While silicon is the most widely used semiconductor material in electronics, a range of semiconductors is used, including germanium, gallium arsenide, silicon carbide, and organic semiconductors. Each material has advantages such as cost-to-performance ratio, high-speed operation, high-temperature tolerance, or the desired response to a signal.
Semiconductors
Semiconductors are useful because engineers control the electrical properties and behavior during the manufacturing process. Semiconductor properties are controlled by adding small amounts of impurities in the semiconductor through a process called doping. Different impurities and concentrations produce different effects. By controlling the doping, the way electrical current moves through a semiconductor can be controlled. In a typical conductor, like copper, electrons carry the current and act as the charge carrier. In semiconductors, both electrons and holes (the absence of an electron) act as charge carriers. By controlling the doping of the semiconductor, the conductivity and the charge carrier are tailored to be either electron or hole based. There are two types of doping:
N-type dopants, typically phosphorus or arsenic, have five electrons, which, when added to a semiconductor, provides an extra free electron. Since electrons have a negative charge, a material doped this way is called N-type.P-type dopants, such as boron and gallium, have three electrons, which result in the absence of an electron in the semiconductor crystal. This creates a hole or a positive charge, hence the name P-type.
Both N-type and P-type dopants, even in minute quantities, make a semiconductor a decent conductor. However, N-type and P-type semiconductors are not special and are only decent conductors. When these types are placed in contact with each other, forming a P-N junction, a semiconductor gets different and useful behaviors.
The P-N Junction Diode
A P-N junction, unlike each material separately, doesn’t act like a conductor. Rather than allowing current to flow in either direction, a P-N junction allows current to flow in only one direction, creating a basic diode. Applying a voltage across a P-N junction in the forward direction (forward bias) helps the electrons in the N-type region combine with the holes in the P-type region. Attempting to reverse the flow of current (reverse bias) through the diode forces the electrons and holes apart, which prevents current from flowing across the junction. Combining P-N junctions in other ways opens the doors to other semiconductor components, such as the transistor.
Transistors
A basic transistor is made from the combination of the junction of three N-type and P-type materials rather than the two used in a diode. Combining these materials yields the NPN and PNP transistors, which are known as bipolar junction transistors (BJT). The center, or base, region BJT allows the transistor to act as a switch or amplifier. NPN and PNP transistors look like two diodes placed back to back, which blocks all current from flowing in either direction. When the center layer is forward biased so that a small current flows through the center layer, the properties of the diode formed with the center layer change to allow a larger current to flow across the entire device. This behavior gives a transistor the capability to amplify small currents and act as a switch that turns a current source on or off. Many types of transistors and other semiconductor devices result from combining P-N junctions in several ways, from advanced, special-function transistors to controlled diodes. The following are a few of the components made from careful combinations of P-N junctions:
DIAC Laser diode Light-emitting diode (LED) Zener diode Darlington transistor Field-effect transistor (including MOSFETs) IGBT transistor Silicon controlled rectifier Integrated circuit Microprocessor Digital memory (RAM and ROM)
Sensors
In addition to the current control that semiconductors allow, semiconductors also have properties that make for effective sensors. These can be made to be sensitive to changes in temperature, pressure, and light. A change in resistance is the most common type of response for a semiconductive sensor. The types of sensors made possible by semiconductor properties include:
Hall effect sensor (magnetic field sensor)Thermistor (resistive temperature sensor)CCD/CMOS (image sensor)Photodiode (light sensor)Photoresistor (light sensor)Piezoresistive (pressure/strain sensors)
