Hey, friends welcome to the Kohiki All About Electronics And Electrical. In this article, we will learn about the photodiode.
- What is Photodiode? How does Photodiode Works?
- Photodiode parameters
- Different modes of operation of Photodiode
What is Photodiode? How does Photodiode Works?
The photodiode is a semiconductor device that converts the incident light into the electrical current and these diodes are used in many applications. For example, they are used in many medical and scientific instruments as well as in the optical communication system.
And even they are used in many consumer electronic devices like television and the ACs to remotely control them. So if you see the symbol of this photodiode then it is very similar to the PN junction diode.
But here it has incoming arrows and this arrow indicates that whenever the light falls on this photodiode then it generates the photocurrent. And if you see the construction also then it is very similar to the PN Junction diode, but here to receive the light, the active area of the photodiode is kept transparent.
And unlike the PN Junction diode, these photodiodes are used in the reverse bias mode. Now we know that whenever the PN Junction is operated in the reverse bias then only a small amount of reverse saturation current flows through this diode. And this current flows due to the minority charge carriers.
Now whenever the light or the photons of sufficient energy falls on this diode then they can knock off the bound electrons of the atom in this depletion region. So actually if you seethe same PN Junction structure then it will look like this and the light or the photons enter from this side into the device. And whenever these photons are absorbed in this depletion region then they can knock off the bound electrons of the atom.
due to that, the additional electron-hole pairs are created in this depletion region. now after the total generated electron-hole pairs some of them will get recombined with each other but the remaining can contribute to the flow of current.
So now let us see how these generated electron-hole pairs are actually contributing to the flow of current. Now as we have discussed in the earlier Articles, the built-in electric field already exists in this depletion region. And due to this built-in electric field, the electrons will get attracted towards the positive terminal and the holes will get attracted towards the negative terminal and due to this electric field, they are able to cross this depletion region.
once they cross this depletion region then they will get attracted by the external force of this applied voltage. And in this way, they are contributing to the flow of current. So if we connect the ammeter in the circuit then we can actually measure this photocurrent. So in this way in the reverse bias mode whenever the light falls on this photodiode then in addition to the small reverse saturation current the photocurrent is also getting generated.
Now typically this reverse saturation current used to be in the range of microampere and as the intensity of the light increases then the photocurrent will also increase.
Here I Including Some Type’s Or parameters
So if we see the V-I characteristic of the photodiode then it will look like this. so here these P1, P2, P3, and P4 represent the intensity of the incident light and as you can see as the intensity of the incident light increases the photocurrent will also increase.
Now as I said earlier in the reverse bias condition if no light is falling on the photodiode then also we will get some current and in the photodiode terminology, this current is known as the dark current. so for any diode, this dark current should be as minimum as possible because this dark current will act as a noise for the generated photocurrent.
Now as Is aid earlier unlike the PN Junction diode, the active area of the diode is kept transparent so that these diodes can accept the incident light.
to generate the photocurrent the energy of the incident photons should be sufficient enough.
Photodiode: Energy Band Diagram
so for the silicon photodiode, the energy of the photon should be greater than 1.1 eV. That means the energy of the incident photon should be greater than the bandgap energy so that this incident light can generate the electron-hole pairs. now we know that this energy can be represented as hc divided by lambda.
That means the generated photocurrent is related to the wavelength of the incident light. So for the silicon photodiode, the wavelength of the incident photon should be between 400 nanometers to 1100 nano-meter. And then and then only it will be able to generate the photocurrent. so for any given diode, with a given intensity of the light how much photocurrent will generate is represented using the term responsivity.
so this responsivity is the ratio of generated photocurrent to the incident light power. And it is represented in the unit of an ampere per Watt.
so this responsivity is one of the main parameters which you often find in the data sheets and this responsivity is the function of wavelength as well as the quantum efficiency of the photodiode. and this responsibility can be expressed by this expression so here the eta is the quantum efficiency, and the lambda is the wavelength of the light.
the quantum efficiency can be expressed by this expression. So here the numerator is the number of electron-hole pairs that are actually contributing to the flow of current and the denominator is the number of incident photons. so in short this responsivity is the function of quantum efficiency and the wavelength.
for any given photodiode if you see the responsivity curve then it will look like this. so as you can see this responsivity is the function of wavelength. And it also depends on the diode material. so for the silicon diode, this responsivity varies from 400 nanometers to 1100 nanometers.
or in other words, we can say that the silicon photodiode works in the visible spectrum. while the indium gallium arsenide or the InGaAs diodes are used for the IR spectrum. And typically they are used in optical communication.
So depending on the operating wavelength one should decide the particular photodiode And this responsivity curve slightly enhances whenever the diode is used in the reverse bias condition because in the reverse bias condition the depletion region gets wider and due to that, the chances of diode getting absorbed in this depletion region will increase. and to further increase the quantum efficiency many times the PIN diodes are used instead of the PN photodiodes.
Pin Photo Diode
so the PIN photodiodes are similar to the PN junction photodiodes but it has a large intrinsic layer between the p and n-type.
And during the reverse bias, virtually this entire intrinsic layer will act as a depletion layer. And due to this enhanced depletion region, the chances of photons getting absorbed in this region will increase.
Then apart from the responsivity, the other parameter for the photodiode is the response time that means how fast is able to respond to the optical pulse and it is a very important parameter in the fast switching applications, particularly if I say it is very important in the optical communication.
So this response time of the diode is represented either using the rise time or the fall time. and the rise time of the diode is defined as 0.35 divided by 3 dB frequency where the 3 dB frequency is the frequency where the signal power becomes half. So usually this rise time is represented in the nanoseconds and this rise time depends on the many parameters but the main parameter is the RC time constant of the circuit.
Where R is the equivalent resistance of the photodiode and the load resistor and C is the equivalent capacitance of the diodes the diode capacitance or the junction capacitance of the diode depends on the area of the diode and the applied reverse bias voltage.
so as the area of the photodiode increases, the response time will increase because as the area increases, the capacitance of the diode will increase. And that is why the large area of diodes is slower compared to the small area of diodes. So for the fast switching applications, one should select the small area photodiodes.
And this Junction capacitance also reduces with the applied reverse bias voltage. So as the reverse bias voltage increases, the depletion region will get wider and due to that, the junction capacitance will reduce. so while selecting the photodiode for the fast switching applications, one should also look for these parameters.
Noise Equivalent Power
Then the other important parameter of the photodiode is the noise equivalent power and it defines the minimum power that the diode is able to detect. or by definition, it is the minimum power which diode is able to detect such that the generated photocurrent is equal to the noise current. or in other words whenever the signal-to-noise ratio is equal to one. And it is represented as the ratio of the noise current to the responsivity of the diode.
because we know that the responsivity is defined as photocurrent to the incident power and this noise equivalent power is defined in the unit of watt per square root of Hertz, because the noise current is the frequency-dependent parameter and the bandwidth increases then this noise current will also increase.
so in general to compare the detectability of the photodiode, the noise is normalized with one hertz and if you are aware the noise current is directly proportional to the square root of the bandwidth. And that is why the unit of this noise equivalent power is equal to what per square root hertz because the unit of responsivity is equal to A / W. so for the better detectability of the diode the value of NEP should be as minimum as possible.
Different modes of operation of Photodiode
now, this photodiode can be operated in two modes. the one is the photovoltaic mode. And the second is the photoconductive mode.
so in the photovoltaic mode, no biasing is applied to the photodiode, and in this mode, the photocurrent increases linearly with the incident light so as you can see over here the op-amp is configured in the trans-impedance amplifier and here the generated photocurrent is converted into the voltage.
now for this photovoltaic mode, the reason for using this op-amp circuit is that this virtual ground actually ensures that the diode remains at the zero biasing voltage because if you directly connect the resistor with this diode then with the photocurrent the biasing voltage across a photodiode will change.
so in this photovoltaic mode, the dark current of the diode is minimum so this mode of operation is suitable for the low noise application. but in this mode the response time of the photodiode is slow. so basically it is not suitable for fast switching applications.
On the other end in the photoconductive mode, the photodiode is reverse bias and due to this reverse bias, the depletion region width will increase. or we can say that due to that the junction capacitance will reduce.
so this photoconductive mode is suited for fast switching applications. but in the reverse bias condition, the dark current will also increase. so there is a trade-off between the two modes and according to the required one should select the particular mode of operation so this is all about the different modes of operation for the photodiode.
so I hope in this Article, you understood what is a photodiode, how a photodiode works, and what are the different modes of operation of the photodiode.
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