Extrinsic Semiconductor: Semiconductors belong to a class of crystalline solids that are intermediate in electrical conductivity between a conductor and an insulator. Semiconductors have a small energy gap between the valence band and the conduction band. Electrons can make the jump up to the conduction band, but not with the same ease as they do in conductors.
Semiconductors are utilised in the manufacture of various kinds of electronic devices. These materials possess the unique ability to control the flow of their charge carriers, making them valuable in applications like cell phones, computers, TVs, etc. Semiconductors are broad of two types:
Intrinsic semiconductors
Extrinsic semiconductors
Intrinsic Semiconductor
An intrinsic semiconductor is the purest form of the semiconductor. It is also known as an undoped semiconductor. The electrons in these materials are bonded to the parent semiconductor atom. Still, at a certain voltage or when heat is applied, these valence electrons leave the parent atom and movely in the lattice. Such electrons constitute current in the intrinsic semiconductor material. At absolute zero temperature, the valence band of an intrinsic semiconductor is filled. When the temperature is raised, and some heat energy is supplied, some valence electrons are lifted to the conduction band, leaving behind holes in the valence band. Thus, an intrinsic semiconductor is a naturally occurring, pure element, and examples are \(Si\,\& \,Ge.\)
Extrinsic Semiconductor
In extrinsic semiconductors, the bandgap is controlled by purposefully adding small impurities to the material. This process is called doping. The conductivity of a semiconductor can be increased several times by doping it with a suitable impurity. The semiconductors obtained when an impurity is added to a pure semiconductor are known as extrinsic semiconductors. The impurities that are added to an intrinsic semiconductor are called dopants. These dopants are chosen suitably regarding the purpose that we want to fulfil from the given extrinsic semiconductor and only a small amount of dopant is sufficient to make a good extrinsic semiconductor. Although while choosing the dopant, we have to make sure that the size of the dopant atom is almost similar to the size of the original atom. Dopant occupies only a few positions in the crystal lattice of the semiconductor. Therefore its presence should not disturb the shape of the original semiconductor.
Extrinsic Semiconductors: Types of Dopants
The conductivity or the number of charge carriers in a semiconductor changes after doping. We commonly use the fourth group elements of the periodic table like Silicon and Germanium for manufacturing semiconductor devices. Thus, the crystals of the tetravalent element of silicon and Germanium can be doped using two types of dopants:
1. Pentavalent (valency \(5\)); like Arsenic \(\left({As} \right),\) Antimony \(\left({Sb} \right),\) Phosphorous \(\left( P \right),\) etc.
2. Trivalent (valency \(3\)); like Indium \(\left({In} \right),\) Boron \(\left( B \right),\) Aluminium \(\left({Al} \right),\) etc.
The pentavalent and trivalent dopants belong to the third and fifth groups, respectively, close to the fourth group of the periodic table. This ensures that the size of the atoms is not much different from the fourth group. Thus, by using these dopants, we get the following two types of extrinsic semiconductors:
An extrinsic semiconductor obtained when a trivalent element is used to dope a pure semiconductor, like \(Si\) and \(Ge,\) is known as a \(p\)-type semiconductor. When a trivalent dopant atom replaces an -\(Si\) atom in an intrinsic semiconductor, three of its electrons form covalent bonds with three neighbouring \(Si\) atoms. However, this process leaves an electron in the \(Si\) atom unbonded. Since there is no electron to bond with the fourth \(Si\) atom; This creates a hole or a vacancy between the trivalent dopant and the fourth silicon atom. The electron from the lattice might jump in to fill the void present in the \(Si\) lattice, thereby creating a vacancy at its original site. Therefore, a hole is now present at the lattice point from where the electron jumped. This hole can now be used for conduction.
The number of holes made available by the dopant atoms depends solely on the doping level and does not depend on the temperature of the surroundings. As the temperature is raised further, more electrons are generated from the \(Si\) atoms, leading to some more holes. The number of electrons generated due to heat supplied to the semiconductor is minimal. It is significantly lower than the number of holes present at any given point.
Thus, in a semiconductor doped by a trivalent impurity, the majority charge carriers are holes while the electrons are the minority charge carriers. Since there are a greater number of holes than electrons, the resulting extrinsic semiconductor is a \(p\)-type semiconductor.
For a \(p\)-type semiconductor,
Number of holes \(\left({{n_h}} \right)\; >>\) Number of electrons \(\left({{n_e}} \right)\)
Here, keep in mind that the crystal maintains overall charge neutrality. The charge of additional charge carries equal and opposite to the ionised cores in the lattice.
n-Type Semiconductor
An extrinsic semiconductor obtained by doping a pure semiconductor like \(Si\) and \(Ge\) with pentavalent elements is an \(n\)-type semiconductor.
When a pentavalent atom is added to a pure semiconductor, four electrons of the \(Si\) atom forms a covalent bond with the four electrons of the dopant. But, the fifth electron of the pentavalent impurity remains weakly attached to the parent atom. Therefore, even for minimal ionisation energy applied to the atom, its electron will be set to move around the lattice. Thereby, this electron can move in the lattice even at room temperature.
Post doping, at room temperature, the ionisation energy of the silicon crystal lattice drops to \(0.05\;\rm{eV}.\) However, the similar ionisation energy for an intrinsic semiconductor is around \(1.1\;\rm{eV}.\) Thus, it becomes easier for an electron to move from the valence band into the conduction band. The temperature of surroundings does not affect the number of electrons. Although the number of electrons created by a dopant atom certainly varies with the semiconductor’s doping level.
As the temperature is raised further, more electrons are generated from the \(Si\) atoms, leading to some more electrons. The number of holes generated due to heat supplied to the semiconductor is minimal. It is significantly lower than the number of electrons present at any given point.
Thus, in a semiconductor doped by a pentavalent impurity, majority charge carriers are electrons while holes are the minority charge carriers. Since there are more electrons than holes, the resulting extrinsic semiconductor is an \(n\)-type semiconductor.
In an \(n\)-type semiconductor,
Number of electrons \(\left({{n_e}} \right) > > \) Number of holes \(\left({{n_h}} \right)\)
The semiconductor’s energy band structure gets modified by the presence of dopants. In the case of extrinsic semiconductors, doping leads to the formation of additional energy states. These are:
1. Energy state due to donor impurity \(\left({{E_D}} \right)\)
2. Energy state due to acceptor impurity \(\left({{E_A}} \right)\)
In the energy band diagram is of an \(n\)-type \(Si\) semiconductor; we can see that the energy level of the donor \(\left({{E_D}} \right)\) is lower than that of the conduction band \(\left({{E_C}} \right).\) Hence, a very small amount of energy is required to move the electrons into the conduction band \(\left({ \cong 0.01\;\rm{eV}} \right).\) The conduction band contains most of the electrons generated by the donor impurities at room temperature.
In the energy band diagram is of a \(p\)-type \(Si\) semiconductor, we can see that the acceptor energy level \(\left({{E_A}} \right)\) is higher than that of the valence band \(\left({{E_V}} \right).\) Hence, a very small amount of energy is required to move the electrons into the acceptor band \(\left({{E_A}} \right)\) from the valence band. Also, most acceptor atoms are already in the ionised state at room temperature, thereby leaving holes in the valence band. Hence, the valence band has the most holes from the impurities.
The electron and hole concentration in a semiconductor in thermal equilibrium is:
\({n_e} \cdot {n_h} = n_i^2\)
Where \({n_e}:\) the electron concentration in the semiconductor.
\({n_h}:\) the hole concentration in the semiconductor.
\({n_i}:\) the concentration of charge carriers in an intrinsic semiconductor. Charge on an Extrinsic Semiconductor: In an intrinsic semiconductor, we have an equal number of holes and electrons, making them electrically neutral. So many students assume that a \(p\)-type semiconductor has a large number of holes and current conduction is mainly due to these holes. So, the total electric charge of a \(p\)-type semiconductor is positive. Similarly, due to electrons being the majority charge carriers in an \(n\)-type semiconductor, the total electric charge of an \(n\)-type semiconductor is negative. But this assumption is wrong. Even though \(p\)-type semiconductors have a large number of holes and \(n\)-type semiconductors have a large number of electrons, the doping will not affect the overall neutrality of a semiconductor. Therefore, the total electric charge of an extrinsic semiconductor is neutral.
Applications of Extrinsic Semiconductors
Extrinsic semiconductors are used as one of the components in the majority of electronic devices. Many devices require a current to flow strictly in one direction, and for that purpose, they have diodes in them. In a diode, both \(n\) and \(p-\)type semiconductors are joined together to form a \(p-n\) junction. Extrinsic semiconductors are extensively used in transistors, bipolar junction transistors and field-effect transistors. Most of the transistors work as switching devices.
Summary
When a pure or intrinsic semiconductor is doped with a certain impurity, we get an extrinsic semiconductor. Whether using a pentavalent or trivalent dopant, an extrinsic semiconductor can either be \(p\)-type or \(n\)-type. In a \(p\)-type semiconductor, holes are the majority charge carriers. In \(n\)-type semiconductors, electrons are majority charge carriers; the doping will not affect the overall neutrality. Therefore, the total electric charge of an extrinsic semiconductor is neutral.
Frequently Asked Questions on Extrinsic Semiconductors
Frequently asked questions related to extrinsic semiconductors is listed as follows:
Q.1. What are Extrinsic Semiconductors? Ans: Extrinsic Semiconductors are created by adding specific impurities to a pure semiconductor. These are of two types \(n\)-type and \(p\)-type.
Q.2. Name the elements that make a good semiconductor. Ans: Semiconductors have their energy bandgap between that of insulator and conductor. The energy band gap of Silicon and Germanium is small, and that’s why these two elements make a good semiconductor. Both are tetravalent elements, i.e., they have four valence electrons.
Q.3. What happens when a pentavalent impurity is added to a pure semiconductor? Ans: When a pentavalent impurity is added to an intrinsic semiconductor, an \(n\)-type extrinsic semiconductor is generated.
Q.4. What is doping? Ans: Doping is a method of selectively increasing carrier concentration by adding selected impurities to an intrinsic semiconductor.
Q.5. Name a few pentavalent dopants. Ans: Pentavalent impurities are atoms with five valence electrons used for the doping of semiconductors to create \(n\)-type semiconductors. A few examples of such dopants are Arsenic \(\left({As} \right),\) Phosphorous \(\left({Pi} \right),\) Antimony \(\left({Sb} \right),\) etc.
Q.6. Who are the major charge carriers in \(n-\)type and \(p-\)type semiconductors? Ans: In \(n\)-type semiconductors, electrons are the major charge carriers. In \(p\)-type semiconductors, holes are the major charge carriers.
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