Unit 6
Semiconductor and Semiconductor
Devices
Semiconductors
, Semiconductors
Resistivity in between conductor and insulators.
Elemental semiconductors: Antimony, Arsenic, Boron, Carbon, Germanium, Selenium,
sulfur and Silicon.
Si, known to be the best among these forming the basis of most IC’s.
Common semiconductor compounds: Gallium, Arsenide, Indium antimonide, oxides of
most metals.
GaAs is widely used in low noise, weak-signal amplifying devices.
At absolute zero, acts as an insulator.
When temp. increases, some of the valence electrons are able to cross the small
forbidden gap and reach to the conduction band- hence increase its conductivity as T
increases.
Forbidden gap is small in semiconductors and wide in insulators, doesnot exists in
metals.
Semiconductor devices: Junction rectifiers, photocells, solar batteries, thermistors,
transistors etc., use either pure or doped semiconductors.
, Intrinsic semiconductors
An intrinsic semiconductor is one whose impurity concentrations are so low that
its electric properties are not affected by them but are solely determined by the
band structure of the material.
An semiconductor (intrinsic) is a solid that at 𝑻 = 𝟎𝑲 has a valence band
completely filled with electrons, separated by a forbidden energy gap 𝑬𝒈 from an
empty conduction band and acts as an insulator; no current can be conducted.
As the temperature rises, the conduction band become partially filled with
electrons and the valence band becomes partly empty. This makes the
conduction possible.
At room temperature, there are enough electrons excited across the forbidden
gap to make both the electron and hole concentrations in the conduction and
valence bands. When a valence electrons crosses the band gap after gaining the
energy at least equal to band gap energy 𝑬𝒈.
The newly entered electron to the conduction band can move freely through out
the solid is termed as free electron. The space left behind in the valence band by
electron excited to the conduction band is called hole. So in intrinsic
semiconductor both the conduction of electron and hole are generated together
called electron-hole pair.
, Electron and Hole (Concentrations) Densities
The most important factor determining the electrical conductivity of
semiconductors is the number of electrons per unit volume, 𝑵𝒆, that are excited
into the conduction band at a given temperature 𝑻.
Let us choose as the zero of energy the top of the valence band as shown in
Fig. 1.
The number of electrons per unit volume in
the conduction band, 𝑵𝒆, can be obtained by
multiplying the density of states in each
incremental energy interval by the probability
of occupancy of that interval. `
FIGURE 1
Schematic of the conduction and
valence bands of 𝑺𝒊. The top of
the valence band is chosen as the
zero energy point; the energy of
the bottom of the conduction
band is 𝑬𝒈 .
Semiconductor and Semiconductor
Devices
Semiconductors
, Semiconductors
Resistivity in between conductor and insulators.
Elemental semiconductors: Antimony, Arsenic, Boron, Carbon, Germanium, Selenium,
sulfur and Silicon.
Si, known to be the best among these forming the basis of most IC’s.
Common semiconductor compounds: Gallium, Arsenide, Indium antimonide, oxides of
most metals.
GaAs is widely used in low noise, weak-signal amplifying devices.
At absolute zero, acts as an insulator.
When temp. increases, some of the valence electrons are able to cross the small
forbidden gap and reach to the conduction band- hence increase its conductivity as T
increases.
Forbidden gap is small in semiconductors and wide in insulators, doesnot exists in
metals.
Semiconductor devices: Junction rectifiers, photocells, solar batteries, thermistors,
transistors etc., use either pure or doped semiconductors.
, Intrinsic semiconductors
An intrinsic semiconductor is one whose impurity concentrations are so low that
its electric properties are not affected by them but are solely determined by the
band structure of the material.
An semiconductor (intrinsic) is a solid that at 𝑻 = 𝟎𝑲 has a valence band
completely filled with electrons, separated by a forbidden energy gap 𝑬𝒈 from an
empty conduction band and acts as an insulator; no current can be conducted.
As the temperature rises, the conduction band become partially filled with
electrons and the valence band becomes partly empty. This makes the
conduction possible.
At room temperature, there are enough electrons excited across the forbidden
gap to make both the electron and hole concentrations in the conduction and
valence bands. When a valence electrons crosses the band gap after gaining the
energy at least equal to band gap energy 𝑬𝒈.
The newly entered electron to the conduction band can move freely through out
the solid is termed as free electron. The space left behind in the valence band by
electron excited to the conduction band is called hole. So in intrinsic
semiconductor both the conduction of electron and hole are generated together
called electron-hole pair.
, Electron and Hole (Concentrations) Densities
The most important factor determining the electrical conductivity of
semiconductors is the number of electrons per unit volume, 𝑵𝒆, that are excited
into the conduction band at a given temperature 𝑻.
Let us choose as the zero of energy the top of the valence band as shown in
Fig. 1.
The number of electrons per unit volume in
the conduction band, 𝑵𝒆, can be obtained by
multiplying the density of states in each
incremental energy interval by the probability
of occupancy of that interval. `
FIGURE 1
Schematic of the conduction and
valence bands of 𝑺𝒊. The top of
the valence band is chosen as the
zero energy point; the energy of
the bottom of the conduction
band is 𝑬𝒈 .
