P-N Junction

One of the crucial keys to solid state electronics is the nature of the P-N junction. When p-type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of material alone. Specifically, current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode. This non-reversing behavior arises from the nature of the charge transport process in the two types of materials.

The open circles on the left side of the junction above represent "holes" or deficiencies of electrons in the lattice which can act like positive charge carriers. The solid circles on the right of the junction represent the available electrons from the n-type dopant. Near the junction, electrons diffuse across to combine with holes, creating a "depletion region". The energy level sketch above right is a way to visualize the equilibrium condition of the P-N junction. The upward direction in the diagram represents increasing electron energy.

Depletion Region

When a p-n junction is formed, some of the free electrons in the n-region diffuse across the junction and combine with holes to form negative ions. In so doing they leave behind positive ions at the donor impurity sites.

Depletion Region Details

	In the p-type region there are holes from the acceptor impurities and in the n-type region there are extra electrons. 在p型区域存在来自受主杂质的空穴，在n型区域存在额外的电子。
	When a p-n junction is formed, some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes. 当形成p-n结时，一些从n区跃迁到导带的电子可以扩散过结并与空穴结合。
	Filling a hole makes a negative ion and leaves behind a positive ion on the n-side. A space charge builds up, creating a depletion region which inhibits any further electron transfer unless it is helped by putting a forward bias on the junction. 填充电极孔会生成负离子，并在n侧留下正离子。由此形成空间电荷区，抑制了进一步的电子转移，除非施加正向偏置电压以帮助电子转移。

Bias effect on electrons in depletion zone

	Equilibrium of junction Coulomb force from ions prevents further migration across the p-n junction. The electrons which had migrated across from the N to the P region in the forming of the depletion layer have now reached equilibrium. Other electrons from the N region cannot migrate because they are repelled by the negative ions in the P region and attracted by the positive ions in the N region. 库仑力阻止了离子进一步迁移过p-n结。在形成耗尽层时，从N区迁移至P区的电子现在已经达到平衡。其他来自N区的电子无法迁移，因为它们被P区中的负离子排斥，又被N区中的正离子吸引。 节点平衡
	Reverse bias An applied voltage with the indicated polarity further impedes the flow of electrons across the junction. For conduction in the device, electrons from the N region must move to the junction and combine with holes in the P region. A reverse voltage drives the electrons away from the junction, preventing conduction. 施加的电压 polarity 进一步阻碍了电子跨结的流动。对于器件的导电，N 区的电子必须移动到结并结合 P 区的空穴。反向电压会将电子驱离结，阻止导电。 反向偏置
	Forward bias An applied voltage in the forward direction as indicated assists electrons in overcoming the coulomb barrier of the space charge in the depletion region. Electrons will flow with very small resistance in the forward direction. 在正向方向施加的电压如图所示，有助于电子克服 depletion 区内空间电荷的库仑势垒。电子在正向方向流动时具有非常小的电阻。 正向偏置