The magic word semiconductor is composed of two words-Semi and Conductor. Semi means not completely while conductor mean something, which can conduct electricity. Everybody is familiar with "Electricity". It is present everywhere; it runs many appliances in your home and outside the home like TV, Bulb, Freeze, and Microwave Oven etc. In simple terms, the current must past through wires so that the electricity can reach all these appliances. So a conductor is nothing but a material having ability to conduct this electricity. Semiconductors conduct electricity to some extent, less than the conductors, how much do you think? Well, it depends on the type of material or it's mixture and size. A semiconductor is a material that has intermediate conductivity between a conductor and an insulator. It means that it has unique physical properties somewhere in between a conductor like aluminum and an insulator like glass. To increase or decrease conductivity Doping process carry out.
Semiconductors are mainly classified into two categories: Intrinsic and Extrinsic. An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes). Where as an extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity.
In a pure (intrinsic) Si or Ge semiconductor, each nucleus uses its four valence electrons to form four covalent bonds with its neighbors (see figure below). Each ionic core, consisting of the nucleus and non-valent electrons, has a net charge of +4, and is surrounded by 4 valence electrons. Since there are no excess electrons or holes In this case, the number of electrons and holes present at any given time will always be equal.
An intrinsic semiconductor. Note each +4 ion is surrounded by four electrons.
Now, if one of the atoms in the semiconductor lattice is replaced by an element with three valence electrons, such as a Group 3 element like Boron (B) or Gallium (Ga), the electron-hole balance will be changed. This impurity will only be able to contributethree valence electrons to the lattice, therefore leaving one excess hole (see figure below). Since holes will "accept" free electrons, a Group 3 impurity is also called an acceptor.
A semiconductor doped with an acceptor. An excess hole is now present.
Because an acceptor donates excess holes, which are considered to be positively charged, a semiconductor that has been doped with an acceptor is called a p-type semiconductor; "p" stands for positive. Notice that the material as a whole remains electrically neutral. In a p-type semiconductor, current is largely carried by the holes, which outnumber the free electrons. In this case, the holes are the majority carriers, while the electrons are the minority carriers.
In addition to replacing one of the lattice atoms with a Group 3 atom, we can also replace it by an atom with five valence electrons, such as the Group 5 atoms arsenic (As) or phosphorus (P). In this case, the impurity adds five valence electrons to the lattice where it can only hold four. This means that there is now one excess electron in the lattice (see figure below). Because it donates an electron, a Group 5 impurity is called a donor. Note that the material remains electrically neutral.
A semiconductor doped with a donor. A free electron is now present.
Donor impurities donate negatively charged electrons to the lattice, so a semiconductor that has been doped with a donor is called an n-type semiconductor; "n" stands for negative. Free electrons outnumber holes in an n-type material, so the electrons are the majority carriers and holes are the minority carriers.
Light can also give this energy boost and create what we call an electron-hole pair: a free electron and a free hole: this phenomenon is called absorption. Photoconductivity is the increase of current in a semiconductor due to the absorption of photons. Light has a dual nature: it behaves as a wave and as a particle. The particle associated with light is called a photon. Photons can have different energies.
When light illuminates a semiconductor:
· the photons with the right energy are absorbed by the material
· the electrons from the valence band have enough energy to jump to the
conduction band
· the conductivity increases due to the higher number of electrons in the
conduction band.
Electroluminescence is the conversion of electrical energy into light. Let's consider electrons in the conduction band. These electrons are in an excited state: they have gained some energy to jump to the conduction band.
Such electrons eventually fall back into the valence band in a lower energy state:
· they release the extra energy that they have
· this energy is emitted as a photon
Photons emitted by electroluminescence come out in random directions: this type of light is called incoherent light. For instance light from a light bulb is incoherent. Stimulated emission is a little bit like electroluminescence except that it is not a spontaneous process: the excited electron is forced into jumping back to the valence band and emitting a photon.
DOPING
In a process called doping, small amounts of impurities are added to pure semiconductors causing large changes in the conductivity of the material. Examples include silicon, the basic material used in the integrated circuit, and germanium, the semiconductor used for the first transistors.
CLASSIFICATION OF SEMICONDUCTOR
Doping process produces two groups of semiconductors: the negative charge conductor (n-type) and the positive charge conductor (p-type).
Positive charge conductor(P-type) :-
In a pure (intrinsic) Si or Ge semiconductor, each nucleus uses its four valence electrons to form four covalent bonds with its neighbors (see figure below). Each ionic core, consisting of the nucleus and non-valent electrons, has a net charge of +4, and is surrounded by 4 valence electrons. Since there are no excess electrons or holes In this case, the number of electrons and holes present at any given time will always be equal.
An intrinsic semiconductor. Note each +4 ion is surrounded by four electrons.
Now, if one of the atoms in the semiconductor lattice is replaced by an element with three valence electrons, such as a Group 3 element like Boron (B) or Gallium (Ga), the electron-hole balance will be changed. This impurity will only be able to contributethree valence electrons to the lattice, therefore leaving one excess hole (see figure below). Since holes will "accept" free electrons, a Group 3 impurity is also called an acceptor.
A semiconductor doped with an acceptor. An excess hole is now present.
Because an acceptor donates excess holes, which are considered to be positively charged, a semiconductor that has been doped with an acceptor is called a p-type semiconductor; "p" stands for positive. Notice that the material as a whole remains electrically neutral. In a p-type semiconductor, current is largely carried by the holes, which outnumber the free electrons. In this case, the holes are the majority carriers, while the electrons are the minority carriers.
Negative charge conductor (N-type) :-
In addition to replacing one of the lattice atoms with a Group 3 atom, we can also replace it by an atom with five valence electrons, such as the Group 5 atoms arsenic (As) or phosphorus (P). In this case, the impurity adds five valence electrons to the lattice where it can only hold four. This means that there is now one excess electron in the lattice (see figure below). Because it donates an electron, a Group 5 impurity is called a donor. Note that the material remains electrically neutral.
A semiconductor doped with a donor. A free electron is now present.
Donor impurities donate negatively charged electrons to the lattice, so a semiconductor that has been doped with a donor is called an n-type semiconductor; "n" stands for negative. Free electrons outnumber holes in an n-type material, so the electrons are the majority carriers and holes are the minority carriers.
Properties
Semiconductors have many useful properties that insulators and conductors do not possess. These properties are based on the fact that an electron can jump from the valence band to the conduction band and vice versa. Temperature can give this little extra energy to an electron and make it jump to the conduction band thus creating a hole in the valence band.Light can also give this energy boost and create what we call an electron-hole pair: a free electron and a free hole: this phenomenon is called absorption. Photoconductivity is the increase of current in a semiconductor due to the absorption of photons. Light has a dual nature: it behaves as a wave and as a particle. The particle associated with light is called a photon. Photons can have different energies.
When light illuminates a semiconductor:
· the photons with the right energy are absorbed by the material
· the electrons from the valence band have enough energy to jump to the
conduction band
· the conductivity increases due to the higher number of electrons in the
conduction band.
Electroluminescence is the conversion of electrical energy into light. Let's consider electrons in the conduction band. These electrons are in an excited state: they have gained some energy to jump to the conduction band.
Such electrons eventually fall back into the valence band in a lower energy state:
· they release the extra energy that they have
· this energy is emitted as a photon
Photons emitted by electroluminescence come out in random directions: this type of light is called incoherent light. For instance light from a light bulb is incoherent. Stimulated emission is a little bit like electroluminescence except that it is not a spontaneous process: the excited electron is forced into jumping back to the valence band and emitting a photon.
-------------------------------------------------------------------------
Please give your comment
Comments