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在前几节中,我们讨论了pn结和金属半导体结。pn结为双极结,因为电子和空穴都充当电荷载流子,而金属半导体结为单极结(也称为单极结),因为电子或空穴充当电荷载流子。
为产生pn结,使用扩散、外延生长或离子注入工艺,用硼(B)或其它p型掺杂剂,对轻掺杂的n型(或p型)半导体衬底进行重掺杂。因此,轻掺杂的n型(或p型)衬底充当串联电阻器。请注意,pn结的电导率调制会导致其串联电阻降低。
典型的pn结二极管由轻掺杂n(n–)区域两侧的重掺杂p型(p+)区域和n型(n+)区域组成。具有极轻掺杂n–区域的二极管称为PIN二极管。典型的pn结二极管具有类似于PIN二极管的结构,尽管pn结二极管的I层掺杂更重。
来自电源的电子通过n+区域流入n–区域。为保持电中性,空穴从p+区域注入到n–区域。这些电子和空穴再结合并最终消失。载流子寿命是指这种再结合所需的平均时间。由于在此期间n–区域中同时存在电子和空穴,因此其呈现低电阻,就好像它是重掺杂区域一样。
载流子寿命越长,电导率调制效应越强,但会增加反向恢复时间(即二极管停止导电所需的时间)。东芝提供一种反向恢复时间缩短(即载流子寿命缩短)的二极管,称为快速恢复二极管(FRD)。与典型pn结二极管相比,FRD的正向电压与正向电流曲线(即,正向电阻更高)更平滑,具体如图2-10所示。