Transcription
We have already completed the Junction Field Effect Transistors, and now we will start the Metal Oxide Semiconductor Field Effect Transistors, which we call MOSFET. Like BJT and JFET, MOSFET is also an active device.
So first, we will try to understand what an active device is. Any type of circuit component with the ability to control the flow of electrons is known as an active device. If there is a circuit component that can control the flow of electrons and, eventually, the current flow, then we call this component an active device. A few examples are bipolar junction transistors, junction field effect transistors, and MOSFETs.
On the other hand, a circuit component with no ability to control the flow of electrons is known as a passive device. It cannot control the flow of electrons, and a few examples are P-N junction diodes, capacitors, transformers, etc. These devices cannot control the flow of electrons.
Now we will move to the classification of MOSFETs. We already know that MOSFETs are classified into two types: the first one is depletion type MOSFET, and the second one is enhancement type MOSFET. We can further classify them into two types. The depletion type can be classified as N-channel depletion type MOSFET and P-channel depletion type MOSFET. In the same way, enhancement type MOSFET can be classified as N-channel enhancement type MOSFET and P-channel enhancement type MOSFET.
The construction is almost the same, except for one important difference. When you see the construction of depletion type MOSFET and enhancement type MOSFET, you will find they are almost the same, but there is one very important difference: initially, there is no channel between the drain and source in the case of enhancement type MOSFET. However, in the case of depletion type MOSFET, the channel is there from the beginning between the drain and source. This is one very important difference between depletion type MOSFET and enhancement type MOSFET.
In this lecture, we will study enhancement type MOSFET, and to explain the construction and working, I will take N-channel enhancement type MOSFET. So let's start. First, we take the parent material, or body, or base of silicon, and then we dope it with trivalent impurities to make it P-type material. This is the parent material or the silicon wafer, and now we will dope it with trivalent impurities to make it P-type material. We call this substrate or body.
In the case of N-channel enhancement type MOSFET, it is of P-type material. The meaning of substrate is layer, and we will fabricate our device over this layer. After this, two N-type wells are created. This is the first N-type well, and this is the second N-type well. As you can see, there is a junction between two N-type materials and P-type material. Depletion regions will be formed; two depletion regions with the same width will be formed.
Out of the two wells, one well will act as the source, and the other N-well will act as the drain. Let's say this N-well is acting as the source, and this N-well is acting as the drain. Like JFET, we have metal contacts in this case as well. This terminal is the source terminal, and the metal contact is there for the second N-well as well, and this terminal is the drain terminal. The substrate or body is also having a metal contact like this, and the terminal we will represent as S or B, where B stands for body and S stands for substrate.
Now we will talk about the gate terminal. But first, let us make one thing clear: when the substrate or body is of P-type material, then the source and drain are N-type materials. When the substrate or body is of N-type material, then the source and drain are made of P-type material. In the case of P-channel enhancement type MOSFET, the substrate is of N-type, and the source and drains are of P-type. So this is a very easy thing to remember.
Now let's talk about the gate terminal. The gate terminal is not in direct contact with the body; it is located over a very thin layer of silicon dioxide. So let's make a very thin layer of silicon dioxide. This is the thin layer of silicon dioxide, and over this layer, a metal contact is placed. This is the metal contact of the gate, and this terminal is the gate terminal. The width of the layer is very small; the silicon dioxide layer width is nearly equal to 1,000 angstroms, and we also call it gate oxide. We make it very thin because we want control over the surface by the gate electrode.
This is the complete construction of N-channel enhancement type MOSFET. The connections we will see in the next lecture. In this lecture, we will discuss how the channel is formed in the enhancement type MOSFET. You can see there is no channel initially between the source and the drain.
Now let's try to understand how the channel is formed. The substrate is made of P-type material, so we already know the majority charge carriers are holes, and the minority charge carriers are electrons. The overall charge neutrality is maintained, which means mobile charge carriers have the same number as the immobile ions. This is very basic, and you already know this. In P-type material, the majority charge carriers are holes, and the minority charge carriers are electrons. The charge neutrality is maintained, meaning the number of mobile charge carriers (electrons and holes) is equal to immobile ions.
Now we will make the gate terminal more positive with respect to the substrate terminal. We will consider the metal contact at the gate terminal as one plate of a capacitor. Simply consider a capacitor: the first plate of the capacitor is the metal contact of the gate, and the second plate of the capacitor is the substrate. The body or substrate is the second plate of the capacitor, and in between, we have silicon dioxide, which is acting as a dielectric between the parallel plate capacitor.
Now we will connect the gate terminal to a higher potential and the substrate terminal to a lower potential. After applying the voltage, positive charges will accumulate over the gate plate, and negative charges will accumulate over the substrate plate.
Now we have to understand how the negative charges are accumulating over the substrate plate. It is very important to understand this. The substrate plate is the body of the MOSFET, and there are minority charge carriers (electrons) that are negatively charged. Initially, when Vgs (this voltage here is Vgs) is low, electrons will accumulate near the surface, and because of this, we have a negative charge on the plate.
But when you increase Vgs, the uncovering of negative immobile ions will take place in the body. We have positive and negative immobile ions, and when you increase Vgs more, the uncovering of negative immobile ions will take place. Because of this reason, holes will be pushed down. There are negative immobile ions with holes, and when the uncovering of negative immobile ions takes place, this means holes are pushed below. Holes are positively charged, and they are pushed down in the body.
So this is how negative charge will accumulate near the surface. We have tried to understand this with the help of a parallel plate capacitor. Positive charge will be at the gate terminal, and now you can see what is happening. This negative charge is due to minority charge carriers (electrons) and the uncovering of negative immobile ions. Due to this region near the surface, it first becomes less P-type. This region near the surface first becomes less P-type, and then it will become N-type due to excess negative charge.
This is the most important point in this lecture: initially, when Vgs is small, minority charge carriers will accumulate near the surface, and because of this reason, a region near the surface will become less P-type. When you increase Vgs more, the uncovering of negative immobile ions will take place, and this region will become N-type due to the excess negative charge present. This process is known as inversion.
Let me write this down: inversion. In inversion, we change N-type to P-type. We invert N-type to P-type by making the gate terminal more positive with respect to the substrate or body terminal.
So what will happen when this region is converted to N-type? A channel is formed between the source and drain. I will show you how the channel is formed between the source and drain. The region near the surface is N-type, so this N-type and N-type will be joined together like this, and the depletion region will be formed like this.
When you make the gate terminal more positive with respect to the substrate terminal, a channel is formed between the source and drain. This is not the complete working; in the next lecture, we will complete the working of N-channel enhancement type MOSFET.
Before I end this lecture, I will explain inversion one more time. When you make Vgs greater than 0 volts, inversion will happen—the inversion of P-type material to N-type material. Because of inversion, a conductive channel is formed between the source and drain. This is the conductive channel.
So we can say that by increasing Vgs and making it more positive, the channel width or the depth in this case will also increase.
This is all for this lecture. See you in the next one. [Music]