Electronic Devices-Revision Notes

 CBSE Class 12 Physics

Revision Notes
Chapter-14
Electronic Devices

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• Intrinsic Semiconductor: The pure semiconductors in which the electrical conductivity is totally governed by the electrons excited from the valence band to the conduction band and in which no impurity atoms are added to increase their conductivity are called intrinsic semiconductors and their conductivity is called intrinsic conductivity. Electrical conduction in pure semiconductors occurs by means of electron-hole pairs. In an intrinsic semiconductor-
   ne = nh = ni
  where ne = the free electron density in conduction band, nh = the hole density in valence band, and ni = the intrinsic carrier concentration.
• Extrinsic Semiconductors: A Semiconductor doped with suitable impurity atoms so as to increase its conductivity is called an extrinsic semiconductor.
• Types of Extrinsic Semiconductors: Extrinsic semiconductors are of two types:

1. n-type semiconductors
2. p-type semiconductors

• n-type semiconductors: The pentavalent impurity atoms are called donors because they donate electrons to the host crystal and the semiconductor doped with donors is called n-type semiconductor. In n-type semiconductors, electrons are the majority charge carriers and holes are the minority charge carriers. Thus,
  ne>>nh$n_e>>n_h$
• p-type semiconductors: The trivalent impurity atoms are called acceptors because they create holes that can accept electrons from the nearby bonds. A semiconductor doped with acceptor type impurities is called a p-type semiconductor. In p-type semiconductor, holes are the majority carriers and electrons are the minority charge carriers Thus,nh>>ne$n_h>>n_e$

  

• Holes: The vacancy or absence of an electron in the bond of a covalently bonded crystal is called a hole. A hole serves as a positive charge carrier.
• Mobility: The drift velocity acquired by a charge carrier in a unit electric field is called its electrical mobility and is denoted by (μ=vdE)$\left(\mu =\frac{v_d}{E}\right)$.  The mobility of an electron in the conduction band is greater than that of the hole (or electron) in the valence band.
• If a potential difference V is applied across a conductor of length L and area of cross-section A, then the total current I through it is given by-
  I=eA(neve+nhvh)$I=eA(n_e{v}_e+n_h{v}_h)$ where ne and nh are the electron and hole densities, and ve and vh are their drift velocities, respectively.
• If μe${\mu }_e$ and μh${\mu }_h$ are the electron and hole mobilities, then the conductivity of the semiconductor will be, ρ=e(neμe+nhμh)$\rho =e(n_e{\mu }_e+n_h{\mu }_h)$ .
• Forward Biasing of a pn-junction: If the positive terminal of a battery is connected to the p-side and the negative terminal to the n-side, then the pn-junction is said to be forward-biased. Both electrons and holes move towards the junction. A current, called forward current, flows across the junction. Thus a pn-junction offers a low resistance when it is forward biased.
• Reverse Biasing of a pn-junction: If the positive terminal of a battery is connected to the n-side and negative terminal to the p-side, then pn-junction is said to be reverse biased. The majority charge carriers move away from the junction. The potential barrier offers high resistance during the reverse bias. However, due to the minority charge carriers a small current, called reverse or leakage current flows in the opposite direction. Thus junction diode has almost a unidirectional flow of current.
  
• Action of a transistor: When the emitter-base junction of an npn-transistor is forward biased, the electrons are pushed towards the base. As the base region is very thin and lightly doped, most of the electrons cross over to the reverse biased collector. Since few electrons and holes always recombine in the base region, so the collector current Ic is always slightly less then emitter current IE.
  IE = IC + IB
  where IB is the base current
• Three Configurations of a Transistor: A transistor can be used in one of the following three configurations:-

1. Common-base (CB) circuit
2. Common-emitter (CE) circuit
3. Common-collector (CC) circuit

• Current Gains of a Transistor: Usually low current gains are defined:-

1. Common base current amplification factor or ac current gain α$\alpha$: It is the ratio of the small change in the collector current to the small change in the emitter current when the collector-base voltage is kept constant.

   α=[δICδIE]VCB=constant$\alpha ={\left[\frac{\delta I_C}{\delta I_E}\right]}_{V_{CB=\text{constant}}}$

2. Common emitter current amplification factor or ac current gain β$\beta$ : It is the ratio of the small change in the collector current to the small change in the base current when the collector emitter voltage is kept constant.

          β=[δICδIB]VCE=constant$\beta ={\left[\frac{\delta I_C}{\delta I_B}\right]}_{V_{CE=\text{constant}}}$

• Relations between α$\alpha$ and β$\beta$: The current gains α$\alpha$  and β$\beta$ are related as-
   β=α1−α$\beta =\frac{\alpha }{1-\alpha }$
• Transistor as an amplifier: An amplifier is a circuit that is used for increasing the voltage, current or power of alternating form. A transistor can be used as an amplifier. Its circuit diagram is shown below:
  
• AC Current Gain: AC current gain is defined as-
  βac or Ai=[δICδIB]VCE= constant${\beta }_{ac}\text{ or }{\text{A}}_i={\left[\frac{\delta I_C}{\delta I_B}\right]}_{V_{CE=\text{ constant}}}$
• DC Current Gain: DC current gain is defined as -
  βdc =[ICIB]VCE= constant${\beta }_{dc}\text{ =}{\left[\frac{I_C}{I_B}\right]}_{V_{CE=\text{ constant}}}$
• Voltage Gain of an Amplifier: It is defined as - 
  Av=δV0δVi= A small change in output voltageA small change in input voltage$A_v=\frac{{\delta V}_0}{{\delta V}_i}=\frac{\text{ A small change in output voltage}}{\text{A small change in input voltage}}$
  i.e. Voltage gain = Current gain X Resistance gain.
• Power Gain of an Amplifier: It is defined as the ratio of the change in the output power to the change in the input power.
• Logic Gate: A logic gate is a digital circuit that has one or more inputs but only one output. It follows a logical relationship between input and output voltage.
• Truth Table: This table shows all possible input combinations and the corresponding output for a logic gate.
• Boolean Expression: It is a shorthand method of describing the function of a logic gate in the form of an equation or an expression. It also relates all possible combinations of the inputs of a logic gate to the corresponding output.
• Positive and Negative Logic: If in a system, the higher voltage level represents 1 and the lower voltage level represents 0, the system is called a positive logic. If the higher voltage represents 0 and the lower voltage level represents 1, then the system is called a negative logic.
• OR Gate: An OR gate can have any number of inputs but only one output. It gives higher output (1) if either input A or B or both are high (1), otherwise, the output is low (0). A + B = Y
  which is read as ‘A or B equals Y’.
• AND gate: An AND gate can have any number of inputs but only one output. It gives a high output (1) if inputs A and B are both high (1), or else the output is low (0). It is described by the Boolean expression- A . B = Y
  which is read as ‘A and B equals Y’.
• NOT Gate: A NOT gate is the simplest gate, with one input and one output. It gives as high output (1) if the input A is low (0), and vice versa. Whatever the input is, the NOT gate inverts it. It is described by the Boolean expression:
  A-=Y$\stackrel{\text{-}}{\text{A}}=Y$
  which is read as ‘not A equals Y’.
• NAND (NOT + AND) gate: It is obtained by connecting the output of an AND gate to the input of a NOT gate. Its output is high if both inputs A and B are not high. If is described by the Boolean expression-
  A.B¯¯¯¯¯¯¯¯¯¯=Y or AB¯¯¯¯¯¯¯¯=Y$\overline{A.B}=Y\text{ or }\overline{AB}=Y$
  which is read as ‘A AND B negated equals Y’.
• NOR (NOT + OR) Gate: It is obtained by connecting the output of an OR gate to the input of a NOT gate. Its output is high if neither input A nor input B is high. It is described by the Boolean expression:
  A+B¯¯¯¯¯¯¯¯¯¯¯¯¯=Y$\overline{A+B}=Y$
  which is read as ‘A OR B negated equals Y’.
• XOR or Exclusive OR gate. The XOR gate gives a high output if either input A or B is high but not when both A and B are high or low. It can be obtained by using a combination of two NOT gates, two AND gates, and one OR gate. It is described by Boolean expression:
  Y=A¯.B+A.B¯$Y=\overline{A}.B+A.\overline{B}$
• Integrated Circuits: The concept of fabricating an entire circuit (consisting of many passive components like R and C and active devices like diode and transistor) on a small single block (or chip) of a semiconductor has revolutionized the electronics technology. Such a circuit is known as Integrated Circuit (IC).