LED, photodiode and solar cell

What are an LED, a photodiode and a solar cell?

In plain words: in an LED an electron that falls into a hole gives up its energy as a flash of light, so the colour depends on the size of the band gap. A photodiode and a solar cell run the same process backwards: light knocks electrons free, and the junction’s field pushes them out as a current. The photodiode uses that current to measure light; the solar cell uses it to deliver power.

Light, photons and the band gap

• Light comes in photons of energy E = hν = hc/λ (NCERT Chapter 11, page 283). A handy form, used on the photon page: E in eV ≈ 1240/λ in nm.
• An electron that drops from the conduction band into a hole in the valence band gives up about the band gap energy E_g. In an LED this comes out as a photon, so the wavelength of the light depends on the band gap (Wikipedia, “Light-emitting diode”).
• The other way round, a photon can free an electron, making an electron-hole pair, in the simple single-photon band-to-band model if hν ≥ E_g (Wikipedia, “Theory of solar cells”). This is an approximate interband threshold; indirect-gap materials also require phonon assistance, and sub-gap absorption by other mechanisms is outside this model (reasoned).

Light-emitting diode and its I-V characteristic

• An LED is a diode that produces light when forward biased; the colour, such as red, green or amber, depends on the material it is made of (NEETS Module 7, page 3-27).
• How it works: forward bias injects electrons into the p-side and holes into the n-side, as in any forward-biased diode. Near the junction they recombine, and in an LED material the energy given up appears as light, a process called electroluminescence (Wikipedia, “Light-emitting diode”). Within its rated operating range and at controlled temperature, more current generally gives more light; efficiency and heating prevent unlimited proportional growth (reasoned).
• Materials: compound semiconductors such as gallium arsenide (GaAs) for infrared and gallium arsenide phosphide (GaAsP) for red (Wikipedia, “Light-emitting diode”). Ordinary bulk Si and Ge are inefficient band-edge light emitters because their gaps are indirect, and their band-edge photons are infrared: photons of their band gaps, 1.1 eV and 0.7 eV (NCERT page 332), are in the infrared (reasoned; see Example 1).
• I-V characteristic: the same shape as a silicon diode’s, but the current starts to rise at a larger voltage. NEETS gives about 1.6 V at about 10 mA (Module 7, page 3-27); Wikipedia’s table gives about 1.6 V to 2.0 V for red LEDs and about 2.5 V to 3.7 V for blue (“Light-emitting diode”). The bluer the light, the larger the photon energy and the larger the threshold: roughly, e times the threshold voltage is about E_g, so a red LED with photons of about 1.9 eV (Example 1) starts to conduct near 1.9 V (reasoned).
• Reverse bias: an LED lights only in forward bias. Its reverse breakdown voltage is low, typically about 5 V, and above it a large current flows and damages the LED (Wikipedia, “Light-emitting diode”). Strictly, the 5 V printed on a datasheet is an absolute-maximum reverse-voltage rating, not a measured breakdown voltage: the Kingbright WP7113ID datasheet lists 5 V as the maximum reverse voltage and allows at most 10 μA of reverse current at 5 V. Follow the manufacturer’s limits.
• In a circuit an LED needs current control, typically a series resistor or a regulated current driver (reasoned; see Example 2). The NCERT Laboratory Manual (Physics, Class 12) suggests plotting the I-V curve of an LED with the circuit of Experiment 16 and comparing the threshold voltages of different coloured LEDs (page 124).
• Advantages over a filament lamp: lower power use, less waste heat, longer life, more robust, smaller and faster to switch (Wikipedia, “Light-emitting diode”). NEETS gives a life of over 10⁵ hours (Module 7, page 3-27); real lifetimes depend on the device, its operating current and temperature (reasoned).
• Symbol: a diode symbol with two arrows pointing away from it, for the light leaving (NEETS Module 7, page 3-27).

Photodiode in reverse bias and its I-V characteristic

• A photodiode is a light-sensitive junction diode operated in reverse bias, with a transparent window over the chip so that light can reach the junction; it conducts in direct proportion to the intensity of the light (NEETS Module 7, page 3-30). Strictly, it can also be used at zero external bias, its photovoltaic mode (Wikipedia, “Photodiode”), and its photocurrent is proportional to the incident optical power only at a fixed spectrum, temperature and active area, within the detector’s linear range (reasoned).
• How it works: a photon with hν ≥ E_g, absorbed in or near the depletion region, creates an electron-hole pair. The junction’s field sweeps the electron to the n-side and the hole to the p-side, giving a photocurrent (Wikipedia, “Photodiode”). The total current is the small dark current, which flows with no light, plus the photocurrent.
• Why reverse bias (reasoned from NCERT pages 335–336): the reverse current is carried by minority carriers and is limited by how many there are. Light creates electron-hole pairs, which multiplies the number of minority carriers many times over while hardly changing the number of majority carriers. So the reverse current changes by a large fraction and is easy to measure; in forward bias, the large current of majority carriers would hide the change. Reverse bias also widens the depletion layer and lowers the junction capacitance, so the response is faster, but it adds dark current and with it more noise, which is why zero bias suits low-noise measurements (Wikipedia, “Photodiode”).
• I-V characteristic (third quadrant): for each light intensity the reverse current is almost independent of the reverse voltage, and it rises in step with the light: at fixed spectrum and within the linear range, double the intensity and the photocurrent doubles (Wikipedia, “Photodiode”; NEETS Module 7, page 3-30).
• Approximate interband cut-off: in the single-photon model, photons with hν ≥ E_g give a photocurrent, so a photodiode responds only to wavelengths up to λ_max = hc/E_g: about 1130 nm for silicon (see Example 3).
• Uses: light meters, optical scanners and card readers, because it responds quickly to changes in light (NEETS Module 7, page 3-30). Symbol: a diode symbol with two arrows pointing towards it.

Solar cell and its I-V characteristic

• A solar cell (photovoltaic cell) is a p-n junction that converts light energy into electrical energy (NEETS Module 7, page 3-32). It works with no external bias; in effect it is a photodiode with a very large junction area (Wikipedia, “Photodiode”).
• How it works: a photon with hν ≥ E_g makes an electron-hole pair; the junction’s field drives the electron to the n-side and the hole to the p-side (Wikipedia, “Theory of solar cells”). Charge builds up, the p-side positive and the n-side negative, and this voltage forward biases the junction (Wikipedia, “Photodiode”). Joined to a load, the cell drives a current through it, leaving by the p-side terminal, like a battery.
• One cell gives about 0.45 V; as with batteries, cells are connected in series for a larger voltage and in parallel for a larger current. They power communications satellites and solar-powered homes (NEETS Module 7, page 3-32). NCERT gives no figure. Wikipedia puts an ordinary single-junction silicon cell at about 0.5–0.6 V open-circuit (“Solar cell”), a typical value but not an upper limit: silicon heterojunction cells, whose surfaces are very well passivated, consistently give over 0.7 V (Wikipedia, “Heterojunction solar cell”). The voltage also depends on the light, the temperature and recombination (reasoned).
• I-V characteristic (fourth quadrant): the voltage is positive (p-side positive) but the current is negative, because it flows out of the p-side, opposite to the current of a forward-biased diode. The curve runs from the short-circuit current I_SC at V = 0 to the open-circuit voltage V_OC at I = 0. The cell gives no power at either end (Wikipedia, “Theory of solar cells”). Many books turn the curve over and draw it in the first quadrant, with the current taken as positive.
• Maximum power is drawn at the knee of the curve, where the rectangle from the origin to a point on the curve, of area V_mI_m, is largest. The fill factor is the ratio of this maximum delivered power to V_OCI_SC, using positive current magnitudes in the power and fill-factor formulas (Wikipedia, “Solar cell”):

• Light intensity: I_SC is the light-generated current, so it grows in proportion to the intensity, while V_OC grows only slowly, with its logarithm (Wikipedia, “Theory of solar cells”).
• Material: semiconductors with band gaps between about 1 eV and 1.5 eV are the best for a single-junction cell (Wikipedia, “Solar cell”). Silicon, with 1.1 eV (NCERT page 332), lies in that range.

Solved examples

Common mistakes to avoid

Quick revision summary

• LED: forward biased; electrons and holes recombine and give out light of photon energy about E_g; threshold about 1.6–2.0 V for red and 2.5–3.7 V for blue; low reverse breakdown, typically about 5 V (a datasheet’s 5 V is a reverse-voltage rating, not a measured breakdown).
• Photodiode: commonly reverse biased, or operated at zero external bias; light with hν ≥ E_g makes electron-hole pairs; the reverse current is nearly independent of the reverse voltage and proportional to the intensity.
• Solar cell: no external bias source; the I-V curve is in the fourth quadrant, from I_SC (proportional to intensity) to V_OC (material- and condition-dependent; 0.6 V is an example, not a maximum); maximum power at the knee.
• Silicon, with a 1.1 eV gap, makes photodiodes for visible light and solar cells, but not visible LEDs, whose band-edge photons are infrared; LEDs use compound semiconductors such as GaAsP.

Questions students ask

Are the LED, photodiode and solar cell in the JEE Main and NEET syllabus?

Yes, the LED, the photodiode and the solar cell are in both syllabuses: Unit 19, Electronic Devices, of the JEE Main syllabus lists the “I-V characteristics of LED, the photodiode, solar cell”, and the NEET (UG) 2026 syllabus lists the same three in its Unit 19. The current NCERT Class 12 Physics Part II, still the 2025–26 reprint, has no section on any of them; the nearest it comes is the generation and recombination of electrons and holes in Section 14.3 (page 328). Their I-V curves are not Unit 20 experiments, though the LED is one of the parts in Unit 20’s identification activity.

What is the difference between an LED and an ordinary diode?

An LED differs from an ordinary silicon diode, beyond the NCERT text, in its material: in a compound semiconductor such as gallium arsenide phosphide the energy given up when an electron and a hole recombine comes out as light, while in silicon it goes mostly into heat. Its wider band gap also gives it a larger forward threshold, about 1.6–2.0 V for red, against NCERT’s 0.7 V for silicon (page 337), and its allowed reverse voltage can be much lower than a rectifier’s, as its reverse breakdown voltage is low (Wikipedia, “Light-emitting diode”).

What is the difference between an LED and a photodiode?

An LED and a photodiode do opposite jobs, beyond the NCERT text: an LED is forward biased and turns electrical energy into light as electrons and holes recombine, while a photodiode is operated in reverse or zero bias and turns light into a current, its reverse current growing in proportion to the light (US Navy, NEETS Module 7, pages 3-27 and 3-30; Wikipedia, “Photodiode”). The two often work as a pair: in an optical coupler an LED’s light drives a photodiode, so a signal passes between two circuits that have no electrical connection (NEETS page 3-32). Reverse biased, an LED can itself work as a light sensor, picking up only a narrow range of wavelengths (Wikipedia, “Light-emitting diode”).

What is the difference between a photodiode and a solar cell?

A photodiode and a solar cell differ, beyond the NCERT text, in use rather than principle: in both, light makes electron-hole pairs that the junction’s field separates, but photodiodes are optimised to measure light (in reverse or zero bias), while solar cells are optimised to deliver power with no external bias source; physical size alone does not define the distinction (Wikipedia, “Photodiode”). Reverse bias makes the photodiode respond fast, with its curve in the third quadrant; the solar cell sets up its own forward voltage and works near the knee of its fourth-quadrant curve. At zero bias, in photovoltaic mode, a photodiode is in effect a tiny solar cell.

What is the efficiency of a solar cell, and why is it low?

The efficiency of a solar cell, beyond the NCERT text, is the electrical power it delivers divided by the light power falling on it (US DOE, “Solar Photovoltaic Cell Basics”): a cell of area 0.0225 m^² in sunlight of 1000 W m^⁻² receives 22.5 W, so if it gives 4.05 W at its maximum power point its efficiency is 18%. The band gap sets the main limit: sub-gap photons do not produce usable band-to-band carriers in the ideal model, and photons above it lose their extra energy as heat, so the detailed-balance ideal single-junction limit under unconcentrated terrestrial sunlight is about 33%, the Shockley–Queisser limit, at E_g ≈ 1.34 eV (Wikipedia, “Shockley–Queisser limit”). Cells stacking several band gaps exceed it, some above 45% (US DOE).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is a photodiode operated in reverse bias?

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   In reverse bias the current is carried by minority carriers and is small. Light creates electron-hole pairs, which multiplies the minority carriers many times over, so the reverse current changes by a large fraction and measures the light. In forward bias the large current of majority carriers would hide the change (reasoned from NCERT pages 335–336).

2. Q2. In which quadrant does the I-V characteristic of a solar cell lie, and why?

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   The fourth: the voltage across the cell is positive, with the p-side positive, but the current flows out of the p-side into the load, so it is negative. The cell is a source of power, not a load.

3. Q3. An LED gives yellow light of 590 nm. What is the energy of each photon, in eV?

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   E ≈ 1240/590 ≈ 2.1 eV.