Semiconductors are materials which exhibit properties of both conductors and insulators. The semiconductors are of two types, Intrinsic and Extrinsic Semiconductors. The intrinsic semiconductors are the one with no doping and therefore called as pure semiconductors. The extrinsic conductors are doped with impurity atoms. Based on the type of impurity added they are classified as: N-type and P-type Semiconductors.
What is an N-type Semiconductor?
A N-type semiconductor is defined as a type of extrinsic semiconductor doped with a pentavalent impurity element which has five electrons in its valence shell. The pentavalent impurity or dopant elements are added in the N-type semiconductor to increase the number of electrons for conduction.
Doping in N-type Semiconductor
The N-type semiconductor is doped with pentavalent impurity elements. The pentavalent elements have five electrons in the valence shell. The examples of pentavalent impurities are Phosphorus (P), Arsenic (As), Antimony (Sb). The pentavalent impurity is added in a very minute fraction in the N-type semiconductor such that the crystal structure of the original intrinsic semiconductor is not disturbed. The pentavalent impurity atom makes covalent bonds with four silicon atoms and one electron is not bonded with any silicon atom. Each pentavalent impurity atom donates one electron to the N-type semiconductor hence it is called as a Donor impurities. Thus, there are more number of electrons in the N-type semiconductor.
N-type Semiconductor Example
An intrinsic semiconductor material like Silicon (Si) has 14 electrons with a configuration of 2,8,4 and Germanium (Ge) has 32 electrons with a configuration of 2,8,18,4. Each atom requires 8 electrons in its valence shell to be stable. Hence intrinsic semiconductor atoms have covalent bonds based on sharing the electrons of a nearby atom to achieve 8 electrons to balance their atomic structure.
Figure 3. N-type semiconductor with donor impurity
A N-type semiconductor is created by doping this pure silicon crystal lattice with a pentavalent impurity element like Antimony (Sb). In an N-type semiconductor the atom of pentavalent impurity element Antimony (Sb) is in between silicon atoms. The Silicon atoms have four electrons in the valence shell. Each of the silicon atom creates a covalent bond with an electron of the prevalent impurity atom.
The Antimony (Sb) impurity element electron form covalent bonds with only four silicon atoms. The fifth electron of the impurity atom is not bonded with any semiconductor atom in the crystal lattice. This electron is loosely bonded to its parent impurity atom. Thus, as external voltage or heat is applied this fifth electrons easily breaks its bond with the parent atom and takes part in conduction.This fifth electron majorly contributes to the current in an N-type semiconductor.In the N-type Semiconductor the electrons become the majority carrier.
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Energy Diagram of n-Type Semiconductor
Figure 4. Fermi level for intrinsic semiconductor
The Energy diagram represents two energy bands, the valence band and the conduction band. The electrons in the valence band in the energy diagram represent the electrons which are in the valence band of the atom and they are still bonded to the parent atom. The electrons in the conduction band in the energy diagram represent atoms which take part in conduction. The energy gap between the valence and conduction band is called as the forbidden band or band gap.
Figure 5. Fermi level for N-type semiconductor
In an N-type semiconductor because of the pentavalent impurity a number of loosely bonded electrons are available in the lattice structure. As the voltage is applied, these electrons break free from the covalent bonds andare ready to conduct. These electrons are depicted in the conduction band.When a certain amount of voltage is applied, these electrons gain energy to cross the forbidden gap and leave the valence bandto enter into the conduction band. A very less number of holes are formed in the valence band as the electron leaves valence band to enter conduction band. The Fermi level is near the conduction band as more number of electrons enter the conduction band.
Conduction through N-type Semiconductor:
The conduction through an N-type semiconductor is majorly caused by the electrons. The pentavalent donor impurity has imparted extra electrons to the lattice structure. As a voltage is applied or the semiconductor is subject to external heat the electrons gain energy. The electrons break covalent bonds and more electrons are released into the conduction band. The electron which breaks away from its covalent bond, leaves a void space or hole in its place.
As the negatively charged electron leaves a hole, this empty space attracts other electrons. Hence the hole is considered to be positively charged. Thus the N-type conductor has two types of carriers,negatively charged electrons and positively charged holes. In an N-type semiconductor the electrons are greater in number and hence they are termed as the majority carriers and the holes are termed as minority carriers as they are less in number. The current constituted in an N-type semiconductor by electrons is called a majority carrier current and the current constituted by holes is called the minority charge current.
When a covalent bonds break and the electrons leaves a hole in its place, some other electron breaks away from its covalent bond and gets attracted towards this hole. Thus the electron and holes move in opposite directions. The electrons get attracted towards positive terminal of the battery and holes are attracted towards negative terminal of the battery. As the electrons and holes travel through the lattice current starts following in the N-type semiconductor.
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The N-type extrinsic semiconductor has majority carrier as electrons and hence has greater conductivity. Because of this reason the N-type Semiconductor in combination with a P-type semiconductor is used to manufacture all the major semiconductor components and devices. The basic components like PN Diode, Bipolar Junction Transistor and Field effect Transistors have their working based on the properties and characteristics of N-type semiconductor.