1 00:00:00,399 --> 00:00:03,580 Now let's look at the electrical view of the MOSFET. 2 00:00:03,580 --> 00:00:08,220 Its operation is determined by the voltages of its four terminals. 3 00:00:08,220 --> 00:00:12,249 First we'll label the two diffusion terminals on either side of the gate terminal. 4 00:00:12,249 --> 00:00:17,000 Our convention is to call the diffusion terminal with the highest voltage potential the "drain" 5 00:00:17,000 --> 00:00:21,380 and the other lower-potential terminal the "source". 6 00:00:21,380 --> 00:00:26,130 With this labeling if any current is flowing through the MOSFET switch, it will flow from 7 00:00:26,130 --> 00:00:28,830 drain to source. 8 00:00:28,830 --> 00:00:34,400 When the MOSFET is manufactured, it's designed to have a particular threshold voltage, V_TH, 9 00:00:34,400 --> 00:00:42,510 which tells us when the switch goes from non-conducting or "open" to conducting or "closed". 10 00:00:42,510 --> 00:00:50,370 For the n-channel MOSFET shown here, we'd expect V_TH to be around 11 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. 12 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 13 00:01:01,120 --> 00:01:03,760 to the voltage of the source and drain. 14 00:01:03,760 --> 00:01:07,650 We'll have specific rules about how to connect up this terminal. 15 00:01:07,650 --> 00:01:12,510 The MOSFET is controlled by the difference between the voltage of the gate, V_G, and 16 00:01:12,510 --> 00:01:18,510 the voltage of the source, V_S, which, following the usual terminology for voltages, we call 17 00:01:18,510 --> 00:01:22,870 V_GS, a shortcut for saying V_G minus V_S. 18 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 19 00:01:28,120 --> 00:01:29,789 threshold voltage. 20 00:01:29,789 --> 00:01:35,360 In this configuration, the switch is open or non-conducting, i.e., there is no electrical 21 00:01:35,360 --> 00:01:37,830 connection between the source and drain. 22 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 23 00:01:43,520 --> 00:01:46,830 red in the diagram) forms at their junction. 24 00:01:46,830 --> 00:01:52,080 This is a region of substrate where the current-carrying electrical particles have migrated away from 25 00:01:52,080 --> 00:01:53,908 the junction. 26 00:01:53,908 --> 00:01:59,450 The depletion zone serves as an insulating layer between the substrate and source/drain. 27 00:01:59,450 --> 00:02:04,490 The width of this insulating layer grows as the voltage of source/drain gets larger relative 28 00:02:04,490 --> 00:02:06,280 to the voltage of the substrate. 29 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 30 00:02:11,989 --> 00:02:14,880 the source and drain terminals, keeping them electrically isolated. 31 00:02:14,880 --> 00:02:22,280 Now, as V_GS gets larger, positive charges accumulate on the gate conductor and generate 32 00:02:22,280 --> 00:02:27,829 an electrical field which attracts the electrons in the atoms in the substrate. 33 00:02:27,829 --> 00:02:32,680 For a while that attractive force gets larger without much happening, but when it reaches 34 00:02:32,680 --> 00:02:37,370 the MOSFET's threshold voltage, the field is strong enough to pull the substrate electrons 35 00:02:37,370 --> 00:02:40,459 from the valence band into the conduction band, 36 00:02:40,459 --> 00:02:45,239 and the newly mobile electrons will move towards the gate conductor, collecting just under 37 00:02:45,239 --> 00:02:50,180 the thin oxide that serves the gate capacitor's insulator. 38 00:02:50,180 --> 00:02:55,260 When enough electrons accumulate, the type of the semiconductor changes from p-type to 39 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 40 00:03:00,819 --> 00:03:03,689 the source and drain terminals. 41 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 42 00:03:08,439 --> 00:03:11,470 from the original p-type material. 43 00:03:11,470 --> 00:03:15,459 The MOSFET switch is now closed or conducting. 44 00:03:15,459 --> 00:03:20,010 Current will flow from drain to source in proportion to V_DS, the difference in voltage 45 00:03:20,010 --> 00:03:23,720 between the drain and source terminals. 46 00:03:23,720 --> 00:03:28,170 At this point the conducting inversion layer is acting like a resistor governed by Ohm's 47 00:03:28,170 --> 00:03:37,079 Law so I_DS = V_DS/R where R is the effective resistance of the channel. 48 00:03:37,079 --> 00:03:41,918 This process is reversible: if V_GS falls below the threshold voltage, the substrate 49 00:03:41,918 --> 00:03:47,668 electrons drop back into the valence band, the inversion layer disappears, and the switch 50 00:03:47,668 --> 00:03:50,329 no longer conducts. 51 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 52 00:03:55,019 --> 00:03:56,339 bottom figures. 53 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 54 00:04:02,359 --> 00:04:06,689 pinches off at the end of the channel near the drain. 55 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 56 00:04:11,269 --> 00:04:16,019 the conducting inversion layer still present next to the source terminal. 57 00:04:16,019 --> 00:04:20,478 How does pinch-off affect I_DS, the current flowing from drain to source? 58 00:04:20,478 --> 00:04:24,909 To see, let's look at some plots of I_DS on the next slide. 59 00:04:24,909 --> 00:04:30,669 Okay, this plot has a lot of information, so let's see what we can learn. 60 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. 61 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. 62 00:04:42,889 --> 00:04:48,800 The first six curves are all plotted on top of each other along the x-axis. 63 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. 64 00:04:56,680 --> 00:05:02,270 This makes sense: the larger V_GS becomes, the more substrate electrons are attracted 65 00:05:02,270 --> 00:05:07,229 to the bottom plate of the gate capacitor and the thicker the inversion layer becomes, 66 00:05:07,229 --> 00:05:10,780 allowing it to conduct more current. 67 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 68 00:05:16,360 --> 00:05:17,639 Law. 69 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. 70 00:05:22,960 --> 00:05:26,960 The slope of the linear part of the curve is essentially inversely proportional to the 71 00:05:26,960 --> 00:05:30,439 resistance of the conducting MOSFET channel. 72 00:05:30,439 --> 00:05:35,330 As the channel gets thicker with increasing V_GS, more current flows and the slope of 73 00:05:35,330 --> 00:05:40,840 the line gets steeper, indicating a smaller channel resistance. 74 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 75 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 76 00:05:52,540 --> 00:05:54,139 V_DS. 77 00:05:54,139 --> 00:06:01,300 Instead I_DS is approximately constant and the curve becomes a horizontal line. 78 00:06:01,300 --> 00:06:07,800 We say that the MOSFET has reached "saturation" where I_DS has reached some maximum value. 79 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 80 00:06:12,889 --> 00:06:16,580 to increase slightly as V_DS gets larger. 81 00:06:16,580 --> 00:06:21,490 This effect is called channel-length modulation and reflects the fact that the increase in 82 00:06:21,490 --> 00:06:26,570 channel pinch-off isn't exactly matched by the increase current induced by the larger 83 00:06:26,570 --> 00:06:28,280 V_DS. 84 00:06:28,280 --> 00:06:30,000 Whew! 85 00:06:30,000 --> 00:06:31,639 MOSFET operation is complicated! 86 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 87 00:06:37,090 --> 00:06:42,870 if we obey some simple rules when designing our MOSFET circuits. 88 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: 89 00:06:48,710 --> 00:06:53,020 with n-type source/drain diffusions in a p-type substrate. 90 00:06:53,020 --> 00:06:59,569 These are called n-channel MOSFETs since the inversion layer, when formed, is an n-type 91 00:06:59,569 --> 00:07:00,569 semiconductor. 92 00:07:00,569 --> 00:07:05,139 The schematic symbol for an n-channel MOSFET is shown here, with the four terminals arranged 93 00:07:05,139 --> 00:07:07,169 as shown. 94 00:07:07,169 --> 00:07:12,070 In our MOSFET circuits, we'll connect the bulk terminal of the MOSFET to ground, which 95 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 96 00:07:16,270 --> 00:07:20,199 the voltage of the source and drain diffusions. 97 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 98 00:07:26,080 --> 00:07:27,830 diffusions in a n-type substrate. 99 00:07:27,830 --> 00:07:33,659 This is called a p-channel MOSFET, which also behaves as voltage-controlled switch, except 100 00:07:33,659 --> 00:07:36,689 that all the voltage potentials are reversed! 101 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 102 00:07:41,870 --> 00:07:46,479 a p-channel switch to be "off" and vice-versa. 103 00:07:46,479 --> 00:07:52,569 Using both types of MOSFETs will give us switches that behave in a complementary fashion. 104 00:07:52,569 --> 00:07:58,319 Hence the name "complementary MOS", CMOS for short, for circuits that use both types of 105 00:07:58,319 --> 00:08:00,039 MOSFETs. 106 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 107 00:08:04,499 --> 00:08:09,610 to use them to build circuits useful for manipulating information encoded as voltages.