Analog electronics, Electronic components, Electronics, Semiconductors

MOSFET: how does it work?

The MOSFET is another type of field effect transistor. Much more used than JFET and it’s the most important component for digital computers.

This is the link to access the post about junction field effect transistor.

JFETClick here


In addition to three terminals: gate, source and drain, MOSFET has the body terminal. In some transistors, body is linked to the source.

The substrate is made of a semiconductor, usually silicon, doped with impurities to have more or less electrons. Part of semiconductor close to source and drain terminals are doped with opposite type of the substrate. The semiconductors close to drain and source terminals (n+) have more impurities than the substrate (p). Source: StackExchange.

MOSFET (Metal oxide semiconductor field effect transistor) has this name due to small insulating film made of silicon oxide (SiO_{2}), put between the gate’s conductor material and substrate. The conductor electrode is made of polycrystalline silicon or polysilicon and doped with high impurity degree.

Types of MOSFET
The p-type MOSFETs (pMOS) have n type substrate, while n-type (nMOS) have type-p substrate. Source: Warosu.

Due to SiO_{2} insulating layer, this device’s input impedance is higher than JFET.

Types of MOSFETs and operation

In addition to n and p types, the MOSFETs can be divided in other two categories: enhancement and depletion types. They have different operation modes.

Enhancement type MOSFET or E-MOSFET

Enhancement type MOSFET
In the figure, above is the enhancement type MOSFET and below, the symbols on schematics. Source: Circuit Crush.

Applying a difference potential between source and drain, there will be no current circulation on MOSFET. Because there are p-n junction and one of them is on reverse polarization, blocking the current. Gate’s polysilicon, oxide and semiconductor form a MOS capacitor, that generates an electric field when MOSFET receives a positive potential on gate, if its a nMOS.

enhancement type MOSFET operation.
The positive potential on gate attract free electrons on substrate. Electrons concentrate close to gate, creating a channel with excess of electrons between drain and source. Some combine with holes, creating a depletion zone. Where there is no free electrons and holes. Source: Electronics Tutorials.

Due to induced channel, there will be current circulation between drain and source. If it were a pMOS, polarities of V_{GS} and V_{DS} would have to be inverted.

Characteristic curves
These are E-MOSFET’s characteristic curves. Source: Electrical Technology.

The relation between current drain (I_{D}) and V_{GS}.


Where k is a constant related to component construction. Can be calculated as follows:

k=\frac{I_{D(ON)}}{\left (V_{GS(ON)}-V_{T} \right )^{2}}

I_{D(ON)} and V_{GS(ON)} can be found on transistor’s datasheet.

Depletion type MOSFET or D-MOSFET

depletion type transistor
In this type, the MOSFET is already built with a channel connecting drain and source, which is doped with the opposite type of substrate. Source: Electrical and Electronics Tutorials.
D-MOSFET representation.
Representation on schematics. Source: Kotak Enterprise.

Despite the name, it can work on depletion and enhancement mode. Depends on the voltage V_{GS}. When there is no potential on gate and a voltage between drain and source, the current I_{DSS} circulates through the channel.

depletion mode
In a nMOS, when V_{GS} is negative, electrons on channel are repelled in direction to substrate and holes are attracted, creating a depletion region. Resulting in a lower drain current.

If potential on gate is positive, electrons are attracted and holes are repelled. Making the channel expand, increasing current until a limit determined by V_{GS}. Must watch the maximum drain current the component can handle.

The output characteristic curve (on left) and transfer curve (on right) of depletion type MOSFET. Source: RF Wireless World.

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