Notes: Optical Devices | Electronic Devices - Electronics and Communication Engineering (ECE) PDF Download

In these devices photon (the basic particle of light) plays a major role. Here we will talk about four groups of devices.

1. Light emitting diodes (LEDs)
2. LASERS
3. Photo detectors
4. Solar cells

Light Emitting Diodes (LEDs):  They are p-n junctions that can emit spontaneous radiation in ultraviolet, visible and infrared regions. They are fabricated from direct band gap materials Like GaAs. ( In these materials momentum of electrons and holes is same in both conduction and valance band, so electron can directly emit a photon). Recombination of holes & electrons gives light, so to sustain recombination the junction is forward biased.

In ordinary diodes recombination energy is thermal .While in GaAs the radiation is Infra- red (0.9 μm) and GaP gives orange, yellow or green light.
Spontaneous emission (means random emission)
They find applications in Pilot lights, fibre optic comm.   opto- isolators (infrared), optical comm.

Liquid Crystal Displays (LCDs): Material used for these devices is Liquid Crystal. Reflective and transmitter type liquid crystals cells are available. They are light reflectors and transmitters not light generators, so consume very small energy.

Its advantages are

• Low power consumptions (10-15 μw per 7 segment display)
• Life time 50,00 Hrs .
• Can have any big size.

Its decay time is 150 m sec. which is very high as compared to rise & fall time of LEDs. They find applications in LCD display, LCD monitors.

Laser Diode: Laser is acronym for light amplification by stimulated emission of radiation. It is a source of highly directional monochromatic (same frequency or energy) and coherent (in phase and reinforcing) beam .It can generate Continuous beam of low power or short burst of high power.
Semiconductor lasers are similar to solid state ruby laser and helium neon gas laser. But they are small and easily modulated. Materials are of direct band gap type with high radiative – transition probability ). To enhance stimulated emission for LASER operation we need population inversion.

Photo Diode: Photo diode is a p-n junction operated under Reverse Bias . They convert optical signal into electrical signals.
Note that when diode is R.B. small current flows due to minority carriers. When light falls on p-n junction it imparts energy to these electrons. So light can control reverse current in diode.
For high frequency operations, the depletion region must be kept thin to reduce transit time. But to increase quantum efficiency, the depletion layer must be sufficiently thick to absorb incident light. The function of photo diode is opposite to LED and LASER diodes.
There are different type of photodiodes namely p-n photodiodes, p-i-n photo diodes and avalanche photo diodes (APD)
(i) p-n photodiode:    Made of p-n junction under reverse bias. When photon strikes the junction it is absorbed and electron excited to conduction band. Electron hole pair is generated in depletion region.
(ii) p-i-n photodiode:  It consists of thick I- layer sand-witched between p & n, so depletion layer width is extended.

• Most of photo carriers are created in the depletion layer where they are collected much faster. True for Ge & Si photodiodes.
• Junction capacitance is smaller, more expensive to fabricate and less Bandwidth.

Avalanche photodiode (APD): 

• Operate at high reverse bias ( 20V or more) avalanche multiplication region
• Fabricated from Si or In GaAs (for high speed)
• Provide gain due to avalanche multiplication, but more noise because of avalanche process.

APDs find application as Detectors in fibre optic comm. Systems

Solar Cells: Consists of p-n junction which operates in forward bias. Si is an important material for solar cell. III-V compound semiconductors and their alloy system provide wide choices for band gap.
PERL cell has efficiency of 24°/0, GaInP / GaAs Tandem cell (30°/0), low cost series interconnected  Si solar cell (5°/0)

Uses

• Useful for both space & terrestrial applications used for long duration power supply for satellites.
• As an alternate energy source at low operating cost and non-polluting.