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III-Nitride Based High Electron Mobility Transistor


By
Wenzhe Guo

February 26, 2017 - Posted in Discussion
​​In 1956, three physicists were awarded the Nobel Prize for the discovery of transistor effect. Transistors have contributed to the development and improvement of integrated circuits and hence the extraordinary progress of information technology. Conventional transistors are realized based on the n-doped or p-doped Si material, which are limited in the high-frequency and high-power application.

High electron mobility transistor (HEMT) was proposed with the replacement of conventional III-V semiconductor materials, such as GaAs, by III-nitride materials. Shown in Figure 1, HEMT of a simple structure consists of source, gate and drain electrodes, AlxGa1-xN/GaN heterostructure channel and a substrate. The unique feature of HEMT is that the channel could be formed with two-dimensional electrons gas (2DEG) inside by AlxGa1-xN with wider bandgap grown on unintentionally doped GaN with narrower bandgap, which is attributed to spontaneous and piezoelectric polarization [1]. As illustrated in Figure 2, for Ga-face AlxGa1-xN, grown along [0001], spontaneous polarization is directed from the top to the substrate, but for N-face AlxGa1-xN, grown along, the direction is opposite. Besides, piezoelectric polarization, caused by the lattice mismatch between two materials, has the same direction as spontaneous polarization with the presence of tensile strain and opposite direction for compressive strain. According to the equation, where σ is the sheet charge density at the interface, P is the total polarization and n is the normal unit vector, the sheet charge density is positive when P has the same direction as n, shown in Figure 2(a)(b)(f), which can attract electrons coming to the interface to form the 2DEG channel. With the 2DEG channel, HEMT is benefited from the much high carrier density and mobility resulting from the high polarization field and good carrier confinement, widely employed in the high-power and high-frequency application.

1.PNG

Figure 1: Schematic of HEMT [2]


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   Figure 2: Polarization induced sheet charge density and directions of the spontaneous and piezoelectric polarization in Ga- and N-face strained and relaxed AlGaN/GaN heterostructures.


Reference:

[1]O. Ambacher, et al, Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures, J. Appl. Phys., 85, 3222 (1999).

[2]S. Seo, High frequency MIS-Based III-Nitride Transistor and Integrated Bio-Sensor Technology, University of Michigan, 2009.










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