Semiconductor are classified into two classes:
- Intrinsic (or pure) semiconductors
- Extrinsic (or impure) semiconductors
A semiconductor in an extremely pure form is known as intrinsic semiconductor. A semiconductor is not truely intrinsic (or pure) unless its impurity content is less than one part in 100 million parts of semiconductor.
In an intrinsic semiconductor, hole-electron pairs are formed at room temperature. Since the electrons and holes move randomly in crystal lattice, there is a possibility of electron meeting a hole.
Due to this, both the hole and electron disappear because the electron takes the position of a hole in a broken covalent bond. The covalent bond is again formed and recombines the electrons and holes.
Before recombination, an electrons and holes travels some distance, which give new term called ‘lifetime‘. The average time an electron and hole remains free is called its lifetime.
When an external electric field is applied across an intrinsic (or pure) semiconductor, the current conduction through the semiconductor takes place by both free electrons and holes. However, in Fig. both charge carriers (of free electrons and holes) move in opposite directions and the currents are in the same directions.
Therefore the total current inside the semiconductor are added.
We notice, the free electrons in this Fig. move towards the positive terminal of the battery while the holes being positively charged move towards the negative terminal of the battery.
The electric current flows through the semiconductor in the same direction as in which the holes are moving. As the holes reach the negative terminal Y, electrons enter the semiconductor near the terminal and combine with holes, thus disappear them.
At the same time, new holes are formed near the positive terminal X, because the loosely held electrons near the positive terminal X are repelled away from their atoms. This net movement of electrons and holes are called ‘drift’.
The movement of electrons is more than that of holes because the probability of energy required by an electron to move to an empty state in conduction band is much higher than the energy required to move to an empty state in valence band.
Thus the current due to movement of electrons is greater than that due to movement of holes. The resultant current applied for the movement of charge carriers is known as drift current.
There is also another current exists in semiconductor is called diffusion current, and it flows as a result of gradient of carrier concentration (i.e. the difference of carrier concentration from one region to another).
Intrinsic semiconductor example
Silicon (Si) and Germanium (Ge) are the two example of intrinsic semiconductor.
Properties of intrinsic semiconductor
- Pure semiconductor.
- Density of electrons is equal to the density of holes.
- Electrical conductivity is low.
- Dependence on temperature only.
- No impurities.
A semiconductor in an impure form is known as an extrinsic semiconductor. Basically due to the bad current conduction capability of intrinsic (pure) semiconductors, at room temperature, are not very useful in electronic devices.
To make use in electronic devices, the current conduction capability of the intrinsic semiconductor should be increased. Practically, this can be done by adding certain specified amount of impurities to the pure (or intrinsic) semiconductor, so that it becomes impure (or extrinsic) semiconductor.
The process of adding impurities to a semiconductor material is known as doping. Before doping, the very first step is to refine a semiconductor in its highly purest form. Then, a doped semiconductor is known as an extrinsic semiconductor.
Generally, the small amount of impurity should be added, for 108 atoms of intrinsic semiconductor just one impurity atom is added. The main purpose of adding impurity to the pure semiconductor is to increase either the member of holes or number of electrons in the crystal.
Depending upon the type of impurity (donor or N-type and acceptor or P-type) added, the extrinsic semiconductor are divided into two classes namely:
- N-type extrinsic semiconductor
- P-type extrinsic semiconductor
N-type Extrinsic Semiconductor
When a small amount of pentavalent* impurities such as arsenic, phosphorus etc is added to a pure semiconductor crystal, then the resulting crystal is known as n-type semiconductor. Since, these pentavalent impurities donate free electrons to the semiconductor crystal therefore such an impurities are also known as donor impurities.
Let us see what happens, if a small amount of pentavalent impurity like arsenic (atomic no. 33) is added to a pure (or intrinsic) germanium semiconductor crystal. Germanium atom has 4 valence electrons while arsenic atom has 5 valence electrons, therefore each arsenic atom forms a covalent bond with surronding 4 germanium atoms.
Hence the fifth valence electron of arsenic atom is left free as shown in Fig. Therefore, for each arsenic atom added, donates one free electron to each germanium atom in the crystalline structure.
Thus, the number of free electrons becomes higher than no. of holes in an N-type semiconductor. That is why electrons are called majority carriers and holes are called minority carriers.
P-type Extrinsic Semicondutor
When a small amount of trivalent impurity like aluminiun, gallium, boron etc. is added to a pure semiconductor, it is called p-type extrinsic semiconductor. Since the holes of trivalent impurity atom accepts the electrons therefore such impurities are also known as acceptor impurities.
Let us consider a small amount of trivalent impurity like boron is added to germanium crystal. Since germanium has 4 valence electrons and boron has three valence electrons, therefore there exists a large number of holes in the crystalline structure as shown in Fig below.
The reason is simple, three valence electrons in boron form covalent bond with four surrounding atoms of germanium. Thus, fourth electron of germanium atom left blank as a hole.
It is, because three valence electrons of boron atom can form only three single covalent bonds with three germanium atoms out of four atoms. Hence, we can say that for each boron atom added each germanium atom accepts one hole in a crytalline structure.
Thus, as a whole the number of holes (+ve charged) increases than the number of electrons (-ve charged), hence, the semiconductor becomes more positive and is known as p-type semiconductor.
That is why in p-type materials holes are called majority carriers and electrons are called minority carries Notice, a small amount of boron provides millions of holes. Representation of p-type semiconductor is shown in Fig.
At the end we conclude that, current conduction in N-type semiconductor is due to excess of free electrons while in P-type, current conduction is due to excess of holes.