Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
An LED is always FORWARD biased when it is glowing. In forward bias, electrons and holes cross the junction and recombine. Each recombination releases energy as one photon of light. The photon energy equals the band gap, E = hc/lambda, so a bigger energy gap gives shorter wavelength (bluer) light. If you connect an LED in reverse bias it does not light up, and a large reverse voltage can even damage it because LEDs have a low reverse breakdown voltage.
A photodiode is used in REVERSE bias on purpose. In reverse bias only a tiny current flows (the reverse saturation current from minority carriers). When light falls on the junction it creates extra electron-hole pairs, which raises this reverse current. Because the dark current is very small, even a small light signal makes a clear, measurable change in current. That is why reverse bias gives better sensitivity for detecting light. In forward bias the large forward current would hide the small light-generated current.
Both make use of light, but their job is opposite. A photodiode is a light DETECTOR: it needs an external reverse-bias battery, and it measures how much light falls on it by the change in reverse current. A solar cell is a light-to-electricity CONVERTER: it needs NO external battery. Sunlight itself creates electron-hole pairs that build up a voltage across the cell, so the cell acts like a small power source. A solar cell also has a large surface area to catch more light, while a photodiode is small and fast.
Only the SOLAR CELL works without an external battery. It is a self-powered device: incident light directly produces a voltage and current (the photovoltaic effect). The Zener diode, LED and photodiode all need an external supply. The Zener and photodiode need a reverse-bias supply, and the LED needs a forward-bias supply to glow.
The solar cell I-V characteristic lies in the FOURTH (IV) quadrant. This is because the cell delivers power outward: the voltage is positive but the current direction is opposite to a normal forward-biased diode, so the working part of the curve sits in the fourth quadrant. This exact fact was tested in NEET 2024.
No. A normal diode can be destroyed by reverse breakdown, but a Zener diode is specially made (heavily doped, thin depletion region) to work safely IN the breakdown region. In breakdown, the Zener holds its voltage almost constant even when current changes a lot. That constant voltage is exactly what makes it useful as a voltage regulator, as long as the current is kept within the rated limit by a series resistor.
An LED is constructed from a p-n junction diode using GaAsP. The energy gap is 1.9 eV. The wavelength of the light emitted will be equal to:
Statement I: Photovoltaic devices can convert optical radiation into electricity. Statement II: Zener diode is designed to operate under reverse bias in breakdown region. Choose the most appropriate answer.
A. For a solar cell, the I-V characteristic lies in the IV (fourth) quadrant of the graph. B. In a reverse biased p-n junction diode, the current measured in microamperes is due to majority charge carriers. Identify the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Only the LED works in forward bias when it is emitting light. The Zener diode and photodiode work in reverse bias, and the solar cell needs no external bias at all.
All four are p-n junction diodes made from semiconductors. The difference is only in how they are doped and how they are used: Zener as a voltage regulator, LED as a light source, photodiode as a light detector, and solar cell as a light-to-electricity converter.
Use this line: Zener and Photodiode are in reverse, LED is forward, Solar cell needs no bias. LED gives out light in forward bias; photodiode takes in light in reverse bias; Zener holds voltage in reverse breakdown; solar cell makes its own voltage from light.
E = hc/lambda, where E is the band gap energy. A quick NEET shortcut is lambda (in Angstrom) = 12400 / E(in eV). A larger energy gap gives a smaller wavelength, meaning bluer light; a smaller gap gives longer wavelength, redder or infrared light.
A solar cell is designed with a large area and shallow junction so that sunlight itself separates enough charges to build a usable voltage (photovoltaic effect). A photodiode is small and is used only to sense light while being powered by an external reverse-bias battery, so it acts as a detector, not a generator.
