imported>Chunbum Park |
imported>John Stephenson |
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| {{Image|Two diode structures.PNG|right|200px|Mesa diode structure (top) and planar diode structure with guard-ring (bottom).}} | | {{:{{FeaturedArticleTitle}}}} |
| | | <small> |
| 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]].
| | ==Footnotes== |
| | | {{reflist|2}} |
| 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.)
| | </small> |
| ==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|...]]
| |
Latest revision as of 09:19, 11 September 2020
The Mathare Valley slum near Nairobi, Kenya, in 2009.
Poverty is deprivation based on lack of material resources. The concept is value-based and political. Hence its definition, causes and remedies (and the possibility of remedies) are highly contentious.[1] The word poverty may also be used figuratively to indicate a lack, instead of material goods or money, of any kind of quality, as in a poverty of imagination.
Definitions
Primary and secondary poverty
The use of the terms primary and secondary poverty dates back to Seebohm Rowntree, who conducted the second British survey to calculate the extent of poverty. This was carried out in York and was published in 1899. He defined primary poverty as having insufficient income to “obtain the minimum necessaries for the maintenance of merely physical efficiency”. In secondary poverty, the income “would be sufficient for the maintenance of merely physical efficiency were it not that some portion of it is absorbed by some other expenditure.” Even with these rigorous criteria he found that 9.9% of the population was in primary poverty and a further 17.9% in secondary.[2]
Absolute and comparative poverty
More recent definitions tend to use the terms absolute and comparative poverty. Absolute is in line with Rowntree's primary poverty, but comparative poverty is usually expressed in terms of ability to play a part in the society in which a person lives. Comparative poverty will thus vary from one country to another.[3] The difficulty of definition is illustrated by the fact that a recession can actually reduce "poverty".
Causes of poverty
The causes of poverty most often considered are:
- Character defects
- An established “culture of poverty”, with low expectations handed down from one generation to another
- Unemployment
- Irregular employment, and/or low pay
- Position in the life cycle (see below) and household size
- Disability
- Structural inequality, both within countries and between countries. (R H Tawney: “What thoughtful rich people call the problem of poverty, thoughtful poor people call with equal justice a problem of riches”)[4]
As noted above, most of these, or the extent to which they can be, or should be changed, are matters of heated controversy.
- ↑ Alcock, P. Understanding poverty. Macmillan. 1997. ch 1.
- ↑ Harris, B. The origins of the British welfare state. Palgrave Macmillan. 2004. Also, Oxford Dictionary of National Biography.
- ↑ Alcock, Pt II
- ↑ Alcock, Preface to 1st edition and pt III.
