81
Review of Basic Device Physics
h. Compute and plot C j –V characteristics for –1.2  V  ≤  V d   ≤  1.2 V
using Equation 2.141 for V d   ≤  FC.f bi and V d   ≥  FC.f bi . Consider
m = 0.36. State any assumptions you make including the fitting
parameter FC. Explain your results.
2.6 In the derivation of the forward I–V characteristic of a pn-junction,
we assumed quasi-equilibrium; that is, we assumed that we could simply subtract V d as a small perturbation on the equilibrium situation.
We will examine the validity of this assumption in this problem.
Consider the diode shown in Figure E2.2 (the contacts are remote).
a. Assuming D n  = 25 cm 2 sec –1 , D p  = 10 cm 2 sec –1 , and L n  = L p  = 10 μm,
calculate the current that flows across the junction at an applied
forward bias of 0.4 V.
b. With V d  = 0, electrons and holes will flow across the junction due
to drift and diffusion, such that the currents due to drift and diffusion exactly cancel each other (I = 0). Estimate the hole diffusion
current that would flow if there were no electric field to stop it.
c. What do your answer in part (a) and (b) tell you about the validity
of our quasi-equilibrium assumption.
p-type
N a = 10
19 cm
−3
n-type
N a = 10
17 cm
−3
FIGURE E2.2
pn-junction I–V characteristics.