5.1 Thermionic Emission and Cathode Ray Tube
1. Thermionic Emission
Thermionic emission is the process of emission of free electrons from the surface of a heated metal filament.
- Factors affecting rate of emission: Filament temperature, surface area of filament, type of metal, and surface coating (e.g., barium/strontium oxide).
2. Cathode Rays
Cathode rays are high-speed electron beams traveling in a vacuum from the cathode (negative electrode) to the anode (positive electrode).
- Properties: Move in straight lines, possess kinetic energy and momentum, cause fluorescence on phosphor screens, and are deflected by electric and magnetic fields.
3. Energy Conversions in Cathode Ray Tube (CRT)
Electrical potential energy supplied by high voltage ($E_p = eV$) is converted entirely into kinetic energy ($E_k = \frac{1}{2}mv^2$) of the accelerating electrons:
$$e V = \frac{1}{2} m v^2$$
- $e$ = Electron charge ($1.6 \times 10^{-19}\text{ C}$)
- $V$ = Accelerating potential difference ($\text{V}$)
- $m$ = Mass of electron ($9.1 \times 10^{-31}\text{ kg}$)
- $v$ = Maximum velocity of electron ($\text{m s}^{-1}$)
5.2 Semiconductor Diode
1. Doping Process and Semiconductors
Pure semiconductors (e.g., Silicon, Germanium) have low conductivity. Doping is the addition of small amounts of specific impurities to increase conductivity.
- n-type Semiconductor: Doped with pentavalent impurities (e.g., Phosphorus, Arsenic). Majority charge carriers are free electrons.
- p-type Semiconductor: Doped with trivalent impurities (e.g., Boron, Indium, Gallium). Majority charge carriers are holes.
2. p-n Junction Diode Operation
A diode allows electric current to flow in one direction only.
- Forward Bias: Positive terminal connected to p-type, negative to n-type. Depletion layer narrows, allowing current to flow easily.
- Reverse Bias: Positive terminal connected to n-type, negative to p-type. Depletion layer widens, blocking current flow.
3. Rectification
Rectification is the process of converting alternating current (a.c.) to direct current (d.c.).
- Half-Wave Rectification: Uses a single diode to allow only positive half-cycles.
- Full-Wave Rectification: Uses a bridge rectifier containing 4 diodes to utilize both half-cycles.
- Smoothing: A capacitor connected in parallel with the load resistor smooths out voltage fluctuations by storing charge during peak voltages and discharging during dips.
5.3 Transistor
A transistor is a semiconductor device with three terminals used as an electronic switch and a current amplifier.
1. Transistor Terminals and Types
- Base ($B$): Controls the switching/amplification.
- Collector ($C$): Collects majority carriers.
- Emitter ($E$): Emits majority carriers.
- Types: npn transistor (arrow on emitter points out) and pnp transistor (arrow on emitter points in).
2. Transistor Current Relationship
$$I_E = I_B + I_C$$
Since $I_B$ is extremely small ($I_B \ll I_C$), $I_E \approx I_C$.
3. Transistor as a Current Amplifier
A small change in base current ($\Delta I_B$) produces a large change in collector current ($\Delta I_C$).
$$\text{Current Amplification Factor } (\beta) = \frac{\Delta I_C}{\Delta I_B}$$
4. Transistor as an Automatic Switch
Functions as a potential divider circuit. The transistor turns ON when the potential difference across the base-emitter junction ($V_{BE}$) exceeds the minimum threshold voltage ($\approx 0.7\text{ V}$).
- Light-Controlled Switch: Uses a Light Dependent Resistor (LDR) to turn on lights automatically at night.
- Heat-Controlled Switch: Uses a Thermistor to trigger fire alarms when temperature rises.