1. PRESENTING ON MOSFET OPERATION
&CHARECTERISTICS
Rafsan Rafin Khan.
Chittagong University Of Engineering & Technology
2.
3. FET
• USED AS SWITCH.
• WIDELY USED IN ELECTRONICS DEVICE.
• AUTO INTENSITY CONTROL OF STREET LIGHT
BY MOSFET
Voltage control device.
Three terminal device.
Unipolar Devices
Field effect relates to the depletion region
formed on the channel in FET
Faster then BJT
Low voltage device.
4. FET
BASICALLY TWO TYPES -
1. JUNCTION FIELD EFFECT TRANSISTOR (JFET)
2. METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR
(MOSFET).
5. MOSFET
No P-N junction structure.
Insulator SiO₂ is used.
Polycrystalline Silicon material is used for the
gate material.
6. Construction :
1. No direct connection to the Gate terminal and
the
Channel.
2. SiO₂ is the insulating layer that
create very high input impedance.
3. For insulated gate its also
called IGFET
4. In some cases Substrate is internally
connected to the source terminal.
10. BASIC OPERATION OF ENHANCEMENT TYPE
MOSFET……
The induced channel allow to pass
electrons from source to drain
More wide channel means more
flow of electron.
More gate to source volt means
more current.
P-type N-Channel
MOSFET
11. The gate and channel act as a parallel
plate capacitor.
The gate voltage must be greater then
zero.
The positive potential at gate will
pressure the holes in the p- substrate to
leave the area.
Electrons will attract to the positive
potential.
If gate voltage increase the concentration
of electrons near the siO2 surface
increase rapidly.
Gate voltage when cross the threshold
voltage then the current pass rapidly.
Threshold voltage(VGST ) -The minimum positive
voltage between gate and source to induce n-channel.
13. CHARACTERISTICS OF ENHANCEMENT
TYPE
• VGS always positive
• As VGS increases Id
increases
• Increasing VDS ,keeping
VGS constant, then ID
goes to saturation .
14. VDS=VGS-VT
For fixed value of VT , higher the value of VGS
higher the saturation level of VDS
For value of VGS less then the threshold voltage,
the drain current is zero .
Also When VGS =VT the current will zero.
VDG=VDS-VGS
When VDG is fixed and greater then VDS, then
increasing VDS will create the channel to pinch off.
15. VT>VGS –cut off region.
VGS-VT>VDS>0 – linear region.
VDS>VGS-VT – saturation region.
18. OPERATION :
If the gate to source voltage is zero the current
will pass through the implemented gate
But if this volt is positive the MOSFET will run
as enhancement type.
Negative volt of VGS will run the circuit as
depletion type.
But negative type voltage limit will not exceed
the pinch off level.
If the negative voltage touch pinch off point
then the current flow will stop.
21. Discussion Characteristic between two types of MOSFET :
If Gate voltage is positive and
more then threshold voltage,
then Enhancement Type &
depletion type will not show
the same characteristic.