Video on np junction physics

     This video looks at the nature of np junctions. It includes both the static case (no bias, no applied voltage, no steady-state current flow), and the case of a bias that can produce flow of electrons from the n-side to the p-side of the junction. (An applied voltage that is negative on the n-side will tend much electrons over toward the p-side.)
      Interestingly, the key thing aspect of the current flow is diffusion of electrons from the n-side to the p-side, and, at the same time, diffusion of hole from the p side to the n side.
      The video is long, but it can be divided into parts.
The first 12 minutes or so (00:00 to 12:00)  has a discussion of the nature of the static junction (no applied voltage, no current flowing).
The rest deals with a biased junction with electrons flowing from the n  to p side.
12:00-17:30  Big picture discussion.
17:30-24:00  The enhanced value of n(x) at x= x_d. The ideal junction approximation and its consequences.
23:40 Minority carried enhancement...
24:00 - 44:00 Calculation of enhanced n(x) for x greater than x_d. Assuming a specific enhancement at x =x_d, what does that imply about n(x) for x greater than x_d?
44:00-50:30   Discussion of diffusion region (x>x_d) length scale. How rapidly does n(x) regress to its equilibrium value?
Please post lots of comments and questions here!