Carrier njection Under Forward Bias在正向偏压下的载流子注入.pptVIP

Chapter 6-2. Carrier injection under forward bias Carrier injection under forward bias (continued) General current and minority carrier diffusion equations Current equations applied to a diode Current equations applied to a diode Diode current equations Forward and reverse bias characteristics Example 1 * Last class, we established the excess minority carrier concentration profile under biased conditions. The excess minority carrier concentration at the edge of the depletion layer will increase under forward biased condition. The minimum carrier concentration decreases exponentially with distance from the depletion layer edge. Carrier injection under forward bias At equilibrium, # of holes diffusing to n-side equals # of holes drifting from n-side. When we apply external forward voltage, VA, holes diffusing (injection) to n-side from p-side increases exponentially. This increases the hole concentration at the edge of the depletion layer on n-side. Similarly, Minority carrier concentration profile under bias x x pn = pn0 + ?pn(x) np= np0 + ?np(x) ?pn(0) ?np(0) np0 + VA ? pn0 Change of axes to x and x (see graph) x axis x axis Simplified equations Find Jn and Jp at the edge of the depletion layer and add them to get the total current. Assumption: No generation or recombination inside the depletion layer. x x Therefore, Similarly, And total current equals, Current due to electrons will be along positive x direction. Shockley equation J Large forward bias (VA kT/q): Large reverse bias (VA – kT/q):