May 20, 2024

Aluminum Nitride (AlN): Ultra-Wide Bandgap (UWBG) Technology Emerging in the Power Semiconductor Industry

All semiconductors operate based on chemical doping of impurity elements. When a doping material is inserted, an n-type or p-type semiconductor can be created, depending on whether the step creates an excess of negative charge carriers, electrons, or positive charges from a shortage of electrons, called holes. Almost all successful devices on the market are sandwiched by such doped semiconductors. The original semiconductor structure is a p-n junction connecting two terminals or diodes.

There are compound semiconductors containing elements from Groups III and V of the periodic table, such as gallium nitride, that have unusual but easily exploitable properties. At the interface where two specific semiconductors meet, such as GaN and AlGaN, they can spontaneously generate a two-dimensional electron gas (2DEG) with extremely strong mobile charge carriers, even without chemical doping. Nitrogen has a higher electronegativity than gallium and aluminum, resulting in a net charge shift or electrically spontaneous polarization, i.e., different domains of opposite charge. Furthermore, mechanical stress caused by lattice mismatch can lead to additional polarization due to the piezoelectric effect. In other words, this effect creates a charge simply by tightening the crystal lattice, another form of doping called polar doping. Both types of polarization simultaneously produce a net positive charge. But to achieve charge neutrality, the same amount of negative charge pops up at the interface, which is exactly what high-conductivity 2DEG is.

AlN junction and polarization-induced (Pi) doping

The paper mentioned above was written by a team of seven co-authors, some of whom are from Nagoya University, including Hiroshi Amano, who won the 2014 Nobel Prize for the invention of blue LEDs. This article describes the realization of diodes by implementing doping-free distributed polarization technology in aluminum nitride or, more precisely, aluminum gallium nitride alloy (AlGaN) consisting of a mixture of AlN and GaN. The basic doping technology is a unique polarization-induced (Pi) doping scheme that produces high-mobility 2DEG without impurity doping. Recently a two-dimensional hole gas (2DHG) has also been reported in undoped GaN/AlN structures. In addition to generating two-dimensional carriers from polarization discontinuities at the heterojunction interface, Pi bodies with constant volume concentrations of three-dimensional electron and hole gases can also be obtained from constant polarization gradients in linear gradient structures, or distributed polar doping (DPD).

Like any other diode, this device has a p-doped region and an n-doped region or junction. Doping in both regions is achieved by a distributed polarization doping technique. Different n-type and p-type polarizations are achieved by creating a gradient in the ratio of aluminum nitride to gallium nitride in the alloy in each doping region. The biggest innovation is whether the doping is n-type or p-type, depending only on the direction of the gradient. The authors demonstrated that diodes based on aluminum nitride alloys were able to withstand electric fields of 7.3 megavolts per centimeter, approximately twice that of SiC or GaN. This value is impressive, but still far from the theoretical value of around 15MV/cm shown in Table 1.

Table 1: Physical properties of WBG and UWBG materials

Future development

After proving that AlN vertical diodes are feasible in polarization-induced doping processes, the next step is to implement vertical structure transistors to compete with SiC MOSFETs or GaN HEMTs. According to IEEE member Takeru Kumabe, a co-author of the Nagoya paper, “AlN-based vertical heterojunction bipolar transistors, consisting of two p-n junctions with good power and area efficiency, are our target devices and the dream we want to realize 1,” Kumabe added: ”To realize the dream, a better understanding of charge mobility, carrier lifetime, critical electric field and intrinsic defects is needed.”

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