Chelmsford Amateur Radio Society Advanced Course Technical Aspects Part-5 - Semiconductors

1. Chelmsford Amateur Radio Society Advanced CourseTechnical AspectsPart-5 - Semiconductors

2. 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

3. Group IV Si Silicon Origins of Semiconductors • Silicon and Germanium are in Group-IV where each atom has 4 electrons in its outer shell

4. 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

5. ‘Spare’ Electron As Si n-type doping 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

6. Junction - N P + - + - + - + Depletion Region 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

7. +I Ge Si Vr Vf -I Diodes • Standard Diodes act as rectifiers • Forward threshold Vf is: Silicon ~0.6v, Germanium ~0.4v • Reverse Breakdown, Vr usually many volts

8. 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

9. +VE +5.1V 0V 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

10. - Collector Emitter - - - + Collector Emitter - - - - IC IE + N P N NPN Base Base IB 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 IB NB: IE = IC + IB

11. IB = 60µA IC IC 40µA 1 mA mA 20µA VCE VBE 0.5 V 12 V Collector Base NPN E Emitter B C PNP Bipolar Transistors • Two types - NPN and PNP • Base-Emitter similar to Diode characteristic • ‘Bias’ current in the base controls Collector-Emitter current • PNP has negative current in the Base for bias

12. 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

13. d p-type Depletion Layer g s Drain n-Channel d Gate Source g Depletion Layer about to pinch off channel G2 D G1 G S Dual Gate Insulated FET s n-Channel p-Channel FETs • FETs are a semiconductor device similar to a Valve • Operates by a field effect due to Voltage (as opposed to Current in a Bipolar) • GDS Terminology refers to Electron flow • n-Channel and p-Channel variants exist • Insulated Gate FETS give NMOS, PMOS, and CMOS - which are all static sensitive

14. +V Output Input Medium Zout ~ 5K Low Zin ~1K Common Emitter • Three circuit configurations are possible:- Common Emitter, Common Collector and Common Base • 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 • Thus the circuit inverts, or is said to give 180° phase change

15. +V Input Output High Zin 50k-2M Low Zout 10-500 Ohms Common Collector • Common Collector is also popularly called Emitter Follower • Output Voltage is similar to input but can supply much more current • So no Voltage gain, but used for current buffering • Collector is Common, as at AC the PSU rails have zero potential

16. +V High Zout 50k Input Output Low Zin 50 Ohms Capacitor ensures no signal on the base 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 • Common base amps amplify voltage - not current

17. IC IC Distorted Output Output VBE VBE Input signal normal bias voltage Input signal low bias voltage 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

18. Amplifier Classes • Class-ABiased well on for high fidelity but also results in low efficiency and high heat dissipation in poweramps • Class-BGives only only half the waveform, so usually used in Push-Pull configurations. Fairly efficient, but can give crossover distortion • Class-ABA variation of above with transistor biased to conduct for more than half a cycle for better fidelity, but modest dissipation • Class-CNonlinear 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

19. +15 V TR1 0.5 Ohms Output bias adjust 0.5 Ohms TR2 Input -15 V Class-B Push-Pull • Class-B only gives half a sine wave • In Push-Pull:- TR1 gives positive half, TR2 give negative half • Need to keep Bases at 0.7V else crossover distortion occurs. • Using diodes to do this gives a degree of tracking vs temperature

20. 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