MOSFET : Get Complete Information

MOSFET : This is my second article about electronic devices that I have liked to write about. I hope you also like this article.  Actually MOSFET called the "Metal Oxide Semiconductor Field Effect Transistor". It means MOSFET is like a transistor,but not a transistor. MOSFET is a very fast switching transistor that has shown great promises for low power up to few KWs involving frequency up to 1 MHz applications. Originally, the devices employed surface-grooved technology, called VMOS but today planner DMOS structure is used for high voltage device. 

MOSFET is a three terminal electronic device. It controls the power of the computer and the electronic circuit. You can see MOSFET on any of the desktops and laptops motherboard. It amplify and switch the electrical signal. Today its usage is going on increasing. We can use it in both analog and digital circuits.

MOSFET Information by www.technopediasite.com

MOSFET : Get Complete Information

MOSFET Symbol and Terminals

The circuit symbol and terminal details shown in below figure. There are three terminals in the MOSFET called the drain (D), Source (S), and gate (G).The current flow is from drain to source. The device has no reverse-voltage blocking capability and it always comes with an integrated reverse rectifier.

Unlike a bipolar transistor a MOSFET is a voltage controlled majority carrier device. When positive voltage applied to the gate the transistor switch on. The gate is isolated by a silicon oxide SiO2 layer, and there fore the gate circuit input impedance is extremely high. This feature allows a MOSFET to be driven directly from CMOS or TTL logic. The gate drive current is therefore very low, it cab ne less than milliampere.

MOSFET circuit symbol

MOSFET Symbol and Terminal

Types of MOSFET

There are two type of MOSFET-

1-  p-channel MOSFET
2-  n-channel MOSFET

How to identify n-channel MOSFET

For detection from multimeter, the red prob of the multimeter is put on the drain and when applying the black prob to the source, the multimeter shows some value and if you put the black prob at the gate, it does not show any value. Such like this you can identify easily P-Channel MOSFET.

How to identify p-channel MOSFET 

To detect the N-Channel Mosfet from the multimeter, put the black prob of the multimeter on the drain and put the red prob on the source, so that the multimeter shows some value and if we do not get any value when we put the red prob at the gate such like this you can identify that This is N-Channel MOSFET.

Safe Operation of MOSFET

 MOSFET Operation

Safe operating area of MOSFET

MOSFET has a positive temperature coefficient of resistance and the possibility of secondary breakdown is almost non-existent. If local heating found in the device, the effect of positive temperature coefficient of resistance forces the local concentrations of current to be distributed over the area, thereby avoiding the creation of local hot spots. The safe operating area of MOSFET is shown in above figure. It is bounded by three limits-

(a) Current limit 
(b) Power dissipation limit
(c) Voltage limit.

The SOA can be increased for pulse operation of the device,shown dashes in image. The switching characteristics of MOSFET are similar to those of BJT. However, MOSFET switch on and off very fast, in less than 50 nanoseconds. Because MOSFET can switch under high voltage and current conditions (practically no secondary breakdown) no current snubbing is required during turn off.

However these MOSFET type devices are very sensitive to voltage spikes appearing across them,and snubber circuits may be required to suppress voltage spikes. MOSFET are still not available in high power ratings,MOSFET with rating of 500V, 10A, 50 n sec are available. These devices can be used in parallel for higher current rating.

Advantages of MOSFET over Transistors

Since the transistor is the MOSFET main competitor, it is good to know their relative advantages and disadvantages. The advantages of MOSFET over the transistor are mentioned below-

(a) No power switching losses

(b) No second breakdown

(c) Much larger gain and hence simple and cheaper drive circuitry.

(d) Better reliability, ruggedness and thermal stability.

(e) High peak current handling capability.

(f) Easy to parallel due to the positive resistance coefficient.

(g) Relatively more linear transfer characteristics.

(h) Much higher switching speed.

Disadvantages of MOSFET

Only disadvantages is a higher conduction drop typically 4.5 V compared to 1 V for the transistor for ratings of 400 V and 10 A.

Applications of MOSFET

MOSFET widely used in below mentioned circuit-

1- MOS integrated circuits
2- Digital circuits
3- CMOS circuits
4- Analog circuits
5- Analog switches
6- TV, radio circuits

Probability of Poor Condition or faculty of MOSFET

1.  Leakage MOSFET : If we are checking a MOSFET and showing value on both the Gate and Source from MOSFET Drain, then it is a leakage MOSFET or it is called leakage MOSFET.

2.  Shot MOSFET : If we are checking a MOSFET and it tells the shot (beep) on both the gate and source from MOSFET Drain, then this is called the shot MOSFET.

3.  Open MOSFET : If we are checking a MOSFET and it tells anything (open) from MOSFET Drain on both Gate and Source, then it is open MOSFET.

Note : If we are checking any MOSFET, then N-Channel or P-Channel MOSFET has to be specially taken care of.

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