Test: Photo Diode - Electronics and Communication Engineering (ECE) MCQ

The Quantum efficiency of a photodetector is defined as the fraction of the incident photons that are absorbed by the photoconductor to the generated electrons that are collected at the detector terminal.
Or Quantum efficiency is defined as the fraction of the number of electrons generated that contributes to the photocurrent to the total number of incident photons.
It is dented by ‘η’ and is given by:

Mathematically, Quantum efficiency will be:

Special Note:
Quantum efficiency is also defined in terms of responsivity (R).
Responsivity (R): It is a measure of the sensitivity of a photodetector to light. It is defined as the ratio of photocurrent I_ph to the incident optical power P₀ for a particular wavelength.
Responsivity 
Responsivity depends on:

• The wavelength of the incident light.
• Applied reverse bias.
• Temperature.

Hence,

E = Energy of photon given as:

• A photodiode is indeed used to detect optical signals.
• A photodiode is a type of photodetector that converts light energy into an electrical current. When photons (light particles) strike the photodiode, they generate electron-hole pairs, resulting in a measurable current flow.
• This current can then be amplified and processed to detect and interpret the optical signal.
• Photodiodes are commonly used in various applications, including optical communication systems, light sensors, barcode readers, optical switches, and many other devices that require the detection of optical signals.

• PIN photodiodes are used in fibre optic network cards and switches.
• They are designed to detect photons and are used in optical detection.
• Reverse current flows through the photodiode when it is sensing light. 
• When photons excite carriers in a reverse-biased p-n junction, a very small current proportional to the light intensity starts to flow.
• The intensity depends on the wavelength of the light.
• Photodiodes convert incident light into electric energy, so they are used as optical detectors.

• In physics and in electronic engineering, the dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons are entering the device;
• it consists of the charges generated in the detector when no outside radiation is entering the detector (the current the photodiode produces with no light hitting the diode).
• It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes.
• Physically, the dark current is due to the random generation of electrons and holes within the depletion region of the device.

The function of battery V_r is to reverse bias the photocurrent is independent of reverse bias voltage
I_p = sensitivity × Incident power
V_out = -I_p (R)
Application:
Since V_out = -I_pR does not contain any function of Reverse bias voltage.
V_out = -1V

The output voltage is given by V­_out = V_cc - I_cR_c
Vout = 5 - I_c (1k) p I_c is in mA]
Also
I_c = β I_B
As the Intensity of light falling on the photodiode increases, the Base current IB increase
as I_B increases I_C Increases
Hence
Output voltage which is given by V_out = 5 - I_c(1) decreases. 

Photocurrent = Sensitivity × intensity × Area
After displacing by 100 μm

Current form the upper part:

I_L = 80 μA
Difference = (120 – 80) μA
= 40 μA

The output voltage of I-V converter is

Photo current generate
I_p = Area × sensitivity × Intensity

I_p = 20 μA
Output of I-V converter

= 2000 mV
= 2 V

The whole power of 100 W will not fall on the active area
Intensity of light at a distance ‘d’ from an isotropic source is given by

Here the photodetector is at 1m distance
So, d = 1m

Power falling on photodetector = Intensity x active area

Output current = Power on photodetector x Responsivity

Hence, the photo-current generated in the detector is 1 mA

Given:
η = 0.75
λ = 830 nm
h = 6.626 x 10^-34 J-sec
e = 1.6 x 10^-19 C
c_m = 2 x 108 m/s
P_in = 100 μW
Concept:
The ratio of output current to input power is given by Responsivity

Calculation:

I = 75.18 μA