Short Questions

1. What are intrinsic and extrinsic semiconductors? An extremely pure semiconductor is called intrinsic semiconductor. For e.g. pure silicon and germanium are intrinsic semiconductors. At 0 Kelvin, conduction band of such semiconductors is completely empty and the valence electrons are shared with other similar atoms forming covalent bonds. As the temperature rises from 0 K, some of the covalent bonds are broken due to thermal energy and the free electron jump to conduction band leaving a space called hole in the valence band. An intrinsic semiconductor has very few no of electrons and holes at certain temperature. In an intrinsic semiconductor both charge carriers electrons and holes are present and number of holes and number of electrons are equal. Since the number of charge carriers is very few in an intrinsic semiconductor the charge carriers are not sufficient at normal temperature. To increase the number of charge carriers in the pure semiconductor by doping suitable impurity atoms. Depending upon the impurity atoms doped the extrinsic semiconductors are divided into P- type semiconductor and N- type semiconductor. In extrinsic semiconductors, the number of electrons and holes are not equal. 2. What are P- type and N- type semiconductors? A semiconductor doped with pentavalent impurity atom is called N- type semiconductor. Addition of pentavalent impurity atoms like As, P, Sb in an intrinsic semiconductor provides extremely large number of electrons in the conduction band. When pentavalent atoms are doped in pure semiconductors like Silicon and Germanium, four of the five valence electrons of the impurity atom forms four covalent bonds with the atoms of semiconductor and fifth one is left as free electron. This free electron is responsible for the conductivity of semiconductor. The no. of free electrons can be controlled by addition of suitable no. of doping agent i.e. pentavalent impurity atoms. The N- type semiconductor contains large no. of free electrons in the conduction band and most of the electrons are from impurity atoms and very few from valence band due to thermal excitation. Due to this the valence band contains very few no. of holes. As a result the Ntype semiconductor contains electrons as majority charge carriers and holes as minority charge carriers. A semiconductor doped with trivalent impurity atom is called P- type semiconductor. Addition of trivalent impurity atoms like Boron, Indium, and Gallium in an intrinsic semiconductor provides extremely large number of holes in the valence band. When a trivalent atom replaces an atom from the pure semiconductors like Silicon and Germanium, three valence electrons of the impurity atom forms three covalent bonds with the atoms of semiconductor and a deficiency of electron is created in the fourth covalent bond. This deficiency of electron called hole is responsible for the conductivity of semiconductor. The no. of holes can be controlled by addition of suitable no. of doping agent i.e. trivalent impurity atoms. The P- type semiconductor contains large no. of holes in the conduction band and most of the holes from impurity atoms and very few electrons are on the conduction band due to thermal excitation. Due to this the valence band contains very large no. of holes and the conduction band contains very few no. of electrons. As a result the P-type semiconductor contains holes as majority charge carriers and electrons as minority charge carriers.

3. Why does the conductivity of semiconductors increases with the increase in temperature? The atoms of the semiconductor are bonded with covalent bonds at absolute zero temperature and there are no free electrons for the conduction at this temperature. In terms of Band theory, at absolute zero, all the valence electrons of the semiconductor are in the valence band and the conduction band is completely empty i.e. there are no electrons in the conduction band at absolute zero temperature. As the temperature increases, the electrons in the valence band gains sufficient kinetic energy to break the covalent bonds and more no of electrons jump to the conduction band from valence band leaving holes in the valence band. As the electrons and the holes are charge carriers, increase in temperature causes increase in no. of charge carriers which in turn increases the conductivity of semiconductor. 4. What is P-N junction diode? When a semiconductor piece doped with trivalent impurity atoms (P-type semiconductor) is fused with the semiconductor piece doped with pentavalent impurity atoms (N- type semiconductor), there exists a common contact between them. The surface of contact between these two types of semiconductors is called P-N junction. A P-N junction is commonly known as P-N junction diode or simply junction diode. In designing a junction diode, a pure semiconductor wafer (piece) is doped with trivalent impurity atoms on one side and pentavalent impurity atoms on the other side heating the wafer at very high temperature. On doing so the side of the wafer doped with trivalent atoms becomes P type and the side doped with pentavalent atoms becomes N type. Holes

Immobile ions + + + + +

Electrons Junction

Depletion layer figure 5. Can holes be created in metals? Justify. No, holes cannot be created in metals. Holes are the empty space left on the valence band, by the electrons after they move to the conduction band after gaining sufficient energy to cross the forbidden gap. In metals (conductors) the forbidden gap is almost missing i.e. conduction band and valence band are overlapped hence no holes can be created in the metals. In other word, when trivalent atom occupies a space within the crystals of tetravalent atom, a deficiency of electron called hole appears in the covalent bonds between the atoms of the tetravalent crystals. Since there are no covalent bonds in the metals no deficiency of electron is created in the metallic crystals and no holes are created in the metals as well. 6. What is depletion layer and barrier potential? When a P type material is kept in contact with N type material to produce a P-N junction, the majority charge carriers from one type material diffuse to the other type material. We

know electrons are the majority charge carrier in N- type material and holes are the majority charge carriers in P-type material. So in forming P-N junction electrons from N type material diffuse towards P-type material and holes from P-type material diffuse towards N type materials. In the process, the electrons and holes recombine near the junction. As a result of this recombination of electrons and holes, a small region near the junction on N side becomes positively charged and a small region near the junction on P side becomes negatively charged. These charged regions then stop further mobility of electrons and holes due to repulsion and act like a barrier. The small oppositely charged region near the junction is called depletion layer and the P.D. between these oppositely charged regions near the junction is called barrier potential.

Figure 7. What is the effect of temperature on Barrier Potential? The Barrier potential arises in a P-N junction due to diffusion and recombination of electrons and holes. As the temperature increases the number of charge carries in P side (holes) and N side (electrons) increases. Some of these thermally generated electrons and holes recombine with the opposite charges in the corresponding sides which help to decrease in the barrier potential of the junction. Thus we can say that the barrier potential decreases with the increase in temperature. 8. Is N-type material charged or is it neutral? Justify your answer. The N-type material is electrically neutral not charged even it contains many electrons. An N- type semiconductor is obtained when a neutral tetravalent semiconductor crystal is doped with neutral pentavalent atoms. As this process is simply the combination of two neutral atoms without gain or loss of electrons there is no charging of the atoms. The N type semiconductor has more electrons only in the conduction band. In total it has equal no. of protons and electrons hence it is electrically neutral. 9. What is forward and reverse biasing of the P-N junction diode? A P-N junction diode is said to be forward biased if the anode (positive terminal) of the battery is connected to the P- side of the diode and cathode of the battery (negative terminal) is connected to N-side. In doing so, the holes are repelled by the positive terminal of the battery towards the junction and the electrons are repelled towards the junction by negative terminal of the battery. This electron and hole recombine near the junction reducing the depletion layer. For each recombined electron hole pair, one electron enters the N side from the negative terminal of the battery and a valence electron leaves P-side to go to positive terminal of the battery. This movement of electron constitute current in the diode. It should be noted that both electron and hole move inside the diode but only electron move in the connecting wire. In the forward biasing of the PN junction diode, the majority charge carriers constitute large amount of current and minority charge carriers constitute a very small amount of current called leakage current which is negligible.

A junction diode is said to be reverse biased if –ve terminal of the battery is connected to P-side and +ve terminal is connected to N-side of the diode. In doing so, the electrons are attracted towards the positive terminal and holes towards the negative terminal and get repelled by the junction thus the depletion layer widens and recombination of electrons and holes becomes impossible. As a result no current flows through the diode. But a small amount of current called leakage current flows through the diode due to minority charge carriers in reverse biasing.

10. Define Knee voltage. In the forward biasing of a P-N junction diode