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1.13 Active Components: Semiconductors, Diodes, and Transistors
ACMA Amateur Radio Standard Recognition Certificate - Theory Exam
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1 Chapter 1: Electrical Principles and Components
1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour
2 Chapter 2: Transmitters and Receivers
2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth
3 Chapter 3: Transmission Lines and Antennas
3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses
4 Chapter 4: Radio Wave Propagation
4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection
5 Chapter 5: Electrical and RF Measurements
5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions
6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)
6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints
7 Chapter 7: Safety and Environmental Hazards
7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety
1.13 Active Components: Semiconductors, Diodes, and Transistors
Key Takeaways Explain This Topic AI
Semiconductor materials (Silicon and Germanium) are doped with impurity atoms to create P-type (hole) and N-type (electron) materials.
P-N junction diodes permit unidirectional forward current flow once barrier potential is exceeded (~0.7V for Silicon, ~0.3V for Germanium).
Specialised diodes serve key RF functions: Zener diodes regulate voltage in reverse breakdown, Varactor diodes act as voltage-variable tuning capacitors, and Schottky diodes provide low drop fast switching.
Bipolar Junction Transistors (BJTs) are current-controlled amplifiers (Ic = β × Ib), available in NPN and PNP configurations.
Field-Effect Transistors (FETs and MOSFETs) are voltage-controlled devices with extremely high input impedance, superior noise performance, and reduced intermodulation distortion in RF stages.
Last updated: July 2026
1.13 Active Components: Semiconductors, Diodes, and Transistors
Scope note: The Standard syllabus (part 4) states that candidates are not required to know the internal workings of a transistor. The semiconductor physics below is background only — what is examinable is the function, symbol, terminal names and typical application of each device.
Modern radio equipment relies almost entirely on active semiconductor devices to amplify microvolt RF signals, mix frequencies, generate stable local oscillator signals, and switch high RF powers. This section covers semiconductor physics, P-N junction diodes, special-purpose diodes, Bipolar Junction Transistors (BJTs), and Field-Effect Transistors (FETs).
1. Semiconductor Physics & P-N Junctions
Pure Group IV elements— Silicon (Si) and Germanium (Ge) —possess four valence electrons per atom, forming covalent bonds in a crystalline lattice. In their pure (intrinsic) state, they act as poor conductors.
Extrinsic Doping
To alter conductivity, trace impurities are added in a process called Doping :
N-type Material : Doped with Pentavalent elements (5 valence electrons, e.g., Phosphorus, Arsenic). Free electrons become the majority charge carriers.
P-type Material : Doped with Trivalent elements (3 valence electrons, e.g., Boron, Gallium). Holes (absence of electrons) become majority charge carriers.
P-N Junction & Barrier Potential
When P-type and N-type materials are joined, free electrons near the junction cross over to recombine with holes, creating a narrow non-conductive Depletion Region .
This establishes an internal Barrier Potential ( V b V_b V b ) :
Silicon (Si) Diode : ≈ 0.7 Volts \approx 0.7\text{ Volts} ≈ 0.7 Volts
Germanium (Ge) Diode : ≈ 0.3 Volts \approx 0.3\text{ Volts} ≈ 0.3 Volts Forward Bias: (+) connected to P-type, (-) connected to N-type (Current Flows when V > Vb)
- Reverse Bias: (-) connected to P-type, (+) connected to N-type (Depletion Region Widens, No Current)
- 2. Special-Purpose Diodes in Radio Engineering
Diode Type Operating Mode & Characteristics Typical Radio Applications
Signal / Rectifier Diode Standard forward biased junction ( V f ≈ 0.7 V V_f \approx 0.7\text{V} V f ≈ 0.7 V ) AC rectification, AM envelope detection
Zener Diode Operated in Reverse Breakdown Region at fixed voltage ( V Z V_Z V Z ) Voltage regulators, reference sources
Varactor (Varicap) Diode Reverse-biased P-N junction acting as voltage-variable capacitor Electronic tuning in VCOs and synthesisers
Schottky Diode Metal-to-semiconductor junction ( V f ≈ 0.2 − 0.4 V V_f \approx 0.2-0.4\text{V} V f ≈ 0.2 − 0.4 V ), fast switching High-frequency RF mixers, balanced modulators
Light Emitting Diode (LED) Forward-biased electroluminescent junction ( V f ≈ 1.8 − 3.3 V V_f \approx 1.8-3.3\text{V} V f ≈ 1.8 − 3.3 V ) Visual status indicators, optical isolators
PIN Diode High intrinsic layer; acts as RF variable resistor at high frequencies High-speed solid-state RF switching & T/R relays
3. Bipolar Junction Transistors (BJT)
A Bipolar Junction Transistor (BJT) is a three-terminal active semiconductor device constructed with two P-N junctions in close proximity, forming either an NPN or PNP sandwich.
Terminals:
Base (B) : Thin, lightly doped centre region that controls current flow.
Collector (C) : Large, moderately doped region that collects majority carriers.
Emitter (E) : Heavily doped region that emits majority carriers. NPN Symbol: Emitter arrow points OUTWARD ("Not Pointing iN")
- PNP Symbol: Emitter arrow points INWARD ("Points iN Proudly")
- Current Amplification & Current Gain ( β \beta β / h F E h_{FE} h FE )
The BJT is a current-controlled current device . A small base current ( I B I_B I B ) controls a significantly larger collector current ( I C I_C I C ): I C = β × I B = h F E × I B I_C = \beta \times I_B = h_{FE} \times I_B I C = β × I B = h FE × I B I E = I C + I B I_E = I_C + I_B I E = I C + I B
Where β \beta β (Beta) or h F E h_{FE} h FE is the DC current gain (typically 50 50 50 to 300 300 300 ).
BJT Operating Regions:
Cut-off Region : V B E < 0.7 V V_{BE} < 0.7\text{ V} V BE < 0.7 V , I B = 0 I_B = 0 I B = 0 , transistor is OFF (open switch).
Active Linear Region : V B E ≈ 0.7 V V_{BE} \approx 0.7\text{ V} V BE ≈ 0.7 V , collector-base junction reverse-biased. Transistor acts as a linear amplifier.
Saturation Region : Base current is high, V C E ≈ 0.2 V V_{CE} \approx 0.2\text{ V} V CE ≈ 0.2 V , transistor is FULLY ON (closed switch).
Common Emitter Amplifier Configuration
The Common Emitter configuration is widely used because it provides high voltage gain, high current gain, and a 180 ∘ 180^\circ 18 0 ∘ phase inversion between input and output.
4. Field-Effect Transistors (FET)
Unlike BJTs, a Field-Effect Transistor (FET) is a voltage-controlled device . Current flowing through a semiconductor channel between Drain (D) and Source (S) is controlled by an electric field applied to the Gate (G) .
Main Types of FETs:
Junction FET (JFET) : Reverse-biased P-N junction controls channel width.
MOSFET (Metal-Oxide-Semiconductor FET) : Gate is insulated from the channel by an ultra-thin silicon dioxide dielectric layer, resulting in near-infinite input resistance ( 10 12 Ω 10^{12}\ \Omega 1 0 12 Ω ).
Advantages of FETs over BJTs in RF Equipment:
Extremely High Input Impedance : Minimal loading on preceding oscillator or filter stages.
Low Noise Figure : Ideal for weak-signal VHF/UHF receiver preamplifiers.
Square-Law Transfer Characteristic : Superior dynamic range and resistance to cross-modulation and intermodulation distortion in receiver front-ends.
Worked Example 1.8: BJT Transistor Biasing Calculation
In an NPN Common Emitter RF preamplifier stage, the collector resistor is R C = 2.2 k Ω R_C = 2.2\text{ k}\Omega R C = 2.2 k Ω connected to a + 12.0 V DC +12.0\text{ V DC} + 12.0 V DC supply. If base bias resistor sets the base current to I B = 20 μ A I_B = 20\ \mu\text{A} I B = 20 μ A and current gain β = 150 \beta = 150 β = 150 , calculate:
The collector current ( I C I_C I C ) in mA.
The collector-to-emitter voltage ( V C E V_{CE} V CE ) in Volts.
Solution: I C = β × I B = 150 × ( 20 × 10 − 6 A ) = 3.00 × 10 − 3 A = 3.00 mA I_C = \beta \times I_B = 150 \times (20 \times 10^{-6}\text{ A}) = 3.00 \times 10^{-3}\text{ A} = 3.00\text{ mA} I C = β × I B = 150 × ( 20 × 1 0 − 6 A ) = 3.00 × 1 0 − 3 A = 3.00 mA V C E = V C C − ( I C × R C ) = 12.0 V − ( 3.00 mA × 2.2 k Ω ) = 12.0 V − 6.6 V = 5.40 Volts V_{CE} = V_{CC} - (I_C \times R_C) = 12.0\text{ V} - (3.00\text{ mA} \times 2.2\text{ k}\Omega) = 12.0\text{ V} - 6.6\text{ V} = 5.40\text{ Volts} V CE = V CC − ( I C × R C ) = 12.0 V − ( 3.00 mA × 2.2 k Ω ) = 12.0 V − 6.6 V = 5.40 Volts