半导体（Semiconductor）是指一种导电性可受控制，范围可从绝缘体至导体之间的材料。无论从科技或是经济发展的角度来看，半导体的重要性都是非常巨大的。今日大部分的电子产品，如计算机、移动电话或是数字录音机当中的核心单元都和半导体有着极为密切的关连。常见的半导体材料有硅、锗、砷化镓等，而硅更是各种半导体材料中，在商业应用上最具有影响力的一种。

材料的导电性是由导带中含有的电子数量决定。当电子从价带获得能量而跳跃至导电带时，电子就可以在带间任意移动而导电。一般常见的金属材料其导电带与价电带之间的能隙非常小，在室温下电子很容易获得能量而跳跃至导电带而导电，而绝缘材料则因为能隙很大（通常大于9电子伏特），电子很难跳跃至导电带，所以无法导电。

Semiconductors are crystalline or amorphous solids with distinct electrical characteristics. They are of high resistance - higher than typical resistance materials, but still of much lower resistance than insulators. Their resistance decreases as their temperature increases, which is behavior opposite to that of a metal. Finally, their conducting properties may be altered in useful ways by the deliberate introduction of impurities ("doping") into the crystal structure, which lowers its resistance but also permits the creation of semiconductor junctions between differently-doped regions of the extrinsic semiconductor crystal. The behavior of charge carriers at these junctions is the basis of diodes, transistors and all modern electronics.