In this video, we will introduce Metal Oxide Semiconductor Field Effect Transistor or MOSFET. The original concept of MOSFET, was developed way back in the 1930's. However, the practical difficulty of device fabrication has prevented from the wide use until 1960's. However, once the technology has improved enough, to produce high-quality MOSFET. MOSFET quickly replaced Bipolar Junction Transistors as the mainstream dominant technology, for semiconductor electronic devices. MOSFET is generally easier to fabricate, it is more suitable for dense integration which has been a main drive in the technological development in the past few decades and also MOSFET is known to consume less power in general compared to Bipolar Junction Transistor. So, the technological drive for MOSFET, has been to reduce the size of the transistor and increase the density of transistors in a given real estate of a semiconductor wafer. The density of the transistor has increased exponentially over the years and this trend is famously known as the Moore's Law. In back in the 70's, they started out with a hundreds of transistors per millimeter squared and now it's the number is many millions of transistors per millimeter squared. Likewise, the feature size or the gate length of the MOSFET device has decreased exponentially. Now, the gate length is approaching ten nanometers. Now, there's a lot of debate about how far the Moore's Law is going to continue. Given the physical limit, we are already approaching zero. Pretty fast and so, it is pretty clear that Moore's Law is not going to continue like this indefinitely. However, at the same time, given the performance and given the cost of these semiconductor devices based on these MOSFET, it is also pretty clear that the MOSFET based semiconductor device technology is going to be the dominant technology for electronic devices for a long time. The MOSFET is a four-terminal device; there is gate, there's source and drain and there is a back contact called the body terminal. So, these are four. The gate electrode is part of the MOS devices metal oxide, this yellow region here is the oxide and the green region is the semiconductor. So, the gate electrode is part of the MOS device, which we discussed in the previous module. This gate region is surrounded or sandwiched by source and drain which is an n-type region. This is built on a p-type substrate. The basic operation is to apply a voltage on the gate, to create an inversion layer in this MOS device and when these inversion layer is formed, then you have a direct electrical connection between the source and drain region and now you can apply a voltage between drain and source to drive a current. So, this is the basic operation with your MOS device. There are two types of MOSFETs; n-channel and p-channel. It depends on what type of substrate that you're using, p-type substrate leads to n-channel device, n-type substrate leads to p-channel device and this is because the gate region, gate devices is a metal oxide semiconductor device. So, if you have an MOS device using p-type substrate, when you create an inversion layer, the inversion layer will be an n-type layer, electron layer. So, hence the n-channel MOSFET and if you use an n-type substrate, then you will create a p-channel by inverting the surface region. Because we primarily discuss the MOS device using p-type substrate for continuity, we will focus in this module. We will focus our discussion on the n-channel MOSFET device using a p-type substrate. Normally, the source and drain regions are electrically disconnected because in source and drain regions are n-type region on a p-type substrate. So, there is no direct electrical connection normally until, you apply a voltage on the gate and create an inversion layer then you have an electrical conduction between source and drain. Now, this type of device is called the enhancement mode MOS device. Normally, when you don't have a bias on the gate, it is electrically disconnected. You have to apply a bias on the gate to create an inversion layer to turn on the channel as it's called, in order to create on the electrical conduction between source and drain. So, this is the most common type of MOSFET. Is called the enhancement mode type. Sometimes though, you can make a device where it is turned on with zero gate bias. You have to apply a voltage on the gate to eliminate, to remove the inversion layer and this type of device is called a depletion mode MOSFET device. In our course, we will focus primarily on the enhancement mode type MOSFET, because it's the most common type. Here is a circuit symbol, there are four terminals as I said already gate, drain, source and substrate. The type of the substrate is indicated by the arrow and also if you have two narrow lines as shown here, that means it's the enhancement type. If you have a thick black line here, that indicates the depletion mode device.
