WEBVTT 00:00:00.399 --> 00:00:03.580 Now let's look at the electrical view of the MOSFET. 00:00:03.580 --> 00:00:08.220 Its operation is determined by the voltages of its four terminals. 00:00:08.220 --> 00:00:12.249 First we'll label the two diffusion terminals on either side of the gate terminal. 00:00:12.249 --> 00:00:17.000 Our convention is to call the diffusion terminal with the highest voltage potential the "drain" 00:00:17.000 --> 00:00:21.380 and the other lower-potential terminal the "source". 00:00:21.380 --> 00:00:26.130 With this labeling if any current is flowing through the MOSFET switch, it will flow from 00:00:26.130 --> 00:00:28.830 drain to source. 00:00:28.830 --> 00:00:34.400 When the MOSFET is manufactured, it's designed to have a particular threshold voltage, V_TH, 00:00:34.400 --> 00:00:42.510 which tells us when the switch goes from non-conducting or "open" to conducting or "closed". 00:00:42.510 --> 00:00:50.370 For the n-channel MOSFET shown here, we'd expect V_TH to be around 00:00:50.370 --> 00:00:55.740 The P+ terminal on the left of the diagram is the connection to the p-type substrate. 00:00:55.740 --> 00:01:01.120 For the MOSFET to operate correctly, the substrate must always have a voltage less than or equal 00:01:01.120 --> 00:01:03.760 to the voltage of the source and drain. 00:01:03.760 --> 00:01:07.650 We'll have specific rules about how to connect up this terminal. 00:01:07.650 --> 00:01:12.510 The MOSFET is controlled by the difference between the voltage of the gate, V_G, and 00:01:12.510 --> 00:01:18.510 the voltage of the source, V_S, which, following the usual terminology for voltages, we call 00:01:18.510 --> 00:01:22.870 V_GS, a shortcut for saying V_G minus V_S. 00:01:22.870 --> 00:01:28.120 The first picture shows the configuration of the MOSFET when V_GS is less than the MOSFET's 00:01:28.120 --> 00:01:29.789 threshold voltage. 00:01:29.789 --> 00:01:35.360 In this configuration, the switch is open or non-conducting, i.e., there is no electrical 00:01:35.360 --> 00:01:37.830 connection between the source and drain. 00:01:37.830 --> 00:01:43.520 When n-type and p-type materials come in physical contact, a depletion region (shown in dark 00:01:43.520 --> 00:01:46.830 red in the diagram) forms at their junction. 00:01:46.830 --> 00:01:52.080 This is a region of substrate where the current-carrying electrical particles have migrated away from 00:01:52.080 --> 00:01:53.908 the junction. 00:01:53.908 --> 00:01:59.450 The depletion zone serves as an insulating layer between the substrate and source/drain. 00:01:59.450 --> 00:02:04.490 The width of this insulating layer grows as the voltage of source/drain gets larger relative 00:02:04.490 --> 00:02:06.280 to the voltage of the substrate. 00:02:06.280 --> 00:02:11.989 And, as you can see in the diagram, that insulating layer fills the region of the substrate between 00:02:11.989 --> 00:02:14.880 the source and drain terminals, keeping them electrically isolated. 00:02:14.880 --> 00:02:22.280 Now, as V_GS gets larger, positive charges accumulate on the gate conductor and generate 00:02:22.280 --> 00:02:27.829 an electrical field which attracts the electrons in the atoms in the substrate. 00:02:27.829 --> 00:02:32.680 For a while that attractive force gets larger without much happening, but when it reaches 00:02:32.680 --> 00:02:37.370 the MOSFET's threshold voltage, the field is strong enough to pull the substrate electrons 00:02:37.370 --> 00:02:40.459 from the valence band into the conduction band, 00:02:40.459 --> 00:02:45.239 and the newly mobile electrons will move towards the gate conductor, collecting just under 00:02:45.239 --> 00:02:50.180 the thin oxide that serves the gate capacitor's insulator. 00:02:50.180 --> 00:02:55.260 When enough electrons accumulate, the type of the semiconductor changes from p-type to 00:02:55.260 --> 00:03:00.819 n-type and there's now a channel of n-type material forming a conducting path between 00:03:00.819 --> 00:03:03.689 the source and drain terminals. 00:03:03.689 --> 00:03:08.439 This layer of n-type material is called an inversion layer, since its type has been inverted 00:03:08.439 --> 00:03:11.470 from the original p-type material. 00:03:11.470 --> 00:03:15.459 The MOSFET switch is now closed or conducting. 00:03:15.459 --> 00:03:20.010 Current will flow from drain to source in proportion to V_DS, the difference in voltage 00:03:20.010 --> 00:03:23.720 between the drain and source terminals. 00:03:23.720 --> 00:03:28.170 At this point the conducting inversion layer is acting like a resistor governed by Ohm's 00:03:28.170 --> 00:03:37.079 Law so I_DS = V_DS/R where R is the effective resistance of the channel. 00:03:37.079 --> 00:03:41.918 This process is reversible: if V_GS falls below the threshold voltage, the substrate 00:03:41.918 --> 00:03:47.668 electrons drop back into the valence band, the inversion layer disappears, and the switch 00:03:47.668 --> 00:03:50.329 no longer conducts. 00:03:50.329 --> 00:03:55.019 The story gets a bit more complicated when V_DS is larger than V_GS, as shown in the 00:03:55.019 --> 00:03:56.339 bottom figures. 00:03:56.339 --> 00:04:02.359 A large V_DS changes the geometry of the electrical fields in the channel and the inversion layer 00:04:02.359 --> 00:04:06.689 pinches off at the end of the channel near the drain. 00:04:06.689 --> 00:04:11.269 But with a large V_DS, the electrons will tunnel across the pinch-off point to reach 00:04:11.269 --> 00:04:16.019 the conducting inversion layer still present next to the source terminal. 00:04:16.019 --> 00:04:20.478 How does pinch-off affect I_DS, the current flowing from drain to source? 00:04:20.478 --> 00:04:24.909 To see, let's look at some plots of I_DS on the next slide. 00:04:24.909 --> 00:04:30.669 Okay, this plot has a lot of information, so let's see what we can learn. 00:04:30.669 --> 00:04:36.789 Each curve is a plot of I_DS as a function of V_DS, for a particular value of V_GS. 00:04:36.789 --> 00:04:42.889 First, notice that I_DS is 0 when V_GS is less than or equal to the threshold voltage. 00:04:42.889 --> 00:04:48.800 The first six curves are all plotted on top of each other along the x-axis. 00:04:48.800 --> 00:04:56.680 Once V_GS exceeds the threshold voltage I_DS becomes non-zero, and increases as V_GS increases. 00:04:56.680 --> 00:05:02.270 This makes sense: the larger V_GS becomes, the more substrate electrons are attracted 00:05:02.270 --> 00:05:07.229 to the bottom plate of the gate capacitor and the thicker the inversion layer becomes, 00:05:07.229 --> 00:05:10.780 allowing it to conduct more current. 00:05:10.780 --> 00:05:16.360 When V_DS is smaller than V_GS, we said the MOSFET behaves like a resistor obeying Ohm's 00:05:16.360 --> 00:05:17.639 Law. 00:05:17.639 --> 00:05:22.960 This is shown in the linear portions of the I_DS curves at the left side of the plots. 00:05:22.960 --> 00:05:26.960 The slope of the linear part of the curve is essentially inversely proportional to the 00:05:26.960 --> 00:05:30.439 resistance of the conducting MOSFET channel. 00:05:30.439 --> 00:05:35.330 As the channel gets thicker with increasing V_GS, more current flows and the slope of 00:05:35.330 --> 00:05:40.840 the line gets steeper, indicating a smaller channel resistance. 00:05:40.840 --> 00:05:47.020 But when V_DS gets larger than V_GS, the channel pinches off at the drain end and, as we see 00:05:47.020 --> 00:05:52.540 in on the right side of the I_DS plots, the current flow no longer increases with increasing 00:05:52.540 --> 00:05:54.139 V_DS. 00:05:54.139 --> 00:06:01.300 Instead I_DS is approximately constant and the curve becomes a horizontal line. 00:06:01.300 --> 00:06:07.800 We say that the MOSFET has reached "saturation" where I_DS has reached some maximum value. 00:06:07.800 --> 00:06:12.889 Notice that the saturated part of the I_DS curve isn't quite flat and I_DS continues 00:06:12.889 --> 00:06:16.580 to increase slightly as V_DS gets larger. 00:06:16.580 --> 00:06:21.490 This effect is called channel-length modulation and reflects the fact that the increase in 00:06:21.490 --> 00:06:26.570 channel pinch-off isn't exactly matched by the increase current induced by the larger 00:06:26.570 --> 00:06:28.280 V_DS. 00:06:28.280 --> 00:06:30.000 Whew! 00:06:30.000 --> 00:06:31.639 MOSFET operation is complicated! 00:06:31.639 --> 00:06:37.090 Fortunately, as designers, we'll be able to use the much simpler mental model of a switch 00:06:37.090 --> 00:06:42.870 if we obey some simple rules when designing our MOSFET circuits. 00:06:42.870 --> 00:06:48.710 Up to now, we've been talking about MOSFETs built as shown in the diagram on the left: 00:06:48.710 --> 00:06:53.020 with n-type source/drain diffusions in a p-type substrate. 00:06:53.020 --> 00:06:59.569 These are called n-channel MOSFETs since the inversion layer, when formed, is an n-type 00:06:59.569 --> 00:07:00.569 semiconductor. 00:07:00.569 --> 00:07:05.139 The schematic symbol for an n-channel MOSFET is shown here, with the four terminals arranged 00:07:05.139 --> 00:07:07.169 as shown. 00:07:07.169 --> 00:07:12.070 In our MOSFET circuits, we'll connect the bulk terminal of the MOSFET to ground, which 00:07:12.070 --> 00:07:16.270 will ensure that the voltage of the p-type substrate is always less than or equal to 00:07:16.270 --> 00:07:20.199 the voltage of the source and drain diffusions. 00:07:20.199 --> 00:07:26.080 We can also build a MOSFET by flipping all the material types, creating p-type source/drain 00:07:26.080 --> 00:07:27.830 diffusions in a n-type substrate. 00:07:27.830 --> 00:07:33.659 This is called a p-channel MOSFET, which also behaves as voltage-controlled switch, except 00:07:33.659 --> 00:07:36.689 that all the voltage potentials are reversed! 00:07:36.689 --> 00:07:41.870 As we'll see, control voltages that cause an n-channel switch to be "on" will cause 00:07:41.870 --> 00:07:46.479 a p-channel switch to be "off" and vice-versa. 00:07:46.479 --> 00:07:52.569 Using both types of MOSFETs will give us switches that behave in a complementary fashion. 00:07:52.569 --> 00:07:58.319 Hence the name "complementary MOS", CMOS for short, for circuits that use both types of 00:07:58.319 --> 00:08:00.039 MOSFETs. 00:08:00.039 --> 00:08:04.499 Now that we have our two types of voltage-controlled switches, our next task is to figure out how 00:08:04.499 --> 00:08:09.610 to use them to build circuits useful for manipulating information encoded as voltages.