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    2021-10-21 17:42:01
    What is a MOSFET?


    I. Working Principle
    II. Advantages
    III. Types 1. Dual-gate MOSFET
    2. Depletion MOSFET
    3. NMOS logic
    4. Power MOSFET

    Judging by the name of the MOSFET, it will actually give people the wrong impression because of the word "metal". However, it does not exist in most of the current components of the same kind. Early metal oxide half field effect transistor gates used metal as a material, but with the advancement of semiconductor technology, modern metal oxide half field effect transistor gates have replaced the metal with polysilicon.

    The metal oxide half field-effect transistor is conceptually an "Insulated-Gate Field Effect Transistor" (IGFET). The gate insulating layer of IGFET may be other materials than the oxide layer used in MOSFETs. Some people prefer to use IGFETs when referring to field-effect transistor components with polysilicon gates, but most of these IGFETs refer to metal oxide half field effect transistors.

    The oxide layer in the MOSFET is located above the channel. Depending on its operating voltage, the thickness of this oxide layer is only tens to hundreds of angstroms (Å). The usual material is silicon dioxide ( SiO2), but some new advanced processes can already use silicon oxynitride (SiON) as an oxide layer.

    Today, silicon is usually the first choice for semiconductor components, but some semiconductor companies have developed processes that use other semiconductor materials. The most famous is the SiGe process (SiGe process) developed by IBM using a mixture of silicon and germanium. Unfortunately, many semiconductor materials with good electrical properties, such as gallium arsenide (GaAs), cannot grow a good-quality oxide layer on the surface, so they cannot be used to make MOSFET components.

    When a large enough potential difference is applied between the gate and source of the MOSFET, the electric field will induce charges on the surface of the semiconductor under the oxide layer, and then a so-called "inversion groove" will be formed. Road" (inversion channel). The polarity of the channel is the same as its drain and source. Assuming that the drain and source are n-type, then the channel will also be n-type. After the channel is formed, the MOSFET can allow current to pass and depending on the voltage applied to the gate, the amount of current that can flow through the channel of the MOSFET will also change under its control.

    I. Working Principle of MOSFET

    The core of MOSFET: metal-oxide layer-semiconductor capacitor

    MOSFET is structured with a metal-oxide layer-semiconductor capacitor as the core (as mentioned earlier, most of today's MOSFETs use polysilicon instead of metal as their gate material), and most of the material of the oxide layer is Silicon dioxide, underneath is silicon as the base, and above it is polysilicon as the gate. Such a structure is exactly equal to a capacitor, the oxide layer plays the role of the dielectric material in the capacitor, and the capacitance value is determined by the thickness of the oxide layer and the dielectric constant of silicon dioxide. The gate polysilicon and base silicon become the two endpoints of the MOS capacitor.

    When a voltage is applied across the MOS capacitor, the charge distribution of the semiconductor will also change. Consider a MOS capacitor formed by a P-type semiconductor (hole concentration is NA). When a positive voltage VGB is applied to the gate and base terminals (as shown in the figure), the concentration of holes will decrease and the concentration of electrons will increase. When the VGB is strong enough, the electron concentration near the gate terminal will exceed the hole. In the P-type semiconductor, the area where the concentration of electrons (with negative charges) exceeds the concentration of holes (with positive charges) is the so-called inversion layer.

    The characteristics of the MOS capacitor determine the operating characteristics of the MOSFET, but a complete MOSFET structure also requires a source that provides a majority carrier and a drain that accepts these majority carriers.

    Circuit of MOSFET

    There are many variations of circuit symbols commonly used in MOSFETs. The most common design uses a straight line to represent the channel, two lines perpendicular to the channel represent the source and drain, and the shorter line parallel to the channel on the left represents the gate. As shown below. Sometimes the straight line representing the channel is replaced with a dashed line to distinguish between enhancement mode MOSFET and depletion-mode MOSFET. In addition, it is divided into NMOSFET and PMOSFET. The circuit symbol is shown in the figure:


    Since the MOSFET on the integrated circuit chip is a four-terminal element, in addition to the gate, source, and drain, there is also a base (Bulk or Body). In the MOSFET circuit symbol, the direction of the arrow extending from the channel to the right can indicate that the component is an N-type or P-type MOSFET. The arrow direction always points from the P terminal to the N terminal, so the arrow pointing from the channel to the base terminal is a P-type MOSFET, or PMOS for short (the channel of this component is P-type); on the contrary, if the arrow points from the base to the channel, it represents the base. Extremely P-type, and the channel is N-type, this component is an N-type MOSFET, referred to as NMOS. In a general distributed MOSFET element (discrete device), the base and source are usually connected together, so the distributed MOSFET is usually a three-terminal element. The MOSFETs in integrated circuits usually use the same common bulk, so the polarity of the base is not marked, and an extra circle is added to the gate of the PMOS to show the difference.

    In this way, MOSFET has 4 types: P-channel enhancement type, P-channel depletion type, N-channel enhancement type, and N-channel depletion type. Their circuit symbols and application characteristic curves are shown in the figure below:


    II. Advantages of MOSFET

    MOSFET was first implemented successfully by D. Kahng and Martin Atalla of Bell Lab. in 1960. The operating principle of this device is the same as the dual-carrier junction type invented by William Shockley and others in 1947. Transistor (Bipolar Junction Transistor, BJT) is completely different, and because of the advantages of low manufacturing cost, small use area, and high integration, it is used in large-scale integrated circuits or very large integrated circuits.

    Due to the gradual improvement of the performance of MOSFET components, in addition to traditional applications such as microprocessors, microcontrollers, and other digital signal processing applications, there are also more and more analog signal processing integrated circuits that can be implemented with MOSFETs. 

    1. Field effect transistors are voltage control elements, while bipolar junction transistors are current control elements. In the case of only allowing less current to be drawn, FETs should be selected; and in the case of low signal voltage and allowing more current from the signal source, bipolar transistors should be selected.

    2. The source and drain of some FETs can be used interchangeably, and the gate voltage can also be positive or negative, which is more flexible than bipolar transistors.

    3. Field effect transistors use majority carriers to conduct electricity, so they are called unipolar devices, while bipolar junction transistors have both majority carriers and minority carriers to conduct electricity. Therefore it is called a bipolar device.

    4. Field effect transistors can work under a very small current and very low voltage conditions, and its manufacturing process can easily integrate many field effect transistors on a silicon chip, so field-effect transistors are in large-scale integrated circuits. Has been widely used.

    III. Types of MOSFET

    1. Dual-gate MOSFET

    Dual-gate MOSFETs are usually used in radio frequency (RF) integrated circuits. Both gates of this MOSFET can control the current. In the application of radiofrequency circuits, the second gate of a double-gate MOSFET is mostly used for gain, mixer, or frequency conversion control.

    2. Depletion MOSFET

    Generally speaking, depletion-mode MOSFETs are less common than the aforementioned enhancement-mode MOSFETs. The depletion type MOSFET changes the impurity concentration doped into the channel during the manufacturing process so that even if the gate of this MOSFET is not applied with voltage, the channel still exists. If you want to close the channel, you must apply a negative voltage to the gate. The largest application of depletion MOSFETs is in "normally-off" switches, while enhanced MOSFETs are used in "normally-on" switches.

    3. NMOS logic

    NMOS with the same drive capability usually occupies a smaller area than PMOS, so if only NMOS is used in the design of logic gates, the chip area can be reduced. However, although NMOS logic occupies a small area, it cannot consume static power like CMOS logic. Therefore, it has gradually withdrawn from the market after the mid-1980s.

    4. Power MOSFET

    Cross-sectional view of a power transistor unit. Usually, a commercially available power transistor contains thousands of such units. Main article: Power transistor

    There is a significant difference in structure between the power MOSFET and the aforementioned MOSFET element. Generally, MOSFETs in integrated circuits have a planar structure, and each endpoint in the transistor is only a few microns away from the chip surface. And all power components are vertical structures so that components can withstand high voltage and high current working environments at the same time. The voltage that a power MOSFET can withstand is a function of the impurity doping concentration and the thickness of the N-type epitaxial layer, and it can pass

    The current is related to the channel width of the component. The wider the channel, the more current it can accommodate. For a MOSFET with a planar structure, the amount of current and breakdown voltage that it can withstand is related to the length and width of its channel. For a MOSFET with a vertical structure, the area of the device is approximately proportional to the current it can hold, and the thickness of the epitaxial layer is proportional to its breakdown voltage.

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