Semiconductor materials are a kind of electronic materials with semiconductor properties (the conductivity is between conductor and insulator, and the resistivity is about 1m Ω· cm ~ 1g Ω· cm), which can be used to make semiconductor devices and integrated circuits. According to the type, it can be divided into element semiconductors and compound semiconductors. Element semiconductors refer to semiconductors formed by single elements of silicon and germanium, and compounds refer to semiconductors formed by compounds such as gallium arsenide and indium phosphide. With the development of wireless communication, high-frequency circuits are more and more widely used. Today, we will introduce the semiconductor materials and processes suitable for RF, microwave and other high-frequency circuits.
Gallium arsenide GaAs
The electron migration rate of GaAs is 5.7 times higher than that of silicon, which is very suitable for high frequency circuits. The electrical characteristics of gallium arsenide components in high frequency, high power, high efficiency and low noise index are much higher than those of silicon components. The empty gallium arsenide field effect transistor (MESFET) or high electron mobility transistor (HEMT / PHEMT) can have 80% power added efficiency under 3 V voltage operation. It is very suitable for medium and long distance wireless communication in high tier The need for long communication time.
Gallium arsenide components adopt special processes because their electron mobility is much higher than that of silicon. In the early stage, it was MESFET metal semiconductor field effect transistor, and later evolved into HEMT (high electron mobility transistor), and PHEMT (interface strained high electron mobility transistor) is currently HBT (heterojunction double carrier transistor). Heterobipolar transistor (HBT) is a gallium arsenide component without negative power supply. Its power density, current drive capability and linearity exceed that of FET. It is suitable for designing microwave amplifiers with high power, high efficiency and high linearity. HBT is the best component. HBT module has advantages in phase noise, high GM, high power density, collapse voltage and linearity. In addition, it can operate with a single power supply, which simplifies the difficulty of circuit design and subsystem implementation. It is very suitable for the development of RF and if transceiver modules, especially microwave signal source and high linear amplifier.
Gallium arsenide production method is very different from the traditional silicon wafer production method. Gallium arsenide needs to be manufactured by epitaxial technology. The diameter of this epitaxial wafer is usually 4-6 inches, which is much smaller than 12 inches of silicon wafer. Lei wafers need special machines. At the same time, the cost of GaAs raw materials is much higher than that of silicon, which eventually leads to the high cost of finished GaAs IC. At present, there are two kinds of epitaxial, one is chemical MOCVD and the other is physical MBE.
Gallium nitride Gan
Among the wide band gap semiconductor materials, gallium nitride (GAN) has developed slowly due to the lack of suitable single crystal substrate materials and high dislocation density. However, after entering the 1990s, with the continuous development of material growth and device technology level, GaN semiconductor materials and devices have developed very rapidly. At present, Gan has become a dazzling new star in the wide band gap semiconductor materials.
The application of GaN semiconductor materials first made a major breakthrough in the field of light-emitting devices. In 1991, Nichia first developed Gan blue light emitting diodes (LEDs) with sapphire as substrate, and then commercialized GaN based blue and green LEDs. Using GaN based blue LED and phosphorescent technology, the company has developed white light-emitting device products with the characteristics of high service life and low energy consumption. In addition, GaN based blue semiconductor lasers were first developed.
The super large screen full-color display made of GaN based high-efficiency blue-green LED can be used for dynamic information display in various indoor and outdoor occasions. As a new type of high-efficiency and energy-saving solid-state light source, high-efficiency white light-emitting diode has a service life of more than 100000 hours, which can save electricity by 5 ~ 10 times compared with incandescent lamps. It achieves the dual purpose of saving resources and reducing environmental pollution. At present, GaN based LED is widely used. You may see it every day, in traffic lights, color video billboards, children’s toys and even flash lights. The success of GaN based LEDs has triggered a revolution in the optoelectronic industry. GaN based blue semiconductor laser is mainly used to make the next generation DVD. It can increase the storage density by more than 20 times compared with the current CD.
Using GaN material, ultraviolet (UV) light detector can also be prepared. It is widely used in flame sensing, ozone detection, laser detector and so on. In addition, in terms of electronic devices, Gan materials can be used to prepare high-frequency and high-power electronic devices, which is expected to play an important role in aerospace, high-temperature radiation environment, radar and communication. For example, in the field of aerospace, high-performance military flight equipment needs sensors, electronic control systems and power electronic devices that can work at high temperature to improve the reliability of flight. GaN based electronic devices will play an important role. In addition, it greatly simplifies the electronic system and reduces the flight weight because it does not need a refrigerator when working at high temperature.
Indium phosphide InP
Indium phosphide is another important III-V compound semiconductor material after silicon and gallium arsenide. Almost with the development and research of the first generation element semiconductor materials such as germanium and silicon, scientists have also begun a lot of exploration work on compound semiconductor materials.
Indium phosphide (INP), as a new type of semi insulating wafer, is of great significance to improve the performance of InP based microelectronic devices. The semi insulating wafer prepared by high temperature annealing process not only maintains the high resistance characteristics of the traditional primary iron doped substrate, but also greatly reduces the iron concentration and significantly improves the electrical properties, uniformity and consistency. At present, the production quality of semi insulating InP substrate needs to be improved.
Primary semi insulating InP is prepared by doping iron atoms during single crystal growth. In order to achieve the purpose of semi insulation, the doping concentration of iron atoms is high, and the high concentration of iron is likely to diffuse with the epitaxy and device process. Moreover, because the segregation coefficient of iron in indium phosphide is very small, InP Single crystal ingot shows an obvious doping gradient along the growth axis, and the iron concentration at the top and bottom differs by more than one order of magnitude, so it is difficult to ensure the consistency and uniformity of the single crystal wafer cut by it. For the single InP wafer cut, due to the influence of the solid-liquid interface during growth, the iron atoms are distributed in concentric circles from the center of the wafer, which obviously can not meet the needs of some device applications. All these factors are the biggest obstacle restricting the production quality of semi insulating InP Single crystal wafer at present.
In recent years, studies at home and abroad have shown that the semi insulating substrate obtained by annealing low resistance undoped InP wafers at high temperature in a certain atmosphere can overcome the above problems. In InP crystals, the formation mechanism of semi insulation can be summarized into two aspects: one is to realize the semi insulation state by adding deep acceptors (elements) to compensate shallow donors, which is the case of primary iron doped semi insulating indium phosphide; The other is to reduce the concentration of shallow donors through the formation of new defects, and compensate the resident deep donors (elements). Undoped semi insulating indium phosphide belongs to this category. This new defect can be formed in the process of high-temperature annealing and irradiation. According to this idea, the relevant researchers of the Institute of semiconductors of the Chinese Academy of sciences have taken three steps to prepare the undoped semi insulating InP substrate: first, the undoped InP Single Crystal with high purity and low resistance (the surface is low resistance) is drawn by liquid sealing Czochralski method, then it is cut into a wafer with a certain thickness and encapsulated in a quartz tube, and finally annealed at high temperature under appropriate atmosphere conditions. The researchers conducted dozens of annealing comparative experiments in pure phosphorus atmosphere and iron phosphide atmosphere. Through comparative test and analysis, it is found that the semi insulating InP wafer annealed in iron phosphide atmosphere not only has few defects, but also has good uniformity.
In order to further study the actual effect of this annealed substrate on adjacent epitaxial layers, the researchers used molecular Cambodia epitaxy technology to grow the same InAlAs epitaxial layers on primary iron doped and semi insulating InP Substrates annealed in iron phosphide atmosphere. The test results show that the latter is more conducive to the growth of epitaxial layers with good crystalline quality. In addition, after implanting the same dose of Si ions into the two substrates and rapid annealing, hall test results show that the latter can greatly improve the activation efficiency of implanted ions.
InP chips are often used to manufacture high-frequency, high-speed and high-power microwave devices and circuits, as well as solar cells for satellites and outer space. In the rapidly developing field of optical fiber communication, it is the preferred substrate material. In addition, InP based devices also have advantages in IC and switching applications. The successful development of this new semi insulating InP chip will play an important role in the field of national defense and high-speed communication. The 13th Research Institute of China Electronics Technology Group has successfully fabricated high electron mobility transistors with a working frequency of 100GHz using this new semi insulating indium phosphide pure phosphorus substrate.
In the 1980s, IBM added Ge to improve Si Materials in order to increase the speed of electron flow, reduce energy consumption and improve functions, but unexpectedly successfully combined Si and Ge. Since IBM announced that SiGe has entered the stage of mass production in 1998, SiGe has become one of the most valued wireless communication IC process technologies in the past two or three years.
According to the material characteristics, SiGe has good high-frequency characteristics, good material safety, good thermal conductivity, mature process, high integration and low cost. In other words, SiGe can not only directly use the existing 200mm wafer process of semiconductor to achieve high integration, so as to create economic scale, but also has the high-speed characteristics comparable to GaAs. With the recent investment of IDM manufacturers, SiGe technology has gradually improved and become more and more practical in the problems of low cut-off frequency (FT) and breakdown voltage.
At present, this process technology developed by IBM has integrated high-efficiency SiGe HBT (heterojunction bipolar transformer) 3.3V and 0.5 μ M CMOS technology, which can use active or passive components to engage in configuration applications in analog, RF and mixed signals.
SiGe not only has the advantages of integration, yield and cost of silicon process, but also has the advantages of class 3 to 5 Semiconductors (such as gallium arsenide (GaAs) and indium phosphide (INP)) in speed. As long as metal and dielectric laminations are added to reduce parasitic capacitance and inductance, SiGe semiconductor technology can be used to integrate high-quality passive components. In addition, the behavior of the device with temperature can be designed by controlling germanium doping. SiGe BiCMOS process technology is compatible with almost all new technologies in the silicon semiconductor VLSI industry, including insulator silicon (SOI) technology and channel isolation technology.
However, in order to replace gallium arsenide, silicon germanium still needs to continue to make efforts in breakdown voltage, cut-off frequency and power consumption.
RF CMOS process can be divided into two categories: bulk silicon process and SOI (silicon on insulator) process. Due to the diode effect between the source and the drain to the substrate of bulk silicon CMOS, many experts believe that it is impossible to make high power and high linearity switches by this process. Unlike bulk silicon, RF switches made by SOI process can connect multiple FETs in series to deal with high voltage, just like GaAs switches.
Although the CMOS process of pure silicon is considered to be only applicable to the design with more digital function requirements, not to the RF IC Design dominated by analog circuits, after more than ten years of efforts, with the improvement of CMOS performance, the cooperation of wafer foundry with process technology below 0.25mm and the trend of wireless communication chip integration, RF CMOS process is not only a hot topic in academic research, but also attracted the attention of the industry. The biggest advantage of using RF CMOS process is, of course, the high integration of RF, fundamental frequency and memory components, and reduce the cost of components at the same time. However, the problem still lies in whether RF CMOS can solve the problems of high noise, low insulation and Q value, and reduce the process cost increased by improving performance, so as to meet the strict requirements of radio frequency circuit of wireless communication.
At present, most of the products that have used RF CMOS to make RF IC are Bluetooth and WLAN RF IC with relatively loose RF specification requirements, such as CSR, Oki, Broadcom and other Bluetooth chip manufacturers have launched Bluetooth transmitter made of CMOS; Intel announced that it has developed a full CMOS process direct conversion dual band wireless transceiver prototype that can support all current Wi Fi standards (802.11a, B and G) and meet the expected requirements of 802.11n, including 5GHz PA, and easily realize the separation of transmitter and receiver functions. WLAN chip manufacturers such as Atheros and envara have also recently launched multi-mode WLAN (. 11b / g / a) RF chipsets with full CMOS process.
The specification of RF IC for mobile phone is very strict, but the ice has been broken. Silicon labs first used digital technology to strengthen the functions of low if to fundamental frequency filter and digital channel selection filter to reduce the problem of too high CMOS noise. For the aero low if GSM / GPRS chipset produced by silicon labs, Infineon immediately followed up and launched a large number of products with RF CMOS technology. After acquiring berkana, Qualcomm also vigorously adopted RF CMOS technology, A number of new RF manufacturers without exception adopt RF CMOS technology, even the most advanced 65 nm RF CMOS technology. The old brand Philips, Freescale, STMicroelectronics and Renesas still adhere to the traditional process, mainly SiGe BiCMOS process. Nokia still uses a large number of STMicroelectronics RF transceivers. European and American manufacturers have always been conservative about new products and lack trust in RF CMOS. However, Korean big manufacturers Samsung and LG, as well as Chinese manufacturers Xiaxin and Lenovo, have adopted a large number of RF CMOS transceivers under cost pressure. At present, the disadvantage may be that the failure rate is slightly higher and the power consumption is slightly higher, and multiple chips are required, which increases the design complexity. But it’s still tolerable.
Other applications include automotive safety radar systems, including 24 GHz radar for detecting blind spots
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