电子信息类专业英语,http:/www.cmpbook.com,机械工业出版社同名教材配套电子教案,温丹丽 高源 制作,Reading Material Field-Effect Transistors,机械工业出版社 http:/www.cmpbook.com,Text The field effect transistor or FET finds its greatest use in integrated circuit, especially in the digital area. The circuitry of a single chip often contains several thousand FETS, which are use not only as active devices but also as resistors and capacitors. Compared with general transistor, the FET has special properties of particular significance. One of these is the extremely high input resistance.,Reading Material Field-Effect Transistors,The MOSFET is the most commonly used FET. The letters MOS represent metal-oxide-semiconductor, and the device is often referred to as a MOS transistor, or simply MOS. MOS transistors are classified as P-channel or n-channel device, depending on the conductivity type of the channel（沟道）region. In addition, these devices can also be classified according to their mode（方式，模式）of operation as enhancement（增强）or depletion（耗尽，耗损）type devices.,机械工业出版社 http:/www.cmpbook.com,Reading Material Field-Effect Transistors,A simplified structure of a P-channel MOSFET is illustrated in Fig.1-5. There are two p+-type regions in the n-type substrate. One is called source region, which is represented electrically isolate（使隔离，使孤立）from the silicon by a very thin insulator（绝缘体）. This insulator is usually an oxide（氧化物）and the most common choice is silicon dioxide SiO2.,Reading Material Field-Effect Transistors,Fig.1-5 Structure of a P-channel MOSFET,Reading Material Field-Effect Transistors,Now let us see how the MOSFET as Fig.1-5 to operate in a circuit. We suppose that the substrate and gate terminals are connected to the source, along with a battery between D and S that makes, VDS6V. Thus VGS initially（最初，开头）is equal to zero. Then suppose VGS is changed from zero to 10V with VDS unchanged. The negative gate attracts positive holes. When VGS is beyond a threshold value Vth, the positive holes are attracted more enough so that a localized inversion layer is formed directly below the gate. This serves as a conducting channel between the source and the drain electrodes（电极）.,Reading Material Field-Effect Transistors,As shown in Fig.1-6, there is a continuous P region from the source to the drain and VDS cause holes to flow from the source through the channel to the drain. This is a majority-carrier current, which flows by the drift process. Because their operation depends on only a single type of charge carrier, FET is uniploar（单极的）transistor. In contrast, the bipolar（双极的） junction transistor requires both hole and electron currents. In this case, we see that with zero gate voltage and VGS positive, there is no conducting channel but when VGS is sufficiently negative, a channel is formed and current flows. The more negative we make VGS, the greater the current. A MOSFET that conducts appreciable only when a nonzero voltage is applied to the gate is called an enhancement-mode field-effect transistor.,Reading Material Field-Effect Transistors,Fig.1-6 principle of a P-channel enhancement MOSFET,Reading Material Field-Effect Transistors,In a depletion-mode MOSFET, a conducting channel exists under the gate with no applied gate voltage. The applied gate voltage controls the current flow between the source and the drain by depleting a part of this channel. The operation of a depletion-mode MOSFET is very similar to that of the enhancement-mode MOS transistor. The depletion-mode MOSEFET transistors are often used in conjunction with enhancement-mode MOS transistors to serve as active loads, to improve the gain or the switching speed of the enhancement devices.,Reading Material Field-Effect Transistors,It is informative to compare the basic operation of a p-channel device with that of a PNP junction transistor. In the p-channel MOSFET the holes flow from the source through the channel to the drain, with the flow controlled by the gate voltage. In the PNP transistor the holes flow from the emitter through the base to the collector, with the flow controlled by the base current. Accordingly, there is a functional correspondence between the source and the emitter, the gate and the base, and the drain and the collector.,Class is over!,Have a rest!,追 求,机械工业出版社 http:/www.cmpbook.com,