Power MOSFETS Flashcards
How do MOSFETS control behaviour?
Through an electric field. Current will be carried by either major or minor carriers.
What are the four terminals of MOSFETS
Gate, body, source, drain.
What is the working principle of an NPN MOSFET?
The drain and source N type regions will form diodes with the P type substrate. When a drain source voltage is applied a high impedance path is formed. To allow the current to flow we must apply a voltage between the gate and the source to provide an N type channel.
How does applying a Gate Source voltage help current flow through the MOSFET
The voltage at the gate will attract electrons at the substrate while repelling holes which creates an impedance region the forms the channel. This allows current to flow between the drain and the source. The current that flows is proportional to the voltage that is applied to the gate.
What is a Depletion Mode MOSFET?
This type of MOSFET is normally on, or conducting even in the absence of a Gate voltage. We need a negative gate source voltage to deplete the N channel of its electrons, switching the MOSFET off.
How does depletion Mode differ between P and N type MOSFETS?
P type the major charge carriers are holes while N type it is electrons. Therefore in P type we have to deplete the channel of holes by applying a positive voltage. N type we have to apply a negative voltage to deplete it of electrons.
What is Enhancement Mode?
The device is normally off when the gate voltage = 0. Current will only flow once the gate voltage reaches above a certain level.
How does the Enhancement Mode differ between P and N type MOSFETs.
For P type we have to apply a negative voltage to attract holes and create the channel but for N type we have to apply a positive voltage to attract the electrons.
How do we control the MOSFET?
By controlling the voltage across the Gate and the Source.
How does the MOSFET work as an amplifier?
When it operates in the saturation region. For the MOSFET to be in the saturation region the gate source voltage must be greater than the threshold voltage and the drain source voltage must be less than the gate source voltage.
The MOSFET will behave like a voltage controlled resistance where the resistance is proportional to the Gate Source voltage.
How does the MOSFET act like a switch?
We drive it back and forth between the Ohmic and cut off regions.
What are the properties of the Cut Off region?
Input and the gate are driven to 0V. The gate source voltage is below the threshold voltage so the channel isn’t big enough to carry current - drain current = 0. Output voltage is equal to the supply voltage. There is maximum output resistance and the MOSFET acts like an open switch.
What are the properties of the Ohmic region?
Input and Gate voltage is driven to the supply voltage. The voltage across the gate and the source is much greater than the threshold voltage. This means we have max drain current. Output voltage Vds = 0. The main output resistance is very small , less than 0.1Ohm. The MOSFET acts like a closed switch.
In reality, what is the threshold voltage representative of?
It is the voltage at which the drain current increases above 250 micro amps.
What are the parasitic capacitances of the MOSFET?
In reality there are small delays in turning the MOSFET on and off. This is because there are virtual capacitances that exist between the drain and the source, the gate and the source, and the gate drain.
