Photodiode Principles and LED Construction Explained
UNIT-5
1.Briefly discuss about Photo diode?
Photodiode:
A photodiode is a semiconductor device that converts light into an electrical
Current. It operates based on the principle of the photovoltaic effect, where the
Absorption of photons generates electron-hole pairs, leading to a current flow
When the device is connected in an electrical circuit
Structure and Symbol:
*Structure: Photodiodes are made of semiconductor materials such as silicon
They typically have a p-n junction, with the p-side connected to the anode and the
N-side to the cathode. Some photodiodes are designed with a PIN structure
(p-type, intrinsic, n-type) to enhance performance
Symbol: : The symbol for a photodiode is similar to that of a regular diode, but with
Arrows pointing towards the diode, indicating incoming light
1. Photovoltaic Effect: When light (photons) strikes the photodiode, it is absorbed by
The semiconductor material, creating electron-hole pairs
2,Modes of Operation:
– **Photovoltaic Mode**: The photodiode is used as a solar cell and generates a
Voltage when exposed to light, typically operated in zero bias or open-circuit condition
– **Avalanche Mode**: In this mode, the photodiode is operated under high reverse bias,
Leading to avalanche multiplication of the photocurrent for increased sensitivity.\
Characteristics
1. **Responsivity**: – The measure of the photodiode’s electrical output per unit of
Light input, usually expressed in A/W (amperes per watt). It depends on the wavelength
Of the incident light
2. **Quantum Efficiency**: – The ratio of the number of electron-hole pairs generated to
The number of incident photons. It is usually expressed as a percentage
4. **Response Time**: – The time it takes for the photodiode to respond to a change in light intensity
It is influenced by the junction capacitance and the load resistance in the circuit
5. **Spectral Response**: – The sensitivity of the photodiode as a function of wavelength
Different photodiodes are optimized for different wavelength ranges
Applications
1. **Light Detection and Measurement**: – Used in optical communication systems, light meters, and photometry
2. **Optical Sensors**: – Common in smoke detectors, medical devices, and barcode scanners
3. **Solar Cells**: – Large-area photodiodes are used in solar panels to convert sunlight into electrical energy
4. **Safety Systems**: – Used in light-based safety systems, such as intruder alarms and automatic door openers
2..Explain the construction and working of LED?
Construction of LED
An LED is a PN junction semiconductor device that emits light when it is forward biased. Its main construction parts are:
Semiconductor chip (PN junction)
Made from direct band-gap materials such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), or Gallium Arsenide Phosphide (GaAsP).
The PN junction is the heart of the LED where light is produced
Anode and Cathode leads
Anode (longer lead): connected to the P-type semiconductor
Cathode (shorter lead): connected to the N-type semiconductor
Reflector cup
Supports the semiconductor chip and reflects light outward to increase efficiency
Encapsulating lens (epoxy resin)
Protects the LED from damage
Focuses and directs the emitted light
The color of the lens often indicates the color of emitted light
Working of LED:
When the LED is forward biased, the P-type is connected to the positive terminal and the N-type to the negative terminal of the supply.
Electrons from the N-region and holes from the P-region move toward the junction
At the junction, electrons recombine with holes
During recombination, energy is released in the form of light (photons)
The color of light depends on the band-gap energy of the semiconductor material used
KEY POINYS:
LEDs operate only in forward bias
They require low operating voltage (≈1.8 V to 3.5 V)
They are highly efficient, small in size, and have a long lifespan
Light intensity increases with forward current (within safe limits)