WEBVTT NOTE RF Semiconductors and Packaging 1 00:00:00.000 --> 00:00:02.292 Materials by frequency. 2 00:00:02.292 --> 00:00:14.619 Question. Why is gallium arsenide (GaAs) useful for semiconductor devices operating at UHF and higher frequencies? 3 00:00:15.519 --> 00:00:19.439 Answer. Higher electron mobility. 4 00:00:20.039 --> 00:00:28.959 Question. Which of the following materials supports the highest frequency of operation when used in MMICs? 5 00:00:29.859 --> 00:00:33.259 Answer. Gallium nitride. 6 00:00:33.859 --> 00:00:47.814 The ladder is silicon → gallium arsenide → gallium nitride, and each step buys frequency. Electrons move faster, so the device can respond faster. 7 00:00:47.814 --> 00:01:05.019 GaN additionally handles high voltage and power density, which is why modern solid-state HF and VHF amplifiers use GaN devices where they once used bipolar or LDMOS silicon. MMICs. 8 00:01:05.019 --> 00:01:12.059 Question. Which is the most common input and output impedance of MMICs? 9 00:01:12.959 --> 00:01:15.795 Answer. 50 ohms. 10 00:01:16.395 --> 00:01:24.937 Question. What characteristics of MMICs make them a popular choice for VHF through microwave circuits? 11 00:01:25.837 --> 00:01:36.317 Answer. Controlled gain, low noise figure, and constant input and output impedance over the specified frequency range. 12 00:01:36.917 --> 00:01:40.821 That constant 50 ohms is the practical point. 13 00:01:40.821 --> 00:01:57.477 A discrete transistor amplifier needs matching networks designed for it; an MMIC is 50 ohms in and 50 ohms out across its whole range, so it drops into a 50-ohm system with no matching at all. 14 00:01:57.477 --> 00:02:04.917 Question. What type of transmission line is often used for connections to MMICs? 15 00:02:05.817 --> 00:02:08.641 Answer. Microstrip. 16 00:02:09.241 --> 00:02:18.481 At these frequencies every track is a transmission line, so the board is designed as controlled-impedance microstrip. 17 00:02:18.481 --> 00:02:25.149 Question. How is power supplied to the most common type of MMIC? 18 00:02:26.049 --> 00:02:33.849 Answer. Through a resistor and/or RF choke connected to the amplifier output lead. 19 00:02:34.449 --> 00:02:49.223 There is no separate supply pin. DC is fed through the output, via a choke or resistor that passes DC while blocking RF, which is why an MMIC has so few connections. 20 00:02:49.223 --> 00:02:58.023 Question. Which of the following noise figure values is typical of a low-noise UHF preamplifier? 21 00:02:58.923 --> 00:03:02.883 Answer. 0.5 dB. 22 00:03:03.483 --> 00:03:19.443 Half a decibel. Worth having as a reference point: it tells you what "good" means, and it tells you that a preamplifier claiming 0.1 dB is either exotic or optimistic. Packaging. 23 00:03:19.443 --> 00:03:25.403 Question. Which of the following device packages is a through-hole type? 24 00:03:26.303 --> 00:03:29.023 Answer. DIP. 25 00:03:29.623 --> 00:03:36.343 Question. What is a characteristic of DIP packaging used for integrated circuits? 26 00:03:37.243 --> 00:03:45.803 Answer. Two rows of connecting pins on opposite sides of package (dual in-line package). 27 00:03:46.403 --> 00:03:53.243 Dual In-line Package, the classic rectangular chip with legs down both sides. 28 00:03:53.243 --> 00:04:03.182 Question. Why are DIP through-hole package ICs not typically used at UHF and higher frequencies? 29 00:04:04.082 --> 00:04:07.672 Answer. Excessive lead length. 30 00:04:08.272 --> 00:04:25.250 The lead-length problem from the electrical principles lesson, in packaging form. A DIP's leads are perhaps 10 mm of wire, which at 450 MHz is a meaningful reactance between the die and the circuit. 31 00:04:25.250 --> 00:04:35.305 Question. Which of the following component package types have the least parasitic effects at frequencies above the HF range? 32 00:04:36.205 --> 00:04:38.836 Answer. Surface mount. 33 00:04:39.436 --> 00:04:55.775 Question. What advantage does surface-mount technology offer at RF compared to using through-hole components? A. Smaller circuit area. B. Shorter circuit board traces. C. 34 00:04:55.775 --> 00:05:04.617 Components have less parasitic inductance and capacitance. D. All these choices are correct. 35 00:05:05.517 --> 00:05:09.037 Answer. All these choices are correct. 36 00:05:09.637 --> 00:05:14.326 Surface mount is not merely a manufacturing convenience. 37 00:05:14.326 --> 00:05:32.914 At RF it is an electrical improvement: no leads means almost no parasitic inductance, which is why microwave circuits are entirely surface mount and why a through-hole prototype of a UHF circuit often will not work at all. 38 00:05:32.914 --> 00:05:47.316 Check yourself. You need a low-noise preamplifier for 1296 MHz. What semiconductor material, and what noise figure would be good? Why does an MMIC need no matching network? 39 00:05:47.316 --> 00:05:51.837 Why is a DIP-packaged op-amp a poor choice at 450 MHz? 40 00:05:57.837 --> 00:06:07.867 Gallium arsenide (or gallium nitride for power), and around 0.5 dB is typical of a good low-noise UHF preamplifier. 41 00:06:07.867 --> 00:06:19.380 Its input and output impedances are constant at 50 ohms across its specified range, which is what the surrounding system already is. 42 00:06:19.380 --> 00:06:27.317 Excessive lead length, the package's leads present significant parasitic inductance at UHF.