Electrical conductivity of semiconductor changes appreciably with temperature variations. At absolute zero, it behaves as an insulator. At room temperature, because of thermal energy, some of covalent bonds of the semiconductor break. The breaking of bonds sets those electrons free, which are engaged in the formation of these bonds. This results in few free electrons. These electrons constitute a small current if potential is applied across the semiconductor crystal. This shows the conductivity for intrinsic semiconductor increases with increase in temperature as given by η = Aexp (− E g / 2kt ) where η is the carrier. Concentration eg is the band gap, T is the temperature and A is constant. In case of extrinsic semiconductors, addition of small amount of impurities produces a large number of charge carriers. This number is so large that the conductivity of an extrinsic semiconductor is many times more than that of an intrinsic semiconductor at room temperature. In n – type semiconductor all the donors have donated their free electrons, at room temperature. The additional thermal energy only serves to increase the thermally generated carriers. This increases the minority carrier concentration. A temperature is reached when number of covalent bonds that are broken is large, so that number of holes is approximately equal to number of electrons. The extrinsic semiconductor then behaves like intrinsic semiconductor.
The most commonly used semiconductor parameters are intrinsic concentration, forbidden energy gap, mobility and conductivity. The effect of temperature on these parameters is discussed below.
Intrinsic concentration (Ni): The number of holes or electrons present in an intrinsic semiconductor at any temperature is called intrinsic carrier concentration (Ni). It depends upon temperature of an intrinsic semiconductor. In N type semiconductor, the number of free electrons (n) does not change appreciably with the increase in temperature, but number of holes (p) increases. In P type semiconductor, the number of free electrons (n) increases with the increase in temperature, but number of holes remains constant.
Forbidden energy gap (EG): The energy required to break a covalent bond in a semiconductor is known as energy gap. It is equal to the difference of energy levels between the conduction band and valence band of the semiconductor crystal structure. The forbidden energy gap decreases with the increase in temperature.
Mobility (µ): The mobility means the movement of charge carriers. The mobility of intrinsic semiconductor decreases with increase in temperature because at higher temperature, the numbers of carriers are more and they are energetic also. This causes an increased number of collisions of charge carriers with the atoms and thus the mobility decreases.
Conductivity (σ): The conductivity of an intrinsic semiconductor depends upon the number of hole electron pairs and mobility. The number of hole electron pairs increases with increase in temperature, while its mobility decreases. However, the increase in hole electron pairs is greater than the decrease in their mobility’s. Therefore, the conductivity of an intrinsic semiconductor increases with increase in temperature. The conductivity of an extrinsic semiconductors decreases with the increase in temperature, the number of majority carriers is nearly constant, but mobility decreases. Thus causes the conductivity to decrease.