Hello. In this video I will present an intuitive overview of the MOS Transistor, a very, very simple overview. And of course, later on in this course, we will go back and examine things in great quantitative detail. The material that I will cover this week comes from the first chapter of the book, but I would like to warn you that it is out of order. In particular, the material in this video can be found near the end of chapter one. So here you see a simplified view of the MOS transistor. It is built on a so-called p-type substrate. P-type meaning that it has a lot of positive mobile carriers which are called holes, and which we will discuss later on. On top of which we diffuse two n-type regions. N-type meaning that they have a lot of negatively charged mobile electrons that are free to move and conduct electricity. Between the n and the p we form pn junctions, and as you go from the source Terminal to the drain terminal here. You can see that there is no path through which the carriers can flow initially. Later on, we will create a path for the current to flow. The length of the, this region, which is called the channel, will be denoted by L and the width of this channel will be denoted by W. On top of the p-type region we have grown an insulator. Typically silicon dioxide, and often referred to simply as oxide. And on top of that we have a conductive region called the gate. Typically the gate is much taller than what I show you. But for economy of space, we would show it as, a shallow region. The acronym MOS stands for metal oxide semiconductor. It originates in the old days of MOS transistors, where the gate was made out of metal. And the insulator was made out of silicone dioxide. And the rest of course is the semiconductor, which is silicon for us. These days, the gate is not necessarily made of metal and the insulator is not necessarily made of silicon dioxide. But the name has stuck. Another acronym we will be using is FET, standing for field-effect transistor. Because as they will see, the transistor action is due to electric fields from the gate to the channel region. And sometimes we'll combine the two acronyms and call the MOS transistor simply MOSFET. The MOSFET can be very small. For example, it can be 100 nanometers on a side. To give you an idea how small that is, take into account that the human hair has a diameter of about 0.1 millimeters or 100 micrometers, which simply means that in one diameter of the human hair You can fit about 1,000 modern MOS transistors. So this is the region, the reason you can have millions of transistors on the chip. The w and l dimensions. This one and this one, for us, will be the actual dimensions of the final fabricated device. And I have to say this, because when you design the transistor. You're looking at the computer screen and you specify the geometric dimension on this so called the mask, which are wm and lm. But when you fabricate the device, certain distortion takes place, denoted here by delta w and delta l and the final fabricated device has width w. And length L. It is these two dimensions that we will be using in this class. So now here you see the MOS transistor biased in a common way. We have shorted the body or substrate to the source, as you see here. And then between the drain and the source, we apply a positive potential. Positive voltage VDS, drain source voltage, and to the gate, we apply positive potential with respect to the source VGS. And if VGS is large enough, positive charges accumulate on the gate as you see here. These positive charges attract negative charges consisting of electrons below the, the insulator. So now, you have an n-type region which has plenty of electrons, you have another region in the channel which has plenty of electrons and, finally, a drain region with plenty of electrons. So, this has formed a bridge between the source and the drain and current can flow. Because you have connected the voltage with a plus here, the minus there, the electrons will be flowing towards the plus. Of the battery. But negative charges going like this are equivalent to positive charges going like that. So if you define, drain current in this direction, then this current will be positive. Of course, if you increase VDS further, you're going to place more positive charges on the gate, which will mean more negative charges in the channel and therefore the current will increase. So the current, the current versus the drain source voltage, this voltage here, if you plot it goes like that. And if you had the larger VGS, then it could go like this. This region is called non saturation to distinguish it from this region, which is called saturation, and we will be discussing why the current tends to saturate like this. Now as we said before, the body is p-type which means that it has plenty of positively charged, carriers called holes. But of course, the positive charges on the gate have attracted negatively charged electrons in the channel. So you see negative charges in a p-type region, which is the opposite from what you expect. This is why we say that inversion has taken place and depending on the value of the gates or voltage, you can have a weak inversion, or moderate inversion or a strong inversion. So there are three regions of inversion. We will discuss each in great detail in this course. Let me now give you an intuitive fluid dynamical analogue for the operation of the inverse transistor. I start with strong inversion. Here is a repetition of the circuit you just saw on the previous slide and here is the IV characteristic for a given value of VGS. This is the fluid-dynamical analogue I was talking about. So the source region here corresponds to a tank over here full of water. And the drain region corresponds to another tank of water, again full of another tank full of water. In between them, there is this separator that in the position shown prevents communication between the source tank and the drain tank. I have a handle on this separator and I can move it up and down. Now, in interpreting what I'm about to say. Keep in mind that water level corresponds to potential energy. And the way potential energy varies for an electron is in the opposite direction that potential itself varies because the electron has a negative charge. So, lower levels of water mean lower potential energy but higher potential. The distance between the source level and the handle corresponds to VGS. In other words, it corresponds to the voltage over here, this one corresponds to this. Now I lower the handle, which means I increase the VGS. And now, water can flow both from source and drain, and fill the channel area. This corresponds to the channel being filled with electrons. However, because I have the same level of water in the source and the drain, after the channel fills with water, there is no net movement of, of, of water anymore. To make water flow, I'm going to lower the level of the water on the drain tank. So I lower the level, compared to the level at the source. This distance here is corresponds to VDS, which is this quantity over here. And now you can see that water will flow. In this direction, and go into the tank. If you have very large tanks or if you connect externally a pump that maintains the water flow you can achieve steady state in which during which water will keep flowing from the source through the channel towards the drain. And you can see that if you increase this distance VDS there, there will be a larger difference between this level. And this level over here. And therefore more water will be flowing. This corresponds to the fact that when you increase VDS, the drain current increases. But if you do it If you continue doing this and if you keep increasing this distance for example here and you bring the level of water to such a low position that it is below this level over here. Then all of the water that can enter the channel region will spill over to the drain region and any further. Decrease of this level will not effect the total water flow. So that's why you reach saturation. For the water flow which corresponds to the saturation in the current. Continue the fluid-dynamic analogue I showed you what strong inversion is like, now weak inversion corresponds to this. Rather than having fluid flowing, you actually have water vapors flowing. So above the surface here at the source, there are water vapors and the density of these vapors decreases as you go up and the same happens over here. If the levels here, at the drain, and the source are different. You have denser water vapors here then you have over there. So there is a diffusion of water vapors. Through the channel. And in fact, as we will see later on in a weakly invet, inverted MOS transistor, even the electrons flow by diffusion. Finally, moderate inversion is something between strong and weak inversion and it corresponds to a little fluid flow. And also vapor flow. And in fact we will see for later on that there is something very similar happening in the MOS transistor. So let's go back to the circuit and plot now the current. Id versus Vds, Vds, using Vds as a parameter. So for each value of VGS we plot one of these curves. The larger the Vd, Vgs, the more the electrons here and the larger the current. So you go from one curve to one above it and so on. It turns out for a typical, set of parameters for a MOS transistor that practically all of this region is strong inversion. And you do have moderate and weak inversion which is compressed near the bottom and you cannot really see. So instead of plotting, in, on a linear scale, I will plot the drain current on a logarithmic scale. And now, things look like this. We have expanded the low current region. And now we can clearly see the weakened inversion region here. The moderate inversion region. And finally, the strong inversion and region. In fact, we have such resolution for low currents, that we can even see a region affected by leakage. And by leakage, I mean. The leakage current of this junction, the p n junction here is reversed biased and there are some, there is some leakage current associated with it. Which we will discuss later on. So in this course, we will learn to predict the shape of these curves under a variety of conditions. For each separate region of inversion. And we, we'll derive models that actually predict the whole thing, in one single piece, as you go from weak to moderate to strong inversion. In this we do, we saw an overview, an intuitive overview of the MOS transistor. And we use a water analogue to, give an intuitive field for the regions of weak, moderate, and strong inversion. In the next video we will look at some realistic MOS transistor structures in popular fabrication processes.
