ELT-114 Electrical Essentials & Networks Page 298 The arrow head of emitter shows the direction of current flow. The arrow head is always situated on the emitter. PNP-Transistor In the figure below the construction of a PNP transistor has been shown. As depicted from the figure, a PNP transistor is constructed with the two P type semiconductor materials, those are separated with the help of a N type base section. The symbol for the PNP transistor has been shown in the figure below. Notice that the arrow head position of the emitter has been reversed. It means that the direction of flow of current has also been reversed than of NPN transistor. Figure below the construction and symbol of PNP transistor. Operation of BJT as Voltage Amplifier: The circuit diagram of BJT voltage amplifier is shown in figure below. The emitter base junction is forward biased and collector base junction is reverse biased. The base current is very small as compared to emitter and collector currents. Because the collector current is equal to the base current multiplied by the current gain, b. Therefor there is an amplification in transistor current. For explanation, let's look at the circuit in figure (a). An AC Voltage, Vin is superimposed on the DC bias voltage VBB by connecting them in
ELT-114 Electrical Essentials & Networks Page 299 series with the bse resistor RB, as shown. The DC bias voltage VCC is connected to the collector through the collector resistor, RC. The AC input voltage produces an AC base current, which results in a much larger AC collector current. The AC collector current produces an AC voltage across RC, thus producing amplified but invested signal. The reproduction of the AC input voltage in the active region of operation is illustrated in figure (b). The input and output wave forms are 180° out of phase. 11.8 Introduction to Integrated Circuits: Normally bipolar junction transistors, diodes and field effect transistors are commonly used electronics component in electronic circuit. These components are interconnected along with required resistors and capacitors to form an electronic circuit. This type of circuit is known as Discrete Circuit as each of the components can be separated from the circuit as when required. Now days there is a new trend of producing electronic circuit where on a semiconductor wafer numbers of diodes, transistors and capacitors etc. are permanently fabricated. As the components in this type of electronic circuit are not separable that is integrated on the semiconductor wafer, this circuit is commonly referred as Integrated Circuit. IC is also popularly known as chip or microchip. This technology was invented in the year of 1950 the by Jack Kilby of Texas Instruments USA and Robert Noyce of Fairchild Semiconductor USA. The first costumer to this new invention was the US Air Force. In the year 2000 Jack Kilby won the Nobel Prize in Physics for miniaturized electronic circuits. One and a half years after Kilby demonstrated his IC design, Robert Noyce of Fairchild Semiconductor Limited came up with his own integrated circuit. His model solved many practical problems which Kilby's device had. It was made up of silicon where as Kilby's was made up of germanium. Jack Kilby and Robert Noyce both received US patents for their part of work on integrated circuits. After several years of legal issues both companies wisely decided to cross license their technology and created a huge global market. 11.9 Types of Integrated Circuits Like electronic circuit, ICs can also be categorized as digital IC and analog IC based on their applications. i. Analog IC In this type of ICs, the input and output both signals are continuous. The output signal level depends upon the input signal level and the output signal level is a linear function of input signal level. Analog ICs are most
ELT-114 Electrical Essentials & Networks Page 300 commonly used as audio frequency amplifier and radio frequency amplifier. Op amps, voltage regulators, comparators and timers are also well-known examples of linear ICs or analog ICs. ii. Digital IC The logic Gates, such as AND gate, OR gate, NAND gate, XOR gate, flip flops, counters; microprocessors are some well-known examples of digital ICs. These ICs operate with binary data such as either 0 or 1. Normally in digital circuit, 0 indicates 0 V and one indicate +5 V. The main components of an IC are transistors. These transistors may be bipolar or field effect depending upon the applications of ICs. As the technology is improving day by day, the number of transistors incorporated in a single IC chip is also increasing. Types of Integration based on no. of Transistors: Depending upon the number of transistors incorporated in a single chip, the ICs are categorized in five groups. Namely; i. Small Scale Integration (SSI) where the number of transistors incorporated in a single IC chip is up to 100. ii. Medium Scale Integration (MSI) where the number of transistors incorporated in a single IC chip is from 100 to 1000. iii. Large Scale Integration (LSI) where the number of transistors incorporated in a single IC chip is from 1000 to 20,000. iv. Very Large Scale Integration (VLSI) where the number of transistors incorporated in a single IC chip is from 20,000 to1000, 000. v. Ultra Large Scale Integration (ULSI) where the number of transistors incorporated in a single chip is from 1000, 000 to 10,000,000. 11.10 Application of Integrated Circuits: Integrated Circuits are used in industry in variety of applications. 1. Logic circuits 2. Flip flops 3. Counters 4. Memory chips 5. Microprocessor chips 6.Micro controller chips 7. Clock chips 8. Calculator chips 9. Digital to Analog Converter Chips 10. Analog to Converter Chips 11. Coders 12 .Encoders 13. Multiplexers 14.Demultiplexers MCQs (Multiple Choice Questions)
ELT-114 Electrical Essentials & Networks Page 301 Q.1: The process by which impurities are added to a pure semiconductor is: (a). Diffusing (b). Drift (c) Doping (d) Mixing Q.2 : A Germanium atom contains. (a). Two electron or bits (b). Three valance electrons (c). Four protons (d). Four valance electron Q3: The type of atom bonding most common in semiconductor is (a). Metallic (b). Ionic(c). Covalent (d). Chemical Q.4: The reverse resistance of an ideal diode is: (a)Low (b) Zero (c) Infinite (d) Very high Q.5 A circuit that removes positive or negative parts of a waveform is called: (a)Clamper (b) Clipper (c) Rectifier (d) Multiplier Q.6: Voltage multipliers are used to produce: (a)Low voltage and low current (b) Low Voltage and High current (c) High voltage and low current (d) High voltage and high current Q.7: Diode acts as a close switch when it is: (a) Not Biased (b) Forward biased (c) Reverse biased (d) none of above Q.8: Pregion of a diode is called _______. (a) Anode (b) Cathode (c) Any of these (d) None of these Q.9: The amount of energy required to produce full conduction across the PN junction is ____. (a) Junction potential (b) Barrier potential (c) Biased potential (d) Diode voltage Q.10 The N region of a diode is called _______. (a) Anode (b) Cathode (c) Barrier potential (d) Biasing Q.11 A semiconductor device with a single PN junction that conducts current in only one direction is ___. (a) Diode (b) Transistor (c) SCR (d) Thyristor Q.12 The boundary between two different types of semi conductive materials is _______. (a) PN junction (b) Diode (c) Triode(d) Tetrode Q.13 The condition in which a diode prevents current is _______. (a) Forward Bias (b) Reverse Bias (c) Conduction (d) Break down Q.14 _______ is a semi conductive material. (a) Silicon (b) Germanium (c) Both a and b (d) neither a nor b
ELT-114 Electrical Essentials & Networks Page 302 Q.15 The most widely used semi conductive material in electronic devices is (a) Germanium (b) Carbon (c) Copper (d) Silicon Q.16 Electron-hole pairs are produced by (a) Recombination (b) Thermal energy (c) Ionization (d) Doping Q.17 In an intrinsic semiconductor _______. (a)No free electrons (b) Free electrons are thermally produced (c) There are only holes (d) as many electrons as holes (e) b and d Q.18 When a diode is forward biased _______. (a) The only current is the hole current (b) The only current is electron current (c) The only current is produced by majority carriers (d) The current is produced both holes & electrons Q.19 For silicon diode, the value of the forward bias voltage typically: (a) > 0.3v (b) > 0.7v (c) Depends on the width of Depletion Region (d) Depends on the Majority Carriers Q.20 When a 60Hz sinusoidal voltage is applied to the input of a half wave rectifier, the output frequency is_______. (a) 120Hz (b) 30Hz (c) 60Hz (d) 0Hz ANSWER KEY Q.1 (c) Q.2 (d) Q.3 (c) Q.4 (c) Q.5 (c) Q.6 (b) Q.7 (c) Q.8 (a) Q.9 (b) Q.10 (b) Q.11 (b) Q.12 (a) Q.13 (a) Q.14 (c) Q.15 (d) Q.16 (b) Q.17 (a) Q.18 (d) Q.19 (b) Q.20 (c) Short Questions 1. Define semiconductor. Name semiconductor materials. 2. Differentiate between intrinsic & extrinsic semiconductor. 3. Define the term doping. 4. Why pentavalent impurities called donor impurities? 5. Why trivalent impurities called accepter impurities? 6. Name pentavalent and trivalent elements. 7. What is meant by a hole? 8. What is the effect of temperature on intrinsic semiconductor? 9. Define PN junction. 10. How a diode is forward biased? 11. How a diode is reverse biased?
ELT-114 Electrical Essentials & Networks Page 303 12. Define depletion region. 13. Define barrier potential. List the values of barrier potential for silicon and germanium diode. 14. List the application of PN junction diode. 15. Define the term rectification. 16. List the types of rectifier. 17. Define multiplier. Enlist its types. 18. In what condition, a diode is used as an open switch? Long Questions 1. Describe the energy band structure of insulator, semiconductor & conductors. 2. Explain the difference between intrinsic and extrinsic semiconductors. 3. Compare P&N type semiconductors. 4. Explain the PN-Junction and its biasing in detail. 5. Explain the construction & working of PN junction diode. 6. Describe the half wave rectifier in detail. 7. Explain the full wave rectifier in detail.