12/15/2023 0 Comments Transistor switch![]() ![]() When the device is ON, there will be a finite on state resistance resulting in a forward voltage drop.Maximum operating frequency is also limited. Finite turn on and turn off times, which limit the switching speed.The blocking voltage during OFF state is also limited. ![]() During ON state, the power handling capabilities are limited i.e., limited conduction current.In a practical situation, a semiconductor device like a MOSFET has the following characteristics. Operating speed of the device has no limits.īut the World isn’t ideal and it is applicable even to our semiconductor switches.OFF state resistance should be infinite.When the device is in ON state, there should be zero voltage drop.In OFF state, there should not be any limit on the blocking voltage.During ON state, there should not be any limit on the amount of current it can carry.Ideal Switch Characteristicsįor a semiconductor device, like a MOSFET, to act as an ideal switch, it must have the following features: A semiconductor device like a BJT or a MOSFET are generally operated as switches i.e., they are either in ON state or in OFF state. Semiconductor switching in electronic circuit is one of the important aspects. In both these regions, the MOSFET is in ON state but the difference is in linear region, the channel is continuous and the drain current is proportional to the resistance of the channel.Ĭoming to saturation region, as V DS > V GS – V TH, the channel pinches off i.e., it broadens resulting in a constant Drain Current. The Gate – Drain bias voltage V GD will determine whether the MOSFET is in linear or saturation region. When V GS > V TH, the device is said to be in triode (or constant resistance) region or saturation region depending on the voltage across drain and source terminals V DS.įor any V GS, if V DS V GS – V TH, then the device enters into saturation region. In order to carry a drain current, there should be a channel between the drain and source regions of the MOSFET.Ī channel is created when the voltage between gate and source terminals V GS is greater than the threshold voltage V TH. ![]() Let us now try to understand how an n-Channel Enhancement Mode MOSFET works. If we consider an N-channel MOSFET, both the Source and Drain are made up of n-type, which sit in a P-type substrate. ![]() Irrespective of the application, a MOSFET has basically three terminals namely Gate, Drain and Source. The structure of a MOSFET varies based on the application i.e., MOSFETs in IC technology are fairly lateral, while the structure of Power MOSFETs is more of a vertical channel. The main difference between Enhancement Mode MOSFET and Depletion Mode MOSFET is that in depletion mode, the channel is already formed i.e., it acts as a Normally Closed (NC) switch and in case of enhancement mode, the channel is not formed initially i.e., a Normally Open (NO) switch. The symbols for each of these types of MOSFETs are shown in the image below. Each of these types are further divided into N-channel MOSFET and P-channel MOSFET. MOSFET can be classified into Enhancement type MOSFET and Depletion type MOSFET. Speaking of terminals, a MOSFET is typically a 3-terminal device, which are Gate (G), Source (S) and Drain (D) (even though there is a 4th terminal called Substrate or Body, it is usually not used in either input or output connection). It is a type of field effect transistor with an insulated gate from the channel (hence, sometimes called as Insulated Gate FET or IGFET) and the voltage at the gate terminal determines the conductivity. A MOSFET or Metal Oxide Semiconductor Field Effect Transistor, unlike a Bipolar Junction Transistor (BJT) is a Unipolar Device in the sense that it uses only the majority carriers in the conduction. ![]()
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