One of the biggest stories in semiconductors over the past decade has been the unexpected overtaking of traditional silicon in power electronics - silicon carbide (sic) and gallium nitride (gan) have surpassed silicon to capture a multi-billion dollar market segment Marketing. As major applications fall into the hands of these upstarts with superior attributes, a question naturally arises.
What is the next new power semiconductor - and will its superior capabilities take major market share away from SIC and GAN?
Attention has focused on three candidate materials: gallium oxide, diamond and aluminum nitride (aln). They all have remarkable properties, but also fundamental weaknesses that have hindered commercial success to date. Now, however, the outlook for ALNs has improved significantly, thanks to several recent breakthroughs, including a technological advance reported by Nagoya University at the latest IEEE International Electronic Devices Conference in San Francisco last December.
Figure 1
How aluminum nitride is catching up (and ahead of?)
sic and gan
iedm The paper describes the fabrication of diodes based on aluminum nitride alloys that can withstand electric fields of 7.3 megavolts per centimeter, a figure approximately that of silicon carbide Or twice the electric field that gallium nitride can withstand. Notably, the device also has very low resistance when conducting current. "This is a remarkable result," said W. Alan Doolittle, IEEE senior member and professor of electrical and computer engineering at Georgia Tech. "Especially the on-resistance of this thing, it's really good." The Nagoya University paper has seven co-authors, including IEEE member Hiroshi Amano, who won the 2014 Nobel Prize for the invention of blue LEDs.
Aluminum nitride has long fascinated semiconductor researchers.
One of the most important properties of a power semiconductor is its band gap. It is the energy, measured in electron volts, required for electrons in a semiconductor lattice to jump from the valence band to the conduction band, where they can move freely in the lattice and conduct electricity. In semiconductors with wide bandgaps, such as gallium nitride (GAN) or silicon carbide (SIC), the bonds between atoms are strong. The material is therefore able to withstand very strong electric fields before the bonds break and the transistor is destroyed. But they all pale in comparison to aln. Aln has a band gap of 6.20 electron volts; for gan the value is 3.40; for the most common sic type it is 3.26.
aln A long-standing problem is doping, the insertion of impurity elements that create an excess charge in a semiconductor so that it can carry electrical current. Strategies for chemically doping aln have only begun to emerge in recent years, are not yet fully mature, and their effectiveness is a controversial topic among researchers. During the doping process, the excess charges can be electrons, in which case the semiconductor is called "n-type", or they can be electron-deficient, called holes, in which case the semiconductor is "p-type" type". Almost all commercially successful devices are composed of doped semiconductors sandwiched together.
But it turns out that impurity doping is not the only way to dope semiconductors.
Some semiconductors based on compounds containing elements from groups iii (aka scandium) and group v (vanadium) of the periodic table of elements (such as the compound gallium nitride) have unusual and remarkable properties. At the boundary where two such semiconductors meet, even without chemical doping, they can spontaneously create a two-dimensional pool of extremely mobile charge carriers, called a two-dimensional pool. Electron gas (2-dimensional electron gas).
It is generated by the internal electric field of the crystal, and the internal electric field of the crystal has several properties: First, the crystals of these III-V semiconductors have unusual polarity: within the unit cells of the crystal, the electron cloud and positively charged nuclei are offset from each other enough to provide each unit cell with distinct negatively and positively charged regions (dipole)). Furthermore, charges can be generated in the lattice of these semiconductors simply by deforming the lattice, a phenomenon known as piezoelectricity.
The story behind the big advances
In the early 2000s, researchers at the University of California, Santa Barbara took advantage of these properties to develop a technology called distributed polarization doping that allowed them to Obtain n-type doping of bulk (3D) gallium nitride without impurities. The group included IEEE Fellow Umesh Mishra (currently chair of the UCS Engineering Department) and his graduate students Debdeep Jena and Huili (Grace) Xing, both now at Cornell University. Jena and Xing are both IEEE academicians. They subsequently demonstrated p-type distributed polarization doping at Cornell University in 2010, and then demonstrated dopant-free two-dimensional hole gas (hole) at Cornell University in 2018. gases).
Figure 2
The Nagoya group built on these previous results by implementing dopant-free distributed electrodes in aluminum nitride (or, more precisely, aluminum gallium nitride (algan) alloys consisting of a mixture of aln and gan). chemical doping technology.
As with any diode, their devices have a p-doped region paired with an n-doped region, with a boundary in between called a junction. For both regions, doping is accomplished by distributed polarization doping. They achieved different polarizations (n-type and p-type) by establishing a gradient in the percentage of aln versus gan in the alloy in each doped region. Whether the doping is n-type or p-type depends only on the direction of the gradient. "The aluminum composition is not a uniform algan composition but varies spatially in a linear manner,"
jena said. The p-doped layer starts with pure gallium nitride on the side adjacent to the anode contact. Moving towards junctions with n-doped layers, the percentage of aluminum nitride in the algan alloy increases until it reaches 95% aln at the junction. Continuing in the same direction, across the n-doped region, the percentage of Aln decreases with increasing distance from the junction, starting at 95% and reaching a nadir of 70%, where the layer is in contact with the pure Aln substrate.
"This is a new concept in semiconductor devices," said Nagoya Devices' Jena. The next step, he added, is to create a diode with a layer of pure aln at the junction, rather than 95% aln. According to his calculations, a layer of aluminum nitride just two microns thick would be enough to block 3 kilovolts. "That's exactly what's going to happen in the near future," he said.
At Georgia Tech, Doolittle agrees that there is still huge room for improvement by including higher levels of pure aluminum nitride in future devices. For example, the breakdown electric field of a Nagoya diode is an impressive 7.3 mv/cm, but the theoretical maximum for an aln device is about 15. More aln will also greatly increase thermal conductivity. Thermal conductivity is critical for power devices, and the thermal conductivity of algan alloys is average, less than 50 watts per meter Kelvin. On the other hand, the value of 320 °C for pure aluminum nitride is very respectable, between gan (250 °C) and sic (490 °C).
jena and doolittle say the ultimate goal is commercial aln power transistors that perform significantly better than existing options, and Nagoya's work will undoubtedly eventually lead to that goal. “It’s just engineering right now,” doolittle said. They all pointed out that Nagoya diodes are vertical devices, which is the preferred orientation for power semiconductors. In a vertical device, current flows from the substrate directly up to the contacts on the top of the device, a configuration that allows maximum current flow.
At least six ARN-based transistors have been demonstrated in recent years, but none of these transistors are vertical devices, and none have characteristics that compete with commercial gan or sic transistors. They also use algan in key components of their devices.
Nagoya paper co-author and IEEE member Takeru Kumabe wrote in an email to IEEE Spectrum: "We believe that using distributed polarization doping technology to demonstrate commercially competitive [power transistors]... is based on Aln's vertical heterojunction bipolar transistor consists of two p-n junctions and has good power and area efficiency. It is our target device and the dream we want to realize.
kumabe added that to realize this dream, the team will focus on a deeper understanding of charge mobility, "carrier lifetimes, critical electric fields and deep defects." It should also develop devices capable of producing high-quality device layers and introducing them during processing Less damaging crystal growth and device fabrication techniques."
"We hope to solve these problems within 3-5 years and achieve commercialization of aln-based power devices in the 2030s," he said.
The world's first aluminum nitride transistor
In April 2022, NTT Corporation announced that it had realized the operation of a transistor using high-quality aluminum nitride (ALN).
transistors are an important part of semiconductor power devices and are used for power conversion in home electronics and electric vehicles. Improvements in their efficiency will help save energy. Aln for ultra-wide bandgap (UWBG) semiconductors has a large breakdown electric field and is therefore a promising semiconductor material for realizing low-loss, high-voltage power devices.
According to reports, NTT has successfully produced high-quality ALN using metal-organic chemical vapor deposition (mOCVD) and developed methods for the formation of ohmic and Schottky contacts. These technologies allowed us to demonstrate the aln transistor for the first time. Furthermore, the Aln transistor exhibits good device characteristics even at a high temperature of 500°C. These results will help realize ultra-low-loss power devices and high-temperature electronic devices.
Figure 3: The relationship between the specific on-resistance and breakdown voltage of each semiconductor material
ntt Corporation stated that semiconductor power devices for power conversion are widely used in home electronics, personal computers and smartphones, as well as database servers and electric car. In recent years, the application of power devices has expanded to high-power operation fields such as photovoltaic power generation and railways. To achieve carbon neutrality, losses in electrical equipment should be further reduced. Silicon (si) is commonly used in semiconductor power devices. By using a wide bandgap semiconductor with a large breakdown electric field, losses can be reduced and breakdown voltage increased. Therefore, wide bandgap semiconductors such as silicon carbide (sic) and gallium nitride (gan) are being developed for power devices. UWBG semiconductors with breakdown fields larger than SIC or GA further improve the performance of power devices (Figure 3). uwbg semiconductors include aln, diamond, and gallium oxide (ga2o3) (table i). For aln power devices, the theoretical power loss is expected to be only 5% of si, 35% of sic, and 50% of gan.
Table 1: Band gap energies and breakdown fields of semiconductor materials
Aln has been used as an insulator since it was first synthesized more than a century ago. In 2002, NTT successfully manufactured the semiconductor ARN for the first time in the world, thus opening up new avenues for semiconductor device applications. Among UWBG semiconductors, the advantage of ALN is that devices can be fabricated on large-scale wafers, and various device structures can be obtained by forming heterojunctions with other nitride semiconductors (such as gan). However, there are few reports on power device fabrication in this regard, and their characteristics need to be improved.
Figure 4: Aln transistor schematic
ntt For the first time, Aln successfully achieved transistor operation with good characteristics using high-quality semiconductors manufactured by mocvd. The current-voltage characteristics of the aln transistor show good ohmic characteristics (Figure 4 and Figure 5) and extremely small leakage current. The breakdown voltage is as high as 1.7kv.
Figure 5: (a) Drain current and drain voltage characteristics of aln transistor; (b) Off-state breakdown characteristics of aln transistor.
ntt emphasized in the press release that they also confirmed that the aln transistor can operate stably at high temperatures (Figure 6). Aln transistors exhibit better performance at high temperatures compared to conventional semiconductor materials. As the ambient temperature increases from room temperature to 500°C, the drain current increases approximately 100 times. In addition, the leakage current remains at an extremely low level of 10 -8 a/mm even at 500°C. As a result, a high drain current on/off ratio of 106 was obtained at 500°C.
Figure 6: Drain current and gate voltage characteristics of aln transistor in the range of room temperature (rt) to 500°c
In ntt's view, to realize aluminum nitride, the following technical problems need to be solved:
Technology to be solved first The problem is crystal growth technology for high quality aln.By developing unique high-temperature MOCVD using a specially designed reactor, the density of residual impurities and crystal defects in ALN is reduced. As a result, ntt achieved high-quality n-type aln semiconductors with the world's highest electron mobility.
The second point is how to achieve good ohmic contact. aln has a large energy barrier to the metal, making it difficult to form ohmic contact on it. ntt uses a compositionally graded layer of algan between the aln and the metal electrode to obtain a good ohmic contact (Figure 7).
Figure 7: Metal/n-type Aln contact structure (a) with and (b) without compositionally graded algan layer. (c) Current-voltage characteristics with and without gradient layer.
The third point is how to achieve good rectification of Schottky contacts. Schottky characteristics are affected by the crystal quality of the semiconductor, the interface state between the semiconductor and the metal electrode, and the contact resistance of the ohmic electrode. As mentioned above, NTT achieves near-ideal Schottky characteristics and good rectification due to high-quality ALN and good ohmic contact.
The establishment of these basic technologies led to the successful operation of the aln transistor. With the efforts of these manufacturers and research institutions, the future of semiconductors is just around the corner.
1.https://spectrum.ieee.org/aluminum-nitride
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