How does a photodiode detector work

The entire current through the diode is the sum of the absence of light and the photocurrent. So the absent current must be reduced to maximize the sensitivity of the device. The operating modes of the photodiode include three modes, namely Photovoltaic mode, Photoconductive mode, an avalanche diode mode. Photovoltaic Mode: This mode is also known as zero-bias mode, in which a voltage is produced by the lightened photodiode.

Photoconductive Mode: The photodiode used in this photoconductive mode is more usually reverse biased. This mode is too fast and displays electronic noise. Avalanche Diode Mode: Avalanche diodes operate in a high reverse bias condition, which permits the multiplication of an avalanche breakdown to each photo-produced electron-hole pair. This outcome is an internal gain in the photodiode, which slowly increases the device response.

The photodiode operates in the mode of photoconductive. When the diode is connected in reverse bias, then the depletion layer width can be increased.

In fact, this biasing will cause quicker response times for the diode. Both the photodiode and phototransistor are used for converting the energy of light to electrical. However, the phototransistor is more responsive as contrasted to the photodiode due to the utilization of the transistor.

The photodiodes time response is very fast as compared with the phototransistor. So it is applicable where fluctuation in the circuit occurs. For better understating, here we have listed out some points of photodiode vs photoresistor. The circuit diagram of the photodiode is shown below. This circuit can be built with a 10k resistor and photodiode. Once the photodiode notices the light, then it allows some flow of current throughout it.

The sum of current that supplies through this diode can be directly proportional to the sum of light noticed through the diode. In any application, the photodiode works in reverse bias mode.

The anode terminal of the circuit can be connected to the ground whereas the cathode terminal is connected to the power source. Once illuminated through light, then current flows from the cathode terminal to the anode terminal.

Once photodiodes are utilized with exterior circuits, then they are allied to a power source within the circuit. So, the amount of current generated through a photodiode will be extremely small, so this value is not sufficient to make an electronic device. Once they are connected to an exterior power source, then it delivers more current toward the circuit. When photodiodes are used with external circuits, they are connected to a power source in the circuit.

The amount of current produced by a photodiode will be very small. This value of current will not be enough to drive an electronic device.

So when they are connected to an external power source, it delivers more current to the circuit. So, battery is used as a power source. The battery source helps to increase the current value, which helps the external devices to have a better performance.

Photodiode operates in reverse bias condition. Reverse voltages are plotted along X axis in volts and reverse current are plotted along Y-axis in microampere. Reverse current does not depend on reverse voltage. When there is no light illumination, reverse current will be almost zero. The minimum amount of current present is called as Dark Current. Once when the light illumination increases, reverse current also increases linearly.

Your email address will not be published. Electronics Tutorials , General. What is a Photodiode? Working, V-I Characteristics, Applications. Avalanche photodiodes APD use impact ionization avalanche effect to create an internal gain in the material. APDs require high reverse bias operation near reverse breakdown voltage. Each photo-generated carrier creates more pairs and so is multiplied by avalanche breakdown.

This creates internal gain within the photodiode, which in turn increases the effective responsivity larger current generated per photon. Figure 3 shows the cross section of the APD.

The typical spectral response range is around — nm. APDs generally have a higher response speed and the ability to detect or measure light in lower levels.

Figure 3. APD Cross-section. Photodiodes can be operated without any voltage bias. Without added voltage across the junction, dark current can be extremely low near zero. This reduces the overall noise current of the system. Thus unbiased P-N or PIN photodiodes are better suited for low light level applications compared to operation with reverse voltage bias. Unbiased photodiodes can also work well for low frequency applications up to kHz. When the photodiode is illuminated, the electric field in the depletion region increases.

This produces the photocurrent which increases with increasing photon flux. This is most commonly seen in solar cells where the generated voltage is measured between the two terminals.

Compared to biased mode, photovoltaic mode has less variation of photocurrent responsivity with temperature. The major downfall with unbiased photodiodes is the slow response speed. Without bias to the system, the capacitance of the photodiode is at a maximum, leading to a slower speed.

When the photodiode is reverse biased, an external voltage is applied to the P-N junction. The negative terminal is connected to the positive P layer, and the positive terminal is connected to the negative N layer. This causes the free electrons in the N layer to pull toward the positive terminal, and the holes in the P layer to pull toward the negative terminal. When the external voltage is applied to the photodiode, the free electrons start at the negative terminal and immediately fill the holes in the P layer with electrons.

This creates negative ions in the atoms with extra electrons. The charged atoms then oppose the flow of free electrons to the P layer. Similarly, holes go about the same process to create positive ions but in the opposite direction. When reverse biased, current will only flow through the photodiode with incident light creating photocurrent. The reverse bias causes the potential across the depletion region to increase and the width of the depletion region to increase.

This is ideal for creating a large area to absorb the maximum amount of photons. The response time is reduced by the reverse bias by increasing the size of the depletion layer. This increased width reduces the junction capacity and increases the drift velocity of the carriers in the photodiode. The transit time of the carriers is reduced, improving the response time.

Unfortunately, increasing the bias current increases the dark current as well. This hinders the performance in low light situations.

If using APDs, the signal to noise ratio will be large regardless because of the gain of the photodiode. The silicon photodiode response is usually linear within a few tenths of a percent from the minimum detectable incident light power up to several milliWatts. Response linearity improves with increasing applied reverse bias and decreasing effective load resistance. Heating the silicon photodiode shifts its spectral response curve including the peak toward longer wavelengths.

Conversely, cooling shifts the response toward shorter wavelengths. The following values are typical for the temperature dependence of responsivity for different wavelength regions A silicon photodiode can be operated in either the photovoltaic or photoconductive mode. In the photovoltaic mode, the photodiode is unbiased; while for the photoconductive mode, an external reverse bias is applied.

Mode selection depends upon the speed requirements of the application, and the amount of dark current that is tolerable. In the photovoltaic mode, dark current is at a minimum.