In the military market, the demand for GaN RF devices is growing rapidly. According to the data of "Overview of the Third Generation Semiconductor Development and Opportunities, Challenges and Countermeasures in China", the demand for GaN RF power modules for fighter radar alone will reach 75 million. At present, the GaN RF power amplifier device has been used to replace GaAs device in the new generation jammer pod and air and missile defense radar (AMDR) of the US Navy.

The development of semiconductor materials is mainly reflected in three aspects:

1) substrate and epitaxial materials develop to large diameter;

2) improvement of material quality and device performance;

3) The decline of cost and price promotes industrial development.

GaN RF deviceAdvantages of radar application

Radar, that is, Radar(radio detection and ranging), is an electronic device that uses electromagnetic waves to detect targets. It is "the eye of the three armed forces and the heavy weapon of the country"! Our eyes can see things around us, because sunlight or reflected light from light hitting objects enters our eyes, or objects directly emit light (such as fireflies, etc.) into our eyes.The principle of radar is the same. Electromagnetic waves are emitted by the radar Transmitter, and the echoes reflected by the target are received by the radar Receiver. After a series of complex signal processing and analysis such as filtering and amplification, the attitude, distance, speed and azimuth of the target can be judged according to the time delay, Doppler frequency shift, angle of arrival and amplitude of the echo signal, which can be used for reconnaissance, guidance, fire control and other functions of carrying facilities.

Of course, radar differs from eyes in that radar actively emits electromagnetic waves and detects echoes, while human eyes can only passively receive light.

In order to see farther and more clearly, the radar needs higher power (capable of transmitting to longer distance) and higher frequency (denser and faster acquisition of reflected wave information) of the transmitting group.

At the same time, in order to adapt to combat in different environments, the radar hopes to be as light and small as possible, so as to realize airborne and shipborne. T/R module is the core of phased array radar. It contains a large number of RF chips, such as power amplifier, low noise amplifier, circulator, phase shifter, etc., which constitute the transmitting channel and receiving channel of T/R module, and is responsible for processing high-frequency electromagnetic signals.

The radar system requires extremely high performance of RF chips.The transmitting channel is responsible for amplifying the excitation signal, so that the power of the excitation signal is greatly enhanced. The greater the signal power, the farther the electromagnetic wave travels in space, and the higher the detection distance and detection accuracy of the radar will be.

With its excellent frequency and power characteristics, GaAs semiconductor devices occupy a dominant position among the solid-state microwave RF power devices of the above radar T/R module, but GaN has some advantages over GaAs.And the Johnson factors of GaN and GaAs (comprehensively evaluating the application of semiconductor materials in power and frequency) are 27.5 and 2.7, respectively:

1) Compared with GaAs (bandgap 1.43eV), the bandgap of GaN material is 3.4eV, which is about 2.4 times that of GaAs. GaN devices have exponentially higher breakdown field strength and can work at higher voltage.

In addition, the wider bandgap makes the intrinsic carrier concentration of GaN material lower exponentially (higher temperature can make the intrinsic carrier concentration of GaN material equal to that of doped carrier), so the working reliability of GaN device at high temperature is better than that of GaAs.

2)GaN material has higher carrier drift speed under large electric field, so the working current is larger. The electron mobility of GaAs material is very high at low electric field and room temperature, reaching 8800cm2/V.s However, when the electric field is slightly larger, its mobility drops sharply and becomes negative, which shows that the carrier drift speed drops sharply.

This is determined by the energy band structure of GaAs. In addition to the minimum value of the conduction band of k=0, there is an energy valley of 0.36eV in the direction of < 100 >. When the electric field is increased, the energy obtained by electrons will be transferred to this energy valley, which is the electron transfer effect.

Although the carrier mobility of GaN is not as good as that of GaAs at room temperature and low electric field, the mobility of GaN decreases with the electric field at 150kV/cm, but it is always positive, which shows that the drift speed increases all the time.

Under the condition of high field strength, the saturated electron drift velocity of GaN (2.46&times; 107cm/s) is several times as large as GaAs (1&times; 107cm/s), even much higher than the saturation drift velocity of GaAs in low electric field (1.8&times; 107cm/s）。

GaN devices can work at higher voltage and current at the same time (the electron drift speed is related to the current density), so they are far superior to GaAs in high power applications, but GaAs will continue to play a role in low and medium voltage applications.

3）In terms of thermal properties, SiC is the best substrate material for GaN RF power devices. The thermal conductivity of SiC is better than that of silicon and sapphire, which helps to realize the high power characteristics of GaN, and the thermal conductivity of SiC is 10 times higher than that of GaAs. Although SiC substrate is expensive at present, it is not a problem for military products with low price sensitivity, and the price of SiC will definitely decrease with the development of its material technology.

The advantages of wide band gap and better thermal performance of GaN make it more obvious compared with GaAs devices: the channel temperature of GaN devices is higher than that of GaAs under the same reliability condition, and the reliability of GaN devices is much higher than that of GaAs under the same channel temperature.

When Qorvo GaN HEMT and GaAs pseudo-modulation doped heterojunction field effect transistor (pHEMT) channel temperature is 150℃, the average lifetime of GaN and GaAs is 1&times, respectively. 09 hours, 1&times; 06 hours. At 1&times; With an average life of 106 hours, GaN(225℃) can work in an environment 75℃ higher than GaAs(150℃).

4) The above advantages in power and heat dissipation reliability make GaN-based RF devices have smaller volume at the same or even higher output power, which is more conducive to the slight lightening of radar system, and thus more suitable for airborne and shipborne occasions.

Unipolar GaN HEMT is the main structure of GaN RF devices, which has higher carrier mobility than MESFETs, MISFETs and other structures, while the difficulty of doping GaN p limits the performance of bipolar GaN HBTs.

Compared with SiC(Johnson factor is 20), GaN has the advantages in RF devices, that it can form heterojunction with other nitride alloys, and AlGaN/GaN HEMT channel carriers have faster mobility and higher concentration than SiC MESFETs. However, SiC has more advantages for power semiconductors that pay more attention to withstand voltage and conduction current.

Related products and research and development progress

GaN microwave RF devices are favored by various departments and services such as DARPA, NASA, and the US Air Force. They regard GaN as a substitute for GaAs, and continue to provide funds for research and development of GaN, and the technology of GaN materials and devices develops rapidly.

The development and maturity of GaN military science and technology products have also prompted the industry to transfer related technologies to 5G and other commercial fields. After all, the demand for defense military science and technology products is small.

However, the large-scale demand and production in the commercial field have also promoted the further improvement of GaN materials and device technology. Include SixN Passivation, Field-Plated GaN HEMTs, Deep-Recessed GaN HEMTs, metal-oxide-semiconductor hemts (moshemts), More materials and device technologies and processes, such as Non-alloyed Ohmic Contacts and T-shaped Gate, are being developed to make GaN HEMT RF devices work at higher frequency (microwave to millimeter band), higher output power and power density, higher power added efficiency (PAE) and better service reliability.

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  The Hof Institute in Flawn reported at the IEDM conference in 2019 that in the frequency range of 1-2GHz, the working voltage of the device doubled from 50V to 100V, the PAE reached 77.3%, and the power density of 125V voltage exceeded 20 W/mm.
  

• Qorvo GaN RF, a Gan RF manufacturer located in Texas, USA, has reached 35.5 dBm saturated output power and 22% PAE in Ka band, and its maximum operating frequency reaches 31GHz.
  

• CREE GaN 2.7GHz-3.8GHz RF device has the highest output power of 88W(3.1GHz) at 50V working voltage.

  
  

GaN microwave RF device is the key technology. Under the technical blockade of foreign countries, scientific research institutions and industrial companies in China, including xidian university, 13 and 55 institutes of CLP, Suzhou Nengxun, Guolian Wanzhong and ZTE, have explored and innovated in this area and made many achievements.

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  The fmax of the 320GHz millimeter wave device in West China is up to 320GHz by using the groove semi-suspended gate technology with high interface quality. With a certain output power density, PAE reaches the highest value in the current international GaN-based HEMT at 30GHz frequency.
  

• Suzhou Nengxun introduced GaN power amplifier tube products with a frequency of 6GHz, a working voltage of 48V and a design power of 7W-65W;
  

• In 2019, the new generation of RF chip projects led by 13 and 55 CLP branches won the "First Prize of National Science and Technology Progress Award".

  
  

AMDR radar

The most dazzling radar based on GaN device technology is the AMDR(Air and Missile Defense Radar) radar developed by Raytheon for the US Navy (now officially named AN/SPY-6).

AMDR solid-state active phased array radar represents the most advanced level of modern radar, including a four-sided array S-band radar (AMDR-S), a three-sided array X-band radar (AMDR-X) and a radar controller (RSC).

AMDR has made many breakthroughs in the history of shipborne radar technology in the world, one of which is the first application of GaN T/R component technology in shipborne radar with large antenna aperture.

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  Compared with the active AN/SPY-1 radar:
  

• Its total power is increased by 2 times, up to about 10MW, and the radar power generated during operation is increased by more than 35 times.
  

• Sensitivity increased by 70 times, signal-to-noise ratio (S/N) increased by 1000 times with high version (up to 30dB),
  

• The detection distance is increased by 2 times (AMDR-S exceeds 400km, AMDR-X exceeds 200km to the air),
  

• The minimum detectable target is reduced to half, and the coverage is increased by 13 times.
  

• Simultaneously processing the number of targets by 30 times and tracking the number of targets by 6 times,
  

• At the same time, the number of guided missiles in flight increased by three times.

  
  

AMDR will be first installed in Allie &middot of the US Navy; On the Burke-class Flight III anti-aircraft and anti-missile destroyer.In addition to AMDR radar, Northrop Grumman also developed and produced GaN-based AN/TPS-80 Ground/Air Task Oriented Radar system for the U.S. marine corps, and the application projects of gan radio frequency devices include "next generation jammer", three-coordinate mobile long-range radar (3DLRR) and "space fence".

Of course, I have to mention that our country was officially launched in the previous period. The Type 055 destroyer, which is "armed with knives" beside the aircraft carrier, was even ranked first among the "five deadliest combat ships" in the world by National Interest magazine of the United States.

Type 55 destroyer uses the latest digital array active phased array 346B radar, and is equipped with four bands of L, S, C and X radars, integrating long-range early warning (L band, 1-2GHz, the center wavelength has reached 22cm), search (S band main radar, 2-4GHz, the center wavelength is 10cm) and missile guidance (C band, 4-8)

In 2019, a major project developed by CLP 14 won the first prize of the National Science and Technology Progress Award, but the official introduction said that "the project is too advanced and inconvenient to display", and the radar equipped on the 055 destroyer is the achievement of CLP 14, the heart of radar research and development in China-"A big country is heavy with weapons, but it is hard to realize it"!

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