# AP Inter 2nd Year Physics Notes Chapter 15 Semiconductor Electronics: Material, Devices and Simple Circuits

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## AP Inter 2nd Year Physics Notes 15th Lesson Semiconductor Electronics: Material, Devices and Simple Circuits

→ The group of closely lying energy levels corresponding to an orbital is called an energy band.

→ The band that contains valence electrons or the highest completely filled band or partially filled band of a solid is called valence band.

→ The lowest unfilled or empty band of a solid is called conduction band.

→ The energy gap between the valence and conduction bands i.e., the difference between the lowest energy of the conduction band and the highest energy of the valence band is called forbidden band or forbidden gap or energy gap.

→ If the conduction and valence bands overlap, the solid is called conductor.

→ If the conduction and valence bands are separated by an energy gap of greater than 5 eV or the forbidden bandwidth is greater than 5 eV, the solid is an insulator. → If the forbidden band width is about 1 eV, the solid is a semiconductor.

→ A vacancy for an electron in a covalent bond is called hole. It is supposed to have positive charge. When an electric field is applied it moves in the opposite direction of an electron. Holes are in valence band. Holes move in the direction of electric field and electrons move in the opposite direction of the electric field.

→ In an intrinsic semiconductor, the hole concentration i.e., number of holes per unit volume (np) is equal to the conduction electron concentration i.e., number of conduction electrons per unit volume (ne).

→ When a semiconductor contains an impurity then the semiconductor is called an extrinsic semiconductor. It increases the conductivity of a semiconductor.

→ Extrinsic semiconductors are of two types : (i) n – type semiconductor and (ii) p – type semiconductor.

→ When a pentavalent impurity is added to Silicon or Germanium an n – type semiconductor is formed. The pentavalent impurities used are Phosphorus, Arsenic and Antimony.

→ A pentavalent impurity produces conduction electrons whose energy levels are just below the conduction band. These energy levels are called donor levels. The pentavalent impurity is called donor impurity.

→ In an n – type semiconductor, electrons are majority charge carriers and holes are minority charge carriers.

→ When a trivalent impurity is added to Silicon or Germanium a p – type semiconductor is formed. The trivalent impurities used .are Boron, Aluminium, Indium and Gallium.

→ A trivalent impurity produces holes whose energy levels are just above the valence band. These energy levels are called acceptor levels. The trivalent impurity is called acceptor impurity.

→ In a P – type semiconductor, holes are majority charge carriers and electrons are minority charge carriers. → Depletion layer is The region near the junction of a p – n junction diode in which there are neither holes nor conduction electrons. It is of width about 1 μm.

→ Barrier Potential is the potential difference developed across the junction of a p -n junction diode due to the electric field in the depletion layer.

→ When a diode is forward-biased, its depletion layer width, potential barrier height and resistance decrease. The current flow is due to the diffusion current of majority carriers. It is of the order of mA.

→ When a diode is reverse – biased, its depletion layer width, potential barrier height and resistance increase. The current flow is due to the drift current of minority carriers. It is of the order of mA for germanium and nA for silicon,

→ Rectifier is a device that converts ac into dc. The maximum efficiency of a half wave rectifier is 40.6%.

→ In a full wave rectifier, the output current which is dc flows during both the halves of the input ac. The maximum efficiency of a full wave rectifier is 81.2%.

→ The voltage at which break down of a p – n junction diode occurs or the voltage at which the current suddenly increases to a large value in a reverse – biased p – n junction diode is called break down voltage or Zener voltage (Vz).

→ The break down in a diode occurs in two different ways : Avalanche break down, Zener break down.

→ Zener diode is a special type of diode that allows current in the forward direction like a p – n junction diode but also in the reverse direction if the voltage is larger than the rated break down voltage or Zener voltage without any damage. Zener diode is used as a voltage regulator.

→ A transistor is a three terminal device. Its three parts are

1. emitter (E)
2. base (B) and
3. collector (C)

The emitter of a transistor is heavily doped and is of intermediate size.
The base of a transistor is least doped and is of least size.
The collector of a transistor is of intermediate doping level and is of largest size.

→ The two types of a transistors are

1. n – p – n transistor and
2. p – n – p transistor.

The three configurations of a transistor are

1. common – emitter (CE) configuration
2. common – base (CB) configuration and
3. common – collector (CC) configuration. → In the normal operation of a transistor, the emitter – base junction is forward – biased and the collector – base junction is reverse – biased.

→ The current flows inside a transistor or a diode is due to both holes and electrons but in the external circuit the current flow is due to electrons.

→ The common-enmitter current gain β is the ratio of the collector current to the base current.
It is between 20 and 100.
A transistor is used as an amplifier

→ Logie- gates are of different types-depending upon the logical function performed by the gate The- differed types are

1. AND gate.
2. OR gate,
3. NOT gate
4. NOR gate and
5. NAND gate.

→ AND gate is a logic gate whose output is high only when both the inputs are high.

→ OR gate is a logic gate whose output is high when any one of the two inputs is high or both the inputs high.

→ NOT gate is a logic gate whose output is an inversion of its input.

→ NOR gate = OR gate + NOT gate i.e.„ OR gate followed by NOT gate is NOR gate.

→ Logic gate is a digital circuit which works according to some logical relationship between input and output voltages.

Formulae

→ Dynamic resistance (rs) = $$\frac{\Delta V}{\Delta I}$$

→ Efficiency of the rectifier (η) = $$\frac{P_{\text {dc }}}{P_{\text {ac }}} \times 100$$

→ Efficiency of Half-wave rectifier (η) = $$\frac{0.406 \mathrm{R}_{\mathrm{L}}}{\mathrm{r}_{\mathrm{f}}+\mathrm{R}_{\mathrm{L}}}$$

→ Efficiency of full-wave rectifier (η) = $$\frac{0.812\mathrm{R}_{\mathrm{L}}}{\mathrm{r}_{\mathrm{f}}+\mathrm{R}_{\mathrm{L}}}$$

→ IE = IB + IC

→ Input resistance (ri) = → Output resistance (r0) =  → Current gain (βdc) = $$\frac{\Delta \mathrm{I}_{\mathrm{C}}}{\Delta \mathrm{I}_{\mathrm{B}}}$$

→ Voltage gain (A0) = $$\frac{V_0}{V_i}$$ $$\frac{\Delta V_{C E}}{\Delta V_{B E}}=\beta \times \frac{R_{\mathrm{I}}}{R_i}$$

→ Power gain (Ap) = Current gain × Voltage gain

→ β = $$\frac{\boldsymbol{\alpha}}{1-\boldsymbol{\alpha}}$$

→ Boolean expressions

1. OR gate, X = A + B
2. AND gate, X = A.B
3. NOT gate, X = $$\overline{\mathrm{A}}$$
4. NOR gate, X = $$\overline{\mathrm{A+B}}$$
5. NAND gate, X = $$\overline{\text { A.B }}$$
6. X – OR gate, X = A ⊕ B = $$\mathrm{A} \overline{\mathrm{B}}+\overline{\mathrm{A}} \mathrm{B}$$