Is Ldr A Photodiode

[Bessie Murrillo]

A photodiode is a semiconductor device with a P-N junction that converts photons (or light) into electrical current. The P layer has an abundance of holes (positive), and the N layer has an abundance of electrons (negative). Photodiodes can be manufactured from a variety of materials including, but not limited to, Silicon, Germanium, and Indium Gallium Arsenide. Each material uses different properties for cost benefits, increased sensitivity, wavelength range, low noise levels, or even response speed.

Figure 1 shows a cross section of a typical photodiode. A Depletion Region is formed from diffusion of electrons from the N layer to the P layer and the diffusion of holes from the P layer to the N layer. This creates a region between the two layers where no free carriers exist. This develops a built-in voltage to create an electric field across the depletion region. This allows for current to flow only in one direction (Anode to Cathode). The photodiode can be forward biased, but current generated will flow in the opposite direction. This is why most photodiodes are reversed biased or not biased at all. Some photodiodes cannot be forward biased without
damage.

The depletion region creates a capacitance in the photodiode where the boundaries of the region act as the plates of a parallel plate capacitor. Capacitance is inversely proportional to the width of the depletion region. Reverse bias voltage also influences the capacitance of the region.

Current passing through the photodiode can only flow in one direction based on the P and N doped materials. If reverse biased, current will not flow through a photodiode without incident light creating photocurrent.

The PIN photodiode is similar to the P-N Junction with one major difference. Instead of placing the P and N layers together to create a depletion region, an intrinsic layer is placed between the two doped layers. This layer is shown in Figure 2. This intrinsic layer is highly resistive and increases the electric field strength in the photodiode. There are many benefits to the added intrinsic layer because the depletion region is greatly increased.

The capacitance of the junction is decreased, and so the speed of the photodiode increased. The increased layer also allows for a larger volume of photon to electron-hole conversion and higher Quantum Efficiency.

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.

Photodiodes can be operated without any voltage bias. APDs are designed to be reversed biased, so this section will be relevant to the P-N and PIN photodiodes. 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. (The reverse biased APD will still provide a higher sensitivity than P-N or PIN photodiodes for low light applications.) Unbiased photodiodes can also work well for low frequency applications (up to 350 kHz). Unbiased mode (where V = 0) can be seen in Figure 4 in between the forward bias mode (in green) and the reverse bias mode (in blue). The plot shows very little, if any, dark current when unbiased, which can be seen by the lack of current at the intersection of the I-V curve at V=0.

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.

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 noise can be a problem for very sensitive systems using P-N or PIN photodiodes. 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. Because a photon is ideally absorbed in the depletion region, the P layer can be constructed to be extremely thin. This can be balanced with the reverse bias to create an optimal photodiode with a faster response time while maintaining as low as noise as possible.

Another benefit to reverse biased operation is the linear output (straight line in blue section of Figure 4) of the photodiode with respect to the illumination. This simply means that the voltage and current change linearly (directly proportional) with increasing optical power. The non-linearity of the forward bias section (in green) can also be seen.

Figure 4 shows the reverse bias section (in blue) with the breakdown voltage next to it (in red). Photodiodes should not be operated beyond the breakdown voltage. This will damage the photodiode.

A monitor photodiode is often integrated into a laser diode package by the laser diode manufacturer. It produces a current partially proportional to the output laser diode optical power. If photodiode current is used as feedback, a control system will try to keep the photodiode current (and therefore the laser diode optical power) constant. The output of the adjustable current source will vary to keep the optical power level the same (this is called Constant Power (CP) Mode). Photodiode current and laser diode output power are related by a transfer function given in the laser diode datasheet.

Photodiodes that are already incorporated into the laser diode system can be limited in options and information. Laser datasheets usually give the maximum reverse voltage and sometimes the sensitivity of the photodiode.

When deciding to reverse bias your photodiode, or not, it all comes down to balancing speed and noise and deciding what is most important. If your application depends on extremely low noise and low dark current, you should choose to not bias your photodiode. If speed is your main concern, you should choose to reverse bias your photodiode as the response time is improved. In other words, if your
application is precision based, photovoltaic mode will better fit your needs. If your application is speed (high) based, photoconductive mode or reversed biased mode will better fit this area.

The type of photodiode may also affect your decision of bias. Certain types of photodiodes can only be reversed biased, and others may have amplification of the response internal to the system. APDs will be effective in low light situations where sensitivity is critical but are expensive, P-N photodiodes are the most basic design and not widely used, and PIN photodiodes are the most common photodiode and the cheapest while having very low noise. As discussed earlier, the materials, size, and cost also affect the type of photodiode needed for the application. Table 1 shows a simplified chart comparing three different photodiodes.

A photodiode is a semiconductor diode sensitive to photon radiation, such as visible light, infrared or ultraviolet radiation, X-rays and gamma rays.[1] It produces an electrical current when it absorbs photons. This can be used for detection and measurement applications, or for the generation of electrical power in solar cells. Photodiodes are used in a wide range of applications throughout the electromagnetic spectrum from visible light photocells to gamma ray spectrometers.

In photoconductive mode the diode is reverse biased, that is, with the cathode driven positive with respect to the anode. This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance and increases the region with an electric field that will cause electrons to be quickly collected. The reverse bias also creates dark current without much change in the photocurrent.

Avalanche photodiodes are photodiodes with structure optimized for operating with high reverse bias, approaching the reverse breakdown voltage. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.[5]

A solaristor is a two-terminal gate-less phototransistor. A compact class of two-terminal phototransistors or solaristors have been demonstrated in 2018 by ICN2 researchers. The novel concept is a two-in-one power source plus transistor device that runs on solar energy by exploiting a memresistive effect in the flow of photogenerated carriers.[8]

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