In all cases, a thermal equilibrium state or a steady state is assumed, depending on the application of voltage.
Abrupt junction assumption, Application of Depletion Approximation

In the NPN BJT of the figure above, creation/recombination within the depletion region is ignored.
The gray area is the depletion area, and the lengths of the depletion area and neutral area are expressed by W (area name) as shown in the figure.

In addition, the length of the doping region is expressed by L (region name) as shown in the figure.
The voltage between Base and Emitter is VBE, and the voltage between Base and Collector is VBC.
Dn = Dp = 10cm^2/s, diffusion distance Lp = Ln = 10 μm (in all areas)

(1) Assuming that VBE = 0.5 V and WE is infinitely long, at the boundary between the Emitter/Base depletion region and the neutral region
What is the ratio of minority carrier concentration? (np, B/pn, E)
(2) In (1), calculate the current density by the hole passing through the B-E depletion region.
(3) Base/Collector depletion regions and neutral regions when VBC = -0.5 V is applied, assuming that WC is infinitely long
What are the concentrations of minority carriers and their proportions at the boundaries of? (assuming no carrier injection at Emitter.) (np, B/pn, E)
(4) In (3), calculate the current density by the hole passing through the BC depletion region.
(5) In the Base/Collector depletion area of (3), calculate the maximum electric field.

  

LE Emitter ND = 1020 cm-3 WE LB WEL EB Base N = 108 cm-3 WB WBC Lc Collector ND=106 cm-3 Wc