5.2 Semiconductor Diode
Electrical energy is transmitted to homes as alternating current (a.c.), but many everyday electronic gadgets, like smartphones, require direct current (d.c.) to operate. A semiconductor diode is a two-terminal electronic component that allows current to flow in one direction only. This unique property makes it essential for rectification—the process of converting alternating current into direct current.
Learning Objectives
- Explain the function of a semiconductor diode.
- Describe the behaviour and characteristics of a diode in forward biased and reverse biased circuits.
- Explain the concepts and circuit arrangements of half-wave rectification and full-wave rectification.
- Describe the function and working principle of a capacitor as a current smoother in a rectification circuit.
Comparison of Diode Biasing and Rectification Circuits
See how forward and reverse bias change the depletion layer, and how half-wave, full-wave and capacitor-smoothed outputs appear on a CRO.
Forward vs Reverse Bias
Rectification Outputs
Short Explanation
Structure of a Semiconductor Diode
A semiconductor diode is formed by joining a p-type semiconductor and an n-type semiconductor, creating a p-n junction. The triangle in the circuit symbol represents the anode (+), while the vertical bar represents the cathode (−).
Forward Bias
Positive terminal of the cell is connected to the anode (p-type) and negative terminal to the cathode (n-type). The depletion layer becomes thinner, junction voltage decreases, resistance becomes very small, and current flows.
Reverse Bias
Positive terminal is connected to the cathode (n-type) and negative terminal to the anode (p-type). The depletion layer becomes thicker, junction voltage increases until it reaches the battery potential difference, resistance becomes very high, and current stops.
Rectification
Rectification is the process of converting alternating current into direct current. A single diode gives half-wave rectification, while four diodes in a bridge arrangement give full-wave rectification.
| Condition | Depletion Layer | Junction Voltage | Resistance | Current |
|---|---|---|---|---|
| Forward bias | Becomes thinner | Decreases | Very small | Flows |
| Reverse bias | Becomes thicker | Increases to battery voltage | Very high | Stops |
Try to Answer First
Answer in your mind, then press “Check Answer”.
What happens to the depletion layer and the diode resistance when a diode is connected in reverse bias?
Check Answer
In a bridge rectifier, which diodes conduct during the positive half cycle, and which conduct during the negative half cycle?
Check Answer
How should a capacitor be connected relative to the load resistor to smooth out rectified current?
Check Answer
Common Mistakes
- Connecting the positive terminal of a d.c. supply to the cathode (n-type) and expecting current to flow; this creates a reverse-biased state with high resistance where no current flows.
- Believing full-wave rectification automatically yields a completely flat, steady direct current; it actually produces a pulsating direct current that still requires a smoothing capacitor.
- Drawing the smoothing capacitor in series with the load resistor instead of in parallel.
- Assuming that when one diode burns out in a four-diode bridge rectifier, no current flows at all; actually, it reverts to half-wave rectification because the other pair can still conduct during one half cycle.
Concept Misunderstandings
Diodes convert a.c. into d.c. by changing the shape of the voltage wave magically.
Diodes simply conduct in one direction and block current in the reverse direction. By routing paths using one or four diodes, alternating current is forced into a unidirectional (rectified) flow.
A capacitor generates electrical energy to smooth the output current.
A capacitor only stores electrical energy when the output voltage rises (charges) and releases that stored energy when the voltage drops (discharges), preventing output current from reaching zero.
Summary
- A semiconductor diode consists of a p-n junction and conducts current in only one direction (anode to cathode).
- Forward bias: Depletion layer thins, junction voltage decreases, resistance is very low, current flows.
- Reverse bias: Depletion layer thickens, junction voltage reaches battery voltage, resistance is very high, current stops.
- Half-wave rectification: 1 diode; conducts during the positive half cycle and blocks the negative half cycle.
- Full-wave rectification: 4 diodes in a bridge arrangement; conducts both half cycles in the same direction through the load.
- Capacitor smoothing: A capacitor connected in parallel with the load resistor charges when voltage rises and discharges when voltage falls, creating a smoothed d.c. output.
Short Activity
Objective: Identify diode biasing, rectification and capacitor smoothing.
A. Objective Quiz
1 What happens to a p-n junction diode when it is connected in forward bias?
2 What is the function of a capacitor connected in parallel to a load resistor in a rectification circuit?
B. Fill in the Blanks
3 The process of converting an alternating current into a direct current using semiconductor diodes is known as .
4 In a full-wave bridge rectifier, the arrangement uses semiconductor diodes to allow current to flow through the load in the same direction during both half cycles.
C. Matching / Drag and Drop
Drag each term to the correct meaning. If using a phone, tap the answer first, then tap the matching box.
