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PIN & avalanche photodiode
Wednesday, 25 October 2006

Photodiodes are diodes in which charge carriers are generated responsive to light incident upon the photodiode. A photodiode is a semiconductor device which converts the photon energy of light into an electrical signal by releasing and accelerating current-conducting carriers within the semiconductor. A photodiode comprises a photoconductivity cell and a junction diode, and is commonly used in photoelectric products, such as cameras and photosensors of scanners. A light-induced current of the photodiode represents a signal, whereas a current present in an absence of light represents noise. When light shines on a photodiode, it produces current at a voltage, which may be used to supply the power to operate a device or a circuit. The photodiode processes signal data by using a magnitude of a signal-to-noise ratio. The photodiode can transform a photo signal into an electric signal with low consumption. Therefore, the photodiode is suitable for being a light-receiving element to be used in the applications of light detection and light communication. With the rapid development of lightwave communications, low-cost, high-performance optical photodiodes are required for a variety of system applications. Among the various techniques for converting an image projected on a target into electric data, targets formed of a semiconductor substrate supporting an array of photodiodes are used. Photodiodes are used in optical communication networks to convert optical signals to electrical signals for processing using electronic circuits. In high bandwidth applications, the photodiodes can often be packaged using flip chip interconnections. The photodiode can be used as a light-receiving element of a mouse, a photo coupler, a photo discontinuation/continuation, a remote controller, or a wireless transmission device. In general photodiode devices are used as imaging sensors in different types of equipment, for example, PC cameras and digital cameras.

A photodiode responds to incident electromagnetic radiation by converting the radiation into electrical energy, thereby enabling measurement of the intensity of the incident radiation. Photodiodes operate based on principals of photoconductivity, which is an enhancement of the conductivity of p-n semiconductor junctions due to the absorption of electromagnetic radiation. The diodes are generally reverse-biased and capacitively charged. In the absence of light, they keep their charge and, when lit, they discharge. A bright image element or pixel can thus be differentiated from a dark pixel and an image in the form of a matrix of data corresponding to the electric charges of each of the pixels can thus be restored. When the photons with energy greater than the band gap of the material are irradiated on the empty area of the diode, the originally-combined electron and electric hole will be separated because the electric forces exerted on them are opposite to each other. And the separated electrons and electric holes will separately flow into the P type and N type junction areas so as to form photo current. By being connected to an outside current amplifier, the photo current can be accurately measured wherein the magnitude of the photo current is directly proportional to the absorbed photons. Generally, in the structure of the diode, an activity layer is formed on the junction between the P type and N type layers. When the activity layer is additionally positioned on the junction between the P type and N type layers, the structure will be called as a PIN photodiode. A PIN photodiode comprises a P-type semiconductor, a N-type semiconductor, and an intrinsic semiconductor layer interposed there-between. The PIN photodiode is typically used as a light-receiving element for converting an optical signal into an electric signal as it possesses excellent characteristics in converting incident photons into electrons, mainly due to its ability to increase the width of a depletion layer of the PIN photodiode.

An avalanche photodiode (APD) is a semiconductor photosensor device capable of light detection. Avalanche diodes are based on reverse-biased p-n junction diodes operated at voltages above the breakdown voltage. An avalanche photodiode is used for converting an input optical signal to an electrical signal, while amplifying the input signal through the avalanche effect by the injection of carriers into an area applied with a high electrical field. Avalanche photodiodes are high-speed, high sensitivity photodiodes utilizing an internal gain mechanism that functions by applying a reverse voltage. Compared to PIN photodiodes, avalanche photodiodes can measure even lower level light and are used in a wide variety of applications requiring high sensitivity. The generic structure of an avalanche photodiode (APD) consists of two electrical contacts separated by a PIN diode. The two electrical contacts are separated by at least three layers of semiconductor material. The avalanche photodiodes typically include a planar-type avalanche photodiode and a mesa-type avalanche photodiode, both of which have a common structure of a multiplicity of stacked layers with an amplifying layer and an absorbing layer on a semiconductor substrate. Avalanche photodiodes achieve amplification of the limited carriers obtained by the photoelectric effect, utilizing the avalanche phenomenon. The avalanche phenomenon is induced by introducing carriers, generated by a photoelectric effect in a light-receiving area, into a high electric field area formed in a semiconductor pn junction. The carriers introduced in such high electric field area collide with neutral semiconductor atoms, thus generating other carriers. This collision process is then repeated and repeated in an avalanche fashion to effectively amplify the limited number of carriers that were initially produced by photon induced free carrier generation. The internal gain provided by avalanche photodiodes allows for the reduction or elimination of more noisy external amplifiers in optical detection systems. For this reason, avalanche photodiodes are attractive for use in detection applications that require optical gain. An avalanche photodiode is especially useful in low, weak or reduced light applications as the avalanche phenomenon utilized by the device provides for a significant degree of amplification.

Photodiodes are widely used in digital imaging devices to convert optical signals into electrical signals. Digital imaging devices are becoming increasingly popular in a variety of applications such as digital cameras, fingerprint recognition, communications systems, digital scanners and copiers and isolation applications. Photodiode arrays of the relevant type include a linear or planar arrangement of a multiplicity of photosensitive elements or sensors, generally designated as pixels, on a semiconductor chip. Each of these pixels generates and provides an analog output signal representing the quantity of light incident on the pixel. Photodiode arrays are used in a number of optical sensor applications in which brightness information is required with spatial resolution. The pinned photodiode has been widely used as an element to produce and integrate photoelectric charges generated in CCD or CMOS image sensors sensing light from an object. A typical photodiode image sensor comprises at least a reset transistor and a photodiode. A complementary metal oxide semiconductor (CMOS) image sensor employs CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits and adopts a switching mode that detects outputs sequentially. The photodiode of a complementary metal oxide semiconductor (CMOS) imaging sensing array is typically reverse-biased. The photodiode is designed to block current flow as long as the voltage stays below a specified value. In recent years, many low cost photodiode CMOS image sensor applications have the active photodiode CMOS image sensor replaced the charge coupled device (CCD). This is because the active photodiode CMOS sensor provides characteristics such as high quantum efficiency, low read noise, high dynamic range and random access.

Avalanche photodiodes (APD) behaves like standard photodiodes, as both APD's and photodiodes convert optical energy into electrical signal. However, APD's additionally incorporate a gain mechanism internal to the device itself, making it more sensitive. Avalanche photodiodes are operated with much higher reverse bias. 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. Avalanche photodiodes are incorporated in many high performance optical communications, imaging and sensing applications because they enable high signal to noise ratio and high-speed operation. An avalanche photodiode has a superior optical sensitivity and can respond to high frequencies, therefore, the APD may be used in a light-receiving module of an optical communication system. On the other hand, the avalanche photodiode shows a temperature dependence in its multiplication factor and, accordingly, affects quality of the optical and the electrical signal in the optical communication system. Avalanche photodiodes are used in optical receivers for converting an optical signal into an electrical signal. Avalanche photodiodes are preferred over many other candidate photodetectors, including PIN diodes, particularly due to their high internal gain characteristics and improved signal-to-noise ratio. The electrical signal output from the APD is coupled to an amplifier for amplification. An optical receiver including an APD has a signal to noise ratio that is a function of several parameters including the APD's excess noise factor.