1.13 Active Components: Semiconductors, Diodes, and Transistors

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 ($V_b$):

• Silicon (Si) Diode: $\approx 0.7\text{ Volts}$
• Germanium (Ge) Diode: $\approx 0.3\text{ Volts}$

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

---

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:

1. Base (B): Thin, lightly doped centre region that controls current flow.
2. Collector (C): Large, moderately doped region that collects majority carriers.
3. 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 ($\beta$ / $h_{FE}$)

The BJT is a current-controlled current device. A small base current ($I_B$) controls a significantly larger collector current ($I_C$):

$I_C = \beta \times I_B = h_{FE} \times I_B$ $I_E = I_C + I_B$

Where $\beta$ (Beta) or $h_{FE}$ is the DC current gain (typically $50$ to $300$).

BJT Operating Regions:

• Cut-off Region: $V_{BE} < 0.7\text{ V}$, $I_B = 0$, transistor is OFF (open switch).
• Active Linear Region: $V_{BE} \approx 0.7\text{ V}$, collector-base junction reverse-biased. Transistor acts as a linear amplifier.
• Saturation Region: Base current is high, $V_{CE} \approx 0.2\text{ V}$, 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 $180^\circ$ 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:

1. Junction FET (JFET): Reverse-biased P-N junction controls channel width.
2. 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 ($10^{12}\ \Omega$).

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 $R_C = 2.2\text{ k}\Omega$ connected to a $+12.0\text{ V DC}$ supply. If base bias resistor sets the base current to $I_B = 20\ \mu\text{A}$ and current gain $\beta = 150$, calculate:

1. The collector current ($I_C$) in mA.
2. The collector-to-emitter voltage ($V_{CE}$) in Volts.

Solution: $I_C = \beta \times I_B = 150 \times (20 \times 10^{-6}\text{ A}) = 3.00 \times 10^{-3}\text{ A} = 3.00\text{ mA}$ $V_{CE} = V_{CC} - (I_C \times R_C) = 12.0\text{ V} - (3.00\text{ mA} \times 2.2\text{ k}\Omega) = 12.0\text{ V} - 6.6\text{ V} = 5.40\text{ Volts}$