114 CHAPTER 3. CHARGE TRANSPORT IN MATERIALS
Initial state has
one electronFinal state has
two electrons +
one holeConduction bandValence band+
–
Figure 3.11: How carriers multiply. The impact ionization process where a high energy
conduction-band electron scatters from a valence-band electron, producing two conduction-band
electrons and a hole.
a distance of one micron. Values of the critical field are given for several semiconductors in
table 3.2. The avalanche process places an important limitation on the power output of devices.
Once the process starts, the current rapidly increases due to carrier multiplication and the control
over the device is lost.^1 The push for high-power devices is one of the reasons for research in
large gap semiconductor devices. It must be noted that in certain devices, such as avalanche
photodetectors, the process is exploited for high gain detection. The process is also exploited in
special microwave devices.
Band-to-band tunneling breakdown
In quantum mechanics electrons behave as waves and one of the outcomes of this is that elec-
trons can tunnel through regions where classically they are forbidden. Thus they can penetrate
regions where the potential energy is larger than their total energy. This process is described by
the tunneling theory. This theory is invoked to understand another phenomenon responsible for
high field breakdown. Consider a semiconductor under a strong field, as shown in figure 3.12a.
At strong electric fields, the electrons in the valence band can tunnel into an unoccupied state in
the conduction band. As the electron tunnels, it sees the potential profile shown in figure 3.12b.
(^1) An analytical treatment of the avalanche breakdown process of ap−njunction is presented in section 4.7