WEBVTT 00:00.480 --> 00:03.280 Hello everyone, I'm Typhoon and welcome to another lecture. 00:03.880 --> 00:06.200 In this lecture you will learn about the mOSFET. 00:07.280 --> 00:13.040 The most widely used transistor in modern computer logic circuitry is the mOSFET here. 00:15.520 --> 00:20.200 Which stands for metal oxide semiconductor field effect transistor. 00:21.080 --> 00:26.920 Among the different types of available, the most commonly used for switching purposes is the enhancement 00:26.920 --> 00:27.800 mode mOSFET. 00:27.840 --> 00:33.720 Now in this lecture, we will focus on understanding the behavior of this type to build a strong foundation, 00:33.720 --> 00:37.920 and the more advanced variations will be reserved for future lectures. 00:38.600 --> 00:43.680 So MOSFETs are typically made from silicon, a material known as semiconductor. 00:43.800 --> 00:50.800 A semiconductor allows electricity to pass through, but not as easily as metals, and to enhance its 00:50.800 --> 00:51.680 conductivity. 00:52.000 --> 00:59.640 Silicon is doped, and this means intentionally introducing impurities, and depending on the type of 00:59.640 --> 01:04.730 impurity, the silicon will favor the movements of either electrons. 01:08.690 --> 01:10.170 Or holes. 01:13.770 --> 01:24.810 So electrons are basically negatively charged, and the holes are effectively the absence of electrons 01:25.210 --> 01:28.570 behaving like a positively charged particles. 01:37.570 --> 01:46.090 Now when a semiconductor conducts mainly via electrons, it is called the n type, and when it conducts 01:46.090 --> 01:50.730 primarily through holes, it is called the p type. 01:51.210 --> 01:56.810 Now, the region through which the current flows in a mOSFET is called the channel. 02:03.580 --> 02:11.060 And this channel is situated between two terminals, the source terminal and the drain terminal. 02:20.100 --> 02:28.300 And the third terminal, called gate, controls whether or not current can flow through this channel. 02:30.380 --> 02:37.620 Now this gate is made from the opposite type of semiconductor than the channel and the separated by 02:37.660 --> 02:41.100 a thin insulating layer. 02:44.620 --> 02:52.260 Now, as I explained, there are two basic types of MOSFETs the n channel and p channel. 02:53.180 --> 03:06.480 In n channel, the source is connected to zero volt ground And the drain is connected to five volts 03:06.680 --> 03:08.840 via a resistor. 03:11.440 --> 03:12.880 And the p-channel. 03:13.200 --> 03:26.600 The source is at five volts, and the drain is connected to zero volts via a resistor. 03:27.000 --> 03:33.960 So it's basically the opposite side and you will see how big a difference this will make here. 03:34.440 --> 03:41.680 Now these diagrams represent simplified versions of actual circuits and meant to introduce the functional 03:41.680 --> 03:45.640 principles of mOSFET, uh, behavior. 03:45.800 --> 03:51.520 And um here you can see the each mOSFET has three terminals. 03:51.880 --> 03:57.440 The gate which is the input terminal controls the switch. 03:57.920 --> 04:06.810 From here the drain Connects to the next stage in a circuit. 04:08.010 --> 04:19.010 This is the output terminal and the source connected to the either the higher or lower potential, depending 04:19.370 --> 04:21.530 on the mOSFET type here. 04:21.570 --> 04:28.330 If the source is zero volt connected to zero volt and the in the p channel, we have a source which 04:28.330 --> 04:30.650 is connected to what? 04:31.370 --> 04:32.770 To five volts. 04:33.370 --> 04:39.410 Now the voltage difference between the gate and the source determines whether the mOSFET is on or off. 04:40.010 --> 04:45.810 In n channel and p channel, MOSFETs work in a complementary way, similar to how we understand complements 04:45.810 --> 04:46.930 in a Boolean logic. 04:47.170 --> 04:51.330 So when used together, they can create faster and more efficient digital switches. 04:52.970 --> 04:55.130 Now we will start explaining. 04:55.170 --> 04:59.450 Firstly the N-channel mOSFET operation. 04:59.490 --> 05:00.330 Now we will. 05:00.650 --> 05:04.860 Let's first focus on the N-channel mOSFET shown here. 05:05.580 --> 05:07.940 The drain is connected to what? 05:08.980 --> 05:13.380 Connected to plus five volts through a resistor R. 05:15.700 --> 05:22.820 And the source is connected to zero volts, which is ground. 05:24.740 --> 05:35.300 When the voltage is high, like five volts relative to the source, the channel becomes conductive and 05:35.300 --> 05:39.580 the mOSFET basically acts like a closed switch. 05:41.700 --> 05:43.140 As shown here. 05:43.140 --> 05:48.060 And the voltage at the drain here drops to zero near zero. 05:53.460 --> 06:01.980 However, the current must pass through the resistor R here, which consumes power by converting electrical 06:01.980 --> 06:08.790 signals Animals into heat, and this is not ideal because it leads to unnecessary power loss. 06:13.470 --> 06:20.350 And when the gate voltage is low, which is zero volts, let's say relative to the source. 06:20.710 --> 06:22.790 Now the channel is not conductive. 06:23.270 --> 06:28.110 So the mOSFET behaves like an open switch like this. 06:30.230 --> 06:32.150 So no current basically flows here. 06:32.150 --> 06:35.470 And the resistor acts as a pull up resistor. 06:35.590 --> 06:39.790 Raising the voltage at the drain to five volts. 06:41.310 --> 06:47.510 Now this behavior is essential for digital logic since it helps represent binary states on. 06:47.910 --> 06:49.950 In this case one is zero volt. 06:50.310 --> 06:57.270 But the off here is five volts, which is weird. 06:58.310 --> 06:59.110 And you will. 06:59.150 --> 07:01.590 Understand why it is like that. 07:04.040 --> 07:05.840 And here we have the P channel. 07:06.920 --> 07:09.120 Now let's examine the p channel mOSFET. 07:09.560 --> 07:11.960 Now here we have the opposite. 07:11.960 --> 07:14.320 The source is connected to five volts. 07:14.840 --> 07:19.720 And drain is connected to zero volts through this resistor R. 07:20.680 --> 07:27.520 And when the gate voltage is equal to the source like for example, if the gate voltage is five volt 07:27.520 --> 07:29.360 and the source is five volts here. 07:31.400 --> 07:33.520 The mOSFET is off. 07:33.560 --> 07:36.640 So it behaves like an open switch. 07:37.160 --> 07:41.160 No current flows through the resistor like that. 07:42.360 --> 07:48.040 And when the voltage is lower than the source for example let's say zero volts. 07:50.440 --> 07:57.080 The channel becomes conductive and the mOSFET acts like a closed switch. 07:58.080 --> 08:03.640 So current flows from source to where where. 08:03.770 --> 08:12.490 drain right and the voltage at the drain rises, and in this case the resistor functions as a pull down 08:12.530 --> 08:18.970 device, ensuring drain is at a low voltage when the mOSFET is off. 08:25.210 --> 08:28.090 So summary of the mOSFET switching. 08:28.090 --> 08:33.610 Is mOSFET act like a controllable switches basically. 08:33.930 --> 08:40.530 So n channel we have here in the mOSFET, turn on when the gate is higher voltage than the source. 08:43.850 --> 08:49.650 And the p channel here turns on when the gate is at a lower voltage than the source. 08:50.290 --> 08:57.530 Using both types together enables faster switching and better power efficiency, especially in complementary 08:57.770 --> 09:02.490 MOS, which are CMOs logic design. 09:04.110 --> 09:08.910 Although the pull up and pull down resistors help set proper voltages at the drain, they introduce 09:08.910 --> 09:10.150 two major problems. 09:11.070 --> 09:12.670 The first is power consumption. 09:13.350 --> 09:20.830 So when a mOSFET is on, current flows through the resistor, in this case here, uh, which as you 09:20.830 --> 09:23.230 know, resistors are not very efficient. 09:23.230 --> 09:25.750 So the current is wasted as heat. 09:26.470 --> 09:34.110 And even though mOSFET gate draws almost no current in a steady state, a short burst of current is 09:34.110 --> 09:35.550 needed to change its state. 09:35.710 --> 09:42.350 And this current typically comes from the drain of a previous, uh, mOSFET, and which is limited by 09:42.350 --> 09:44.550 the resistor in its path. 09:46.870 --> 09:53.670 And if the resistor value is high, which to basically reduce the power, the less current flows and 09:53.670 --> 09:55.870 the switching becomes slower. 09:56.070 --> 09:57.910 Now this creates a trade offs. 09:59.150 --> 10:04.430 Uh, if you use small resistors it will allow faster switching, but it will consume more power. 10:04.430 --> 10:10.760 And if you use the large resistors, it will reduce power consumption, but it will slow down the system. 10:12.000 --> 10:18.080 So basically in a high speed computer logic, finding the right balance between speed and energy efficiency 10:18.080 --> 10:20.880 is critical, as you have learned so far. 10:21.040 --> 10:29.720 And this dilemma ultimately led to the development of resistor less logic families like c MOS, where 10:29.760 --> 10:37.120 pairs of MOSFETs are used together to eliminate the need for pull up, pull down resistors entirely. 10:39.280 --> 10:42.280 Yeah, that was that was it with our lecture. 10:42.400 --> 10:47.920 And in the next lecture we will explore how these transistors are combined to form the building blocks 10:48.120 --> 10:52.560 of logic gates like and or and not. 10:53.520 --> 10:59.920 And how the switching behavior we have studied so far translates to Boolean algebra operations in the 10:59.920 --> 11:00.480 hardware. 11:01.120 --> 11:02.480 Now thank you for watching. 11:02.520 --> 11:05.000 I'm Tiffany and I'm meeting you in the next lecture.