藍色LED：閃耀諾貝爾，照亮21世紀

On the 7th of October 2014， Isamu Akasaki， Hiroshi Amano and Shuji Nakamura were awarded the Nobel Prize in Physics for the invention of efficient blue lightemitting 1）diodes （LEDs）. Red and green LEDs were created in a number of laboratories during the 1950s and 60s but it took another three decades to finally produce efficient blue LEDs， which begs the question—why were they so hard to make？

Light-emitting diodes are electronic devices which are illuminated by the movement of 2）electrons in a 3）semiconductor material. LEDs are able to emit light with a range of wavelengths from the 4）infrared to the 5）ultraviolet.

A semiconductor is a material with an 6）electrical conductivity which is somewhere between a conductor such as copper， and an 7）insulator such as rubber. They are usually made from a poor conductor which is then “doped” by adding atoms of another material to it.

LEDs are typically made from aluminum-gallium-arsenide（AlGaAs） which in its pure form does not contain any free electrons to conduct electrical current. As a result， AlGaAs is doped with either free electrons or “8）electron holes” in order to change the material’s balance and make it more conductive.

Semiconductors can be classified into two types of material； N-type and P-type：

？ N-type semiconductors contain extra negatively charged electrons and， as a result， the free electrons flow from negatively charged areas to positively charged areas.？ P-type semiconductors have extra holes， allowing free electrons to jump between the holes and moving from negatively charged areas to positively charged areas as a result.

A diode consists of a section of an N-type semiconductor attached to a section of a P-type semiconductor （known as a p-n junction） with two 9）electrodes placed at either end of this arrangement. When current flows across a diode， the negatively charged electrons move in one direction in the material and the positively charged holes move in the opposite direction. As the holes exist in lower energy states， a free electron will lose energy when it falls to a hole and emit this energy in the form of a 10）photon of light.

The size of the fall in energy determines the energy the photon has when it is emitted， which in turns determines the colour of the light the diode emits. An emitted photon with a large amount of energy will have a shorter wavelength than light emitted with a lower amount of energy.

The first diode which was able to emit electrically produced light was created in 1907 by H.J. Round whilst he was experimenting with a 11）cat’s-whisker detector. Round applied a potential difference across a silicon carbide （SiC） crystal. He found that the colour of light emitted varied depending on the voltage which was applied across the crystal.

During the 1920s and 1930s， the phenomena of 12）electroluminescence was studied by Soviet physicist who published several journal articles on the subject.

In 1947， the electronic 13）transistor was invented at Bell Telephone Laboratories thanks in part to the advancement in understanding of semiconductors and p-n junctions.

Infrared LEDs were created in 1962 using p-n junctions made from GaAs. By the end of the 1960s， red and green LEDs were being manufactured in different countries using p-n junctions made from GaP. The development of a blue LED however proved far more difficult to scientists.

The first attempts at the emission of blue light from a diode used ZnSe and SiC， but did not produce efficient light emission. The material which enabled the development of blue LEDs was gallium nitride （GaN）.

In 1974 Isamu Akasaki began studying gallium nitride and took up a professorship at Nagoya University to continue his research alongside Hiroshi Amano. In 1986 the 14）MOVPE technique was used in order to produce GaN with high crystal quality and good 15）optical properties. Shuji Nakamura later developed a similar method in order to grow GaN at low temperatures.

A key step in the development of blue LEDs was the development of 16）heterojunctions in the early 1990s by research groups led by Akasaki and Nakamura. In 1994， Nakamura used a double heterojunction InGaN/AlGaN to produce a device with a quantum efficiency of 2.7%， which opened the door for efficient blue LEDs to be easily produced.

Illumination technology is currently undergoing a major revolution， with light bulbs and 17）fluorescent tubes being replaced by LEDs. White LEDs currently have an energy efficiency of around 50% when converting electricity into light. This is a massive improvement on the 4% energy efficiency of conventional light bulbs which were first invented in 1879 by Thomas Edison.

White LEDs have lifetimes of around 100，000 hours and are quickly becoming more affordable as market demand increases. Replacing conventional light bulbs with LEDs will drastically reduce the planet’s energy requirement for light， as between 20% and 30% of the world’s electricity consumption is as a result of lighting.

Presently， LED technology is used in the back-lit screens of many mobile phones， laptops and television screens. Blue GaN diode lasers find applications in the technology which underpins the data storage on Blu-ray Discs， which are predicted to supersede DVDs.

One day， AlGaN/GaN LEDs could find applications in water purification， as their UV light may be able to destroy viral and bacterial DNA. And for countries with poor electrical 18）infrastructure， many believe solar powered white LEDs will replace the use of kerosene lamps at night.

2014年10月7日，赤崎勇、天野浩和中村修二因發明了高效能的藍色發光二極管（LED）而被授予諾貝爾物理學獎。紅色和綠色的LED在上世紀五六十年代已經由多個實驗室創造出來，但高效能的藍色LED卻要經歷又一個三十年才最終被制造出來，這不禁讓人心生疑問—為什么藍色LED這么難造？

發光二極管是一種靠電流在半導體材料中流動而發光的電子器件。LED能夠發出波長范圍從紅外線到紫外線的光。

半導體是一種導電性介乎銅導體和橡膠絕緣體之間的一種材料。通常是通過向某種導電性較差的導體中摻雜其它材料的原子制作而成。

LED一般用鋁砷化鎵做成，這種材料在其純態時因不含任何自由電子而無法導電。于是，人們通過向鋁砷化鎵中摻入自由電子或“電子空穴”來改變材料的平衡，使之更具導電性。

半導體按材料類型可分成兩種—N型和P型：

？ N型半導體帶有額外的帶負電荷的電子，因此，自由電子流就能從帶負電荷的區域流向帶正電荷的區域。

？ P型半導體帶有額外的電子空穴，允許自由電子在電子空穴之間跳躍，從而實現從帶負電荷的區域流向帶正電荷的區域。

二極管由一塊包含了N型半導體和P型半導體的基片（被稱作PN結）以及位于該組合兩端的電極組成。當電流通過二極管時，帶負電荷的電子就會在材料中向某一方向移動，而帶正電荷的空穴則會以反方向移動。由于電子空穴處于較低能階，自由電子就會在掉入電子空穴時以光子的模式釋放出能量。……