MOSFET- Depletion Type MOSFET Explained (Construction, working and Characteristics Explained)

Summary

This video from ALL ABOUT ELECTRONICS delves into the world of depletion-type MOSFETs, a specific type of insulated gate field effect transistor (IGFET). The creator provides a comprehensive look at both n-channel and p-channel MOSFETs, breaking down their construction, working principles, and characteristics. The video explains how the gate terminal is isolated by an insulating layer, resulting in high input impedance and low power consumption. Viewers learn about concepts such as the pinch-off voltage, the impact of positive and negative gate-source voltages (Vgs), and the differences in behavior between n-channel and p-channel MOSFETs. The video concludes with insights on the symbols differentiating these components.

Highlights

• The gate in a depletion MOSFET is insulated, offering much higher input impedance than JFETs, which is great for minimizing power consumption. 🔋
• N-channel depletion MOSFETs utilize n-type material for the channel and p-type for the substrate, influencing electron flow. 🧲
• Pick Vgs determines electron attraction or recombination affecting current flow through the channel. ↔️
• Pinch-off voltage occurs when Vgs is negative enough for the drain current to cease. ⚠️
• In a p-channel MOSFET, the roles and polarities are reversed, affecting how holes contribute to current flow. 💡
• MOSFET symbols show gate isolation and direction of conventional current, with arrows indicating n-channel or p-channel. ➡️

Key Takeaways

• Depletion-type MOSFETs are a type of IGFET with high input impedance, ideal for low power applications. 📉
• These MOSFETs have an isolated gate terminal, often using an insulating SiO2 layer. 🔍
• Understanding the construction involves n-type or p-type material channels and metallic contacts for terminals. ⚡
• The behavior of these MOSFETs can vary significantly depending on whether Vgs is positive or negative. 🔄
• Symbols for MOSFETs reveal details about the channel conduction type and biasing. 📐

Overview

The video kicks off with a brief reminder of previous discussions on field effect transistors, quickly zooming in on IGFETs, especially the MOSFET due to its insulated gate design. The video emphasizes how this design leads to high input impedance, making depletion-type MOSFETs particularly suitable for applications requiring minimal power.

With engaging clarity, the host unravels the construction intricacies of both n-channel and p-channel depletion-type MOSFETs, illustrating how channel material impacts behavior. The explanation of how voltage differences between the gate and source influence electron or hole movement offers a clear understanding of operational dynamics.

The journey through the video also maps out the graphical characteristics that define MOSFET operations, underlining critical concepts like pinch-off and enhancement regions. The final touches involve a visual lesson on distinguishing MOSFET symbols, equipping viewers with the know-how to identify different types on sight.

Chapters

• 00:00 - 01:00: Introduction to IGFET and MOSFET In this introduction, the YouTube channel 'ALL ABOUT ELECTRONICS' revisits the concept of Field Effect Transistor (FET) and its various types. While the Junction Field Effect Transistor (JFET) was discussed in detail in previous videos, this chapter focuses on exploring the second type of FET, specifically the IGFET and MOSFET.
• 01:00 - 02:30: Construction of n-channel Depletion Type MOSFET The chapter provides an overview of the construction of n-channel depletion type MOSFETs. It starts with the introduction of IGFET, explaining that it stands for insulated gate field effect transistor, where the gate terminal is isolated from the channel using an insulating layer. It highlights that MOSFET, the most common IGFET, stands for metal-oxide-semiconductor field-effect transistor and can be classified into depletion and enhancement types.
• 02:30 - 07:00: Working and Characteristics of Depletion Type MOSFET The video explains the construction and working of an n-channel depletion type MOSFET. The channel is made of n-type material, while the substrate is composed of p-type material. Metallic contacts are used to connect the drain and source terminals to the n-channel. It also mentions the gate terminal but doesn't provide details in this segment.
• 07:00 - 09:30: p-channel Depletion Type MOSFET The chapter explains the structure of a p-channel depletion type MOSFET, emphasizing the lack of direct connection between the N-channel and the gate terminal due to isolation by an SiO2 layer. It describes the components, including metallic contacts for the drain, gate, and source terminals, along with the insulating layer and the semiconductor material channel.
• 09:30 - 11:30: Electronic Symbols of MOSFETs In this chapter, the focus is on the electronic symbols and basic characteristics of MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). A key point discussed is that due to the presence of an insulating layer, there is no current flow through the gate terminal, resulting in a very high input impedance. This property of MOSFETs makes them suitable for applications that require minimal power consumption.
• 11:30 - 12:00: Conclusion The chapter begins with an explanation of a type of MOSFET, specifically focusing on the initial conditions where the gate and source terminals are connected together and grounded. It assumes Vgs equals zero volts and a positive voltage is applied between the drain and source terminal. The effect of the applied voltage is discussed, where electrons in the N channel are attracted towards the positive terminal.

MOSFET- Depletion Type MOSFET Explained (Construction, working and Characteristics Explained) Transcription

• 00:00 - 00:30 Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS. In the earlier video of the field effect transistor, we have briefly discussed about the different types of FET. And in detail we have already discussed about the JFET. So in this video let us see the second type of FET, which is
• 00:30 - 01:00 known as IGFET and here this IGFET stands for insulated gate field effect transistor so in this IGFET, the gate terminal is isolated from the channel using this insulating layer and the MOSFET is the most common type of IGFET. So here this MOSFET stands for metal-oxide-semiconductor field-effect transistor and this MOSFET can be further classified as either depletion type or enhancement type of MOSFET. so in
• 01:00 - 01:30 this video we will learn about the depletion type of MOSFET and first of all let us see its construction. so if you see this n-channel depletion type of MOSFET then the channel is made up of n-type material and the substrate is p-type material. And through the metallic contacts the drain and the source terminals are connected to this n- channel and similarly the gate terminal
• 01:30 - 02:00 is also connected through this metallic contact. But if you observer here there is no direct connection between this N channel and this gate terminal. And the gate terminal is isolated from the channel using this Sio2 layer. So if you see the structure of this MOSFET then it consists of the metallic contacts for this drain, gate and the source terminals. then this insulating layer and the conducting channel which is made up of the semiconductor material. And that
• 02:00 - 02:30 is why this MOSFET is known as the metal-oxide-semiconductor field-effect transistor. Now due to this insulating layer there will not be any flow of current through this gate terminal. Or we can say that the input impedance of this gate terminal is very high and in fact it is even higher than the JFETs. And that is why these MOSFETs are used in the application where the minimum power consumption is required. All right, so now let's see the working of this depletion
• 02:30 - 03:00 type of MOSFET. so initially let us assume that the gate and the source terminals are connected together. And together they are connected to the ground terminal. That means initially let us assume that this Vgs is equal to zero volt. And the positive voltage is applied between this drain and the source terminal. So as soon as we apply the positive voltage then the electrons in this N channel will get attracted towards this positive terminal. So if you
• 03:00 - 03:30 observe from the source terminal, the electron starts moving towards the drain terminal. And in this way the current will establish in this N channel. And if we keep on increasing the voltage between this drain and the source terminal then the current which is flowing through the channel will increase. And this process will continue until all the electrons in this channel will contributes in the flow of current. And then after if we increase this
• 03:30 - 04:00 voltage then the current which is flowing through the channel will become constant so if you see the direction of the conventional current then it will flow from the drain terminal towards the source terminal. And for the Vgs is equal to zero, if you see the output or the drain characteristic then it will look like this. That means as we keep on increasing the value of voltage VDS then the drain current ID will increase. And after certain voltage, the drain current ID will become constant. And the value of the saturation current for vgs
• 04:00 - 04:30 is equal to zero is known as the Idss. Now let's see what happens when the voltage vgs is negative. So the negative voltage at the gate terminal will push the electrons towards the substrate and at the same time the holes in this p-type substrate will also get attracted towards these electrons. So in short, due to the negative voltage at the gate terminal the electrons in the channel
• 04:30 - 05:00 will get recombined with this holes. And the rate of the recombination will depend on the applied negative voltage. so as we increase this negative voltage then the rate of recombination will increase. And that will reduce the number of free electrons which is available in this n-channel. And effectively it reduces the flow of current. So as you can see from the graph, as the value of Vgs will become more and more negative, then the value of drain current will reduce. And at one voltage this drain current will become zero. so this voltage
• 05:00 - 05:30 Vgs is known as the pinch-off voltage. so if you see the drain or the output characteristic of the MOSFET then it looks quite similar to the JFET. But this MOSFET also works for the positive values of Vgs. So now let us see what happens when we apply the positive voltage. so whenever we apply the positive voltage at this gate terminal then the electrons which are
• 05:30 - 06:00 minority carriers in this p-type substrate will also get attracted towards this n-channel. And due to that, the number of free electrons in this N channel will increase. so effectively we can say that the flow of current in this n-channel will increase. so for the positive value of voltage vgs the drain current ID will be even more than this Idss. Now for the JFET we had already seen the transfer characteristic
• 06:00 - 06:30 and we had seen that this transfer characteristic defines the relationship between the input and the output quantity. so basically it defines the relationship between the drain current ID and the voltage vgs for the fixed value of Vds. so now similarly let us see the transfer characteristic of this depletion type of MOSFET. so if you see the transfer characteristic then it will be similar to the JFET. But now you will also get the value of current ID for the positive values of Vgs. So due to that the curve will get extended towards
• 06:30 - 07:00 the right-hand side. Now as we have seen whenever this vgs is positive then the number of free electrons in the channel will increase and due to that this region where the Vgs is positive, is known as the enhancement region and this region where the vgs is negative is known as depletion region. But still the relationship between this current ID and the voltage vgs can be expressed by the
• 07:00 - 07:30 same expression which was used for the JFET. That means drain current Id is equal to Idss times 1 minus Vgs divided by Vp, whole square. So using this expression we can find the value of drain current ID for the given value of Vgs. Alright so, so far whatever discussion that we did was only for the N channel MOSFET. So similarly let us briefly talk about the p-channel type of
• 07:30 - 08:00 MOSFET so in case of a p-channel depletion type of MOSFET the channel is made up of p-type semiconductor material and the substrate is n-type. And for the P channel MOSFET, now the polarity of the applied voltage will also get reversed that means this voltage Vds will be negative and this voltage Vgs will be positive but first of all let us see how the current will flow whenever vgs is equal to 0. So when Vgs is equal to 0 and
• 08:00 - 08:30 Vds is applied in this fashion that means when Vds is negative then the holes in this p-type channel will get attracted towards the negative terminal and the flow of holes will be established in in this fashion. And in this case the conventional current will also flow in the same direction. now whenever we apply the positive value of voltage vgs then the holes will be pushed towards the n-type substrate and at the same time the electrons in this
• 08:30 - 09:00 n-type substrate will also get attracted towards the p-type channel. And due to that this holes and the electrons will get recombined and as we keep on increasing this voltage Vgs then the number of holes in this p-type channel will reduce and effectively the flow of current in this p-type channel will reduce. So if you see the drain or the output characteristic of this p-channel MOSFET then it will look like this. but here this voltage VDS is negative and
• 09:00 - 09:30 the voltage vgs is positive. So as you can see as we keep on increasing this voltage Vgs then the drain current ID will reduce and at the pinch off voltage this drain current ID will become zero. and whenever this vgs is negative then the value of drain current ID will be even higher than this Idss. And similarly if you see the transfer characteristic then it
• 09:30 - 10:00 will look like this. So now let us see the electronic symbols of this n-channel and p-channel MOSFETs. So if you see the symbols of this depletion type of MOSFET then they resembles the actual construction of the MOSFET. so it consists of a three terminals that is gate, drain and the source terminals. And further if you observe this symbol there is a space between this gate terminal and this channel. So this line which connects the drain and the source
• 10:00 - 10:30 terminal represents the channel. And the space between this gate terminal and the channel represents that the gate terminal is isolated from the channel. Now in some MOSFETs this substrate pin is also available externally so in that case the MOSFETs are represented by these symbols. But whenever it is internally connected to the source terminal then these symbols are used. Now if you observe the n-channel and the p-channel MOSFETs then the only
• 10:30 - 11:00 difference between the two symbol is the direction of the arrow. so if it is going inward then it indicates the n-channel MOSFET and if it is going outwards then it represents the p-channel MOSFET. And basically it indicates the direction of the flow of current whenever the PN Junction which is formed by the channel and the substrate is forward biased. So in case of the N channel MOSFET whenever this PN Junction is forward biased then the current will flow in this direction
• 11:00 - 11:30 and similarly for the P channel MOSFET whenever this PN Junction is forward bias then current will flow in the outward direction. So basically by the direction of the arrow we can differentiate these two symbols. So I hope in this video you understood the construction, working and the different characteristic of this depletion type of MOSFET. So similarly in the upcoming video we will learn about the enhancement type of MOSFET. So if you
• 11:30 - 12:00 have any question or suggestion do let me know here in the comment section below. If you like this video hit the like button and subscribe channel for more such videos