Chemistry Reference
In-Depth Information
this case the width of the space charge layer at a strong reverse bias becomes very thin
so that the bound electrons in the valence band can tunnel through the band gap into
the conduction band. Since the probability of the process is a strong function of the
thickness of the barrier, tunneling is only significant in highly doped material in which
the depletion layer is narrow.
In the case of avalanche breakdown as shown in Fig. 1.17b, free carriers are able
to gain enough kinetic energy from the field in the space charge layer and break the
covalent bonds in the lattice at collision. In this process, each carrier colliding with
the lattice atoms creates two carriers, which can participate in further avalanching
collisions, leading to a sudden multiplication of carriers in the space charge region. In
the energy-band diagram of Fig. 1.17b, this process can be represented as the excita-
tion of carriers from the valence band to the conduction band by absorption of the
kinetic energy of the free carriers moving in the lattice under a high field.
The field required for breakdown to occur and the mode of breakdown depend
on doping level. As the dopant concentration increases, the width of the space charge
layer decreases and the probability of tunneling increases rapidly so that Zener break-
down becomes more likely than avalanche breakdown. Zener breakdown is, in general,
involved in the electrode processes on and materials under a reverse bias.
Interface tunneling (shown in Fig. 1.17c), can also generate a large current at a
reverse bias for a semiconductor electrode/electrolyte interface when the energetic posi-
tion of a redox couple is favorable relative to the bands. Interface tunneling has been
