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MOSFET Metal-Oxide-Semiconductor Field-Effect Transistor


INTRODUCTION The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a Voltage controlled device.    This means that a voltage at the gate control the current flows from the drain to the source. There are three terminals: Gate - connected to the input device. Drain - connected to the positive, since electrons drain away to the positive. Source - the source of the electrons

Slide 4: 

MOSFET is most common field effect transistor in both digital and analog circuits. Uses channel of n or p-type semiconductor, named NMOSFET and PMOSFET A MOSFET is most commonly used in the field of power electronics.


CHARACTERISTICS Voltage controlled device. Input resistance is very high (1012 ohm). The input current is very small about 1 pA (1 × 10-12 A). Very sensitive to static electricity. The output resistance of signal MOSFET would be in the range 10 to 50 k ohms. while in a power MOSFET it would be somewhat lower.

Basic MOSFET (n-channel) : 

Basic MOSFET (n-channel) The gate electrode is placed on top of a very thin insulating layer. There are a pair of small n-type regions just under the drain & source electrodes. If apply a +ve voltage to gate, will push away the ‘holes’ inside the p-type substrate and attracts the moveable electrons in the n-type regions under the source & drain electrodes. Increasing the +ve gate voltage pushes the p-type holes further away and enlarges the thickness of the created channel. As a result increases the amount of current which can go from source to drain — this is why this kind of transistor is called an enhancement mode device

Basic MOSFET (p-channel) : 

Basic MOSFET (p-channel) These behave in a similar way, but they pass current when a -ve gate voltage creates an effective p-type channel layer under the insulator. By swapping around p-type for n-type we can make pairs of transistors whose behaviour is similar except that all the signs of the voltages and currents are reversed. Pairs of devices like this care called complimentary pairs.

Slide 10: 

In an n-channel MOSFET, the channel is made of n-type semiconductor, so the charges free to move along the channel are negatively charged (electrons). In a p-channel device the free charges which move from end-to-end are positively charged (holes).

MOSFET Operation : 

MOSFET Operation The MOSFET can be categorized into three separate modes when in operation. The first is the cut-off mode VGS < Vt, where Vt is the threshold voltage. In the example shown Vt = 1V. In this mode the device is essentially off, and in the ideal case there is no current flowing through the device. The second mode of operation is the linear region when VGS > Vt and VDS < VGS − Vt. Essentially, the MOSFET operates similar to a resistor in this mode with a linear relation between voltage and current.

Slide 12: 

Lastly, saturation mode occurs when VGS > Vt and VDS > VGS − Vt. In this mode the switch is on and conducting, however since drain voltage is higher than the gate voltage, part of the channel is turned off. This mode corresponds to the region to the right of the dotted line, which is called the pinch-off voltage. Pinch-off occurs when the MOSFET stops operating in the linear region and saturation occurs. In digital circuits MOSFETS are only operated in the linear mode, while the saturation region is reserved for analogue circuits.


DIFFERENCES BJT is a current controlled device High leakage current. Used as amplifier,Current control. Low input resistance. MOSFET is voltage controlled device MOSFET's are better due to the low lekages current. Used for high power amplication and switching High input resistance


USES OF MOSFETS High power devices like motors and light bulbs give a large current output for a very tiny current input.  So a MOSFET can act as the interface between an integrated circuit that can give only a tiny current, and the motor that takes a big current.  In complimentary pairs they are used in hi-fi power amplifiers.  They produce less distortion as they are more linear than bipolar transistors. Integrated circuits, as they can be made very compact.

Advantages of a MOSFET : 

Advantages of a MOSFET Less affected by temperature Very much smaller switching current Switching time is about 10 times faster than a bipolar transistor

Disadvantages : 

Disadvantages Higher resistance than a bipolar transistor. Can be destroyed by high voltages, especially static electricity.



Slide 18: 

BJT is a Bipolar Junction Transistor, while MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor. A BJT has an emitter, collector and base, while a MOSFET has a gate, source and drain. BJTs are preferred for low current applications, while MOSFETs are for high power functions. In digital and analog circuits, MOSFETs are considered to be more commonly used than BJTs. The operation of MOSFET depends on the voltage at the oxide-insulated gate electrode, while the operation of BJT is dependent on the current at the base.



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