The article contains fundamental notes on "Junction and field effect transistors (BJT, FET and MOSFETS)" topic of "Basic Electronics Engineering in Electrical Engineering" subject. Also useful for the preparation of various upcoming exams like GATE Electrical Engineering(EE)/ IES/ BARC/ ISRO/ SSC-JE /State Engineering Services exams and other important upcoming competitive exams.
1.Basics of Mosfet:
A Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a field effect transistor (FET with an insulated gate) where the voltage determines the conductivity of the device. It is used for switching or amplifying signals. The ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. MOSFETs are now even more common than BJTs in digital and analog circuits.
Figure 1: Basic MOSFET structure
There are three possible regions for the working of the MOSFET.
- Triode Region
- Cut-off Region
- Saturation Region
2.Drain Current Equation:
Where μn = mobility of electron
Cox = Capacitance of oxide layer
VDS = drain to source voltage
2.1 Triode region
VDS < VGS – Vt , if VDS (mV)
2.2 Current Saturation
VDS ≥ VGS – Vt
(VDS)Sat = VGS – Vt
→ gm should be more, so kn should be more μn → faster, gain → higher.
→ A good MOSFET should have high value of kn
3.Operating Condition for MOSFET:
3.1.Operating Condition for N channel Enhancement type MOSFET:
Operating region | Required condition |
Cut off region | VGS < VTN |
Triode region | VGS > VTN, VDS < VDS(sat) (or) VDS < VGS – VTN |
Saturation region | VGS > VTN, VDS > VDS(sat) (or) VDS > VGS –VTN |
3.2.Operating Condition for P channel Enhancement type MOSFET:
Operating region | Required condition |
Cut off region | VSG < |VTP| |
Triode region | VSG > |VTP|, VSD < VSD(sat) or VSD < VSG + VTP |
Saturation region | VSG > |VTP|, VSD > VSD (sat) or VSD > VSG + VTP |
3.3.Operating Condition for N channel depletion type MOSFET:
Operating region | Required condition |
Cut off region | VGS < VTN |
Triode region | VGS > VTN, VDS < VDS(sat) (or) VDS < VGS – VTN |
Saturation region | VGS > VTN, VDS > VDS(sat) (or) VDS > VGS – VTN |
3.4.Operating Condition for P channel depletion type MOSFET:
Operating region | Required condition |
Cutoff region | VSG < VTP |
Triode region | VSG > VTP, VSD < VSD(sat) (or) VSD < VSG + VTP |
Saturation region | VSG > |VTP|, VSD > VSD(sat) (or) VSG + VTP |
- MOS Transconductance:
As a voltage-controlled source, a MOS transistor can be characterized by its transconductance
Various dependencies of gm
5.Different biasing methods of MOSFET:
There are four biasing methods for MOSFET: -
- Drain to gate bias
- Voltage divider bias
- Fixed bias
- Self bias
5.1.Drain to gate bias configuration: -
Drain to gate bias Configuration
DC Equivalent
DC Analysis-
Gate current , IG =0
So, we have voltage drop across resistance RG = VRG = 0
Therefore, we get a direct connection between drain and source i.e.
VD = VG
Note:
Drain to gate bias always enables MOSFET in saturation region
For output circuit, we have
VDS = VDD – IDRD
5.2. Voltage divider bias configuration:
Voltage Divider Configuration
DC Equivalent
DC – ANALYSIS:
Using voltage divider, gate voltage is obtained by:
Applying KVL is loop 1, we get
VG – VGS – IDRS = 0
VGS = VG – IDRS …. (1)
Assume that MOSFET is in saturation, so we have ID = Kn (VGS – VTN)2
By solving the quadratic equation, determine the value of VGS or ID, then apply KVL in source to drain loop
VDD – IDRD – VDS – ISRS = 0
VDS = VDD – ID (RS + RD)
If VDS > VGS – VTN, then the transistor is indeed biased in saturation region, as we have assumed.
However, if VDS < VDS (sat), then transistor is biased in the non saturation region
Therefore from equation (1)
VGS = VG – IDRS
5.3.Fixed bias configuration:
Fixed bias Configuration
DC Equivalent
(In DC model, RG is short and input impedance is very high i.e. (IG ≃ 0))
DRAWBACK OF FIXED BIAS:
It is a dual battery design which makes it expensive and more space occupied bias Configuration.
5.4.Self bias configuration:
Self Bias Configuration
DC Equivalent
DC ANALYSIS:
0 = VGS + IDRS
VGS = – IDRS
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