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Chelmsford Amateur Radio Society Advanced Course Technical Aspects Part-5 - Semiconductors Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 1 Solid State Devices • Semiconductors form the basis of all modern solid state devices - diodes, transistors, analogue and digital integrated circuits etc • Common Semiconductors are Silicon and Germanium • Advance Course requires a knowledge of – Semiconductor theory – Diodes, including Zeners and Varicaps – Bipolar & FET Transistors – Amplifiers - Circuits, Classes, Efficiency Note: RF Amps inc. Valves covered in Transmitter Course Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 2 Origins of Semiconductors • Silicon and Germanium are in Group-IV where each atom has 4 electrons in its outer shell Group IV Si Silicon Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 3 Semiconductors • In pure (intrinsic) Silicon and Germanium all four outer electrons pair with neighbours in the crystal lattice leaving none free - making them insulators • By doping these materials with very small amounts of impurities, Electron-rich (N-type) or Electron-poor versions (P-type) can be created • N-type dopants have one extra electron and come from Group-V – Phosphorous, Arsenic, Antimony • P-Type dopants have one less electron and come from Group-III – Boron, Aluminium, Indium Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 4 N-type and P-type • Example shown is Silicon with n-type doping by Arsenic • Arsenic has an extra electron not used for pairing up in covalent bonds, and is free to move under bias • In p-type - a positive ‘hole’ is left due to a shortage which behaves similar to a real electron ‘Spare’ Electron As Si n-type doping Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 5 P-N Junction - The Diode • A junction between P-type an N-type material will have a charge across it - a potential barrier • Near the junction some electrons fill the holes nearby, making the region devoid of charge, the depletion layer, which is an insulator • If bias is applied, the depletion layer narrows until electrons will flow easily from N to P • If reverse bias is applies the depletion region will widen and stop flow Junction N - - + + P + + Depletion Region Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 6 Diodes • Standard Diodes act as rectifiers • Forward threshold Vf is: Silicon ~0.6v, Germanium ~0.4v • Reverse Breakdown, Vr usually many volts +I Ge Si Vr Vf -I Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 7 Varicap Diodes • When Diodes are reverse biased the depletion layer acts as the insulating layer of a capacitor • Varicap Diodes also known as Varactors exploit this • Higher reverse voltages widen the depletion layer, driving the capacitive plates apart, lowering the capacitor value • Typical values are of the order of pF Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 8 Zener Diodes • In normal diodes, little current flows when reverse biased up to the point of catastrophic breakdown • Zener Diodes have a well defined reverse breakdown voltage which can act as a voltage reference for PSU regulators • Current in the diode must be limited to avoid excess heat dissipation +VE +5.1V 0V Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 9 Bipolar Transistors • Ordinary transistors are known as bipolars - two P-N junctions • Apply ‘bias’ current in the base to control Collector-Emitter current • Small Signal Gain or ‘Beta’ is the ratio of IC/IB IC = ß x I B Collector Collector IC - N- + P + - - N- Emitter Chelmsford Amateur Radio Society Advanced Licence Course NPN IE Base IB Emitter Base NB: IE = IC + IB Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 10 Bipolar Transistors • Two types - NPN and PNP Collector • Base-Emitter similar to Diode characteristic Base • ‘Bias’ current in the base controls Collector-Emitter current • PNP has negative current in the Base for bias 40µA 1 mA mA Emitter IB = 60µA IC IC NPN 20µA E B C PNP 0.5 V VBE Chelmsford Amateur Radio Society Advanced Licence Course 12 V Murray Niman G6JYB VCE Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 11 Bias Issues • Bias determines the operating point of a transistor • Transistors are temperature dependent and have variable gain • Circuits need to be designed to be relatively independent of this and give stable operation • This issue is known as bias stability Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 12 FETs d p-type • FETs are a semiconductor device similar to a Valve Depletion Layer g • Operates by a field effect due to Voltage (as opposed to Current in a Bipolar) • GDS Terminology refers to Electron flow s • n-Channel and p-Channel variants exist n-Channel • Insulated Gate FETS give NMOS, PMOS, and CMOS - which are all static sensitive Drain G Gate D G2 S G1 d g Depletion Layer about to pinch off channel Source n-Channel Chelmsford Amateur Radio Society Advanced Licence Course p-Channel Dual Gate Insulated FET Murray Niman G6JYB s Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 13 Common Emitter • Three circuit configurations are possible:- +V Common Emitter, Common Collector and Common Base Output • Common Emitter refers to Emitter being Common to Input & Output • A rise in the input voltage turns on the device harder lowering the voltage on the collector and output Input Low Zin ~1K Medium Zout ~ 5K • Thus the circuit inverts, or is said to give 180° phase change Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 14 Common Collector • Common Collector is also popularly called Emitter Follower +V • Output Voltage is similar to input but can supply much more current Input • So no Voltage gain, but used for current buffering • Collector is Common, as at AC the PSU rails have zero potential Chelmsford Amateur Radio Society Advanced Licence Course High Zin 50k-2M Murray Niman G6JYB Output Low Zout 10-500 Ohms Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 15 Common Base • Base is common to input and output - thus Common Base • A positive input voltage will decrease VBE and reduce IC; causing the Collector voltage to rise, so output is in phase with input • Mainly used for RF frequencies - eg in IF Amplifier chains +V High Zout 50k Input Low Zin 50 Ohms Output Capacitor ensures no signal on the base • Common base amps amplify voltage - not current Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 16 Amplifier Class & Bias • Class-A, B, AB and C are defined by the bias and operating region of the transistor • Higher Classes aim to reduce wasted Output Current and improve efficiency IC IC Distorted Output Output VBE Input signal normal bias voltage Chelmsford Amateur Radio Society Advanced Licence Course VBE Input signal low bias voltage Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 17 Amplifier Classes • Class-A Biased well on for high fidelity but also results in low efficiency and high heat dissipation in poweramps • Class-B Gives only only half the waveform, so usually used in Push-Pull configurations. Fairly efficient, but can give crossover distortion • Class-AB A variation of above with transistor biased to conduct for more than half a cycle for better fidelity, but modest dissipation • Class-C Nonlinear but efficient - high distortion needs filtering - Useful for constant amplitudes such as FM and GSM mobile phones • Other Classes exist but are out of scope: D, E, F, G, H, S etc Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 18 Class-B Push-Pull +15 V • Class-B only gives half a sine wave TR1 • In Push-Pull:TR1 gives positive half, TR2 give negative half 0.5 Ohms • Need to keep Bases at 0.7V else crossover distortion occurs. Output bias adjust 0.5 Ohms • Using diodes to do this gives a degree of tracking vs temperature TR2 Input Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB -15 V Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 19 Amplifier Efficiency, • Principal efficiency definition, usually expressed as a percentage. Collector/Drain/Anode Efficiency: = PRFout / PDCin • For info, other definitions are: • Power Added Efficiency: Pae = (PRFout - PRFin) / PDCin • Overall Efficiency: Overall = PRFout / (PDCin+PRFin) – a good criterion (esp for a multistage amp) but not often quoted Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 5: v1.2, 26-Oct-2006 (3) Technical Aspects - Semiconductors 20