MOSFET

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Metal Oxide Semiconductor Field Effect Transistor.

These are special types of transistors that are voltage controlled rather then current controlled as BJTs are.

mosfets have 3 external connectors knows as the Gate, Drain and the Source (G D S).

Mosfets come in 2 types, the N channel mosfet (most common) and the P channel mosfet.

Mosfet types
N-channel P-channel
N-channel-mosfet.gif P-channel-mosfet.gif

the N type mosfet requires a positive charge on the gate to switch it on while the P type requires a negative charge on the gate to turn it on.

Mosfets have an electricaly isolated gate made of silicon dioxide (SiO2) This means that the gate effectivly acts as a capacitor between Gate and Source pins (Cgs) there is also a gate to drain capacitance (Cgd, miller) and a drain to source (Cds) output capacitance.

Mosfets have a logarithmic decreasing resistance between Source and Drain as you linearly increase the gate voltage, This resistance is called Rds on datasheets. Typically this value is less then an ohm when the gate is fully positive (usually 12 to 20v max) To avoid heating of the mosfet, they are often driven in either the fully on or fully off states to minimise power wasted due to resistive heating. The voltage to start turning on a mosfet can be found in the datasheet under the name Vgs(th). On resistance is specified at a given Vgs well above Vgs(th).

Mosfets also have a maximum allowed voltage drop (Vds) which occures when the mosfet is open circuit, and a maximum allowed current (Ids) which occures when the mosfet is closed circuit. Mosfets also have a maximum power rating (W) which is less then its maximum voltage times its maximum current. Considerations must be taken when chosing a mosfet that these 3 conditions are always higher then the expected during both the on and off states.

A basic circuit that demonstrates how the mosfet can be switched:
Mosfet-basic-circuit.GIF

Driving a MOSFET

Since a MOSFET is voltage-controlled it is fairly simple to drive. It is common that they need 12-20 V of gate voltage but some types like the BUZ11 can be driven directly from a microcontroller.

When a mosfets gate is rapidly switched the charge and discharge current of the capacitive gate can become significant and special considerations must be taken in circuit design such that the required current for the needed risetime is present. Such a task is usually done using specifically designed chips (ICs) knows as mosfet gate drivers. These convert low current signals into higher current signals that is able to rapidly charge and discharge the mosfet gate causing rapid turn on time and reduce losses.

Using 2 or more MOSFETs to increase the current handling

see Paralleling Semiconductors

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