Unit 6
Semiconductor and Semiconductor
Devices
Semiconductor Devices
, Semiconductor Devices
We focus, in this chapter, how two types (n-type & p-
type) of semiconductors are used to create the
semiconductor devices such as diodes and
transistors-that are fundamental to the performance
of such functions as rectification, amplification, and
switching in electronic circuits.
These devices are the basic building blocks of
computers and other digital instruments.
,METAL-METAL JUNCTION: THE CONTACT POTENTIAL
let us consider what happens when two dissimilar metals are joined
together.
It is now clear that the meaning of the work
function is simply the energy difference between
the vacuum level and the Fermi level. The
electrons at 𝑬𝑭 require the least photon energy
to be ejected to the outside. An electron below
the Fermi level requires a photon of
correspondingly higher energy to be ejected. If
this higher energy photon is absorbed by an
FIGURE 1
electron at the Fermi level, however, it will have
Energy diagram for the electrons in
more energy than ∅, the required energy to
the conduction band of a metal. The
leave the metal. This extra energy is the kinetic
zero energy level is chosen to be the
energy of the ejected electron. Because kinetic
energy outside the metal (vacuum
energy is a function of the velocity, when
level). The electrons at 𝑬𝑭 need the
photons with energy 𝒉𝝊 > ∅ are used, the
least amount of energy to come out
emitted electrons will have a distribution of
of the metal. This minimum energy
velocities. The fact that the work function is not
was first introduced as the work
the same for all metals implies that the position
function ∅ of the metal.
of the Fermi level relative to the vacuum level is
different for different metals.
, METAL-METAL JUNCTION: THE CONTACT POTENTIAL
Let us now consider two metals,𝟏 and 𝟐, with work functions ∅𝟏 and ∅𝟐 , respectively,
and let ∅𝟏 < ∅𝟐 . When the two metals are far apart, their conduction bands will look
as in Fig. 2.
FIGURE 2
Energy diagram for the electrons
in the conduction band of two
dissimilar isolated metals with
different work functions. Note
that 𝑬𝑭𝟏 is closer to the vacuum
level than is 𝑬𝑭𝟐 , and therefore
∅𝟏 < ∅𝟐 .
If these metals are placed in contact, electrons are free to flow from one to the other.
Because the electrons near the Fermi level of 𝒎𝒆𝒕𝒂𝒍 𝟏 have higher energy than
those in 𝒎𝒆𝒕𝒂𝒍 𝟐, there will be a net flow of electrons from 𝒎𝒆𝒕𝒂𝒍 𝟏 to 𝒎𝒆𝒕𝒂𝒍 𝟐. This
flow will continue until the highest occupied energy level is the same in both metals,
that is, until both metals have a common Fermi level.
Figure 3a illustrates that the Fermi levels of the two metals in contact are both
changed from their values in isolation to a common level.
Semiconductor and Semiconductor
Devices
Semiconductor Devices
, Semiconductor Devices
We focus, in this chapter, how two types (n-type & p-
type) of semiconductors are used to create the
semiconductor devices such as diodes and
transistors-that are fundamental to the performance
of such functions as rectification, amplification, and
switching in electronic circuits.
These devices are the basic building blocks of
computers and other digital instruments.
,METAL-METAL JUNCTION: THE CONTACT POTENTIAL
let us consider what happens when two dissimilar metals are joined
together.
It is now clear that the meaning of the work
function is simply the energy difference between
the vacuum level and the Fermi level. The
electrons at 𝑬𝑭 require the least photon energy
to be ejected to the outside. An electron below
the Fermi level requires a photon of
correspondingly higher energy to be ejected. If
this higher energy photon is absorbed by an
FIGURE 1
electron at the Fermi level, however, it will have
Energy diagram for the electrons in
more energy than ∅, the required energy to
the conduction band of a metal. The
leave the metal. This extra energy is the kinetic
zero energy level is chosen to be the
energy of the ejected electron. Because kinetic
energy outside the metal (vacuum
energy is a function of the velocity, when
level). The electrons at 𝑬𝑭 need the
photons with energy 𝒉𝝊 > ∅ are used, the
least amount of energy to come out
emitted electrons will have a distribution of
of the metal. This minimum energy
velocities. The fact that the work function is not
was first introduced as the work
the same for all metals implies that the position
function ∅ of the metal.
of the Fermi level relative to the vacuum level is
different for different metals.
, METAL-METAL JUNCTION: THE CONTACT POTENTIAL
Let us now consider two metals,𝟏 and 𝟐, with work functions ∅𝟏 and ∅𝟐 , respectively,
and let ∅𝟏 < ∅𝟐 . When the two metals are far apart, their conduction bands will look
as in Fig. 2.
FIGURE 2
Energy diagram for the electrons
in the conduction band of two
dissimilar isolated metals with
different work functions. Note
that 𝑬𝑭𝟏 is closer to the vacuum
level than is 𝑬𝑭𝟐 , and therefore
∅𝟏 < ∅𝟐 .
If these metals are placed in contact, electrons are free to flow from one to the other.
Because the electrons near the Fermi level of 𝒎𝒆𝒕𝒂𝒍 𝟏 have higher energy than
those in 𝒎𝒆𝒕𝒂𝒍 𝟐, there will be a net flow of electrons from 𝒎𝒆𝒕𝒂𝒍 𝟏 to 𝒎𝒆𝒕𝒂𝒍 𝟐. This
flow will continue until the highest occupied energy level is the same in both metals,
that is, until both metals have a common Fermi level.
Figure 3a illustrates that the Fermi levels of the two metals in contact are both
changed from their values in isolation to a common level.
