Unit 1: Semiconductor Diodes
1. Conductivity, Mobility, and Drift Velocity
In semiconductors, the flow of charge is determined by the behavior of electrons and holes. Drift Velocity is the average velocity attained by charge carriers due to an electric field.
Mobility (μ) is defined as the magnitude of drift velocity per unit electric field. Conductivity (σ) of a semiconductor depends on the concentration of both electrons and holes and their respective mobilities.
2. Formation and Characteristics of P-N Junction Diode
A P-N junction is formed by joining p-type and n-type semiconductor materials. At the interface, a Depletion Region is created where mobile charge carriers are absent.
The V-I Characteristics show how current varies with applied voltage. In forward bias, current increases exponentially after reaching the Knee Voltage. In reverse bias, only a very small leakage current flows until breakdown occurs.
3. Current Flow Mechanism in Biasing
- Forward Bias: The positive terminal is connected to the p-side. This narrows the depletion region, allowing majority carriers to cross the junction easily.
- Reverse Bias: The positive terminal is connected to the n-side. This widens the depletion region, preventing majority carrier flow and allowing only minority carrier drift.
4. Static and Dynamic Resistance
The resistance of a diode is not constant because its V-I curve is non-linear.
- Static (DC) Resistance: The resistance at a specific DC operating point. R = V / I.
- Dynamic (AC) Resistance: The resistance offered to a changing signal, calculated as the reciprocal of the slope of the V-I curve. r = ΔV / ΔI.
5. Half-wave and Full-wave Rectifiers
Rectifiers convert alternating current (AC) into pulsating direct current (DC).
6. Filter Circuits (C and π-filter)
Filters are used to remove the AC components (ripples) from the rectifier output to provide smooth DC.
- C-Filter (Capacitor Filter): A capacitor is connected in parallel with the load. It charges during the peak and discharges through the load, smoothing the voltage.
- π-Filter: Consists of two capacitors and an inductor (or resistor) in a 'π' shape, providing superior ripple reduction compared to a simple C-filter.
7. Zener Diode and Voltage Regulation
A Zener Diode is a heavily doped diode designed to operate in the reverse breakdown region without damage.
Breakdown Mechanisms:
- Zener Breakdown: Occurs in heavily doped junctions at low voltages due to high electric fields pulling electrons from valence bands.
- Avalanche Breakdown: Occurs in lightly doped junctions at high voltages due to collision-induced ionization.
Voltage Regulation: The Zener diode maintains a constant voltage across the load even if the input voltage or load current fluctuates.
8. LED and Solar Cell
- LED (Light Emitting Diode): A forward-biased P-N junction that emits light when electrons recombine with holes, releasing energy as photons.
- Solar Cell: A P-N junction that converts light energy directly into electrical energy through the photovoltaic effect.
Exam Focus Corner
Frequently Asked Questions
- Define Ripple Factor. It is the ratio of the RMS value of the AC component to the DC component in the rectifier output.
- How does a Zener diode act as a regulator? In the breakdown region, the voltage across the Zener remains constant over a wide range of current.
Common Mistakes
- Biasing: Confusing forward and reverse bias connections. Remember: P to Positive = Forward.
- Efficiency: Forgetting that full-wave efficiency is exactly double that of half-wave.
Exam Tips
Tip: Always draw the V-I characteristics when explaining diodes. Use bold labels for the Knee Voltage and Breakdown Voltage. For rectifiers, the output waveforms are just as important as the circuit diagram.