The construction of the MOSFET is very different when compared to the construction of the JFET. The basic structure of the MOSFET is shown in the above figure. The conductivity of the channel due to electrons or holes depends on N-type or P-type channel respectively. The channel is formed between the drain and source in the opposite type to the substrate, such as N-channel is made with a P-type substrate and P-channel is made with an N-type substrate. Generally, this oxide layer is called as ‘Inversion layer’. In enhancement mode MOSFETs, the conductivity increases by increasing the oxide layer, which adds the carriers to the channel. Here, we can observe that a broken line is connected between the source and drain, which represents the enhancement mode type. The symbols of both N-channel and P-channel enhancement mode MOSFETs are shown below. The Enhancement mode MOSFET is equivalent to “Normally Open” switch and these types of transistors require a gate-source voltage to switch ON the device. The conductivity of the channel in depletion MOSFETs is less compared to the enhancement type of MOSFETs. The depletion mode MOSFET transistors are generally ON at zero gate-source voltage (VGS). ![]() In this type of MOSFETs a thin layer of silicon is deposited below the gate terminal. The arrow symbol indicates the type of channel, such as N-channel or P-channel. ![]() The continuous thick line connected between the drain and source terminal represents the depletion type. In the above symbols, we can observe that the fourth terminal (substrate) is connected to the ground, but in discrete MOSFETs it is connected to source terminal. The symbols for depletion mode of MOSFETs in both N-channel and P-channel types are shown above. MOSFETs are basically classified in to two forms. MOSFETs are also available in both types, N-channel (NMOS) and P-channel (PMOS). MOSFETs also have three terminals, namely Drain (D), Source (S) and Gate (G) and also one more (optional) terminal called substrate or Body (B). In present days, the MOSFET transistors are mostly used in the electronic circuit applications instead of the JFET. The small voltage at the gate terminal controls the current flow through the channel between the source and drain terminals. Like JFET, the MOSFET also acts as a voltage controlled resistor when no current flows into the gate terminal. Due to the insulation between gate and source terminals, the input resistance of MOSFET may be very high such (usually in the order of 1014 ohms. In these transistors, the gate terminal is electrically insulated from the current carrying channel so that it is also called as Insulated Gate FET (IG-FET). The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is one type of FET transistor. Metal Oxide Semiconductor Field Effect Transistor or MOSFET.Junction Field Effect Transistor or JFET. ![]() In case of a FET, the voltage at the Gate (a terminal in FET equivalent to Base in BJT) determines the current flow between the other two terminals. The other and very important difference is that a BJT is essentially a current controlled device meaning the current at the base of the transistor determines the amount of current flowing between collector and emitter. The first difference is that in BJT, both the majority and minority charge carriers are responsible for current conduction whereas in FETs, only the majority charge carriers are involved. There are two main differences between BJT and FET. To learn more about a basics of transistor and its history, read the Introduction to Transistors tutorial. Transistors come a variety of shapes, sizes and designs but essentially, all transistors fall under two major families. They are semiconductor devices that act as either an electrically controlled switch or a signal amplifier. Transistors, the invention that changed the World.
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