EC-6.5 Optoelectronic Devices: LED, Photodiode and Solar Cell
You can explain how one junction can emit light, detect light or generate power depending on how it is biased and built, and choose the right device for a given job.
Before:EC-1. Circuit AnalysisEC-5. Physics and Materials for ElectronicsUnlocks:S4-1. Analog CircuitsS5-4. Linear and Digital Integrated CircuitsPE1-1. VLSI TechnologyPE1-2. Fiber Optic CommunicationPE2-1. CMOS Analog and Digital IC DesignEC-12. Electrical Energy, Machines and Power Electronics
Three devices, one structure. A junction under forward bias can give up its recombination energy as light; the same junction under reverse bias produces a current when light creates carriers in it; and with no bias at all it produces a voltage from the same effect. That symmetry is worth seeing plainly, because it is easy to meet these as three unrelated components. It also explains why the material matters so much here: only a direct gap emits light usefully, and the gap size sets the colour.
Work through these
One junction, three jobs, decided by bias and construction
Forward biased it emits, reverse biased it detects, and unbiased under illumination it generates. Recognising that the three devices are one structure used three ways makes each of them easier to reason about.
NPTEL: Introduction to Semiconductor Devices · CourseLight emission, and why the gap decides the colour
An electron recombining across the gap releases that energy as a photon, and photon energy is what the eye reads as colour. A wider gap gives bluer light, which is why colours arrived in the order they historically did.
Why silicon cannot do this, and what is used instead
An indirect gap needs a lattice vibration to take up momentum as well, so the energy leaves as heat rather than light. Emitting devices are made from direct-gap compounds, which is a materials decision rather than a circuit one.
NPTEL: Solid State Devices · CourseThe photodiode: reverse bias, and current proportional to light
Light absorbed in the depletion region creates carrier pairs that the field sweeps out, giving a current that follows the illumination closely. Reverse bias widens the region and speeds the response, which is why detectors are biased that way.
The solar cell, and why it is the same device with no bias
The same light-generated carriers separated by the built-in field produce a voltage with nothing applied, and a current when a load is connected. The device is optimised for area and efficiency rather than speed.
NPTEL: Introduction to Semiconductor Devices · CourseDriving an emitter properly: current, not voltage
The exponential relation means a small voltage error becomes a large current error, so an emitting device is driven with a current source or a series resistor. Connecting one straight across a supply destroys it.
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