thescienceofelectronicsanalogdevices

thescienceofelectronicsanalogdevices内容摘要：

he junction the diffusion of holes and electrons results in a region with excess, immobile negative charge in the ptype material. and a region with excess. immobile positive charge in the ntype material. These regions of excess, immobile charge adjacent to the junction prise what is called a spacecharge layer (SCL) 空间电荷层 . Return Next Previous Contents End Junctions and Contacts (4) The spacecharge regions (SCR空间电荷区 ) to each side of the Junctions are charged with polarity opposite to that of the mobile carriers that have diffused out of the respective regions. As the diffusion process continues and the charged regions increase in size, they exert an increasing attractive force on the majority carriers. This force opposes the diffusive flow. When the pn junction is in equilibrium, in the neighborhood of the junction there are two types of charge transports: 1. Drift 漂移 2. Diffusion 扩散 Return Next Previous Contents End Junctions and Contacts (5)  1. Drift 漂移 The mobile holes and free electrons are forced by the buildup electric field (内建电场 ). Once the current carriers enter the SCR, the holes drift to ptype region, and the electrons drift towards to ntype region.  2. Diffusion 扩散 The mobile holes and free electrons are forced by concentration gradient. The holes diffuse from ptype region to ntype region, and the electrons diffuse from ntype region to ptype region. Notice: the drift and the diffusion are localized in the neighborhood of the junction. Far from the junction the ptype and ntype regions are neutral and homogeneous, unaffected by the presence of the junction. Return Next Previous Contents End Junctions and Contacts (6) The vi Characteristic of the pn junction 1. Forward bias The applied electric field by the voltage source is opposite to the buildup electric field of the pn junction, then the width of the spacecharge region bees narrower as the figure. A forwardbias voltage weakens the field in the spacecharge layer, allowing diffusion of majority carriers across the junction to the side where they are in the minority. Thus, under forwardbias conditions, the concentrations of minority carriers near the junction increase substantially. This increase in minority carrier concentration near a forwardbiased junction, by diffusion across the junction, is called injection of minority carriers. The current increases rapidly with applied forward bias voltage. Return Next Previous Contents End Junctions and Contacts (7) 2. Reverse bias The applied electric field by the voltage source strengthens the electric field in the SCL, then the width of the spacecharge region bees wider as the figure. As the field increases, it opposes the diffusion of majority carriers so strongly that the diffusive ponents of charge transport are virtually stopped. That is, the field is directed so as to hold majority carriers in their respective neutral regions and to prevent their diffusion across the SCL. Also, the field direction is such that it attracts minority carriers from their respective neutral regions and moves them by drift across the SCL, a much smaller negative value of i, or reverse current, flows through the pn junction. Return Next Previous Contents End Junctions and Contacts (8) Although increasing the reverse v。