WEBVTT NOTE The Diode Zoo 1 00:00:00.000 --> 00:00:13.500 Six diodes, six properties. Each specialised diode exists because one property of a PN junction was exaggerated or exploited. 2 00:00:13.500 --> 00:00:23.868 Diode, Zener; Key property, constant voltage drop under varying current; Use, voltage reference. 3 00:00:23.868 --> 00:00:39.096 Diode, Schottky; Key property, lower forward voltage drop; metal-semiconductor junction; Use, VHF/UHF mixer or detector; efficient rectifier. 4 00:00:39.096 --> 00:00:59.076 Diode, Varactor; Key property, capacitance varies with reverse bias; Use, voltage-controlled capacitance. Diode, PIN; Key property, low junction capacitance; Use, RF switch, attenuator. 5 00:00:59.076 --> 00:01:16.680 Diode, Point-contact; Key property, very low capacitance, tiny junction; Use, RF detector. Diode, LED; Key property, band gap sets the forward voltage; Use, light. 6 00:01:16.680 --> 00:01:22.115 Question. What is the most useful characteristic of a Zener diode? 7 00:01:23.015 --> 00:01:29.307 Answer. A constant voltage drop under conditions of varying current. 8 00:01:29.907 --> 00:01:38.617 That is what makes a reference: the current through it can wander and the voltage across it barely moves. 9 00:01:38.617 --> 00:01:49.697 Question. Which characteristic of a Schottky diode makes it a better choice than a silicon junction diode for use as a power supply rectifier? 10 00:01:50.597 --> 00:01:54.692 Answer. Lower forward voltage drop. 11 00:01:55.292 --> 00:02:01.172 Question. Which of the following is a Schottky barrier diode? 12 00:02:02.072 --> 00:02:06.392 Answer. Metal-semiconductor junction. 13 00:02:06.992 --> 00:02:27.232 No P-type material at all, metal against semiconductor. The result is roughly 0.3 V of drop instead of 0.7, and essentially no charge storage, so it switches extremely fast. Both properties matter: 14 00:02:27.232 --> 00:02:32.692 Question. Which of the following is a common use of a Schottky diode? 15 00:02:33.592 --> 00:02:39.192 Answer. As a VHF/UHF mixer or detector. 16 00:02:39.792 --> 00:02:49.272 Fast switching means it still behaves as a diode at frequencies where an ordinary silicon junction has become a capacitor. 17 00:02:49.272 --> 00:02:57.680 Question. What type of semiconductor device is designed for use as a voltage-controlled capacitor? 18 00:02:58.580 --> 00:03:01.980 Answer. Varactor diode. 19 00:03:02.580 --> 00:03:16.679 The depletion-region capacitance from the previous lesson, used deliberately. It is what tunes a PLL's VCO and what a reactance modulator varies to produce FM. 20 00:03:16.679 --> 00:03:23.639 Question. What characteristic of a PIN diode makes it useful as an RF switch? 21 00:03:24.539 --> 00:03:28.339 Answer. Low junction capacitance. 22 00:03:28.939 --> 00:03:46.758 A PIN diode has an intrinsic layer between P and N, which lowers its capacitance so that when it is off, it is genuinely off at RF, an ordinary diode's junction capacitance would let the signal straight through. 23 00:03:46.758 --> 00:03:53.994 Question. What is used to control the attenuation of RF signals by a PIN diode? 24 00:03:54.894 --> 00:03:59.254 Answer. Forward DC bias current. 25 00:03:59.854 --> 00:04:08.730 At RF a forward-biased PIN behaves as a current-controlled resistor, so a DC bias sets an RF attenuation. 26 00:04:08.730 --> 00:04:19.974 That is a genuinely useful trick: an electronically variable attenuator with no moving parts, used in every AGC-controlled front end. 27 00:04:19.974 --> 00:04:27.822 Question. What property of an LED's semiconductor material determines its forward voltage drop? 28 00:04:28.722 --> 00:04:31.260 Answer. Band gap. 29 00:04:31.860 --> 00:04:49.140 The band gap sets both the photon energy, hence the colour, and the forward voltage. That is why a blue LED drops around 3 V and a red one around 1.8: same physics, read two ways. 30 00:04:49.140 --> 00:04:55.140 Question. What is a common use for point-contact diodes? 31 00:04:56.040 --> 00:04:59.269 Answer. As an RF detector. 32 00:04:59.869 --> 00:05:18.909 A tiny metal whisker on a semiconductor, so almost no junction capacitance. The original crystal-set detector, and still used where extremely low capacitance matters more than robustness. What kills a diode. 33 00:05:18.909 --> 00:05:25.281 Question. What causes a junction diode to fail from excessive current? 34 00:05:26.181 --> 00:05:29.565 Answer. Excessive junction temperature. 35 00:05:30.165 --> 00:05:34.495 Not the current itself, the heat it produces at the junction. 36 00:05:34.495 --> 00:05:46.066 Which is why datasheets specify maximum junction temperature and thermal resistance, and why a diode's current rating depends entirely on how well it is heat-sunk. 37 00:05:46.066 --> 00:05:55.223 The same logic as the pass transistor in the regulators lesson: current is only dangerous through the heat it makes. Figure E6-2. 38 00:05:55.223 --> 00:06:03.244 This question refers to one of the exam's circuit diagrams, which you will need to look at in the written lesson. 39 00:06:03.244 --> 00:06:12.124 Question. In Figure E6-2, which is the schematic symbol for a Schottky diode? 40 00:06:13.024 --> 00:06:15.904 Answer. 6. 41 00:06:16.504 --> 00:06:35.168 The specialised diodes all modify the plain diode's cathode bar: Zener, bar bent at both ends, like a Z. Schottky, bar drawn as a square-cornered S. Varactor, a capacitor symbol added alongside the bar. 42 00:06:35.168 --> 00:06:53.185 Tunnel, bar with a different distinctive bend. The triangle-and-bar is always there; the bar's decoration says which kind. Check yourself. You need a mixer diode for 432 MHz. Which type, and why? 43 00:06:53.185 --> 00:07:05.196 What would you use to build an electronically variable RF attenuator? Why do a red and a blue LED have different forward voltages? 44 00:07:11.196 --> 00:07:27.955 Schottky, a metal-semiconductor junction with low forward drop and no charge storage, so it still switches at UHF. PIN diodes, whose RF resistance is set by forward DC bias current. 45 00:07:27.955 --> 00:07:36.011 Different band gaps, which set both the photon energy (colour) and the forward voltage.