In an unbiased p-n junction, holes diffuse from the p-region to n-region because

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Important Points to Remember in Chapter -1 - Semiconductor Electronics: Materials, Devices and Simple Circuits from NCERT PHYSICS PART 2 TEXTBOOK FOR CLASS 12 Solutions

1. Conductors, insulators and semiconductors:

(i) Metals have low resistivity ( $10^{- 2}$ to $10^{- 8} Ω m$ )

(ii) Insulators have very high resistivity ( $10^{11} Ω m^{- 1}$ to $10^{19} Ω m^{- 1}$ )

(iii) Semiconductors have intermediate values of resistivity. ( $10^{- 5} Ω m^{- 1}$ to $10^{6} Ω m^{- 1}$ )

(iv) Semiconductors are elemental $(Si, Ge)$ as well as compound ( $GaAs, CdS$ , etc.).

(v) Pure semiconductors are called intrinsic semiconductors. $n_{e} = n_{h}$ for intrinsic semiconductors.

2. Extrinsic semiconductor:

(i) Semiconductors which are doped with some impurities are known as extrinsic semiconductors.

(ii) Extrinsic semiconductors are of two types: $n$ -type and $p$ -type.

(iii) In n-type semiconductors, $n_{e} > > n_{h}$ while in p-type semiconductors $n_{h} > > n_{e}$

(iv) n-type semiconductors Si or Ge is obtained by doping with pentavalent atoms (donors) like $A s, S b, P$ , etc., while $p$ -type $Si$ or $Ge$ can be obtained by doping with trivalent atom (acceptors) like $B, A l, I n$ etc.

(v) $n_{e} n_{h} = n_{i}^{2}$ in all cases. Further, the material possesses an overall charge neutrality.

3. Energy band:

(i) There are two distinct band of energies called valence band and conduction band, in which the electrons in a material lie. 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 $(E_{g})$ between the top of valence band $(E_{V})$ and the bottom of the conduction band $E_{C}$ .

(ii) For insulators $E_{g} > 3 e V$ , for semiconductors $E_{g}$ is $0.2 e V$ to $3 e V$ while for metals $E_{g} \approx 0$ .

4. Diode:

(i) By changing the external applied voltage, junction barrier of p-n junction diode can be changed.

(ii) Diodes along with the help of a capacitor or a suitable filter, can rectify AC voltage to a DC voltage.

(iii) After a certain (breakdown) voltage, the current suddenly increases in a Zener diode. This property has been used to obtain voltage regulation.

(iv) Applications of diodes: Photodiodes, Solar cells, Light emitting diode and Diode Laser, etc.

5. Transistor:

(i) The important transistor parameters for CE-configuration are:
input resistance, $r_{i} = {(\frac{Δ V_{B E}}{Δ I_{B}})}_{V_{C E}}$
output resistance, $r_{o} = {(\frac{Δ V_{C E}}{Δ I_{C}})}_{I_{B}}$
current amplification factor, $β = {(\frac{Δ I_{C}}{Δ I_{B}})}_{V_{C E}}$

(ii) The voltage gain of a transistor amplifier in common emitter configuration is: $A_{v} = β \frac{R_{C}}{R_{B}}$
where $R_{C}$ is resistances in collector and $R_{B}$ is resistances in base sides of the circuit.