What are the semiconductor materials used in RF microwave power transistors?

Last Update Time: 2020-01-14 10:46:34

       Different types of transistors are used in power amplifiers for the EMC field. The typical transistors and their operating characteristics are briefly described below. Because different kinds of semiconductor materials have different characteristics, the designers of power amplifiers need to be based on actual conditions. Demand selection and design. The semiconductor materials used in the RF microwave power amplifier mainly include the following.

       Bipolar junction transistor (BJT)

       Bipolar junction transistor, BJT is a triode, an electronic device with three terminals, made of a semiconductor with three different degrees of doping. The charge flow in the transistor is mainly due to the diffusion of carriers at the PN junction. Drift motion.

       The operation of such a transistor involves both the flow of electrons and holes, so it is called bipolar, so it is also called a bipolar carrier transistor. Commonly used transistors and silicon transistors can be controlled by current. In a certain range, bipolar transistors have an approximately linear characteristic. This range is called "amplification area" and the collector current is approximately equal to N times of the base current. A bipolar transistor is a relatively complex nonlinear device. If the bias voltage is improperly distributed, it will distort its output signal. Even if it operates in a specific range, its current amplification factor is affected by factors including temperature. The maximum collector dissipated power of a bipolar transistor is the maximum power that the device can work under certain temperature and heat dissipation conditions. 

       If the actual power is greater than this value, the temperature of the transistor will exceed the maximum allowable value, causing the device performance to drop, even Cause physical damage. It can operate from up to 28 volts and operates at frequencies up to several GHz. In order to prevent sudden failure due to thermal breakdown, the bias voltage of the transistor must be carefully designed, because once the thermal breakdown is triggered, the entire transistor will be destroyed immediately. Therefore, amplifiers employing this transistor technology must have protection circuitry to prevent this thermal breakdown from occurring.

       Metal oxide semiconductor field effect transistor (MOSFET)

       MOSFET FETs are unipolar transistors that work only with the drift of a single type of carrier. Metal oxide semiconductor field effect transistors can be classified into an electron-rich N-channel type and a hole-based P-channel type according to the difference in channel polarity, and are generally referred to as N-type gold oxide half field effect transistors. (NMOSFET) and P-type MOSFETs have no fatal shortcomings of BJT, such as thermal runaway.

         In order to be suitable for high-power operation, in the late 1970s, an insulated gate field effect transistor with a vertical channel, namely a VMOS transistor, was developed, which is called a V-channel MOS field effect transistor, which is a newly developed high-efficiency power after the MOSFET. The device has excellent characteristics such as high withstand voltage, large operating current and high output power. The channel length of a vertical MOS field effect transistor (VMOSFET) is controlled by the thickness of the epitaxial layer, and is therefore suitable for short channelization of MOS devices, thereby improving the high frequency performance and operating speed of the device. VMOS tubes operate in the VHF and UHF bands, which are 30MHz to 3GHz. Packaged VMOS devices can deliver up to 1 kW in the UHF band, hundreds of watts in the VHF band, and can be powered from 12V, 28V or 50V. Some VMOS devices can operate at voltages above 100V.

        Lateral diffusion MOS (LDMOS)

        Lateral Double-diffused MOSFET (LDMOS):

        This is a lateral conductive MOSFET to reduce the channel length. The device fabricated by two diffusions is called LDMOS. In high-voltage power integrated circuits, high-voltage LDMOS is often used to meet the requirements of high voltage resistance and power control. RF power circuit.

        Compared with transistors, LDMOS has obvious advantages in key device characteristics such as gain, linearity, and heat dissipation performance, and is widely used because it is easier to be compatible with CMOS processes. LDMOS can withstand the standing wave ratio higher than the bipolar transistor, can operate at higher reflected power without being destroyed; it can withstand the over-excitation of the input signal and has high instantaneous peak power. The LDMOS gain curve is smoother and allows multi-carrier RF signal amplification with less distortion. 

        The LDMOS transistor has a low and unchanged intermodulation level to the saturation region. Unlike the bipolar transistor, the intermodulation level is high and varies with the power level. This main characteristic allows the LDMOS transistor to perform high. The power of the bipolar transistor is good and linear. LDMOS transistors have better temperature characteristics. The temperature coefficient is negative, thus preventing the effects of heat dissipation.

       Due to these characteristics, LDMOS is especially suitable for UHF and lower frequencies. The source of the transistor is connected to the bottom of the substrate and directly grounded, eliminating the influence of the inductance of the bonding wire that produces negative feedback and reduces the gain, so it is a very Stable amplifier.

      The high breakdown voltage of LDMOS and the lower cost compared to other devices make LDMOS

The first choice in 900MHz and 2GHz high power base station transmitters. LDMOS transistors are also widely used in many EMC power amplifiers in the frequency range of 80MHz to 1GHz.

      LDMOS devices with output powers exceeding 1.7 GHz have existed, and semiconductor manufacturers are developing high-power LDMOS devices with a higher frequency range that can operate at 3.5 GHz and above.

      GaAs metal semiconductor field effect transistor (GaAs MESFET)

      Gallium arsenide, chemically crystalline GaAs, is an important semiconductor material. A group III-V compound semiconductor with high electron mobility (5 to 6 times that of silicon), a wide forbidden band width of 1.4eV (silicon is 1.1eV), low noise, etc., GaAs is more suitable than the same Si component. Work in high frequency and high power applications. Because of these characteristics, GaAs devices are used in wireless communications, satellite communications, microwave communications, radar systems, etc., and can operate at higher frequencies, up to the Ku band.

      The breakdown voltage is lower compared to LDMOS. Usually powered by a 12V supply, the lower impedance of the device results in lower impedance of the device, making the design of the broadband power amplifier more difficult.

     GaAs MESFETs are a common choice for electromagnetically compatible microwave power amplifier designs and are widely used in amplifiers in the 80MHz to 6GHz frequency range.

GaAs twin high electron mobility transistor (GaAs pHEMT)

      GaAs pHEMT is an improved structure for high electron mobility transistors (HEMTs), also known as -modulated doped heterojunction field effect transistors (PMODFETs), with a higher electron areal density (approximately 2 times higher); The electron mobility here is also higher (9 % higher than in GaAs), so the performance of PHEMT is superior. PHEMT has a double heterojunction structure, which not only improves the temperature stability of the device threshold voltage, but also improves the output volt-ampere characteristics of the device, resulting in a device with greater output resistance, higher transconductance, and larger Current handling capability and higher operating frequency, lower noise, etc. With this material, a power amplifier with a frequency of up to 40 GHz and a power of several W can be realized.

      In the field of EMC, this material can be used to achieve a 1.8 GHz to 6 GHz power amplifier with a power of 200 W.

      Gallium Nitride High Electron Mobility Transistor (GaN HEMT)

      Gallium Nitride (GaN) HEMT is a new generation of RF power transistor technology. Compared with GaAs and Si-based semiconductor technology, GaN combines higher power, higher efficiency and wider bandwidth to achieve specific GaAs. MESFET devices are 10 times more powerful, with a breakdown voltage of 300 volts and can operate at higher operating voltages, greatly simplifying the design of broadband high power amplifiers.

At present, the cost of gallium nitride (GaN) HEMT devices is about 5 times that of LDMOS, and it has been widely used in the 80MHz to 6GHz power amplifiers in the EMC field.

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