Conductors, semiconductors, and insulators are three types of materials. Semiconductors are those materials whose conductivity lies between that of conductors and insulators. Most electronic devices are based on semiconductors. At absolute zero(0°K) temperature, the semiconductors behave as insulators. As temperature increases, the conductivity increases. In the old days, only vacuum tubes were used. But nowadays semiconductor (solid-state) devices have replaced vacuum tube devices, because of their number of advantages. Semiconductor devices have a small size and their cost is less. They don’t require heating filaments and they can be operated at low voltages. They consume a very small amount of power.
Difference between Conductors, Semiconductor and Insulators:
|1. These are good conductors of electricity.||1. Conductivity lies between that of conductors and insulators.||1. No electrical conductivity.|
|2. Conductivity decreases with the increase of temperature. They have a positive temperature coefficient (PTC) of resistance.||2. Conductivity increases with the increase of temperature. They have a negative temperature coefficient (NTC) of resistance.||2. They have a positive temperature coefficient (PTC) of resistance.|
|3. Free electrons are charge carriers.||3. Free electrons and holes are charge carriers.||3. Charge carriers are absent.|
|4. eg.- Copper, Aluminum, Gold, Silver, etc.||4. eg.- Germanium and Silicon.||4. eg.- Plastic, rubber, wood, cotton, etc.|
|5. In conductors, the valence band (V.B.) overlaps the conduction band (C.B). The forbidden energy gap (EG) is zero. Therefore, small applied voltage is large.||5. In, Semiconductors, there is a small forbidden energy gap of about 1 eV. If this energy is given externally, valence electrons go to C.B. and become free electrons, to provide conduction||5. In insulators, the forbidden energy gap is about 6eV. Such a large amount of energy cannot be supplied externally. Hence conduction is not possible.|
Types of Semiconductors:
- Intrinsic Semiconductors;
- Extrinsic Semiconductors
Intrinsic Semiconductors :
Intrinsic Semiconductors are pure semiconductors. When germanium or silicon atoms combine to form a solid, they arrange themselves in an orderly pattern called a crystal. Ge and Si are tetravalent, i.e., there are four valence electrons. For a stable atom, eight valence electrons are necessary. By sharing one electron each with neighboring four atoms the central atom gets eight electrons in its valence orbit. Therefore, it becomes stable.
The forces that hold the atoms together are called ‘covalent bonds’. Such pure form of germanium or silicon is called ‘intrinsic semiconductor. An intrinsic semiconductor behaves as an insulator at absolute zero (0°K) temperature.
If the temperature is increased beyond 0°K, the heat energy breaks some covalent bonds. Some valence electrons go from the valence band to the conduction band and they become free electrons. The energy required for this purpose is equal to the forbidden energy gap (EG). The value of EG is 1.12 eV for Si and 0.72 eV for Ge. When an electron goes from the valence band to the conduction band, a hole is created in the valence band. The number of holes and free electrons always remains the same in an intrinsic semiconductor. The conduction is provided by free electrons in the conduction band and by holes in the valence band.
Extrinsic Semiconductors :
In an intrinsic semiconductor, there aren’t enough free electrons and holes to produce sufficient current. The process of adding impurity atoms to a crystal to increase either the number of free electrons or holes is called doping. When a crystal of intrinsic semiconductor has been doped, it is called an extrinsic semiconductor.
There are two types of extrinsic semiconductors : (1) p-type and (2) n-type
p-type semiconductors :
There are four valence electrons in germanium and silicon. Therefore, they are tetravalent. The impurity of a trivalent material can be added to the pure semiconductor This is a doping process. Such a doped semiconductor is called a p-type semiconductor. Boron (B), aluminum (Al), indium (In), gallium (Ga) are trivalent materials. Only one or two impurity atoms are added to 106 Si or Ge atoms.
In a trivalent material, three electrons are present in the outer orbit. They form three covalent bonds. The absence of an electron in the fourth bond is called a hole. By controlling the amount of impurity added, the number of holes can be controlled.
The conductivity of a p-type semiconductor is mainly due to holes. With an increase in temperature, some electron-hole pairs are generated. In a p-type semiconductor, holes are majority charge carriers and free electrons are minority charge carriers. Each hole may accept an electron during recombination. Therefore, trivalent impurity atoms are known as acceptor atoms.
n-type semiconductors :
If impurities of pentavalent material are added to pure semiconductor, it becomes an n-type semiconductor. Arsenic (As), antimony (Sb), phosphorus (P) are pentavalent materials.
In a pentavalent material, five electrons are present in the outer orbit. They form four covalent bonds. The remaining electron goes to the conduction band and becomes a free electron. There is a large number of electrons produced mostly by doping. There are only a few holes, created by thermal energy. As temperature increases, electron-hole pairs are generated. In an n-type semiconductor, free electrons are majority charge carriers and holes are minority charge carriers.
Pentavalent atoms are called donor atoms because they produce conduction-band (free) electrons.