from DRGP DREAM DOT
Semiconductor is a material which has electrical conductivity to a degree between conductor and insulator. There are several ways of defining a semiconductor. Historically, the term semiconductor has been used to denote materials with a much higher conductivity than insulators, but a much lower conductivity than metals measured at room temperature. This definition is not complete. What really identified the semiconductor is the temperature dependence of the conductivity. Semiconductor shows conductivity at high temperature but at low temperature they transformed into insulator. There are two type of semiconductor – Intrinsic semiconductor and Extrinsic semiconductor.
Intrinsic Semiconductor is a pure semiconductor material with no impurity atoms and no lattice defects in the crystal. Intrinsic semiconductor has small conductivity. Its small conductivity is due to free electrons and holes created in due to temperature or heat. The concentration of holes and free electrons are equal in intrinsic semiconductor. If a voltage is applied, then both the electron and the hole can contribute to a small current flow. Example: pure silicon and germanium. To understand the conductivity of intrinsic semiconductor first we understand the crystal structure of intrinsic semiconductor.
Consider an atomic structure of an intrinsic semiconductor material like silicon and germanium. Any atom is said to be completely filled and stable, if it contains 8 electrons. But the outermost shell of an intrinsic semiconductor like Si, Ge has only four electrons. Each of these four electrons from a covalent bonds with valence electrons of the neighboring atoms. The atoms in this fashion align themselves to form a three dimensional uniform pattern called a crystal. No free electrons are available at absolute zero temperature. Hence an intrinsic semiconductor behaves as a perfect insulator at absolute zero temperature.
Conductivity At Room Temperature
Intrinsic semiconductor behave as a perfect insulator at absolute zero temperature. But at the room temperature, the number of valence electrons absorb the thermal energy, due to which they broke the covalent bond and drift to the conduction band. Such electrons become free to move in the crystal. Due to band breaking a covalent bond, a vacancy is created in the broken covalent bond. Such a vacancy is called a hole. Whenever an electron becomes free, the corresponding hole gets generated. So free electrons and holes get generated in pairs. So concentration of free electron and holes in intrinsic semiconductor is equal. Thus in an intrinsic semiconductor both holes as well as free electrons are the charge carriers. Because electron has negatively charged, hole is positively charged. Such a generation of electron hole pairs due to thermal energy is called thermal generation.
Conduction Of Electron And Holes
Under the influence of applied voltage there is electron as well as hole motion in one particular direction. It produces current. The free electrons are negatively charged so feel repulsion from the negative terminal and attracted towards the positive terminal. Thus there is an electric current due to the movement of electrons in conduction band. This is called electron current.
There are electrons present in the valence band which are involved in forming the covalent bonds. In valence band holes are present due to escape of electrons from valence to conduction band. Under the influence of applied voltage, the electron of conduction band try to fill the holes present. This can be explain by following diagram.
In this way holes moves towards the negative terminal of battery. Such a movement of holes in valence band produce hole current. So the current due to movement of free electrons in the conduction band is an electron current. The current due to movement of holes in the valence band is a hole current. Total of both current is equal to current in an intrinsic semiconductor.
Conductivity Of Intrinsic Semiconductor
The property called conductivity indicates the ease with which a material can carry the current. In intrinsic semiconductor very few electron-hole pairs get generated at room temperature. Hence the conductivity is very low. This low conductivity has very little practical significance. Conductivity of instrinsic semiconductor depends upon two thing temperature and forbidden energy gap. As temperature increases the conductivity of intrinsic semiconductor increases. As forbidden energy gap increases the conductivity of intrinsic semiconductor increases. For intrinsic semiconductor number of holes and number of electrons are same.