts on A1GaN/CaN heterostructures were fabricated. Some samples were thermally treated in a furnace with N2 ambience at 600 ~C for different times （0.5 h, 4.5 h, 10.5 h, 18 h, 33 h, 48 h, and 72 h）, the others were thermally treated for 0.5 h at different temperatures （500 ~C, 600 ~C, 700 ~C, and 800 ~C）. With the measured current-voltage （I-V） and capacitance-voltage （C V） curves and by self-consistently solving Schrodinger＇s and Poisson＇s equations, we found that the relative permittivity of the A1GaN barrier layer was related to the piezoelectric and the spontaneous polarization of the A1GaN barrier layer. The relative permittivity was in proportion to the strain of the A1GaN barrier layer. The relative permittivity and the strain reduced with the increased thermal stress time until the A1GaN barrier totally relaxed （after 18 h at 600 ~C in the current study）, and then the relative permittivity was almost a constant with the increased thermal strcss time. When the sample was treated at 800 ~C for 0.5 h, the relative permittivity was less than the constant due to the huge diffusion of the contact metal atoms. Considering the relation between the relative permittivity of the A1GaN barrier layer and the converse piezoelectric effect, the conclusion can be made that a moderate thermal stress can restrain the converse piezoelectric effect and can improve the stability of A1GaN/GaN heterostructure devices.
