Optoelectronic Devices
Light-Emitting Diodes (LEDs)
Optoelectronics:
Combines optics and electronics.
Devices include: LEDs, photodiodes, optocouplers, and laser diodes.
LED Function:
Converts electrical energy into light (electroluminescence).
When forward biased, electrons cross the pn junction, releasing energy as photons (light).
LED colors depend on the semiconductor material’s energy band gap:
Colors: Red, green, yellow, blue, orange, white, and infrared (invisible).
LED Characteristics:
Low-power LED:
Longer lifespan, lower energy consumption, faster switching.
Common uses:
Indicators on panels, routers, remote controls (infrared), etc.
LED Voltage and Current:
Series current through a current-limiting resistor:
Typical forward voltage drop: 1.5 - 2.5 V at 10 - 50 mA.
Nominal voltage drop: 2 V (used for analysis/troubleshooting).
Brightness Control:
Luminous Intensity (\(I_V\)): Measured in candelas (cd) or millicandelas (mcd).
Example: TLDR5400 (red LED) has a forward voltage of 1.8 V and an intensity of 70 mcd at 20 mA.
Intensity drops to 3 mcd at 1 mA.
Voltage Difference:
If \(V_S \gg V_D\), LED brightness remains constant.
If \(V_S \approx V_D\), brightness varies noticeably.
Best Method: Use a current source to drive the LED for constant brightness.
Other Optoelectronic Devices
Beyond LEDs, other optoelectronic devices use the photonic action of a pn junction.
These devices source, detect, and control light in various applications.
Seven-Segment Display
Structure:
Consists of seven rectangular LEDs (segments A through G).
Each segment forms part of a digit or letter.
Operation:
By grounding specific segments, different digits (0-9) or letters (A, b, C, d, E, F) can be displayed.
Example:
Grounding segments A, B, and C displays the digit "7".
Grounding segments A, B, C, D, and G displays the digit "3".
Photodiode
Operation:
In reverse bias, the photodiode detects light.
Incoming light dislodges valence electrons, generating free electrons and holes.
The stronger the light, the larger the reverse current.
Key Characteristics:
Light-sensitive.
Reverse current increases with light intensity.
Typical reverse current is in the tens of microamperes.
Symbol:
The schematic symbol includes arrows representing incoming light.
Optocoupler (Optoisolator)
Components:
Combines an LED on input side and a photodiode on output side.
Operation:
Input voltage drives the LED, emitting light.
The photodiode detects the light, creating a reverse current.
The reverse current generates a voltage across an output resistor, with the output voltage varying with the input.
Applications:
Provides electrical isolation between input and output circuits.
Useful in high-voltage applications where circuit potentials may differ by several thousand volts.
Other Variants:
Some optocouplers use phototransistors, photothyristors, or other photo devices for the output.
Laser Diode
Coherent Light:
Unlike LEDs, laser diodes emit coherent light with light waves in phase.
How it Works:
Uses a mirrored resonant chamber to reinforce light waves at a single frequency.
Produces a narrow beam of intense, focused, and pure light.
Types:
Produces visible light (red, green, blue) or infrared (invisible) light.
Applications:
Telecommunications, broadband communication using fiber-optic cables.
Consumer electronics like CD/DVD players, laser printers.
Medical, aerospace, defense, and industrial systems.
New applications in machine vision systems, sensors, and security systems.