Electronics >> Metal Oxide Semiconductor FETs (MOSFETs)
Field Effect Transistors (FETs) are unipolar devices. Their operation depends only on one type of charge, free electrons (N-type material) or holes (P-type material).
The FETs are the preferred device for switching application, because there is no minority carrier in an FET (junction), therefore it can switch off faster, since no stored charge has to be removed from the junction.
(FETs have majority carriers, but not minority carriers)
N-type material are majority carriers
P-type material are minority carriers
The two types of unipolar transistors are:
- JFETs (junction field effect transistors)
- MOSFETs (metal-oxide semiconductor FETs)
View JFETs: Junction Field Effect Transistors (JFETs)
Metal Oxide Semiconductor FET
(MOSFET)
The main characteristic that differentiates a MOSFET device from a JFET is that the gate in MOSFETs are insulated from the channel by a layer of silicon dioxide. This causes the gate current to be even smaller than what we would have in a JFET device.
MOSFETs are also called IGFET, which stands for Insulated Gate FETs.
The types of MOSFET:
- Depletion-mode MOSFET
- Enhancement-mode MOSFET (E-MOSFET)
The enhancement-mode MOSFETs are used in power switching (turning large current on/off)
N-channel and P-channel MOSFET Schematic Symbols
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Depletion-mode MOSFET
Depletion-mode MOSFET is a piece of N-type material with a small P-type region on the right, and on the left side of the channel a thin layer of silicon dioxide (insulator) is deposited to create an insulated gate.
The electrons flowing from source to drain must travel through the channel between the gate and the substrate.
Depletion-mode MOSFET with a negative gate voltage.
The VDD supplies free electrons to flow from the source to drain. These electrons flow through the narrow channel on the left of the P-type substrate. (The gate voltage controls the width of the channel, and as a result it controls the flow of the source, drain current of the device.)
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Enhancement-mode MOSFET
(E-MOSFET)
An E-MOSFET does not have an N-channel between the source and the drain.
When the gate is positive it will attract free electrons into the P-type region. The free electrons will combine with the holes located next to the silicon dioxide.
At the time that the gate becomes positive enough, all the holes touching the silicon dioxide are filled with free electrons, and they begin to flow from the source to the drain of the device. This effect is the same as creating a thin layer of N-type material next to the silicon dioxide that the gate is connected to. The thin conducting layer that is created is called the N-type inversion layer.
While the N-type inversion layer exists, free electrons can flow easily from source to drain.
The amount of the minimum gate voltage that is required to create the N-type inversion layer is called the threshold voltage, (Vth).
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