Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
In reverse bias the current is very small (only microamperes) because it comes from minority carriers. When light falls on the junction, it creates extra electron-hole pairs, so the reverse current increases in a measurable, clear way. A small change on top of a small current is easy to detect. In forward bias the current is already large (milliamperes), so the tiny extra current made by light would be hidden and hard to measure. That is why a photodiode is always operated in reverse bias when used to detect light.
Yes. A photodiode is connected to an external battery that keeps it in reverse bias. The battery does not create the signal; light creates the extra carriers. The battery only sets up the reverse bias so the light-generated current can be read. This is the key difference from a solar cell, which works with NO external battery and generates its own voltage from light.
MINORITY carriers. In reverse bias the normal (dark) current is a small reverse saturation current carried by minority carriers (electrons in the p-side, holes in the n-side). Light gives energy to electrons, making extra electron-hole pairs near the junction. These extra carriers add to the minority-carrier flow, so the reverse current rises. NEET has directly tested that reverse-bias current is due to minority carriers, not majority carriers.
The reverse current increases when the light gets brighter. More light means more photons, which create more electron-hole pairs per second, which means more current. On the I-V graph, each light intensity gives a separate reverse-current curve: brighter light sits lower (larger reverse current). The relationship is nearly linear, which is why photodiodes are used to measure light intensity.
Both use light and a p-n junction, but a photodiode DETECTS light while a solar cell GENERATES power. A photodiode works in reverse bias with an external battery and gives a current that depends on light intensity. A solar cell works with no external battery, generates its own voltage and current from light, and its I-V curve lies in the fourth quadrant. Photodiode = sensor; solar cell = power source.
Consider the following statements A and B and identify the correct 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.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Along with the Zener diode, LED and solar cell, the photodiode is a special-purpose diode. It is a p-n junction designed so that light can reach the junction and change its current.
Yes, but with only a tiny current. Without light, a reverse-biased photodiode passes a small dark current (reverse saturation current) from minority carriers. When light falls on it, this current increases. The photodiode reads light by measuring this increase.
The increase in reverse current with light intensity. Because brighter light gives a larger and nearly linear increase in current, a photodiode can measure how much light is present. This is why it is used in cameras, light meters and optical sensors.
Yes. For the light to create electron-hole pairs, the photon energy (E = hc / lambda) must be greater than the band gap energy Eg of the semiconductor. Light with too long a wavelength (too little energy) will not generate carriers, so it will not add to the current.
No, they are opposite. An LED uses electric current (in forward bias) to make light. A photodiode uses light to make current (in reverse bias). One converts electricity to light, the other converts light to electricity.
