LEDs and Optoelectronics

Optoelectronics is the part of electronics that turns electricity into light and light back into electricity. Building on article 6, this article explains how light-emitting diodes (LEDs) make their colors and how to choose and drive them, then covers the parts that detect light: photodiodes, solar cells, phototransistors and optocouplers.

1. A short recap

• Ohm's law links voltage, current and resistance (recap). In a series circuit one current flows through every part and the voltages add (recap).
• The band gap is the forbidden energy range between a solid's valence band and its conduction band, measured in electron volts (eV). Silicon's gap is about 1.12 eV (recap); gallium arsenide's is about 1.42 eV and gallium nitride's about 3.4 eV (recap).
• A PN junction conducts when forward biased and blocks when reverse biased; its depletion region holds an electric field (recap).
• An LED gives off light when electrons and holes recombine at its junction. Its band gap sets the color, and it needs a series resistor to limit its current (recap).
• A bipolar transistor lets a small base current control a much larger collector current (recap).
• A battery's voltage falls as it discharges (recap).

2. Light, color and photons

Light is an electromagnetic wave. Its wavelength, λ (lambda), is the distance between wave crests. Visible light runs from about 400 nm (violet) to 750 nm (red).^[1] A nanometer (nm) is a billionth of a meter. Just shorter than violet is ultraviolet (UV); just longer than red is infrared (IR). We cannot normally see either.^[1]

Light also comes in tiny packets of energy called photons. A photon's energy depends only on its frequency f (how many crests pass each second):^[2]

E = hf = hc ÷ λ

Here h is Planck's constant and c is the speed of light. With energy in electron volts and wavelength in nanometers, hc is about 1240 eV·nm, so E ≈ 1240 ÷ λ.^[2] A 940 nm infrared photon carries about 1.32 eV; a 465 nm blue one carries about 2.67 eV (both calculated; Figure 1).

That is the link to the band gap. When an electron falls across a gap, it can give off one photon whose energy is about the gap energy, so the photon's wavelength is about 1240 ÷ E_g.^[3] A wider gap gives bluer light.

3. How an LED makes light

An LED is a PN junction built from layers of compound semiconductor. Forward bias drives electrons from the n-type side and holes from the p-type side into a thin active layer between them. When an electron meets a hole they recombine: the electron drops into the empty place in the valence band, and the energy it loses can leave as a photon.^[4]​^[5] Light made this way is called electroluminescence.^[6]

Efficient blue LEDs also needed stacks of alloys with slightly different compositions, called heterojunctions, and very thin layers called quantum wells.^[5] In laser diodes, such layers confine the electrons and holes to a thin active region.^[7]

Direct and indirect band gaps

Electrons in a crystal carry momentum as well as energy; a photon carries almost none. In a direct band gap material such as gallium arsenide (GaAs), the conduction band's lowest point sits at the same momentum as the valence band's highest point, so an electron can drop straight across and emit a photon.^[5] In an indirect band gap material they sit at different momenta. Recombination then needs a phonon, a quantum of the crystal's vibration, to supply the missing momentum, and that limits the efficiency (Figure 2).^[5]

Silicon has an indirect gap, so radiative recombination (the kind that gives off light) is extremely low; recombination through crystal defects and other non-light processes dominates instead.^[8] That is why a silicon diode does not glow.

4. Materials, colors and a short history

Each color needs a material with the right band gap, and mixing elements in different ratios tunes it.^[5]

• Gallium arsenide (GaAs): direct gap of 1.4 eV, in the infrared.^[5] Kingbright's WP7113F3C is a GaAs infrared LED at 940 nm.^[9]
• Gallium arsenide phosphide (GaAsP): adding phosphorus shortens the wavelength into the visible red. The gap stays direct while less than 45% of the arsenic is replaced.^[5] Kingbright's standard red WP7113ID is GaAsP on a GaP substrate.^[10]
• Gallium phosphide (GaP): an indirect gap of 2.2 eV,^[5] still used for dim yellow-green indicator LEDs such as the WP7113SGD.^[11]
• Aluminum gallium indium phosphide (AlGaInP): bright red, orange and yellow, such as the WP7113SURCK (red) and WP7113SYD (yellow).^[12]​^[13]
• Indium gallium nitride (InGaN): alloys of gallium nitride (GaN) with indium, used for green, blue, white and near-UV LEDs.^[5]​^[14]​^[15]​^[16]

History. Light from a semiconductor was first reported in 1907.^[4] After infrared GaAs LEDs, Nick Holonyak Jr. at General Electric demonstrated the first visible-light LED, a red GaAsP device, on October 9, 1962.^[17] Red LEDs were soon lighting watches, calculators and on/off indicators, but blue took three more decades, because good GaN crystals and p-type GaN were so hard to make. Isamu Akasaki, Hiroshi Amano and Shuji Nakamura made bright blue LEDs in the early 1990s and shared the 2014 Nobel Prize in Physics.^[4]

How white LEDs work

A single LED gives nearly one color.^[18] White light needs a mix. Most white LEDs use a blue InGaN chip coated with a phosphor, a material that absorbs some of the blue light and glows at longer wavelengths (yellow, or red and green). The leftover blue plus the glow looks white (Figure 3).^[5]​^[6] Others mix red, green and blue LEDs, or combine both methods.^[18]

5. Reading an LED datasheet

An LED datasheet lists (Table 1):

• Forward voltage (V_F): the voltage across the LED at a stated test current. It rises with the band gap, from about 1.2 V for infrared to about 3.3 V for blue, and the maximum can be well above typical: 4.0 V for a blue LED rated 3.3 V.^[9]​^[15]
• Forward current (I_F): about 20 mA is typical for a standard LED.^[6] The absolute maximum DC current is often 25 or 30 mA, with a higher peak current allowed only for short pulses (160 mA at a 1/10 duty cycle and 0.1 ms pulses for the WP7113ID).^[10]​^[11]
• Reverse voltage (V_R): often only 5 V,^[10] far less than a rectifier diode's, so an LED must never face much reverse voltage.^[6]
• Luminous intensity (I_v): brightness in one direction, in candelas (cd) or millicandelas (mcd, thousandths of a candela). The eye is more sensitive to some colors than others, most of all to greenish-yellow in daylight, so the candela is defined using green light and other colors are weighted by the eye's sensitivity.^[19]
• Viewing angle (2θ½): the full angle across which the intensity is at least half of its peak.^[10] A narrower beam reads more millicandelas from the same light.
• Luminous flux, in lumens (lm): the total visible light in all directions, the figure used for lamps.^[19] Efficacy is lumens per watt of electrical input. In 2014 the Nobel committee compared a record of just over 300 lm/W with 16 for ordinary incandescent bulbs and about 70 for fluorescent lamps.^[4]

As an efficacy example, a Cree XP-G3 white LED in one brightness grade gives at least 164 lm at 350 mA, with a typical 2.70 V at an 85 °C junction.^[20] That is 2.70 V × 0.35 A = 0.945 W, or about 174 lm/W (calculated).

Wavelengths are the dominant wavelength (the color the eye perceives) unless marked as peak. Note how V_F roughly tracks the photon energy, and that the indirect-gap GaP green gives 40 mcd against 14,000 mcd for the direct-gap InGaN green, a ratio of 350 (calculated) that the different viewing angles cannot explain.

6. Kinds of LED

• Through-hole LEDs have two wire leads and an epoxy lens, commonly 3 mm (called T-1) or 5 mm (T-1¾) across.^[22]​^[23]
• Surface-mount (SMD) chip LEDs solder flat to a board. Their size codes give length and width: the APT1608EC is 1.6 × 0.8 mm and the APT2012EC is 2.0 × 1.25 mm.^[24]​^[25] The same sizes in hundredths of an inch are 0603 and 0805,^[26] so check which system a datasheet uses.
• High-power LEDs for lighting take amps, not milliamps. The XP-G3 sits on a 3.45 mm package, takes up to 2,000 mA and spreads its light over about 125°.^[20] It needs a path for heat (section 15).
• RGB LEDs put red, green and blue chips under one lens, so mixing their brightness gives other colors.^[6] An addressable LED such as the WS2812B also contains a small control chip. Each takes 24 bits of color data (256 levels per color) over a single data line at 800 kbit/s and passes the rest along the chain.^[27] Driving them is a job for a microcontroller.
• Seven-segment displays arrange seven bar-shaped LEDs, labeled a to g, plus a decimal point, to show digits. In a common-cathode display such as the SC56-11EWA, all the segments' cathodes join at one pin, and each segment's anode has its own pin (Figure 5).^[28]
• Infrared LEDs, often at 940 nm, are used in remote controls, optical counters and camera illumination.^[22]​^[9] The eye cannot see them, so their output is rated as power per solid angle (milliwatts per steradian) instead of millicandelas.^[9]
• Ultraviolet LEDs near 395 nm are used for curing glues and inks and for checking banknotes.^[16] Their light is mostly invisible and can harm the eyes (section 15).^[29]

7. Driving LEDs

The series resistor, and its limits

Article 6 sized a resistor for one LED: the resistor takes the supply voltage minus V_F, and R = (V_supply − V_F) ÷ I (recap). LEDs in series share one resistor, but LEDs in parallel each need their own, because their forward voltages never match exactly.^[6]

The weak spot is variation in V_F. Two white LEDs (3.3 V typical each) from 12 V at 20 mA need R = (12 − 6.6) ÷ 0.020 = 270 Ω. If both LEDs sit at their 4.0 V maximum,^[21] the current falls to (12 − 8.0) ÷ 270 ≈ 14.8 mA (both calculated). With three LEDs at 4.0 V, the 12 V would leave nothing for the resistor at all (calculated), so always leave headroom.

Temperature matters too. V_F falls as the LED warms, by about 1 to 4 mV per °C,^[30] and 3.0 mV/°C for the white LED above.^[21] Fed from a fixed voltage, a warming LED draws more current and heats further; in the example, the resistor holds a 40 °C rise to less than 1 mA extra (calculated).

Constant-current drivers

A constant-current driver measures the LED current, usually as the voltage across a small sense resistor, and adjusts its output to hold that current steady whatever V_F does.^[31] Power LEDs need one. An XP-G3 at 1,000 mA drops about 2.87 V;^[20] from 5 V, a series resistor would waste 2.1 W, as much as the LED uses (calculated). Switched-mode drivers avoid that loss by switching current through an inductor (recap).^[32] Article 15, Voltage Regulators, covers the circuits.

Dimming with PWM

Analog dimming lowers the current, but the color can shift. Pulse-width modulation (PWM) switches the full current on and off and varies the fraction of time it is on, the duty cycle (Figure 6).^[32]​^[31] At 50% duty, full current flows half the time.^[31] To avoid visible flicker, TI recommends above 100 Hz in one note and about 200 Hz in another.^[31]​^[32] A transistor can do the switching for a power LED (recap).

Multiplexing

Instead of a driver pin for every segment, a multiplexed display joins each segment letter across all digits and lights one digit at a time, cycling so fast that persistence of vision makes the whole display look lit.^[33] In a four-digit display each digit is lit a quarter of the time (calculated), so drivers often use pulses above the DC current rating, within the datasheet's peak limit (for the SC56-11EWA, 160 mA at a 1/10 duty cycle and 0.1 ms pulses).^[28]

8. Photodiodes

A photodiode runs the LED process backward. A photon with at least the band-gap energy lifts an electron into the conduction band and leaves a hole: an electron-hole pair. The depletion region's field sweeps the pairs apart, and that flow is the photocurrent.^[6] Photons below the gap energy pass straight through. Silicon's 1.12 eV gap sets a limit near 1240 ÷ 1.12 ≈ 1,100 nm (calculated), and the BPW34 photodiode responds from 430 to 1,100 nm.^[34]

It works in two modes (Figure 7):

• Photovoltaic mode: no bias. The diode makes its own voltage, like a tiny solar cell. This mode has low noise but responds less linearly over a wide range of light levels.^[6] A BPW34 gives about 350 mV with no load in moderate infrared light (1 mW/cm²).^[34]
• Photoconductive mode: reverse bias (or zero bias into a current amplifier). The current is proportional to the light over many decades and responds faster.^[6] Reverse bias also adds a small dark current, the leakage that flows with no light.^[35] A transimpedance amplifier turns the current into a voltage.^[6]

Responsivity (Hamamatsu calls it photosensitivity) is photocurrent per watt of light, in amps per watt (A/W).^[35] Each photon frees at most one electron, so the limit is λ ÷ 1240 A/W (λ in nm), about 0.77 A/W at 950 nm (calculated). The BPW34 gives about 50 µA from 1 mW/cm² at 950 nm on its 7.5 mm² area,^[34] about 0.67 A/W (calculated).

Speed depends on capacitance, which reverse bias reduces.^[6] The BPW34 measures 70 pF at 0 V and 25 pF at 3 V, and switches in about 100 ns.^[34] A PIN photodiode adds an undoped (intrinsic) layer between P and N and can reach tens of gigahertz; an avalanche photodiode uses high reverse voltage so each photon releases many electrons.^[6] Uses include barcode readers, rotary encoders, color and UV sensors^[35] and remote-control receivers.^[36]

9. Solar cells

A solar cell is a photodiode built to deliver power to a load, working in photovoltaic mode.^[6]​^[35] The current is collected by metal contacts, the grid-like lines on the cell (preview in article 2).^[37] Silicon is by far the most common material.^[37] The U.S. Department of Energy puts a silicon cell's theoretical maximum efficiency at about 32%, set by how much sunlight lies above silicon's band gap, and commercial panels at about 18 to 22%.^[38]

One silicon cell gives only about 0.5 V, so cells are wired in series: for charging 12 V batteries, a panel has 36 of them.^[6] A panel's DC can charge batteries (recap) or feed an inverter that makes AC (recap; solar systems).

10. Phototransistors

A phototransistor is a bipolar transistor whose base-collector junction is exposed to light. The photocurrent there acts as base current, and the transistor multiplies it, much as light in an optocoupler "acts like a base current to the transistor".^[39]

The gain is large. Under the same light, a BPW85 phototransistor passes 0.8 to 8 mA,^[23] while a BPW34 photodiode gives about 50 µA:^[34] 16 to 160 times more (calculated). The spread is so wide that parts are sorted into bins (BPW85A, B and C). The cost is speed: about 2 µs to switch, twenty times slower than the BPW34.^[23]​^[34]

11. Photoresistors

A photoresistor, or light-dependent resistor (LDR), has no junction. It is a strip of cadmium sulfide whose resistance falls as light increases. It is cheap and simple but slow and imprecise (recap). Article 18, Sensors, will compare it with the other light sensors.

12. Optocouplers and optoisolators

An optocoupler (or optoisolator) carries a signal between two circuits using light, with no electrical connection between them.^[39] The 4N35 contains a gallium arsenide infrared LED and a silicon NPN phototransistor facing each other inside one package (Figure 8).^[40] The LED's light turns on the transistor. Solid-state relays use the same idea (recap).

Key ratings:

• Isolation voltage: the voltage the barrier withstands in testing. The 4N35's is 5,000 V rms, with at least 7 mm along the package surface (creepage) between the two sides.^[40]
• Current transfer ratio (CTR): the output current divided by the input current, as a percentage. The 4N35 guarantees at least 100% at 10 mA input and 25 °C, but only 40% across −55 to +100 °C.^[40] Design for the worst case.
• Speed: phototransistor optocouplers are slow. The 4N35's switching time is about 10 µs.^[40]

Why isolate? First, safety. A mains power supply's output side is kept apart from the mains by a transformer (recap), yet its controller on the mains side must know the output voltage. Commonly a TL431 reference on the output side drives an optocoupler's LED, and the phototransistor reports back.^[39] The optocoupler's high-voltage isolation barrier helps the supply meet safety standards.^[41] Second, noise: with no shared wire, ground noise cannot flow between circuits. Vishay lists ground isolation and noise rejection among the 4N35's uses.^[40]

Worked example (calculated). A 3.3 V logic pin that can supply 10 mA drives the 4N35’s LED (about 1.3 V)^[40] through R = (3.3 − 1.3) ÷ 0.010 = 200 Ω. A 4.7 kΩ pull-up to 5 V needs at most about 1.1 mA to pull the output low, and even 40% CTR gives 4 mA, so the output switches fully.

13. Remotes, slot sensors, fiber and lasers

Infrared remote controls. The remote's IR LED, such as the 940 nm TSAL6200, sends bursts of pulses.^[22] The receiver module, such as the TSOP382 series, contains a PIN photodiode, an amplifier and a filter tuned to the burst frequency (38 kHz for the TSOP38238). An IR filter covers it, it resists interference from lamps, and it hands a clean digital signal to a microcontroller.^[36]

Optical interrupters (slot sensors). An IR emitter faces a phototransistor across a slot (3.1 mm in the TCST2103). Anything in the slot blocks the beam and switches the output; uses include counters and encoders (Figure 9).^[42]

Fiber optics. Optical fiber guides light along a hair-thin glass core, trapped by total internal reflection at the surrounding cladding. Multimode fiber (50 or 62.5 micrometer core) is used with LEDs at 850 and 1,300 nm for slower networks and with lasers for gigabit speeds; singlemode fiber (about 9 micrometers) uses lasers at 1,310 and 1,550 nm.^[43] An LED’s broad spread of wavelengths smears fast pulses.^[44]

Laser diodes. A laser diode is a further development of the LED.^[6] Light inside its cavity triggers stimulated emission of more light, amplifying it ("laser" stands for light amplification by stimulated emission of radiation).^[7]​^[44] The result is coherent light, a single color with the waves in step, forming a tight beam.^[6] Below a threshold current there is essentially no laser output; above it, output grows roughly in proportion to the extra current (Figure 10),^[7] so laser diodes need regulated current.^[6] They are found in disc players, barcode readers and laser pointers.^[45]

14. Testing

• Polarity. On a new through-hole LED, the longer lead is the anode (+).^[46] The cathode (−) is the shorter lead, and the lens rim has a flat side next to it.^[6]
• Diode test. A meter's diode-test mode may not reach a blue or white LED's forward voltage, so it reads "OL" (no measurable current) even when the LED is good (recap). The dependable test is a battery and a resistor sized as in section 7.
• Infrared LEDs. Point the LED or remote at a digital camera or phone camera and press a button: a working emitter shows as a light on the screen. Many phones have filters that block IR, so first check that your camera can see a known-good remote.^[47]
• Photodiodes and solar cells. A voltmeter across one should read a few hundred millivolts in light (BPW34)^[34] and near zero when covered.
• Optocouplers. An IR LED’s low forward voltage (about 1.3 V in the 4N35) is within diode-test range. Then drive the LED at 10 mA, load the output through a resistor from a supply, and check that the output switches and that output current divided by input current meets the minimum CTR.^[40]

15. Safety

Eyes. Article 6 covered the blue-light hazard and the laser classes (recap). In practice:

• High-power LEDs. Cree warns: "Do not look at an exposed lamp in operation. Eye injury can result."^[20] Point them away when testing.
• UV LEDs. One maker warns that their intense, mostly invisible light "may be harmful to eyes, even for brief periods", and calls for UV-protective goggles for everyone nearby.^[29] The FDA says never to look directly into a UVC (shortest-wave UV) source, even briefly; it can burn skin and injure eyes.^[48]
• Laser diodes. Laser products fall under a federal performance standard and carry a hazard class; LEDs are not covered by it.^[45] Never look into a beam.
• Infrared lies outside what the eye responds to,^[1] so you cannot tell by looking whether a powerful emitter is on. Treat IR illuminators like bright lights.

Heat. Cree calls improper thermal management "a main cause of LED failures". Its power LEDs turn roughly 50 to 60% of their input into heat, and it suggests designing for 75%.^[30] So the XP-G3 at 1 A (2.87 W) needs a path for about 2.2 W of heat (calculated). Warming changes light output, color and forward voltage, which recover on cooling, but lost lifetime does not, and above the maximum junction temperature, typically 150 °C, damage can be permanent.^[30] Mains LED lamps and drivers also carry dangerous voltages (recap).

The whole story in brief

• A photon's energy is E = hf ≈ 1240 ÷ λ (eV, nm). An LED's band gap sets its photon energy and color.
• Bright LEDs need direct-gap compounds (GaAs, AlGaInP, InGaN); silicon's indirect gap makes heat instead.
• White LEDs are usually blue chips with a phosphor.
• Small LEDs use a series resistor with headroom; power LEDs use constant-current drivers; PWM dims.
• Photodiodes are fast; phototransistors trade speed for gain; solar cells are photodiodes that deliver power.
• Optocouplers pass signals across an insulating gap. Check the isolation voltage and design for the worst-case CTR.
• Protect your eyes from power LEDs, UV and lasers; keep power LEDs cool.

Coming up in this section: 15 Voltage Regulators.

Sources

All links were checked in October 2026. External links open in a new tab.

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13. Kingbright, WP7113SYD 5 mm super bright yellow LED data sheet (PDF): https://www.kingbrightusa.com/images/catalog/SPEC/WP7113SYD.pdf
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40. Vishay, 4N35, 4N36, 4N37 Optocoupler, Phototransistor Output data sheet (PDF): https://www.vishay.com/docs/81181/4n35.pdf
41. Würth Elektronik, "ANP113: Feedback loop compensation of a current-mode flyback converter with optocoupler" (PDF): https://www.we-online.com/components/media/o760916v410%20ANP113a%20EN.pdf
42. Vishay, TCST2103, TCST2202, TCST2300 Transmissive Optical Sensor with Phototransistor Output data sheet (PDF): https://www.vishay.com/docs/81147/tcst2103.pdf
43. The Fiber Optic Association, FOA Reference Guide, "Optical Fiber": https://www.thefoa.org/tech/ref/basic/fiber.html
44. The Fiber Optic Association, FOA Reference Guide, "Fiber Optic Transmitter Sources: Lasers and LEDs": https://thefoa.org/tech/ref/appln/LEDs-lasers.html
45. U.S. Food and Drug Administration, "Laser Products and Instruments": https://www.fda.gov/radiation-emitting-products/home-business-and-entertainment-products/laser-products-and-instruments
46. SparkFun Learn, "Light-Emitting Diodes (LEDs)": https://learn.sparkfun.com/tutorials/light-emitting-diodes-leds/all
47. Sony Electronics support, "Test if your remote control sends an infrared (IR) signal": https://www.sony.com/electronics/support/articles/00025283
48. U.S. Food and Drug Administration, "Ultraviolet (UV) Radiation": https://www.fda.gov/radiation-emitting-products/tanning/ultraviolet-uv-radiation