Semiconductors Notes PDF

# Basic Science (Physics): Conductors, Insulators and Semiconductors ## 2.4 Conductors, Insulators and Semiconductors Materials can be classified into three groups on the basis of their conductivity. 1. **Conductors**: The substances which conduct electricity are called conductors. Its electrical...

# Basic Science (Physics): Conductors, Insulators and Semiconductors ## 2.4 Conductors, Insulators and Semiconductors Materials can be classified into three groups on the basis of their conductivity. 1. **Conductors**: The substances which conduct electricity are called conductors. Its electrical conductivity is very high. - e.g. Copper, Silver, Aluminium (Metals) etc. 2. **Insulators:** The substances which do not conduct electricity are called insulators. - Its electrical conductivity is very low. - e.g. glass, wood, mica etc. 3. **Semiconductors:** The substances whose electrical conductivity lies between the conductivity of conductors and insulators are called semiconductors. - Its conductivity depends on the temperature. - e.g. Germanium (Ge), Silicon (Si) ## 2.4.1 Energy Bands in Solid OR Formation of Energy Bands in Solid In an isolated (single) atom, the electrons in each orbit have definite energy. But in the case of solids, all the atoms are close to each other, so the energy levels of the outermost orbit electrons are affected by the neighboring atoms. When two isolated atoms are brought close to each other, the outermost orbit electrons of two atoms are shared or interact with each other i.e. electrons in the outermost orbit of one atom experience an attractive force from the nearest atom. Due to this, the energy level of electrons are changed to a value which is higher or lower than that of the original energy level of the electrons. Thus in solids, the electrons in the same orbit exhibit different energy levels. The grouping of these different energy levels is called energy band. All the first orbit electrons have slightly different energies which forms a band called 1st energy band. Similarly 2nd, 3rd energy bands are formed. The Fig. 2.4.1 shows an energy band diagram of solid. | | | |---|---| | Conduction band | Empty or partially filled band | | Energy (eV) | $E_g$ = Forbidden energy gap | | Valence band | Completely or partially filled band | | | Completely filled inner band | **Fig. 2.4.1: Energy band diagram of solid** The electrical properties of materials depend on the number of electrons in the outermost (valence) orbit. These electrons are called valence electrons and the corresponding band is called the valence band. The next permitted energy band beyond the valence band is called conduction band and the energy gap between the valence band and the conduction band is called forbidden energy gap. ## Definitions: 1. **Energy band:** The energy level of all electrons in a particular orbit of an atom is called its energy band. OR The range of energies of electrons in a particular orbit is called its energy band. 2. **Valence band:** The band of energy occupied by the valence electrons is called the valence band. It has the highest energy band. The valence band is partially or completely filled with electrons. 3. **Conduction band:** The band of energy occupied by the free electrons or conduction electrons is called the conduction band. It is lowest unfilled energy band. It is empty or partially filled with electrons. 4. **Forbidden energy gap:** The energy gap between the valence band and the conduction band is called the forbidden energy gap or forbidden band. The energy of forbidden band is the difference between energies of bottom of conduction band ($E_c$) and top of valence band ($E_v$). $E_g = E_c - E_v$ ## 2.4.2 Energy Bands in Conductors, Insulators and Semiconductors 1. **Conductors:** The substances which conduct electricity are called conductors. - e.g. Copper, Aluminium, Silver etc. | | | |---|---| | Energy (eV) | Conduction band **Overlapping** Valence band | **Fig. 2.4.2: Energy band diagram of conductor** In conductors, there is no forbidden energy gap between the valence band and the conduction band. i.e. the valence band and conduction band overlap each other. The valence band may be partially empty and conduction band is partially filled. The electrons from the valence band freely enter into the conduction band. Therefore large amount of current can flow through it. Electrical conductivity of conductor is high. 2. **Insulators:** The substances which do not conduct electricity are called insulators. - E.g. glass, wood, mica etc. In case of insulators, the valence band is completely filled with electrons while the conduction band is empty. The energy gap between the valence band and the conduction band is very large ($E_g > 5$ eV). Therefore electrons can not jump from the valence band to the conduction band i.e. no electrons are available for the conduction. Its electrical conductivity is very small. | | | |---|---| | Energy (eV) | Conduction band $E_g > 5$ eV Valence band | **Fig. 2.4.3: Energy band diagram of insulators** 3. **Semiconductors:** The substances whose electrical conductivity lies between conductivity of the conductor and insulator are called semiconductors. - e.g. Germanium (Ge), Silicon (Si) | | | |---|---| | Energy (eV) | Conduction band $E_g = 1$ eV Valence band | **Fig. 2.4.4: Energy band diagram of semiconductor** In semiconductor, the valence band is completely filled with electrons while the conduction band is nearly empty. The energy gap between the valence band and the conduction band is small as compared to insulators. In case of Ge and Si, the band gap is 0.7 eV and 1.1 eV respectively. | | | | |---|---|---| | **Sr. No.** | **Conductors** | **Insulators** | **Semi - conductors** | | 1 | There is no band gap in conductor. | Band gap is large in insulators. | Band gap is small in semiconductors. | | 2 | Its conductivity is very high. | Its conductivity is very low | Its conductivity is lies between conductors and insulators. | | 3 | | | | | 4 | Valence band may be partially empty. | Valence band is completely filled. | Valence band is completely filled. | | 5 | Conduction band may be partially filled. | Conduction band is empty. | Conduction band is nearly empty. | | 6 | e.g. Copper, Al | e.g. wood, mica, glass | e.g. Si, Ge | | 7 | As temperature increases, conductivity decreases | As temperature increases, conductivity increases | As temperature increases, conductivity increases | | 8 | Current flows due to free electrons but large. | Current flows due to electrons but small. | Current flows due to free electrons and holes. | ## 2.4.3 Classification of Semiconductors | | | |---|---| | **Semiconductor** | | | **Intrinsic (pure)** |**Extrinsic (impure)** | | 1. Pure semiconductor | | | 2. e.g. Si, Ge | | | 3. Forth group of element | | | 4. At absolute zero temp act as insulator | | | 5. At room temperature acts as conductor | | | 6. As temp, increases, conductivity increases and resistivity decreases | | | 7. n = n_h | | | 8. Current in intrinsic semiconductor is small | | | 9. Conductivity depends on temperature | | | | | | **N-Type** | **P-Type** | | **(Pure + Pentavalent impurity)** | **(Pure + Trivalent impurity)** | | 1. Si/Ge + As/Sb/P/B: | 1. Si/Ge + Al/In/B/Ga | | 2. Impurities are called doner impurities | 2. Impurities are called accepter impurities | | 3. | 3. | | 4. Electrons are majority charge carriers | 4. Holes are majority charge carriers | | 5. Holes are minority charge carriers | 5. Electrons are minority charge carriers | | 6. Doping increases number of electrons | 6. Doping increases number of holes | ## 2.4.3.1 Intrinsic Semiconductor An extremely pure semiconductor is called intrinsic semiconductor. e.g. Silicon (Si), Germanium (Ge) etc. | | | |---|---| | | Covalent bond | **Fig. 2.4.5: Structure of Intrinsic semiconductor** Consider a pure silicon semiconductor crystal. Every 'Si' atom has four valence electrons. Each Si atom shares its four valence electrons with its four neighboring atoms. Also take one electron from each neighbour and form covalent bonds. Thus, all atoms complete their octet as shown in Fig. 2.4.5. At absolute zero temperature, the valence band is completely filled and the conduction band is empty. Therefore at low temperature, it acts as an insulator. As the temperature increases (at room temperature), due to thermal energy a few covalent bond breaks and electron - hole pair forms. Thus at room temperature, it acts as a conductor. In intrinsic semiconductor, the number of electrons is equal to the number of holes ($n_e = n_h$). When potential difference is applied across the semiconductor, these electrons and holes contribute to current. ($I = I_e + I_h$). But current in intrinsic semiconductor is small. As the temperature of semiconductor increases, conductivity increases and resistivity decreases. ## 2.4.3.2 Extrinsic Semiconductors The doped (impure) semiconductor is called extrinsic semiconductor. There are two types of extrinsic semiconductors 1. **n-type semiconductor** When small amount of pentavalent impurity such as Antimony (Sb), Arsenic (As), Phosphorous (P) is added to a pure semiconductor, it is known as n-type semiconductor. Consider a pentavalent impurity like Arsenic (As) is addend in pure silicon crystal. The 'As' atom has five valence electrons. Out of five valence electrons, four valence electrons from 'As' and four from Si forms covalent bonds and completes the octet as shown in Fig. 2.4.6. Thus 'As' atom remains one electron as a free electron i.e. donates one electron. Therefore pentavalent impurities are called donor impurities. | | | |---|---| | | Free electron | **Fig. 2.4.6: Structure of n-type semiconductor** The n-type semiconductor also has few electrons and holes produced due to thermally broken bonds. Thus in n-type semiconductor $n_e > n_h$, i.e. in n-type semiconductor, electrons are majority charge carriers and holes are minority charge carries. As this semiconductor has large number of electrons and conductivity is due to negatively charged electrons; it is called n-type semiconductor. 2. **p-Type semiconductor** When small amount of trivalent impurity such as Indium (In), Galium (Ga), Aluminium (Al), Boron (B) is added to a pure semiconductor, it is known as p-type semiconductor. Consider a trivalent impurity like Indium (In) is added in a 'Si' crystal. The indium atom has three valence electrons. The three valence electrons from 'In' atoms and three valence electrons from Si' atoms forms covalent bonds. But the bond between In atom and 4^th Si neighbouring atom has a vacancy as shown in Fig. 2.4.7. This vacancy is called a hole. It has a tendency to accept the electron in its close vicinity. Therefore, trivalent impurities are called an accepter impurities. | | | |---|---| | | Hole | **Fig. 2.4.7: Structure of p-type semiconductor** The p-type semiconductor also has few electrons and holes produced due to thermally broken bonds. Thus in p-type semiconductor $n_h > n_e$, i.e. in p-type semiconductor, holes are majority charge carriers and electrons are minority charge carriers. As this semiconductor has large number of holes and conductivity is due to positively charged holes, it is called p-type semiconductor. ## Definitions: 1. **Doping of semiconductor:** The process of adding an impurity into a pure semiconductor is called doping. 2. **Intrinsic semiconductor:** An extremely pure semiconductor is called intrinsic semiconductor. e.g. Ge, Si 3. **Extrinsic semiconductor:** The doped semiconductor is called extrinsic semiconductor. - e.g. p-type and n-type semiconductors. 4. **n-type semiconductor:** When pentavalent impurity is added to a pure semiconductor, it is called as n-type semiconductor. - e.g. Ge + As, Ge + P. 5. **p-type semiconductor:** When trivalent impurity is added to a pure semiconductor, it is called p-type semiconductor. - e.g. Ge + In, Ge + Al, Ge + Ga. 6. **Donor impurity:** The impurity which donates electrons when doped into a pure semiconductor is called donor impurity. - e.g. Arsenic (As), Antimony (Sb), Phosphorous (P), Bismuth (Bi) etc. 7. **Acceptor impurity:** The impurity which accepts electrons when doped into a pure semiconductor is called an acceptor impurity. - e.g. Aluminium (Al), Indium (In), Galium (Ga), Boron (B) etc. ## 2.4.3.3 Minority and Majority Charge Carriers i) **Minority charge carriers:** The charge carriers that are present in small quantities are called minority charge carriers. The minority charge carriers carry a very small amount of electric current in the semiconductor. In p-type semiconductors, electrons are the minority charge carriers. In N-type semiconductors, holes are the minority charge carriers. ii) **Majority charge carriers:** The charge carriers that are present in large quantities are called majority charge carriers. The majority charge carriers carry large amount of electric current in the semiconductors. In p-type semiconductors, holes are the majority charge carriers. In N-type semiconductors, electrons are majority charge carriers. ## 2.4.3.4 Difference between Intrinsic and Extrinsic Semiconductors | **Sr. No** | **Intrinsic semiconductors** | **Extrinsic semiconductors** | |---|---|---| | 1 | It is pure semiconductor. | It is impure (doped) semiconductor. | | 2 | Its electrical conductivity is low. | Its electrical conductivity is high. | | 3 | In this semiconductor, the number of electrons is approximately equal to number holes. ($n_e = n_h$) | In this semiconductor, the number of electrons is not equal to number of holes ($n_e ≠ n_h$) | | 4 | Its conductivity is depends on temperature. | Its conductivity is depends on temperature and quantity of impurity added in it. | | 5 | At absolute zero temperature, it acts as insulator. | At absolute zero temperature, its acts as conductor. | | 6 | e.g. Si, Ge | e.g. Si/Ge + Trivalent or pentavalent impurity (Si + As or Si + In) | | 7 | Fermi level lies at the centre of forbidden energy gap. | Fermi level does not lies at the centre of energy gap. (Depends on impurity). | ## 2.4.3.5 Difference between n-type and p-type Semiconductor | **Sr. No.** | **n-type semiconductor** | **p-type semiconductor** | |---|---|---| | 1 | When pentavalent impurities are added in a pure semiconductor, n-type semiconductor is formed. | When triavalent impurities are added in a pure semiconductor, p-type semiconductor is formed. | | 2 | Electrons are majority charge carriers. | Holes are majority charge carriers. | | 3 | Holes are minority charge carriers. | Electrons are minority charge carriers. | | 4 | Impurities are called donor impurities. | Impurities are called accepter impurities. | | 5 | Doping increases number of free electrons. | Doping increases number of holes. | | 6 | Majority carriers move from lower to higher potential. | Majority carriers move from higher to lower potentials. | | 7 | e.g. i) Ge + As ii) Ge + Sb | e.g. i) Ge + Al ii) Ge + In | | 8 | Electron density ($n_e$) is greater than hole density ($n_h$). | Hole density is greater than electron density. | ## 2.4.3.6 Effect of Temperature on Semiconductor (Intrinsic or Extrinsic) As the temperature of a semiconductor increases, its conductivity increases and resistivity decreases.