P-n photodiode

Figure 3.14 also allows us to analyze the two operational regimes of a p-n photodiode. In the first regime, the applied voltage is negative, so that expression (3.5) can be written, in a first order approximation, as... 

Cross section of a p-n photodiode (a) and potential energy diagram or the junction region b). Absorption of a photon with energy in excess of the band gap produces charge separation of the resultant electron-hole pair to produce a voltage across the depletion layer. 

The simplest structure is the photodiode, shown in Figure. This structure relies on just a single p-n junction and is easily realized in commercial CMOS processes. Double junction p-n photodiodes can also be created. Figure(a) shows the schematic of the cross-section of this type of photodetector, and a physical mask-level CAD layout of this is shown in Figure 10. Again, the light sensitive areas can be made to be any size, but the depth is not under the designer s control when a commercial CMOS process is used for fabrication. 

Figure 10. Side view of a p-n photodiode type-detector, (b) Top view as designed in a CAD program...

The silicon diode (photodiode) detector consists of a strip of p-type silicon on the surface of a silicon chip (n-type silicon). By application of a biasing potential with the silicon chip connected to the positive pole of the biasing source, electrons and holes are caused to move away from the p-n junction. This creates a depletion region in the neighbourhood of the junction which in effect becomes a capacitor. When light strikes the surface of the chip, free... 

High-energy radation can be imaged with a-Si H, either directly or via a converter [3], A thick film is required for direct detection, due to the weak interaction of the radiation with the material. A converter usually is a phosphor, which emits in the visible, and thin a-Si H films are needed. X-rays with an energy up to 100 keV eject the electrons from the inner atomic core levels to high levels in the conduction band. The emitted electrons create electron-hole pairs due to ionization. These pairs can be detected in the same way as in p-i-n photodiodes. 

Special UV-enhanced Si photodiodes can be made by positioning the p-n junction close to the surface. Then, quantum efficiencies of 50% can be achieved for A... 

A phototransistor or photodiode may also be used to detect visible fight. Both devices have p-n junctions. In the photodiode the photon ejects an electron from the p semiconductor to the n semiconductor. The electron cannot cross back across the p-n junction and must travel through the circuitry, an ammeter to return to the p material. In a phototransistor, usually an npn type, the base (p-type semiconductor) is enlarged and photosensitive. Photons dislodge electrons that act as if a potential was applied to the base. This results in an amplified flow of electrons proportional to the number of photons striking the base (Fig. 5.11). 

Photodiodes make use of the unique properties of semiconductors, such as silicon. Silicon can be doped with impurities to make it either electron rich (an n-type semiconductor) or electron poor (a p-type semiconductor). When an n-type semiconductor is in contact with a p-type semiconductor, electronic changes occur at the boundary, or junction. A photodiode is a p-n junction constructed with the top p layer so thin that it is transparent to fight. Light shining through the p layer creates additional free electrons in the n layer that can diffuse to the p layer, thus creating an electrical current that depends on the intensity of the fight. This small current is easily amplified and measured. 

Typical photodiode detectors consist of a p layer which is made of an electron deficient material an n layer which is electron abundant and a depletion region, the p-n junction, located between the two layers. At equilibrium, when no light or current is applied to the system, the p-n junction is in electrostatic equilibrium and the alignment of electrons and electron holes on the two sides of thejunction region creates a contact potential voltage. As incident light strikes the surface of the diode, the... 

In one of the most common types of photodiodes used for time-resolved work, the p-i-n photodiode (see Figure 12.24), the depletion layer thickness (i for intrinsic) is fabricated to obtain this optimum performance. Manufacturers usually give full specification sheets detailing, active area, time/frequency response, responsivity amps/watt (AAV) at a given wavelength, dark current, depletion layer capacitance, and bias volts such that with minimal external electronics devices can be made operative. 

We now turn to photoconductive and photodiode detectors, both of which are semiconductor devices. The difference is that in the photoconductive detector there is simply a slab of semiconductor material, normally intrinsic to minimize the detector dark current, though impurity doped materials (such as B doped Ge) can be used for special applications, whereas by contrast, the photodiode detector uses a doped semiconductor fabricated into a p-n junction. 

When a semiconductor is bombarded with photons equal to or greater in energy than the band gap, an electron-hole pair is formed. The current that results is a direct function of the incident light intensity. Photoconductive devices consist of a p-n junction called a photodiode, or a p-i-n junction commonly used in a photodetector, an... 

As described earlier, when a p-n junction is forward biased, it can emit light. When it is reverse biased, it can serve as a photodiode to measure light intensity. It occurred to us that by carving a trench in a diode, one portion could be forward biased and the other portion reverse biased so that the light emitted from one region of the structure could be detected by the other region, as shown in the... 

Figure 7 Schematic diagrams of a trench structure carved in a p-n diode (a) side view of the trench structure that illustrates how hght is produced by tlie forward-biased diode (LED) and detected by the reverse-biased diode (photodiode) (b) top view of these LED/ photodiode pairs on a wafer. (Adapted from Ref. 14.)...

Figure 3.16 Schematic of a photodiode p-n junction. Reprinted with kind permission of Prentice-Hall International.

Fig. 17. Schematic representation for an a-Si H solid-state color image sensor. 1 and C, represent an a-Si H photodiode, Qv and Pb represent n-MOSFET swiches and Ct represents p-n junction capacitance. [From T. Tsukada et al.. Solid-state color imager using a-Si H photoconductive film. Technical Digest—International Electron Device Meeting. Copyright 1981 IEEE.]...

Photodetector — Device used to detect photons. After a long period having only thermal photodetectors, quantum photodetectors based on photocurrent were developed and are used quite widely in applications such as photographic meters, flame detectors and lighting control. In the late 1950s the p-i-n photodiode, simply referred to as photodiode, was developed and now is one of the most common photodiodes. There are several types of photodetectors, the most adequate depending on the specific application, like photoconductors, p-i-n photodiodes, Schottky-barrier photodiodes, charge-coupled... 

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