what is Diode? and what is its 功用??

Diode = 二極體，有好多種類既，佢既功用就係整流，檢波，降壓等，由於佢有單一方向通電功能，又可以作爲電路保護作用。（參考以下説明）


圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Dioden2.jpg/150px-Dioden2.jpg


圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/d/d5/Diode-closeup.jpg/180px-Diode-closeup.jpg





電子元件當中，二極體是一種具有兩個(也只有兩個)電極的裝置(除了熱游離二極體thermionic diodes可能還會多一到兩條加熱用的輔助端)。二極體有兩個主要的電極可以使想要的訊號通過，大部分的使用上是應用其整流的功能。而變容二極體varicap diode則用來當作電子式的可調電容器。 大部分二極體所具備的電流方向性我們通常稱之為「整流rectifying」功能。二極體最普遍的功能就是只允許電流由單一方向通過(稱為順向偏壓)，反向時阻斷 (稱為逆向偏壓)。因此，二極體可以想成電子版的逆止閥。然而實際上二極體並不會表現出如此完美的開與關的方向性，而是較為複雜的非線性電子特徵－這是由特定類型的二極體技術決定的。二極體使用上除了用做開關的方式之外還有很多其他的功能。

圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/4/44/PnJunction-J.PNG/200px-PnJunction-J.PNG


半導體的pn接合面和能階結構的示意圖




圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/PnJunction-Diode-ForwardBias.PNG/200px-PnJunction-Diode-ForwardBias.PNG

正向偏壓時的PN結二極體



圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/c/cd/PnJunction-Diode-ReverseBias.PNG/200px-PnJunction-Diode-ReverseBias.PNG

反向偏壓時的PN結二極體

In electronics, a diode is a two-terminal device (except that 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 current property. The varicap diode is used as an electrically adjustable capacitor.
The directionality of current flow most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current to pass in one direction (called the forward biased condition) and to block it 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 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.

Diode is an electronic device which allows electric current to flow in one direction only. It can be used in rectifier to convert AC to DC.