Solid State Physics
285
If there is reverse bias |V | > ~ |V B | (see Fig. 8.10), Eq. (8.74) for the current is
no longer valid. For |V| > |V B |, a very rapid increase in the current is observed
(Fig. 8.10). There are two reasons for this increase: (i) the large field at the
junction speeds up the few electrons in the p-region near the junction to such
high velocities that they knock out some of the valence electrons into the
conduction band. This process continues repeatedly and a large current is quickly
built-up. (ii) If the potential difference across the boundary is sufficiently large,
the conduction band on the n-side will overlap the valence band on the p-side
(see Fig. 8.9). In this case, it was suggested by Zender that the electrons in the
valence band on the p-side will tunnel across the boundary into the conduction
band on the n-side. The diodes based on these two effects are known as avalanche
diodes or Zener diodes. They are very useful in voltage stabilization circuits.
If the impurity concentration is very high (of the order of one part in a
thousand), the Fermi energy may move into the valence band on the p-side and
into the conduction band on the n-side [Fig. 8.1(a)]. In this case, there will be
vacant levels above ε f in the valence band on the p-side and electrons below ε f
in the conduction band on n-side. When the reverse bias is applied, many
electrons will move from the valence band on the p-side into the conduction
band on the n-side, giving rise to a large current [Fig. 8.11(b)]. For a small
forward bias, electrons from the conduction band on the n-side can move not
p-type
n-type
e f
(a)
(b)
(c)
I
C
D
V
(d)
Fig. 8.11 The characteristics of the tunnel or Esaki diode; (a) energy bands without
bias, (b) energy bands with reverse bias, (c) energy bands with forward bias,
(d) current as a function of V showing negative resistance between C and D.
285
If there is reverse bias |V | > ~ |V B | (see Fig. 8.10), Eq. (8.74) for the current is
no longer valid. For |V| > |V B |, a very rapid increase in the current is observed
(Fig. 8.10). There are two reasons for this increase: (i) the large field at the
junction speeds up the few electrons in the p-region near the junction to such
high velocities that they knock out some of the valence electrons into the
conduction band. This process continues repeatedly and a large current is quickly
built-up. (ii) If the potential difference across the boundary is sufficiently large,
the conduction band on the n-side will overlap the valence band on the p-side
(see Fig. 8.9). In this case, it was suggested by Zender that the electrons in the
valence band on the p-side will tunnel across the boundary into the conduction
band on the n-side. The diodes based on these two effects are known as avalanche
diodes or Zener diodes. They are very useful in voltage stabilization circuits.
If the impurity concentration is very high (of the order of one part in a
thousand), the Fermi energy may move into the valence band on the p-side and
into the conduction band on the n-side [Fig. 8.1(a)]. In this case, there will be
vacant levels above ε f in the valence band on the p-side and electrons below ε f
in the conduction band on n-side. When the reverse bias is applied, many
electrons will move from the valence band on the p-side into the conduction
band on the n-side, giving rise to a large current [Fig. 8.11(b)]. For a small
forward bias, electrons from the conduction band on the n-side can move not
p-type
n-type
e f
(a)
(b)
(c)
I
C
D
V
(d)
Fig. 8.11 The characteristics of the tunnel or Esaki diode; (a) energy bands without
bias, (b) energy bands with reverse bias, (c) energy bands with forward bias,
(d) current as a function of V showing negative resistance between C and D.