Plenty of research and development has already been done on AlGaN/GaN heterostructure field-effect transistors. This can be attributed to the fact that they are outstanding candidates for high-frequency and high power applications. For such applications, it has been noted that mobility of electrons comprising of the two-dimensional electron gas is a key factor for the heterostructure field-effect transistors performance. Previous studies have shown that the gate in AlGaN/GaN heterostructure field-effect transistors splits the source-to-drain channel into three sections: the gate-source region, the gate-drain region and the field-effect section. Consequently, the principle that the electron mobility under the gate region is modulated by the gate bias and the resistance under the free-contact area is constant has been applied in almost all electron mobility calculations.
Because of the converse piezoelectric effect, the polarization charges of the AlGaN barrier layer under the gate region can be changed by the gate bias. The varying polarization charge distribution has a significant influence on the electron mobility in the three sections, which leads to the resistance in the free-contact area to be changed by the gate bias. Unfortunately, very few studies consider this influence during electron mobility calculations.
Researchers led by professor Zhaojun Lin at Shandong University in China, proposed developed a novel method that considers the resistance variation in the free-contact area with different gate biases. They hoped to confirm the accuracy of their new method by comparing the results of the calculated and measured trans-conductance values. Their research work is now published in the journal, Superlattices and Microstructures.
The experimental procedure commenced by growing AlGaN/GaN heterostructures by molecular beam epitaxy on a sapphire substrate. The density of the two-dimensional electron gas and electron mobility of the grown heterostructures were then measured. The team then formed source and drain ohmic contacts. They then derived the specific resistivity by measuring the transmission line method patterns. Eventually, nickel/gold was deposited to form the Schottky contact.
From the empirical study undertaken, the authors of this paper were able to observe that based on the measured capacitance voltage and current-voltage curves, the new method employed iteration calculation with altered scattering mechanisms. They also realized that when compared to the electron mobility computed by the outmoded method, the results from the new showed an apparent variance, particularly for the device with a larger gate length. This dissimilarity was seen to originate from the device with the greater gate length that has a stronger polarization Coulomb field scattering.
The Zhaojun Lin and colleagues study has indeed presented a novel technique for determining the electron mobility. This novel technique takes into consideration the resistance variation in the free-contact region versus the gate bias. The comparison between the experimental and calculated trans-conductance values has been undertaken and the results reflect the correctness and necessity of this novel technique. Therefore, this novel technique will aid in precise determination of the electron mobility of the two dimensional electron gas in AlGaN/GaN heterostructure field-effect transistors.
Peng Cui, Zhaojun Lin, Chen Fu, Yan Liu, Yuanjie Lv. A method to determine electron mobility of the two dimensional electron gas in AlGaN/GaN heterostructure field-effect transistors. Superlattices and Microstructures volume 110 (2017) page 289-295.
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