CZ:Featured article/Current: Difference between revisions

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{{Image|Two diode structures.PNG|right|200px|Mesa diode structure (top) and planar diode structure with guard-ring (bottom).}}
{{Image|Apollo 11 image 2.jpg|right|300px|The first manned landing on the moon was successfully accomplished by the Apollo 11 mission on July 20, 1969. Astronaut Neil Armstrong took this photograph of fellow astronaut Edwin ("Buzz") Aldrin walking on the Moon's surface during lunar landing.}}  


A '''[[semiconductor diode]]''' is a two-terminal device that conducts current in only one direction, made of two or more layers of which at least one is a semiconductor. An example is the ''pn''-diode, made by joining a ''p''-type semiconducting layer to an ''n''-type semiconducting layer. For a discussion of dopant impurities and the terminology ''p-'' and ''n-''type. see [[Semiconductor#Dopant_impurities|dopant impurities]].
The '''[[Apollo program]]''' was a series of human spaceflight missions undertaken by the United States, during the years 1961–1974, using the Apollo spacecraft and Saturn space launch vehicle. It was conducted by the National Aeronautics and Space Administration (NASA) and was devoted to the goal, expressed in a 1961 address to the U.S. Congress by U.S. President John F. Kennedy, of "... landing a man on the Moon and returning him safely to the Earth ..." within the decade of the 1960s. That goal was successfully achieved by the Apollo 11 mission in July 1969.


The figure shows two of the many possible structures used for ''pn-''semiconductor diodes, both adapted to increase the voltage the devices can withstand in reverse bias. The top structure uses a mesa to avoid a sharp curvature of the ''p<sup>+</sup>-''region next to the adjoining ''n-''layer. The bottom structure uses a lightly doped ''p-''guard-ring at the edge of the sharp corner of the ''p<sup>+</sup>-''layer to spread the voltage out over a larger distance and reduce the electric field. (Superscripts like ''n<sup>+</sup>'' or ''n<sup>−</sup>'' refer to heavier or lighter impurity doping levels.)
The program continued until 1975 with five subsequent Apollo missions which also landed astronauts on the Moon, the last in December 1972. In the six successful Apollo spaceflights, twelve men walked on the Moon. As of 2011, these are the only times that humans have landed on another celestial body.  
==Types==
Semiconductor diodes come in a large variety of types:
*''pn''-diode: The ''pn'' junction diode consists of an ''n''-type semiconductor joined to a ''p''-type semiconductor.
*Zener diode: The Zener diode is a special type of ''pn''-diode made to operate in the reverse breakdown region, and used often as a voltage regulator. The breakdown voltage in these diodes is sometimes called the ''Zener voltage''. Depending upon the voltage range designed for, the diode may break down by either Zener breakdown, an electron tunneling behavior, or by avalanche breakdown.
*Schottky diode: The Schottky diode is made using a metal such as aluminum or platinum, on a lightly doped semiconductor substrate.
*Tunnel diode: Like the Zener diode, the tunnel diode (or Esaki diode) is made up of heavily doped ''n-'' and ''p''-type layers with a very abrupt transition between the two types. Conduction takes place by electron tunneling.
*Light-emitting diode: The light-emitting diode is designed to convert electrical current into light.
*Photodiode: The photodiode is the inverse of the light-emitting diode, acting as a photodetector, converting incident light to a detectable electric current.
*''pin''-diode: The ''pin''-diode is made of three layers: an intrinsic (undoped) layer between the ''p''- and ''n''-type layers. Because of its rapid switching characteristics it is used in microwave and radio-frequency applications.
*Gunn diode: The Gunn diode is a ''transferred electron device''  based upon the Gunn effect in III-V semiconductors, and is used to generate microwave oscillations.
*Varactor: a ''pn''-junction used in reverse bias as a voltage-variable capacitor for tuning radio receivers. The term ''varactor'' also is used for devices that behave like back-to-back Zener diodes.


[[semiconductor diode|...]]
Equipment that was originally produced for the Apollo program was used for the later Skylab program during 1973–1974 and the joint U.S.−Soviet mission (Apollo−Soyuz Test Project) in 1975. Therefore, those subsequent programs are thus often considered to be part of the Apollo program.
 
Despite the many successes, there were two major failures, the first of which resulted in the deaths of three astronauts, Virgil "Gus" Grissom, Ed White and Roger Chaffee, in the Apollo 1 launchpad fire. The second was an explosion on Apollo 13, in whose aftermath the deaths of three more astronauts were averted by the efforts of flight controllers, project engineers, and backup crew members.
 
The Apollo program was named after the Apollo (Greek mythology)|Greek god of the Sun.
 
==Background==
 
The Apollo program was originally conceived early in 1960, during the administration of U.S. President|President Dwight D. Eisenhower|Eisenhower, as a follow-up to America's Mercury program. While the Mercury capsule could only support one astronaut on a limited Earth orbital mission, the Apollo spacecraft was intended to be able to carry three astronauts on a circumlunar flight and perhaps even on a lunar landing. The program was named after the Greek god of the Sun by NASA manager Abe Silverstein, who later said that "I was naming the spacecraft like I'd name my baby." While NASA went ahead with planning for Apollo, funding for the program was far from certain, particularly given Eisenhower's equivocal attitude to manned spaceflight.
 
[[Apollo program|...]]

Revision as of 09:05, 26 June 2011

(PD) Photo: Astronaut Neil Armstrong, National Aeronautics and Space Administration
The first manned landing on the moon was successfully accomplished by the Apollo 11 mission on July 20, 1969. Astronaut Neil Armstrong took this photograph of fellow astronaut Edwin ("Buzz") Aldrin walking on the Moon's surface during lunar landing.

The Apollo program was a series of human spaceflight missions undertaken by the United States, during the years 1961–1974, using the Apollo spacecraft and Saturn space launch vehicle. It was conducted by the National Aeronautics and Space Administration (NASA) and was devoted to the goal, expressed in a 1961 address to the U.S. Congress by U.S. President John F. Kennedy, of "... landing a man on the Moon and returning him safely to the Earth ..." within the decade of the 1960s. That goal was successfully achieved by the Apollo 11 mission in July 1969.

The program continued until 1975 with five subsequent Apollo missions which also landed astronauts on the Moon, the last in December 1972. In the six successful Apollo spaceflights, twelve men walked on the Moon. As of 2011, these are the only times that humans have landed on another celestial body.

Equipment that was originally produced for the Apollo program was used for the later Skylab program during 1973–1974 and the joint U.S.−Soviet mission (Apollo−Soyuz Test Project) in 1975. Therefore, those subsequent programs are thus often considered to be part of the Apollo program.

Despite the many successes, there were two major failures, the first of which resulted in the deaths of three astronauts, Virgil "Gus" Grissom, Ed White and Roger Chaffee, in the Apollo 1 launchpad fire. The second was an explosion on Apollo 13, in whose aftermath the deaths of three more astronauts were averted by the efforts of flight controllers, project engineers, and backup crew members.

The Apollo program was named after the Apollo (Greek mythology)|Greek god of the Sun.

Background

The Apollo program was originally conceived early in 1960, during the administration of U.S. President|President Dwight D. Eisenhower|Eisenhower, as a follow-up to America's Mercury program. While the Mercury capsule could only support one astronaut on a limited Earth orbital mission, the Apollo spacecraft was intended to be able to carry three astronauts on a circumlunar flight and perhaps even on a lunar landing. The program was named after the Greek god of the Sun by NASA manager Abe Silverstein, who later said that "I was naming the spacecraft like I'd name my baby." While NASA went ahead with planning for Apollo, funding for the program was far from certain, particularly given Eisenhower's equivocal attitude to manned spaceflight.

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