Diode Applications (Rectifiers and Special Purpose Diodes)
Application Of Junction Diode As A Rectifier (Half-wave, Full-wave)
The most fundamental application of a p-n junction diode arises from its property of allowing current to flow easily in the forward bias direction while blocking current flow almost entirely in the reverse bias direction. This unidirectional conduction characteristic makes the diode suitable for use as a rectifier, a circuit that converts alternating current (AC) into pulsating direct current (DC).
AC voltage periodically reverses its polarity. A rectifier circuit uses one or more diodes to ensure that current flows in only one direction through the load, regardless of the polarity of the input AC voltage.
Half-wave Rectifier
A half-wave rectifier circuit rectifies only one half of the input AC voltage cycle.
Circuit Diagram
A simple half-wave rectifier consists of a single p-n junction diode connected in series with a load resistor ($R_L$) across the secondary coil of a transformer (to step up or step down the AC voltage as needed).
Circuit diagram of a half-wave rectifier.
Working
Let the input AC voltage across the secondary of the transformer be $v_{in}(t) = V_m \sin(\omega t)$.
- During the positive half-cycle of the input AC voltage (when the top end of the transformer secondary is positive relative to the bottom end): The diode is forward-biased. The voltage across the diode is positive. If $v_{in}$ exceeds the diode's knee voltage (typically around 0.7 V for silicon), the diode conducts and allows current to flow through the load resistor $R_L$. The output voltage across $R_L$ is approximately equal to the input voltage ($v_{out} \approx v_{in}$).
- During the negative half-cycle of the input AC voltage (when the top end of the transformer secondary is negative relative to the bottom end): The diode is reverse-biased. The voltage across the diode is negative. The diode acts as an open switch and almost completely blocks the current flow (only a small reverse saturation current flows, which is usually negligible). The output voltage across $R_L$ is zero (or very close to zero).
Input AC voltage and output pulsating DC voltage waveforms for a half-wave rectifier.
The output voltage and current through the load thus consist only of the positive half-cycles of the input AC voltage. This is a pulsating DC output.
Advantages and Disadvantages
- Advantage: Simple circuit with only one diode.
- Disadvantages:
- Only half of the AC cycle is utilised, leading to low efficiency (maximum theoretical efficiency $\approx 40.6\%$).
- The output DC voltage is low for a given input AC voltage.
- The output is highly pulsating, requiring significant filtering to get a smooth DC voltage.
Full-wave Rectifier
A full-wave rectifier circuit rectifies both halves of the input AC voltage cycle, providing a more continuous output. There are two main types: the center-tapped full-wave rectifier and the bridge rectifier.
Center-tapped Full-wave Rectifier
This circuit uses two diodes and a center-tapped transformer.
Circuit Diagram
It requires a transformer with a secondary coil tapped exactly in the middle. Two diodes (D1 and D2) are connected to the ends of the secondary coil, and their cathodes (or anodes, depending on convention) are connected together to the load resistor $R_L$. The other end of $R_L$ is connected to the center tap of the transformer secondary.
Circuit diagram of a center-tapped full-wave rectifier.
Working
Let the input AC voltage across the entire secondary coil be $v_{in}(t) = V_m \sin(\omega t)$. The center tap divides the secondary voltage into two equal halves, $v_{1}(t) = V_m \sin(\omega t)$ across the upper half (say, D1) and $v_{2}(t) = -V_m \sin(\omega t) = V_m \sin(\omega t + \pi)$ across the lower half (say, D2).
- During the positive half-cycle of the input AC voltage (when the top end A is positive, center tap O is at reference, and bottom end B is negative): Diode D1 is forward-biased (positive voltage at anode A relative to cathode connection at O). Diode D2 is reverse-biased (negative voltage at anode B relative to cathode connection at O). D1 conducts, and current flows through $R_L$ from P to Q. D2 does not conduct.
- During the negative half-cycle of the input AC voltage (when the top end A is negative, center tap O is at reference, and bottom end B is positive): Diode D1 is reverse-biased (negative voltage at anode A relative to O). Diode D2 is forward-biased (positive voltage at anode B relative to O). D2 conducts, and current flows through $R_L$ from P to Q (in the same direction as during the positive half-cycle). D1 does not conduct.
Input AC voltage and output pulsating DC voltage waveforms for a center-tapped full-wave rectifier.
The output voltage across $R_L$ is the positive half-cycle from D1 during the positive half of $v_{in}$, and the inverted negative half-cycle from D2 during the negative half of $v_{in}$. The current flows through the load in the same direction during both half-cycles.
Bridge Rectifier
The bridge rectifier is another type of full-wave rectifier that does not require a center-tapped transformer. It uses four diodes connected in a bridge configuration.
Circuit Diagram
Four diodes (D1, D2, D3, D4) are connected to form a bridge. The input AC voltage is applied across one diagonal of the bridge (terminals A and B). The output DC voltage is taken across the other diagonal (terminals C and D), where the load resistor $R_L$ is connected.
Circuit diagram of a bridge rectifier.
Working
Let the input AC voltage be applied across terminals A and B. Current flows from the positive terminal to the negative terminal through the bridge and the load.
- During the positive half-cycle of the input AC voltage (when A is positive relative to B): Current flows from A, passes through diode D1, then through the load resistor $R_L$ (from C to D), then through diode D3, and finally to B. Diodes D2 and D4 are reverse-biased and do not conduct. Current flows through the load in the direction C $\to$ D.
- During the negative half-cycle of the input AC voltage (when A is negative relative to B): Current flows from B, passes through diode D2, then through the load resistor $R_L$ (from C to D, in the same direction as before), then through diode D4, and finally to A. Diodes D1 and D3 are reverse-biased and do not conduct. Current flows through the load in the direction C $\to$ D.
The output voltage and current through the load have the same shape as the center-tapped full-wave rectifier output, consisting of rectified positive and negative half-cycles of the input AC voltage, ensuring unidirectional current through the load.
Input AC voltage and output pulsating DC voltage waveforms for a bridge rectifier (same as center-tapped).
Advantages and Disadvantages of Full-wave Rectifiers (Center-tapped and Bridge)
- Advantages:
- Both halves of the AC cycle are utilised, resulting in higher efficiency (maximum theoretical efficiency $\approx 81.2\%$, double that of half-wave).
- The average output DC voltage is higher than that of a half-wave rectifier for the same peak input voltage.
- The output is less pulsating (higher ripple frequency), making it easier to filter to get a smooth DC voltage.
- Disadvantages:
- Require more diodes (2 for center-tapped, 4 for bridge).
- Center-tapped type requires a more expensive center-tapped transformer, and the diodes must withstand twice the peak reverse voltage compared to a bridge rectifier.
- Bridge rectifier has slightly higher power loss because the current passes through two diodes in series during each half-cycle (compared to one diode in half-wave and center-tapped).
Rectified outputs (from half-wave or full-wave rectifiers) are pulsating DC. For applications requiring a smooth DC voltage (like powering electronic circuits), filter circuits (usually involving capacitors) are used after the rectifier to reduce the voltage ripple.
Special Purpose P-N Junction Diodes
Besides their primary use as rectifiers, p-n junctions can be specially fabricated or operated under specific conditions to perform various other functions. These are known as special purpose diodes. Their operation often relies on phenomena beyond basic rectification, such as controlled breakdown or interaction with light.
Zener Diode (Voltage Regulator)
A Zener diode is a heavily doped p-n junction diode that is specifically designed to operate reliably in the reverse breakdown region without being destroyed. Standard diodes are usually damaged by prolonged operation in breakdown, but Zener diodes are built to handle the resulting current.
I-V Characteristics
The forward characteristic of a Zener diode is similar to that of a regular diode. However, its reverse characteristic is significantly different. When reverse-biased, it exhibits a sharp breakdown at a specific voltage called the Zener voltage ($V_Z$). After breakdown, the voltage across the diode remains almost constant over a wide range of reverse currents.
I-V characteristic curve of a Zener diode. It operates in the reverse breakdown region.
Application as a Voltage Regulator
The ability of a Zener diode to maintain an almost constant voltage across its terminals in the reverse breakdown region makes it ideal for use as a voltage regulator. A voltage regulator circuit provides a stable DC output voltage despite variations in the input voltage or the load current.
Zener diode as a voltage regulator. It maintains constant output voltage $V_{out} = V_Z$.
Working:
- The Zener diode is connected in reverse bias, in parallel with the load resistor ($R_L$), and in series with a current-limiting resistor ($R_S$) from the input unregulated DC supply ($V_{in}$).
- The input voltage $V_{in}$ must be greater than the Zener voltage $V_Z$. This ensures that the Zener diode operates in the breakdown region.
- In the breakdown region, the voltage across the Zener diode is approximately constant ($V_Z$), regardless of the current flowing through it (within limits). Since the Zener is in parallel with the load, the output voltage across the load $V_{out} = V_Z$, which is a regulated, stable voltage.
- If the input voltage $V_{in}$ increases, the current through $R_S$ increases. This excess current flows through the Zener diode, while the voltage across it remains $V_Z$. The voltage across $R_S$ absorbs the voltage fluctuation ($V_{in} - V_Z$). The output voltage $V_{out}$ remains constant at $V_Z$.
- If the load current through $R_L$ changes (e.g., decreases), the current through $R_S$ remains relatively constant (since $V_{in}$ and $V_Z$ are stable). The Zener current adjusts ($I_Z = I_S - I_L$), absorbing the change in load current, while maintaining $V_{out}$ at $V_Z$.
Optoelectronic Junction Devices (LED, Photodiode, Solar Cell)
These are semiconductor devices whose operation is based on the interaction between light and semiconductors.
Light Emitting Diode (LED)
A Light Emitting Diode (LED) is a p-n junction diode that emits light when it is forward-biased.
Principle: When a p-n junction is forward-biased, electrons from the n-side and holes from the p-side diffuse across the junction. In the depletion region and the nearby areas, these injected minority carriers recombine with the majority carriers. In certain semiconductor materials (like Gallium Arsenide Phosphide - GaAsP), when an electron falls from a higher energy level in the conduction band to a lower energy level (hole) in the valence band, the excess energy is released as a photon of light. This process is called electroluminescence.
The energy of the emitted photon ($h\nu$) is approximately equal to the band gap energy ($E_g$) of the semiconductor material ($h\nu \approx E_g$). The colour (wavelength) of the emitted light depends on the band gap energy, which is determined by the material composition.
LED Symbol and basic diagram showing light emission.
Uses: Indicators in electronic devices, displays (seven-segment, alphanumeric, video walls), lighting (energy-efficient illumination), traffic signals, remote controls.
Photodiode
A photodiode is a p-n junction diode that generates electric current when light falls on it. It is typically operated under reverse bias.
Principle: When a photon with energy greater than or equal to the band gap energy of the semiconductor strikes the p-n junction, it is absorbed and creates an electron-hole pair in the depletion region or near it. These electron-hole pairs are separated by the strong electric field in the depletion region (electrons are swept to the n-side, holes to the p-side). This separation of charge carriers creates a small current that flows in the external circuit in the reverse direction. The magnitude of this reverse current is proportional to the intensity of the incident light.
Photodiode Symbol and basic diagram under reverse bias, showing current flow due to light.
Uses: Light detectors, light meters, optical communication receivers, security alarms, automatic switches (e.g., street lights).
Solar Cell (Photovoltaic Cell)
A solar cell is essentially a large area photodiode that converts light energy directly into electrical energy. It works on the photovoltaic principle and is operated without any external bias (neither forward nor reverse).
Principle: When photons with energy greater than the band gap energy are incident on the solar cell, they generate electron-hole pairs, primarily in the depletion region and the doped regions near it. The built-in electric field at the junction separates these carriers (electrons move to the n-side, holes to the p-side). This separation of charges accumulates at the terminals, creating a potential difference across the p-n junction (voltage generation). If an external load is connected, the generated voltage drives a current through the load, delivering power.
Basic diagram of a solar cell converting light into electricity.
Uses: Generating electricity from sunlight (solar panels for homes, industries, satellites, calculators, watches).
These special purpose diodes demonstrate the versatility of the p-n junction, enabling various functionalities from voltage regulation and light emission to light detection and energy conversion, forming the basis of numerous modern electronic and optoelectronic devices.