1. Subject Name: Electronic Circuits Subject Code:10cs32
Prepared By: Supriya.V.Sullad
Department: CSE
Date:
4/10/2019

2. Overview Bipolar Junction Transistor V/S Field Effect Transistor
Junction Field Effect Transistor
Metal Oxide Field Effect Transistor
JFETs V/S MOSFETs
Handling MOSFETs
Biasing MOSFETs
FET Applications
VMOS Devices
CMOS Devices
IGBTs
4/10/2019

3. BJT V/S FET BJTs are Current Controlled Devices,FETs are
Voltage Controlled Devices.
BJTs are Bi-Polar Devices, FETs are Unipolar
Devices.
Input Impedance of FET is very High
(100MΩ),while BJT has less than 1MΩ.
Gain of FET is smaller compared to BJT
FETs have poor sensitivity to the changes in
the input
Fets are more sensitive to handle than BJT’s
4/10/2019

4. Junction Field Effect Transistor
There are Two PN Junctions formed between the semiconductor channel and the semiconductor layers.
The three terminals are Source,Gate and Drain.
4/10/2019

5. When Vgs =0 ,Id increases linearly with
increase in Vds untill Vds reaches
Saturation effect referred as Pinch-Off
voltage (Vp).
For N-Channel FET,Voltage Vgs is
negative,while gate terminal is made
more negative than the source.
Voltage Vgs is positive for P-channel
FET.
4/10/2019

6. The drain resistance rd in the saturation region is given by:
Rd=r0/(1-Vgs/Vp)2
The relation between output current Id
and Vgs is gven by
Id=Idss(1-Vgs/Vp)2
As in the equation,there is square law
relation between Id and Vgs.
Since JFETs are usefull in Radio and
TV recievers.
4/10/2019

7. Metal oxide semiconductor Field Effect Transistor
There are two types of mosfets based on their Construction and mode of Operation:
Depletion Mosfet and Enhancement Mosfet.
In Depletion Mode for a N-Channel device, there is a channel that exists between Two N+ layers .
4/10/2019

8. In Depletion Region,to drive the Current to the Drain,If gate is 0/-ve and Vds is +ve ,Holes of P-subsrate arrracts to to–wards the Junction and Current flows due to recombination of both holes and electrons.This reduces the no. of electron current flow to the Drain.
4/10/2019

9. Enhancement Mosfet
4/10/2019

10. In Enhancement Region,there is no Physical Channel that exixts between the two N+ regions for N type Mosfet.
Vgs and Vds both are given +ve potential such that,electrons from N+ region are attraced towards the gate and the drain Current Flows.While holes of P-Substrate move away from Sio2 layer.As Vgs is increased there is enhanced flow of electron current through the Drain Treminals.
Vds(sat)=Vgs-Vt
4/10/2019

11. Differences between JFET’s and MOSFET.
Id=K(Vgs-Vt)2
Differences between JFET’s and MOSFET.
The input resistance offerred by Mosfet is more.
Jfets have higher Drain Resistance than Mosfet
The Lekage current of Mosfet is smaller than Fet.
Mosfet’s are easier to construct and are more widely used than JFETs.
4/10/2019

12. Handling MOSFETs
The person handling Mosfet should always touch ground before handling the device
This allows discharge of the charge before handling the device.
By Connecting Zener diodes back-to-Back,between the gate and source ,it ensures that Vgs never exceeds the specified maximum rating of the device.
4/10/2019

13. Depletion and Enhancement Mosfet’s
4/10/2019

14. In Depletion mode,Vgs is given +ve voltage.
Biasing Mosfet
In Depletion mode,Vgs is given +ve voltage.
In Enhancement method there exists a Feedback between Drain and the gate terminal, via the resistor Rg.
In this case,
Vdd-IdRd-Vgs=0
Vgs=Vdd-IdRd
Vds=Vdd-IdRd
Hence Vds=Vgs
4/10/2019

15. Voltage Variable Resistors(VVR) Oscillators
FET APPLICATIONS
Fet is used as an
Amplifier
Analog Switch
Multiplexers
Current Limiters
Voltage Variable Resistors(VVR)
Oscillators
4/10/2019

16. VMOSFET DEVICE
4/10/2019

17. VMOS Devices VMOS is a Vertical Structure Mosfet.
The operation of VMOSFET is similar to E-MOSFET i.e no channel exists between the two N+ layers.
For +ve Vgs ,a N-Channel is formed closed to the gate in the vertical direction.
VMOS devices have smaller channel lengths and larger contact area between Channel and N+ doped layers.
4/10/2019

18. CMOS INVERTER
4/10/2019

19. VMosfets have reduced resistance region,power dissipation
VMOS devices have Faster Switching times
CMOS DEVICES:
(Complementary Metal Oxide Semiconductor field effect Transistor)
It has both P-type and N-type E-Mosfet’s diffused on the same chip.
Have higher input impedance and lower Power Consumption and slower switching speed compared to BJT’s.
4/10/2019

20. Cmos device works as an inverter
When the input voltage is at Low level,the Vg2 S2 of the P-Channel E-Mosfet is equal to –Vss.And the Mosfet is in the ON State.
When the input is at High logic equal to Vss,the VgsS2 of P-Channel E-Mosfet is at 0V and Mosfet is OFF State.
4/10/2019

21. IGBT Construction
4/10/2019

22. Insulated Gate Bipolar Transistor(IGBT)
IGBTs have Fast Switching times as MOSFET’s and Lower turn on voltages and larger blocking voltages as BJTs
The Construction of IGBT is similar to VMOSFET except that N+ drain is replaced by P+collector layer forming a vertical PNP Transistor.
The initial offset in the characteristics is around 0.7V because of the ON –State voltage
While the Steep is due to current flow which is due to both electrons and holes.
4/10/2019

23. IGBTs offers slow Switching speeds especially during Turn-Off.
While IGBT’s are susceptable to a gate insulation damage by electro static discharge.
4/10/2019

24. Thank you
4/10/2019