# Responsivity

By BYJU'S Exam Prep

Updated on: September 25th, 2023

Responsivity plays an important role in enhancing the performance of the photodetector. Photodetectors play a significant role due to diverse applications across the electromagnetic spectrum, such as in short and long-range communication, visible and infrared camera sensors, and biomedical imaging.

In this article, you will learn about responsivity, its derivation, the factors on which it depends, and the typical responsivities of some materials, which will help you understand the relationship between responsivity and wavelength.

Download Complete Electronic Devices Formula Notes PDF

Table of content

## What is Responsivity?

Responsivity is a measure of input-output gain of the detector in a fibre optic system. In the case of a photodetector, it is the measure of electrical output per optical input. Mostly photodetectors are linear functions of input incident power. The responsivity of a photodetector is typically expressed as units of amperes per watt or volts per watt of radiant power.

## Responsivity Formula

It is typically highest in a wavelength region where the photon energy is above bandgap energy, then declining sharply in the bandgap region, where absorption decreases. It can be derived as follows:-

Rλ= IP/P

Where,

• Rλ= Responsivity
• IP = Output Photo Current
• P = Incident Optical Power

Since hυ = Energy of Photon,

P = rp

Where

• rp = Photon flux = P/hυ = Photons/sec

Hence the electron rate comes out to be,

re = ηrp = ηP/hυ

Therefore, the output current is

Ip= eηP/hυ = eηλ/hc = eηλ/1.24

h = Plank’s Constant = 6.63 × 10-34 joule-sec

### Responsivity Unit

The unit of Responsivity is Amperes per watts(A W1) or volts per watts (V W1).

## Responsivity of Typical Photodetectors

The relationship between responsivity and wavelength can be understood from the table below. The unit of wavelength is in nanometers, and the unit of responsivity is ampere per watts.

 Photodetector Wavelength (nm) Responsivity Silicon PN 550-850 0.41-0.7 Silicon PIN 850-950 0.6-0.8 InGaAs PIN 1310-1550 0.85 InGaAs APD 1310-1550 0.80 Germanium 1000-1500 0.70

The photodetector’s responsivity depends on the wavelength of incident radiation and the material’s bandgap. The below-given diagram shows the responsivity vs wavelength graph. The different range of responsivity is shown: maximum, average, and minimum.

## Responsivity Questions in Gate Exam

Question 1: The quantum efficiency of the photodiode is 50% with a wavelength of 0.90* 10-6. Determine the responsivity of photodiodes.