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What is MOSFET?

When you opened the Croduino set for beginners, you must have found one MOSFET transistor among many given components. You must have wondered what is MOSFET and what is its purpose? In this article, we will explain MOSFET in detail, its basic characteristics and work priciple in order for you to understand how this component works and what it can be used for.

What is MOSFET?

MOSFET (engl. Metal–Oxide–Semiconductor Field-Effect Transistor) is a name for a special type of unipolar FET  (engl. Field Effect Transistor) transistor. MOSFET has three pins better known as Source, Drain and Gate. Source and Drain are electrodes derived from layers of semiconductor which are connected into the so-called "channel". Using the control electrode (Gate), we control the flow of current on that channel. MOSFET transistor's main feature is the fact that, unlike bipolar transistors which are current-controlled, MOSFET is voltage-controlled. This means that, the current flow (between Source and Drain electrode) is being controlled by certain amount of voltage brought to the Gate electrode of MOSFET. It is enabled by adding a special thin layer of oxide material, most commonly silicon dioxide (SiO2 - primary glass-making material) around the control electrode, by which it is electrically isolated from the "channel".

MOSFET's work can also be seen as a variable resistor where using the difference between the Source and Gate electrode, we can control the resistance between Source and Drain electrode. When there is no voltage between Source and Gate electrode, the resistance between Source and Drain pin is very high ( it can reach values up to MΩ) and it can be seen as if the circuit is open, i.e. the current does not flow through the circuit. But, by bringing certain amount of voltage between the Source and Gate electrode, specified resistance is reduced, thereby enabling the current flow, i.e. we can say that the circuit is closed.                                        The appearance of static charge can easily damage the isolating oxide layer. Since the oxide layer acts as a very thin dielectric of the capacitor, low static charge can create charge large enough to break through the layer and damage the transistor. Usage requires careful handling to prevent ruining the transistor by static electricity.

 

Types of MOSFET transistors

There are two different types of MOSFET transistors: P (engl. positive) and N (engl. negative) channel MOSFET. Their function is the same, but primarily they differ in the internal structure. P-channel MOSFET's internal structure is made so that the conductive channel is made out of P type, while the N-channel MOSFET's conductive channel is made of N type. For proper operation of the P-channel MOSFET it is necessary to bring negative voltage to the control electrode (Gate). For the N-channel MOSFET it is necessary to bring positive voltage to the control electrode (Gate), in order for it to function properly. The image below shows the internal structure of P and N-channel MOSFET.

Along with that, there are two more types of MOSFET transistors: enhancement and depletion. By the enhancement type of MOSFET, the conducting channel between the Source and Drain electrode is only generated by applying the voltage on the Gate electrode. When talking about the depletion MOSFET transistors, there is a quite opposite characteristic. When there is no voltage on the Gate electrode, the resistance on the conducting channel is minimal. By increasing voltage on the Gate electrode, the resistance on the conducting channel also increases. The image below shows symbols of all types and kinds of MOSFET transistors. Pay attention to the fact that each type has a different symbol that is displayed in the circuit.

What is MOSFET used for?

Now, you must be wondering what are MOSFET transistors used for, do I really need them and where can I find their use? MOSFET is mostly used as an electronic switch or amplifier where by using certain voltage it leaks or controls flows of a different voltage current. Various integrated circuits, such as microprocessors and memory devices, contain from thousands to one million integrated MOSFET transistors on each device, providing basic switch functions necessary for the implementation of logical operations and data storage. Different electronic circuits which contain MOSFET transistors are widely used in applications such as power sources, variable frequency drives and other applications for power supplying where each device can control electric energy of up to 1000W. Radio frequency amplifiers which operate up to very high frequencies (UHF spectrum) use MOSFET transistors as oscillators. It is apparent that the usage of MOSFET transistors is very wide. If you are interested in an example of a project which includes MOSFET transistors, check our tutorial on How to use MOSFET as a switch?.

You want to know more about MOSFET transistors? Then you should certainly check the simulation in which you can see its behavior within the circuit, change parameters of the circuit and see how they affect MOSFET's work.

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