Science Fair Project Encyclopedia
Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium and gallium arsenide. Semiconductor devices have replaced thermionic devices in most applications. They utilize electronic conduction in the solid state, as opposed to the vacuum state or gaseous state. Semiconductor devices are available as discrete units (such as those sold in electronics stores) or can be integrated along with a large number — often millions — of similar devices onto a single chip, called an integrated circuit (IC).
Semiconductor device fundamentals
(See also semiconductor for complementary information on semiconductor physics)
If a semiconductor is pure and if it is unexcited by an input such as an electric field, it allows only very small values of electric current to exist within itself, and it is practically an insulator. The main reason why semiconductors are so useful is that the conductivity of semiconductors can be manipulated by addition of impurities (doping), by introduction of an electric field, by exposure to light, or by other means. For example, CCDs, the primary unit of digital cameras, rely on the fact that semiconductor conductivity increases with exposure to light. Transistor operation, which will be discussed below, depends on the fact that semiconductor conductivity can be increased by the presence of an electric field.
Current conduction in a semiconductor occurs via movable or "free" electrons and holes. Holes aren't real particles; in a sense which requires some knowledge of semiconductor physics to understand: a hole is the absence of an electron. Nevertheless, this absence, or hole, can be treated as a positively-charged counterpart to the negatively-charged electron. Indeed, the precise meaning of "free electrons" also requires a background in semiconductor physics to understand. For descriptive ease, "free electrons" are often simply denoted "electrons," but it should be understood that the majority of electrons in a solid, which aren't free, do not contribute to conductivity.
If a semiconductor crystal is perfectly pure, with no impurities, and it is held at a temperature near absolute zero with no excitations (e.g. electric fields or light), it will contain no free electrons and no holes, and thus will be a perfect insulator. At room temperature, thermal excitations produce some free electrons and holes in pairs, but most semiconductors at room temperature are insulators for practical purposes.
Doping a semiconductor such as silicon with impurity atoms, such as boron and phosphorus, creates unequal numbers of free electrons and holes. Very low levels of doping cause a semiconductor to conduct, yet because the number of carriers is small, the cloud of carriers can easily be swept from the volume of the semiconductor with e-fields, producing depletion zones. When a doped semiconductor contains excess holes it is called "p-type," and when it contains excess free electrons it is known as "n-type." The semiconducting material in devices is almost always carefully doped for engineering purposes. In fact, junctions between n-type and p-type semiconductors, called p-n junctions, are the fundamental elements of many semiconductor devices, such as the p-n diode and the bipolar junction transistor.
An electric field can also create a unequal number of free electrons and holes in a semiconductor. This is the basis for "field effect transistors" like the MOSFET. Exposure to light generally creates electron/hole pairs in a semiconductor, which change its conductivity and allows the light to be sensed, as with components called photocells.
By far, silicon (Si) is the most widely used semiconductor material as of 2004. Its combination of low raw material cost, reasonable speed, relatively simple processing, and a useful temperature range make it currently the best compromise among the various competing materials. Silicon is currently fabricated into boules that are large enough to allow the production of 300mm (approximately 12 inch) wafers.
Germanium (Ge) was a widely used early semiconductor material but its lower melting point makes it less useful than silicon. Today, germanium is often alloyed with silicon for use in very-high-speed SiGe devices; IBM is a major producer of such devices.
Gallium arsenide (GaAs) is also widely used in high-speed devices but so far, it has been difficult to form large-diameter boules of this material, limiting the wafer diameter to a few hundred millimeters and making mass production of GaAs devices significantly more expensive than silicon.
Other less common materials are also in use or under investigation.
Silicon carbide (SiC) has found some application as the raw material for blue light emitting diodes (LEDs) and is being investigated for use in semiconductor devices that could withstand very high operating temperatures and environments with the presence of significant levels of ionizing radiation. IMPATT diodes have also been fabricated from SiC.
The most important semiconductor device in use today is the MOSFET, a type of transistor whose operation depends on a "gate," from which originates an electric field that controls the conductivity of a "channel." MOSFETs, as well as other transistor types, are used as the building blocks of logic gates. Their role in a microprocessor is somewhat analogous to that of neurons in the brain. In digital circuits like microprocessors, transistors act as on-off switches; in the MOSFET, for instance, the input gate voltage determines whether the switch is on or off.
Transistors also serve very crucial operations in analog circuits. Transistors used for analog purposes do not act as on-off switches; rather, they respond to a continuous range of inputs with a continuous range of outputs. For example, transistors are the workhorses of amplifier circuits , which produce an amplified, but otherwise identical (ideally), version of the input. For years the bipolar junction transistor, or BJT, was the transistor of choice for analog circuits. However, the MOSFET has much more desirable properties for digital circuits, and since it is difficult to integrate BJTs and MOSFETs onto a single chip, MOSFETs are now commonly used for both analog and digital purposes.
List of simple semiconductor devices
While the transistor is the most important semiconductor device, there are literally dozens of other families of semiconductor devices. Some of the great many semiconductor devices in current usage include the following:
- Avalanche diode (avalanche breakdown diode)
- Diode (rectifier diode)
- Gunn diode
- IMPATT diode
- Laser diode
- LED (Light Emitting Diode)
- PIN diode
- Schottky diode
- Tunnel diode
- Zener diode
- Bipolar transistor
- Darlington transistor
- Field effect transistor
- IGBT (Insulated Gate Bipolar Transistor)
- SCR (Silicon Controlled Rectifier)
- Unijunction transistor
- Hall effect sensor (magnetic field sensor)
Discrete semiconductor devices
The type designators of semiconductor devices are often manufacturer specific. Nevertheless, there have been attempts at creating standards for type codes, and a subset of devices follow those. For discrete devices, for example, there are three standards: JEDEC JESD370B in USA, Pro Electron in Europe and JIS in Japan.
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