6.5
6.5.1
is the bottom of the conduction band, E C = 0 eV. Using Eq. (6.11) we calculate the Fermi energy in the intrinsic
c-Si:
The Fermi energy in the n-type doped c-Si wafer is calculated from Eq. (6.6a):
We notice that the doping with P atoms has resulted in the shift of the Fermi energy towards the CB. Note that
when n > N C , E F > E C and the Fermi energy lies in the CB.
Transport properties
In contrast to the equilibrium conditions, under operational conditions a net electrical
current flows through a semiconductor device. The electrical currents are generated in a
semiconductor due to the transport of charge by electrons and holes. The two basic
transport mechanisms in a semiconductor are drift and diffusion.
Drift
Drift is charged particle motion in response to an electric field. In an electric field the
force acts on the charged particles in a semiconductor, which accelerates the positively
charged holes in the direction of the electric field and the negatively charged electrons in
the opposite direction. Because of collisions with the thermally vibrating lattice atoms and
ionized impurity atoms, the carrier acceleration is frequently disturbed. The resulting
motion of electrons and holes can be described by average drift velocities v dn and v dp for
electrons and holes, respectively. In the case of low electric fields, the average drift
velocities are directly proportional to the electric field ξ as expressed by
The proportionality factor is called mobility µ. It is a central parameter that characterizes
electron and hole transport due to drift. Although the electrons move in the opposite
direction to the electric field, because the charge of an electron is negative the resulting
electron drift current is in the same direction as the electric field. This is illustrated in
Figure 6.8.
6.5.1
is the bottom of the conduction band, E C = 0 eV. Using Eq. (6.11) we calculate the Fermi energy in the intrinsic
c-Si:
The Fermi energy in the n-type doped c-Si wafer is calculated from Eq. (6.6a):
We notice that the doping with P atoms has resulted in the shift of the Fermi energy towards the CB. Note that
when n > N C , E F > E C and the Fermi energy lies in the CB.
Transport properties
In contrast to the equilibrium conditions, under operational conditions a net electrical
current flows through a semiconductor device. The electrical currents are generated in a
semiconductor due to the transport of charge by electrons and holes. The two basic
transport mechanisms in a semiconductor are drift and diffusion.
Drift
Drift is charged particle motion in response to an electric field. In an electric field the
force acts on the charged particles in a semiconductor, which accelerates the positively
charged holes in the direction of the electric field and the negatively charged electrons in
the opposite direction. Because of collisions with the thermally vibrating lattice atoms and
ionized impurity atoms, the carrier acceleration is frequently disturbed. The resulting
motion of electrons and holes can be described by average drift velocities v dn and v dp for
electrons and holes, respectively. In the case of low electric fields, the average drift
velocities are directly proportional to the electric field ξ as expressed by
The proportionality factor is called mobility µ. It is a central parameter that characterizes
electron and hole transport due to drift. Although the electrons move in the opposite
direction to the electric field, because the charge of an electron is negative the resulting
electron drift current is in the same direction as the electric field. This is illustrated in
Figure 6.8.