14.10 Summary

• Semiconductors are the basic materials used in the present solid-state electronic devices like diodes, transistors, ICs, etc.
• Lattice structure and the atomic structure of constituent elements decide whether a particular material will be an insulator, metal, or semiconductor.
• Metals have low resistivity $\left(10^{-2}\right.$ to $\left.10^{-8} \Omega \mathrm{~m}\right)$, insulators have very high resistivity ( $>10^8 \Omega \mathrm{~m}^{-1}$ ), while semiconductors have intermediate values of resistivity.
• Semiconductors are elemental $(\mathrm{Si}, \mathrm{Ge})$ as well as compound (GaAs, CdS, etc.).
• Pure semiconductors are called ‘Intrinsic semiconductors’. The presence of charge carriers (electrons and holes) is an ‘intrinsic’ property of the material and these are obtained as a result of thermal excitation. The number of electrons $\left(n_e\right)$ is equal to the number of holes $\left(n_h\right)$ in intrinsic conductors. Holes are essentially electron vacancies with an effective positive charge.
• The number of charge carriers can be changed by ‘doping’ of a suitable impurity in pure semiconductors. Such semiconductors are known as extrinsic semiconductors. These are of two types (n-type and p-type).
• In n-type semiconductors, $n_e \gg n_h$ while in p-type semiconductors $n_h \gg n_e$.
• n-type semiconducting Si or Ge is obtained by doping with pentavalent atoms (donors) like As, Sb, P, etc., while p-type Si or Ge can be obtained by doping with trivalent atom (acceptors) like $\mathrm{B}, \mathrm{Al}$, In etc.
• $n_e n_h=n_i^2$ in all cases. Further, the material possesses an overall charge neutrality.
• There are two distinct band of energies (called valence band and conduction band) in which the electrons in a material lie. Valence band energies are low as compared to conduction band energies. All energy levels in the valence band are filled while energy levels in the conduction band may be fully empty or partially filled. The electrons in the conduction band are free to move in a solid and are responsible for the conductivity. The extent of conductivity depends upon the energy gap $\left(E_g\right)$ between the top of the valence band $\left(E_V\right)$ and the bottom of the conduction band $E_C$. The electrons from the valence band can be excited by heat, light, or electrical energy to the conduction band and thus, produce a change in the current flowing in a semiconductor.
• For insulators $E_g>3 \mathrm{eV}$, for semiconductors $E_g$ is 0.2 eV to 3 eV, while for metals $E_g \approx 0$.
• p-n junction is the ‘key’ to all semiconductor devices. When such a junction is made, a ‘depletion layer’ is formed consisting of immobile ion cores devoid of their electrons or holes. This is responsible for a junction potential barrier.
• By changing the external applied voltage, junction barriers can be changed. In forward bias ( n -side is connected to the negative terminal of the battery and p-side is connected to the positive), the barrier is decreased while the barrier increases in reverse blas. Hence, the forward bias current is more $(\mathrm{mA})$ while it is very small $(\mu \mathrm{A})$ in a p-n junction diode.
• Diodes can be used for rectifying an ac voltage (restricting the ac voltage to one direction). With the help of a capacitor or a suitable filter, a dc voltage can be obtained.
• There are some special-purpose diodes.
• Zener diode is one such special-purpose diode. In reverse bias, after a certain voltage, the current suddenly increases (breakdown voltage) in a Zener diode. This property has been used to obtain voltage regulation.
• p-n junctions have also been used to obtain many photonic or optoelectronic devices where one of the participating entity is ‘photon’: (a) Photodiodes in which photon excitation results in a change of reverse saturation current which helps us to measure light intensity; (b) Solar cells which convert photon energy into electricity; (c) Light Emitting Diode and Diode Laser in which electron excitation by a bias voltage results in the generation of light.
• There are some special circuits which handle the digital data consisting of 0 and 1 levels. This forms the subject of Digital Electronics.
• The important digital circuits performing special logic operations are called logic gates. These are: OR, AND, NOT, NAND, and NOR gates.

POINTS TO PONDER

• The energy bands $\left(E_C\right.$ or $\left.E_V\right)$ in the semiconductors are space delocalised which means that these are not located in any specific place inside the solid. The energies are the overall averages. When you see a picture in which $E_C$ or $E_V$ are drawn as straight lines, then they should be respectively taken simply as the bottom of conduction band energy levels and top of valence band energy levels.
• In elemental semiconductors ( Si or Ge ), the n-type or p-type semiconductors are obtained by introducing ‘dopants’ as defects. In compound semiconductors, the change in relative stoichiometric ratio can also change the type of semiconductor. For example, in ideal GaAs the ratio of Ga: As is $1: 1$ but in Ga-rich or As-rich GaAs it could respectively be $\mathrm{Ga}_{1.1} \mathrm{As}_{0.9}$ or $\mathrm{Ga}_{0.9} \mathrm{As}_{1.1}$. In general, the presence of defects control the properties of semiconductors in many ways.
• In modern-day circuits, many logical gates or circuits are integrated in one single ‘Chip’. These are known as Integrated circuits (IC).