Electronic Devices

2.3-1 MSM Photodetector

Photodetectors are devices used for detection of light and converted electric signal from optical radiation with the source of visible, infrared, or ultraviolet wavelength. The important issues of these detectors are its signal to noise ratio, spatial resolution, ability to operate through a range of high to low input light levels, and spectral response. They are often used in sensing objects or encoding data by a change in transmitted or reflected light. There are many types of photodetectors which may

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be appropriate in a particular case ^[32]:

A. Photodiode is a semiconductor device with p-n or p-i-n junction, where detects of light and generates a photocurrent. A particularly sensitive device is avalanche

photodiodes, which can potentially provide higher gain bandwidth performance.

B. The metal-semiconductor-metal (MSM) photodetector is containing two Schottky barrier contacts of doped semiconductor material. The MSM device can be used as a photodetector by shining light on the top surface of the structure.

When light impinges the electrodes of the semiconductor, it generates electric carriers, and is collected by the electric field and thus can form a photocurrent.

C. Phototransistor is similar to photodiode but relatively more complicated to fabricate and generally require sizeable chip area. However, it is attractive for detection applications since it can achieve high gain through transistor action.

2.3-2 PN Diode

In electronics, a diode is a two-terminal device (thermionic diodes may also have one or two ancillary terminals for a heater). Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property. The varicap diode is used as an electrically adjustable capacitor.

The unidirectionality most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve.

Real diodes do not display such a perfect on-off directionality but have a more

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complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.

Early diodes included “cat’s whisker” crystals and vacuum tube devices (also called thermionic valves). Today the most common diodes are made from semiconductor materials such as silicon or germanium.

Most modern diodes are based on semiconductor p-n junctions. In a p-n diode, conventional current is from the p-type side (the anode) to the n-type side (the cathode), but not in the opposite direction. Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction.

2.3-3 Transistor

In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much larger than the controlling (input) power, the transistor provides amplification of a signal. The transistor is the fundamental building block of modern electronic devices, and is used in radio, telephone, computer and other electronic systems. Some transistors are packaged individually but most are found in integrated circuits.

The bipolar junction transistor, or BJT, was the first transistor invented, and through the 1970s, was the most commonly used transistor. Even after MOSFETs became available, the BJT remained the transistor of choice for many analog circuits

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such as simple amplifiers because of their greater linearity and ease of manufacture.

Desirable properties of MOSFETs, such as their utility in low-power devices, usually in the CMOS configuration, allowed them to capture nearly all market share for digital circuits; more recently MOSFETs have captured most analog and power applications as well, including modern clocked analog circuits, voltage regulators, amplifiers, power transmitters, motor drivers, etc.

2.3-4 solar cell

A solar cell or photovoltaic cell is a device that converts sunlight directly into electricity by the photovoltaic effect ^[33]. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified. Assemblies of cells are used to make solar panels, solar modules, or photovoltaic arrays. Photovoltaics is the field of technology and research related to the application of solar cells in producing electricity for practical use. The energy generated this way is an example of solar energy (also called solar power).

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather, by diffusing an n-type dopant into one side of a p-type wafer.

If a piece of p-type silicon is placed in intimate contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). When the electrons diffuse across the p-n

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junction, they recombine with holes on the p-type side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately on either side of the junction which this diffusion creates. The electric field established across the p-n junction creates a diode that promotes charge flow, known as drift current, that opposes and eventually balances out the diffusion of electron and holes. This region where electrons and holes have diffused across the junction is called the depletion region because it no longer contains any mobile charge carriers. It is also known as the "space charge region".