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- 2. • What is MOSFET? • • MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. It is a type of Field Effect Transistor and it is voltage controlled device. It is also called as Insulated Gate Field Effect Transistor (IGFET). It is used for switching or amplifying electronic signals in the electronic devices. It is the most commonly used transistor and it can be used in both analog and digital circuits.
- 5. Construction of MOSFET: • The metallic gate terminal in the MOSFET is insulated from the semiconductor layer by a SiO2 layer or dielectric layer. • The MOSFET consists of three terminals, they are source(S), Gate (G), Drain (D) and the body which is called as substrate. The substrate is connected to the source internally.
- 6. Construction of N channel and P channel Enhancement MOSFET
- 7. • In N channel Enhancement MOSFET the source and drain are of N type semiconductor which is heavily doped and the Substrate is of P type semiconductor. • Majority charge carriers are electrons. The source and drain terminals are physically separated in Enhancement mode. • In P channel Enhancement MOSFET the source and drain are of P type semiconductor which is heavily doped and the Substrate is of N type semiconductor. Majority charge carriers are holes.
- 8. Construction of N channel and P channel Depletion MOSFET
- 9. • In the N Channel depletion MOSFET a small strip of semiconductor of N type connects the source and drain. The source and drain are of N type semiconductor and the Substrate is of P type semiconductor. Majority charge carriers are electrons. The source and drain are heavily doped. • • In the P Channel depletion MOSFET a small strip of semiconductor of P type connects the source and drain. The source and drain are of P type semiconductor and the Substrate is of N type semiconductor. Majority charge carriers are holes. •
- 10. Working of Enhancement MOSFET:
- 11. • In the Enhancement MOSFET the source and the drain are not connected physically so in the symbol lines are broken and in the Depletion mode line is continuous. In the N type the arrow points inside and in the P type arrow points outside.
- 12. N-Channel Enhancement MOSFET Working • To get the drain current first we have to create a channel for the free movement of electrons. • To create a channel we have to apply a voltage between the gate and the source terminal keeping the Gate at a higher potential. This voltage is called VGS. • Now the gate is at higher potential. The free electrons will move toward the gate terminal. As discussed earlier we have a Silicon Dioxide layer at the top. Hence these free electrons will accumulate near the Gate region and will not escape. The silicon dioxide layer also acts as a dielectric. It will allow more free electrons to accumulate near the gate terminal in less applied voltage at the gate terminal.
- 13. • Now on increasing VGS further, a high electric field is developed.. • The free electrons generated will fill the holes near the gate region. This way holes are pushed away from the gate terminal increasing N- type behavior near the gate terminal. • A time will come when an N-channel is created between the two N wells. The VGS voltage at which the channel is created is called the threshold voltage or VT. We can conclude from this discussion when VGS > VT an N channel is induced near the gate terminal as shown in the figure below.
- 15. • A channel is created still we are not getting any current. • Let us see how to get the drain current. Apply a voltage source between the drain and the source keeping the drain at a higher potential. • This voltage is called VDS. On applying this voltage current will start flowing from drain to source. This current is called drain current or ID. • We can conclude from this discussion, when VGS > VT and VDS > 0, the current ID flows from drain to source as shown in the figure below.
- 17. • On increasing the positive voltage at the drain terminal a reverse bias is formed at the PN junction near the drain terminal. • This will result in a thick depletion region near the PN junction. Hence on increasing VDS further, you will see the channel near the drain terminal is becoming narrow. • The drain current will face more resistance near the drain terminal. A situation will reach when the drain current becomes constant and will not increase further. • This situation is called the pinch-off situation and the drain current is called the saturation current. The voltage at which we will get saturation current is called saturation voltage. We can conclude from this discussion, that pinch-off is reached when VGS > 0 (constant) and VDS = VDS(SAT), ID = ID(SAT) as shown in the figure below.
- 18. N Channel Enhancement MOSFET Working
- 19. • Now is there any way to increase the drain current beyond saturation? The answer is yes. Increasing ID further increases the value of VGS. This will increase the width of the complete N-channel. Hence VGS is controlling voltage
- 20. Characteristics of N-Channel E-MOSFET • Characteristics of n-channel E-MOSFET refers to the curves which relate the current and voltage of device with each other. There are mainly two types of characteristics in n-channel E-MOSFET: • Drain Characteristics: These curves provide the relationship between drain current (ID) and drain-to-source voltage (VDS). When different values of drain current and drain-to-source voltage are plotted on graph, it gives respective values of gate-to-source voltage (VGS). These characteristics are also called as V-I characteristics of a curve. • From the graph shown below, it is observed that when the positive value of VGS is increased, the current ID will also increase. This graph consists of two regions: non-saturated region and saturated region. The non-saturated region of the curve is also called as ohmic region, in this region when drain current is increased then subsequently the value of drain-to-source voltage also increases. • Ohmic region lasts till when the value of drain-to-source voltage reaches a threshold value called as threshold voltage (VTN). After this voltage saturation of n-channel E-MOSFET takes place. Hence, the region of curve after threshold voltage is achieved is called as saturated region
- 22. • Cut off region: • No current flows through it and the MOSFET is off. • Ohmic region: • Drain current increases when the drain source voltage increases. Used as amplifier in this region. • Saturation region: • Drain current is constant for drain source voltage. Used as switch in this region. This occurs when the drain source voltage reaches pinch off voltage.
- 23. N-channel Enhancement MOSFET V-I Characteristic Now we can plot VI characteristics very easily. In the VI characteristics, you will see the plots of VDS vs ID for various values of VGS. From the graph, it is clear that the current ID will become constant at a specific value of VDS. current ID increases only when the value of VGS is increased.
- 24. • Transfer characteristics: These curves provide the relationship between drain current (ID) and gate-to-source voltage (VGS). When different values of drain current and gate-to-source voltage are plotted on X- axis and Y-axis respectively, it provides different values of drain-to-source voltage (VDS). These curves are also called as transconductance curves. • From the transfer characteristics of n-channel E-MOSFET shown below it is observed that when the value of gate-to-source voltage is below the threshold voltage (VTN) then no drain current flows. When gate-to-source voltage is increased, and it reaches to threshold voltage then drain current (ID) starts flowing.
- 26. N-Channel Depletion MOSFET Structure
- 27. • Since the channel is already present. connect the Gate terminal to the ground and apply a positive voltage at the drain and source. On applying positive VDS, the electrons in the N channel will move towards the positive drain terminal, and the drain current will start flowing from drain to source. On increasing VDS further, keeping VGS 0, a time will come where ID will become constant, and that value of drain current is called saturation current. • We can conclude from this discussion that when VGS = 0 and VDS > 0, current ID flows from drain to source, and on increasing VDS further, ID = IS = IDSS, as shown in the figure below.
- 29. • The effect of gate voltage on N channel depletion MOSFET. • Apply VGS < 0. Holes from the P-type substrate will attract towards the negative gate terminal and recombine with electrons in the N channel, forming electron-hole pairs. • On increasing negative potential at the gate, more electron-hole combinations will occur, decreasing the number of free electrons in the N channel. As a result, ID decreases. A time will come when the drain current will become zero. The negative gate voltage at which the drain current is zero is called pinch-off voltage or VP. • We can conclude from this discussion at pinch-off, VGS = VP, VDS > 0, and ID = 0, as shown in the figure below.
- 30. Effect of Gate Voltage on MOSFET
- 31. Now we will see the effect of positive gate voltage on the drain current. • On applying VGS > 0, the minority carriers in the p-type substrate, i.e. electrons, will get attracted towards the gate terminal, thereby increasing the concentration of electrons in the N-channel. As a result, the drain current will increase and exceed the saturation current. We can conclude from this discussion, when VGS > 0 and VDS > 0, then ID > IDSS, as shown in the figure below.
- 33. • N channel Depletion MOSFET V-I Characteristics • As discussed earlier, depletion-type MOSFET worked for both positive and negative gate voltages. Now we can plot VI characteristics very easily. In the VI characteristics, you will see the plots of VDS vs ID for various values of VGS.
- 35. • The graph shows that the current ID will flow for both positive and negative values of VGS. You can see from the graph that the drain current is less than the saturation current for the negative value of gate voltage, whereas for the positive value of gate voltage, the drain current exceeds the saturation current. VGS = VP is also represented in this graph for which drain current is zero irrespective of drain to source voltage.
- 36. Difference Between Depletion MOSFET and Enhancement MOSFET Difference between D-MOSFET (Depletion MOSFET) and E-MOSFET (Enhancement MOSFET) is given below:
- 38. • A depletion-mode MOSFET (D-MOSFET) operates by naturally having a conducting channel when no gate voltage is applied, and applying a negative gate voltage "depletes" the channel, reducing its conductivity, while in enhancement mode, a positive gate voltage is applied to create a conducting channel, essentially "enhancing" the current flow through the device; essentially, a D-MOSFET can operate in both depletion and enhancement modes depending on the polarity of the gate voltage applied. • Key points about D-MOSFET in Depletion and Enhancement modes: • Depletion Mode: • When no gate voltage is applied, the channel is already formed and current can flow freely. • Applying a negative gate voltage repels electrons from the channel, creating a depletion region and reducing the channel's conductivity, thus lowering the current flow. • This mode is useful for applications where a high initial current is needed, like in power supplies. • Enhancement Mode: • Applying a positive gate voltage attracts electrons to the channel, creating a stronger conducting path and increasing the current flow. • This mode is similar to a typical enhancement-mode MOSFET, where the channel is induced by the gate voltage