WEBVTT NOTE Semiconductor Materials and Transistors 1 00:00:00.000 --> 00:00:12.670 Doping. Pure silicon conducts poorly. Adding a trace of another element changes that, and which element decides what kind of semiconductor you get. 2 00:00:12.670 --> 00:00:19.470 Question. Which of the following semiconductor materials contains excess free electrons? 3 00:00:20.370 --> 00:00:22.957 Answer. N-type. 4 00:00:23.557 --> 00:00:32.330 Question. What is the name given to an impurity atom that adds holes to a semiconductor crystal structure? 5 00:00:33.230 --> 00:00:36.213 Answer. Acceptor impurity. 6 00:00:36.813 --> 00:00:54.693 N-type has excess electrons (negative carriers), from a donor impurity. P-type has excess holes (positive carriers), from an acceptor impurity. The letters name the sign of the majority carrier. 7 00:00:54.693 --> 00:01:02.634 Question. Why does a PN-junction diode not conduct current when reverse biased? 8 00:01:03.534 --> 00:01:15.306 Answer. Holes in P-type material and electrons in the N-type material are separated by the applied voltage, widening the depletion region. 9 00:01:15.906 --> 00:01:29.062 Reverse bias pulls both carrier types away from the junction, widening the carrier-free depletion region until nothing crosses. Forward bias pushes them together and current flows. 10 00:01:29.062 --> 00:01:45.506 That widening depletion region is also a capacitance that varies with voltage, which is the varactor diode in the next lesson, and the source of the stray capacitance that limits every transistor's high-frequency performance. 11 00:01:45.506 --> 00:01:52.604 Question. In what application is gallium arsenide used as a semiconductor material? 12 00:01:53.504 --> 00:01:57.545 Answer. In microwave circuits. 13 00:01:58.145 --> 00:02:16.465 Electrons move faster in GaAs than in silicon, so GaAs devices work at frequencies where silicon has run out. That is why microwave low-noise amplifiers are GaAs FETs. Bipolar transistor parameters. 14 00:02:16.465 --> 00:02:22.505 Question. What is the beta of a bipolar junction transistor? 15 00:02:23.405 --> 00:02:30.685 Answer. The change in collector current with respect to the change in base current. 16 00:02:31.285 --> 00:02:34.614 Current gain: β = ΔI_C / ΔI_B. 17 00:02:34.614 --> 00:02:59.024 A small base current controls a much larger collector current, and beta is the ratio, typically 50 to 300, and notoriously variable between devices, which is why the biasing arrangements from the amplifiers lesson exist. 18 00:02:59.024 --> 00:03:12.958 Question. What is the term for the frequency at which the grounded-base current gain of a bipolar junction transistor has decreased to 0.7 of the gain obtainable at 1 kHz? 19 00:03:13.858 --> 00:03:17.618 Answer. Alpha cutoff frequency. 20 00:03:18.218 --> 00:03:24.094 Alpha is the common-base current gain, always slightly less than 1. 21 00:03:24.094 --> 00:03:37.863 Alpha cutoff is where it has fallen to 0.7 of its low-frequency value, the −3 dB point again, and it marks the practical upper frequency limit of the device. 22 00:03:37.863 --> 00:03:45.996 Question. Which of the following indicates that a silicon NPN junction transistor is biased on? 23 00:03:46.896 --> 00:03:55.136 Answer. Base-to-emitter voltage of approximately 0.6 volts to 0.7 volts. 24 00:03:55.736 --> 00:04:08.490 The forward drop of the base-emitter junction, which is just a diode, with the silicon figure from the General components lesson. Field-effect transistors. 25 00:04:08.490 --> 00:04:19.879 Question. How does DC input impedance at the gate of a field-effect transistor (FET) compare with that of a bipolar transistor? 26 00:04:20.779 --> 00:04:26.019 Answer. An FET has higher input impedance. 27 00:04:26.619 --> 00:04:39.539 Much higher. A bipolar's base draws current; a FET's gate is voltage-controlled and draws essentially none, the insulating layer from the General track. 28 00:04:39.539 --> 00:04:46.419 Question. What is a depletion-mode field-effect transistor (FET)? 29 00:04:47.319 --> 00:04:57.084 Answer. An FET that exhibits a current flow between source and drain when no gate voltage is applied. 30 00:04:57.684 --> 00:05:15.256 Depletion mode conducts with zero gate voltage and you turn it off with gate bias. Enhancement mode is off at zero and you turn it on. Most modern MOSFETs are enhancement mode; JFETs are always depletion mode. 31 00:05:15.256 --> 00:05:24.216 Question. What is the purpose of connecting Zener diodes between a MOSFET gate and its source or drain? 32 00:05:25.116 --> 00:05:29.999 Answer. To protect the gate from static damage. 33 00:05:30.599 --> 00:05:46.777 The gate insulating layer is a few tens of nanometres thick and punctures permanently at a few tens of volts, well within what static electricity delivers. Back-to-back Zeners clamp the gate before that happens. 34 00:05:46.777 --> 00:05:56.744 Modern MOSFETs usually have them built in, which is why they survive handling that would have destroyed a 1970s part. Figure E6-1. 35 00:05:56.744 --> 00:06:11.312 This question refers to one of the exam's circuit diagrams, which you will need to look at in the written lesson. Two symbols the pool asks you to identify: Symbol 1: P-channel junction FET. 36 00:06:11.312 --> 00:06:27.490 Symbol 4: N-channel dual-gate MOSFET. Reading FET symbols is mechanical once you know what to look at: Arrow direction on the gate gives the channel type, pointing in for N-channel, out for P-channel, on a JFET. 37 00:06:27.490 --> 00:06:34.467 A gate bar separated from the channel means an insulated gate: a MOSFET rather than a JFET. 38 00:06:34.467 --> 00:06:47.808 Two gate connections means a dual-gate device, used as a mixer or as a gain-controlled amplifier, where the second gate takes the AGC or the local oscillator. Check yourself. 39 00:06:47.808 --> 00:07:02.759 A transistor's base current changes by 0.1 mA and its collector current by 15 mA. Beta? A FET conducts with no gate voltage applied. Which mode? Why is gallium arsenide used where silicon is not? 40 00:07:08.759 --> 00:07:29.959 15 / 0.1 = 150. Depletion mode, you turn it off with gate bias rather than on. Electrons move faster in GaAs, so it works at microwave frequencies where silicon devices have run out of gain.