Photodiode

A photodiode is a semiconductor component that detects radiation from the optical domain and transforms it into an electrical signal.

Working principle

When a semiconductor is exposed to a luminous flux, the photons are absorbed provided that the energy of the photon E (ph) is greater than the width of the forbidden band Eg.

 This corresponds to the necessary energy that the electron must absorb so that it can leave the valence band (where it serves to ensure the cohesion of the structure) towards the conduction band, thus making it mobile and able to generate a Electric power.

The existence of the forbidden band implies the existence of an absorption threshold such that E(ph)=Eg. During the absorption of a photon, two phenomena can occur:

Photoemission: this is the exit of the electron from the photosensitive material. The electron can only exit if it is excited near the surface.

Photoconductivity: the electron is released inside the material. The electrons thus released contribute to the electrical conductivity of the material.

When photons enter the semiconductor with sufficient energy, they can create excess electrons and holes in the material. An increase in current is then observed.

 

There is a generation of electron-hole pairs in the depletion region, which dissociate under the action of the electric field; with the electron joining the N region, the hole in the P region. This current is called the transit current or photocurrent.

These two contributions add up to create the photocurrent Iph which is added to the reverse current of the junction.

photodiode Symbol

Characteristics of a PIN photodiode

Equivalent diagram

A photodiode can be represented by a current source in parallel with an ideal diode, a capacitor, a resistor Rsh in series with a resistor Rs, as shown in the following figure:

The current source represents the photocurrent Iph generated by the received light. The diode, through which the dark current Id carries, represents the reverse-biased component in the absence of illumination.

The capacitor of capacitance Cj is the transition capacitance, the series resistance RS comes from the resistance of the contacts and that of the non-depleted region.

The shunt resistance Rsh corresponds to the slope of the characteristic I-V at the origin, i.e. in V = 0. An ideal photodiode should have infinite Rsh. In the practice, Rsh can vary from ten megohms to a few gigohms. Resistor RL is the load resistance at the edges of the photodiode.

 The shunt resistance is considered when we want to evaluate the noise current in the absence of polarization (photovoltaic mode)

Dark current

Dark current is the current flowing in the absence of all light sources. Its origin is thermal and generates thermal noise. Its expression at a given temperature T, is:  

Id = Is = cte. exp (-Eg / 2kBT)

Eg being the band gap of the semiconductor and kB the Boltzmann constant.

fig 2 :Current-voltage characteristics of photodiode

Sensitivity (S):

defines the ratio of the photocurrent Iph to the energy flow (or received optical power) and is expressed in (A / W).

For a sensitive area of 0.0675cm² the sensitivity S in A/W:

S = 1.037 A/W

Passband:

allows you to know if the current can follow a sudden variation in the photon flux, and what is the proportionality factor between the rise or fall time of the photocurrent and those of the luminous flux.

Photodiode operating modes

Ther are two modes:

photovoltaic or photoconductive.

- The selection of the photodiode operating mode depends on the requirements of

speed of application and amount of dark current that is tolerable