MOSFET working: Animation with Java applet simulation

MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) are of four different types or modes. They may be enhancement or depletion mode, may be N channel or P channel. How does a MOSFET work? Here i-St@r provides Java applet tutorial of N channel enhancement mode MOSFETs. This article explains the exact working principle of MOSFET with an interactive animation using java applet technology (MOSFET working animation).

MOSFETs are used in your motherboards, for example to generate CPU Voltage, memory Voltage etc. So it is essential for an electronic hobbyist to know the working of a MOSFET. MOSFETs are voltage controlled power devices whereas bipolar transistors are current controlled devices. Gate controlling is clearly demonstrated in this java applet example of MOSFET.

MOSFET working

If no positive voltage is applied between gate and source, the MOSFET is always non-conducting.

The conductivity of the pathway from source to drain is controlled by providing a voltage between gate and body of the semiconductor.

The electric field produced by this applied voltage is spread through out the insulating layer of SiO2 (Silicon Di Oxide).

What happens if we increase VDS while VGS=0V in MOSFET?

Applying VDS with 0 VGS will effect in zero current because the PN junction between N doped drain and P doped substrate is reverse biased.

The only effect of increasing VDS with zero VGS is to enlarge the width of depletion region close to the drain end.

Even though there is no channel, a small reverse saturation current will flow through the reverse biased PN junction.

Effect of VGS in the formation of Inversion layer in MOSFET

Rising VGS produces an electric field that repels holes from the P doped substrate and attracts mobile electrons from the N doped source and drain regions.

Thus an N type conducting channel is formed between the source and drain region once VGS>VT. (VT: - Threshold voltage). This layer is called inversion layer.

Increase VDS by keeping VGS as constant

The voltage between gate and channel is smaller at the drain end than at the source end when increasing VDS.

With VGS=VT, the inversion layer is merely just formed. The channel therefore is said to be pinched off due the reduced field strength at the drain end.

In this situation there is no current flow.

Increase VGS; keep VDS constant

A superior VGS boosts the number of mobile electrons inside the channel, increasing the width of inversion layer.

Increase VDS : Ohmic region and in channel length modulation in MOSFET- For little VDS, the drain current increases linearly with VDS, this is called ohmic or linear region.

While increasing VDS the reverse bias effect is greater at the drain end, as a result the inversion layer is tapering deeper at the source end than at the drain end.

At the pinch off point, the inversion layer depth at the drain end is shrinking to zero.

Additional increase of VDS causes the pinch off point to shift towards the source, dropping the effective length of channel. This effect is called channel length modulation.

Saturation is attained for large value of VDS. At saturation MOSFET acts as a current source, with ID free of VDS and controlled by VGS (Hence the name voltage controlled device).

Did you feel bored while reading this article? Don’t worry the Java applet interactive animation of MOSFET will help you to understand these concepts easily.

How to run Java applet interactive animation of MOSFET

To run the java applet on your PC, you need to perform the following 2 steps.

For detailed information you can read: What is Java applet? How it is useful to study electronics.

If you are already done with these two, you can skip the steps and run the applet by clicking the following link.

MOSFET Java applet interactive animation

Please follow the link:
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/loader.swf?device=mosfet.swf

Copyright: This applet and images are taken from http://www-g.eng.cam.ac.uk and they are the copyright holders.

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