BEEIE UNIT IV PPT.ppt
- 1. 1 Mrs.R.DEEBIKA, Assistant Professor/EEE, Kongunadu College of Engineering and Technology 20BE204-BASIC ELECTRICAL, ELECTRONICS AND INSTRUMENTATION ENGINEERING Unit IV Electronic Devices and Circuits
- 2. Unit IV Electronic Devices and Circuits PN Junction – Forward and Reverse Bias – Zener Diode – Bipolar Junction Transistor – Characteristics -Introduction to operational Amplifier: Inverting Amplifier – Non Inverting Amplifier.
- 3. ENERGY BAND STUCTURES AND CONDUCTION IN INSULATORS, SEMICONDUCTORS AND METALS A very poor conductor of electricity is called an insulator; an excellent conductor is a metal; and a material whose conductivity lies between these two extremes is a semiconductor. A material may be classified as one of these three depending upon its energy-band structure.
- 4. ENERGY BAND STUCTURES AND CONDUCTION IN INSULATORS, SEMICONDUCTORS AND METALS
- 5. ENERGY BAND STUCTURES AND CONDUCTION IN INSULATORS, SEMICONDUCTORS AND METALS An INSULATOR is a material having extremely poor electrical conductivity. The forbidden energy gap is large. Hence no electrical conduction is possible. Number of free electrons in an insulator is very small, roughly about 107 electrons/m3.
- 6. ENERGY BAND STUCTURES AND CONDUCTION IN INSULATORS, SEMICONDUCTORS AND METALS The conduction in METALS is only due to the electrons. A metal has overlapping valence and conduction bands. VB is only partially filled and the CB extends beyond the upper end of filled valence band.
- 7. Classification of Semiconductor 1. Intrinsic Semiconductor(Pure) 2. Extrinsic Semiconductor(Impure) Intrinsic Semiconductor A semiconductor in an extremely pure form is known as Intrinsic semiconductor. The silicon(Si) and germanium(Ge) are two important intrinsic semiconductors. A missing electron in the valence band leaves a vacant space there, which is known as a hole. Holes also contribute to electric current.
- 8. In an intrinsic semiconductor, even at room temperature, electron- hole pair are created. When an electric field is applied across an intrinsic semiconductor, the current conduction takes place due to free electrons and holes.
- 9. Extrinsic Semiconductor The current conduction capability of intrinsic semiconductor should be increased and this can achieved by adding a small amount of impurity.
- 10. This can be achieved by adding a small amount of impurity to the intrinsic semiconductor, so that it becomes impure or extrinsic semiconductor. This process of adding impurity is known as doping. Depending upon the type of impurity added, extrinsic semiconductors can be divided in to two types. 1. N – type semiconductor. 2. P – type semiconductor. N – Type Semiconductor A small amount of pentavalent impurities such as arsenic, antimony or phosphorus is added to the pure semiconductor (germanium or silicon crystal) to get N-type semiconductor.
- 11. With the addition of pentavalent impurity a large number of free electrons are made available in the conduction band thereby increasing the conductivity of N – type semiconductor. As a result of doping, the number of free electrons far exceeds the number of holes in N – type semiconductor.
- 12. P-Type semiconductor A small amount of trivalent impurity such as aluminium or boron is added to the pure semiconductor to get the p- type semiconductor. The addition of trivalent impurity a large number of holes are made available in the valence band. These positively charged holes increase the conductivity of P-type semiconductor.
- 13. As the number of holes is very much greater than the number of free electrons in a P-type material, holes are termed as majority carriers and electrons as minority carriers. Conductivity of Semiconductor In a pure semiconductor, the number of holes is equal to the number of electrons. Thermal agitation continues to produce new electron hole pairs and the electron hole pair disappears because of recombination. With each electron-hole pair created, two charge carrying particles are formed. One is negative which is free electron with mobility. The other is positive i.e., the hole with mobility.
- 14. PN Junction Diode In a piece of semiconductor material, if one half is doped by P-type impurity and the other half is doped by N-type impurity, a PN junction is formed. The plane dividing the two halves or zones is called PN junction. The N-type material has high concentration of free electrons, while P-type material has high concentration of holes. At the junction there is a tendency for the free electrons to diffuse over to the P-side and holes to the N-side. This process is called diffusion. Anode Cathode
- 15. V–I Characteristics of a diode under forward bias The V–I characteristics of a PN unction diode are shown below
- 16. As the forward voltage is increased, for < , the forward current is almost zero (region OA) because the potential barrier prevents the holes from P-region and electrons from N-region to flow across the depletion region in the opposite direction. For > , the potential barrier at the junction completely disappears and hence, the holes cross the junction from P- type to N-type and the electrons cross the junction in the opposite direction, resulting in relatively large current flow in the external circuit. Under Reverse Bias Condition When the negative terminal of the battery is connected to the P-type and positive terminal of the battery is connected to the N-type of the PN junction, the bias applied is known as reverse bias.
- 17. Operation A holes which form the majority carriers of the P-side move towards the negative terminal of the battery and electrons which form the majority carrier of the N-side are attracted towards the positive terminal of the battery. Electrons forming covalent bonds of the semiconductor atoms in the P- and N-type regions may absorb sufficient energy from heat and light to cause breaking of some covalent bonds.
- 18. Under the reverse bias condition, the thermally generated holes in the P-region are attracted towards the negative terminal of the battery and the electrons in the N-region are attracted towards the positive terminal of the battery. The minority carriers, electrons in the P-region and holes in the N- region, wander over to the junction and flow towards their majority carrier side giving rise to a small reverse current. This current is known as reverse saturation current. The V-I characteristics under reverse bias
- 19. The magnitude of reverse saturation current mainly depends upon junction temperature because the major source of minority carriers is thermally broken covalent bonds. The reverse voltage at which the junction breakdown occurs is known as Breakdown Voltage.
- 20. PN Diode Applications Rectifiers in D.C. power supplies. Switch in digital logic circuits used in computers. Clamping network used as D.C. restorer in TV receivers and voltage multipliers. Clipping circuits used as wave shaping circuits used in computers, radars, radio and TV receivers. Demodulation (detector) circuits.
- 21. Zener Diode Equivalent Circuit of Zener Diode Model
- 22. Zener Diode V-I Characteristics of Zener Diode
- 23. V-I Characteristics of Zener Diode Zener diodes are manufactured to have a very low reverse bias breakdown voltage Since the breakdown at the zener voltage is so sharp, these devices are often used in voltage regulators to provide precise voltage references. The actual zener voltage is device dependent. For example, you can buy a 6V zener diode.
- 24. Breakdown in PN Junction Diodes The diode equation predicts that, under reverse bias conditions, a small constant current, the saturation current, k , flows due to minority carriers, which is independent of the magnitude of the bias voltage.
- 25. BIPOLAR JUNCTION TRANSISTOR Introduction A Bipolar Junction Transistor (BJT) is a three terminal semiconductor device in which the operation depends on the interaction of both majority and minority carriers and hence the name bipolar. BJT is used in amplifier and oscillatory circuits, and as a switch in digital circuits. It has wide application in computers, satellites and other modern communication systems.
- 26. CONSTRUCTION The BJT consists of a silicon (or germanium) crystal in which a thin layer of N-type silicon is sandwiched between two layers of P-type silicon. This transistor is referred to as PNP. Two types of BJT are NPN and PNP transistor.
- 27. The three portions of the transistor are Emitter, Base and Collector, shown as E, B and C, respectively. The arrow on the emitter specifies the direction of current flow when the EB junction is forward biased.
- 28. TRANSISTOR BIASING The emitter-base junction is forward biased and collector-base junction is reverse biased. Due to the forward bias on the emitter-base junction an emitter current flows through the base into the collector. Though, the collector-base junction is reverse biased, almost the entire emitter current flows through the collector circuit.
- 29. OPERATION OF NPN TRANSISTOR The forward bias applied to the emitter base junction causes a lot of electrons from the emitter region to crossover to the base region. The base is lightly doped with P-type impurity, the number of holes in the base region is very small and hence the number of electrons that combine with holes in the P-type base region is also very small. Hence a few electrons combine with holes to constitute a base current. The remaining electrons crossover in to collector region to constitute a collector current. Base and collector current summed up i.e.,
- 30. The magnitude of emitter current related by OPERATION OF PNP TRANSISTOR • The forward bias applied to the emitter – base junction of a PNP transistor causes a lot of holes from the emitter region to crossover to the base region as the base is lightly doped with N – types impurity.
- 31. A few holes combined with electrons to constitute a base current and remaining holes crossover in to the collector region to constitute a collector current. Collector and base current summed up i.e., The magnitude of emitter current are related by
- 32. Types of Configuration The transistor has input, output and common to input and output terminal. A transistor can be connected in three configurations. i. CB configuration. ii. CE configuration. iii.CC configuration.
- 33. CB configuration In this configuration, emitter is the input terminal, collector is the output terminal and base is the common terminal. Input Characteristics To determine the input characteristics, the collector-base voltage is kept constant at zero. The emitter current is increased from zero in suitable equal steps by increasing .
- 34. • When collector base voltage is equal to zero and the emitter-base junction is forward biased as shown in the characteristics, the junction behaves as a forward biased diode so that emitter current increases rapidly with small increase in emitter-base voltage. • When collector base voltage is increased keeping emitter base voltage constant, the width of the base region will decrease.
- 35. Output characteristics To determine the output characteristics, the emitter current is kept constant at a suitable value by adjusting the emitter-base voltage. Then collector base voltage is increased in suitable equal steps and the collector current is noted for each value of emitter current.
- 36. Saturation Region Collector Base voltage VCB is negative. CB junction is forward biased and a small change in VCB results in larger variation in collector current. Active Region Collector current is almost constant EB – Forward biased CB- Reverse biased Cut off region IE =0, Both junction are reverse biased
- 37. CE configuration Input characteristics To determine the input characteristics, the collector to emitter voltage is kept constant at zero volt and base current is increased from zero in equal steps by increasing base emitter voltage in the below circuit.
- 38. The value of base emitter voltage is noted for each setting of base current. This procedure is repeated for higher fixed values of and the curves of . are drawn. The input characteristics thus obtained are shown below.
- 39. Output characteristics To determine the output characteristics, the base current is kept constant at a suitable value by adjusting base-emitter voltage, The magnitude of collector-emitter voltage is increased in suitable equal steps from zero and the collector current is noted for each setting
- 40. CC configuration
- 41. Input characteristics To determine the input characteristics, is kept at a suitable fixed value. The base-collector voltage is increased in equal steps and the corresponding increase in is noted.
- 42. Output characteristics The output characteristics are the same as those of the common emitter configuration.
- 43. FIELD EFFECT TRANSISTORS Introduction The FET is a device in which the flow of current through the conducting region is controlled by an electric field. Hence the name Field Effect Transistor (FET). It is also said to be unipolar device. The FET can be classified into two types. a) Junction FET(JFET) b) Metal Oxide Semiconductor FET(MOSFET) or Insulated Gate FET (IGFET)or Metal Oxide Silicon Transistor(MOST). JFET has been classified in to two types 1. N-channel JFET with electrons as the majority carriers. 2. P-Channel JFET with holes as the majority carriers.
- 44. Construction of N-channel JET It has N-type bar which is made of silicon. Ohmic contacts are made at the two ends of the bar are called drain and source. Source (S) This terminal is connected to the negative pole of the battery. Electrons which are the majority carriers in the N-type bar enter the bar through this terminal. Drain (D) This terminal is connected to the positive pole of the battery. The majority carriers leave the bar through this terminal.
- 45. Gate (G) Heavily doped P-type silicon is diffused on both sides of the N- type silicon bar by which PN junctions are formed. These layers are joined together called Gate. Channel The space between gate through which majority carriers pass.
- 46. Basic Construction of JFET
- 47. Circuit Symbols and notations
- 50. i. As is increased from zero, increases along OP, and the rate of increases of with decreases as shown below ii. When becomes maximum. When is increased beyond the length of the pinch-off or saturation region increases. Drain Characteristics
- 51. When is negative and is increased, the gate is maintained at a negative voltage less than the negative cut-off voltage, the reverse voltage across the junction is increased. The drain current is controlled by the electric field that extends into the channel due to reverse biased voltage applied to the gate. Hence, this device known as Field Effect Transistor.
- 52. MOSFET
- 53. MOSFET
- 54. Enhancement MOSFET Construction The construction of an N-channel enhancement MOSFET is shown below
- 55. Two highly doped regions are diffused in a lightly doped substrate of P-type silicon substrate. One region is called the source S and the other one is called the drain D. The metal area of the gate, in conjunction with the insulating oxide layer of and the semiconductor channel forms a parallel plate capacitor. This device is called the insulated gate FET. Operation If the substrate is grounded and a positive voltage is applied at the gate, the positive charge on G induces an equal negative charge on the substrate side between the source and drain regions. The direction of the electric field is perpendicular to the plates of the capacitor through the oxide.
- 56. The negative charge of electrons which are minority carriers in the P-type substrate forms an inversion layer. The drain current is enhanced by the positive gate voltage as shown
- 57. Depletion MOSFET The construction of an N-channel depletion MOSFET and circuit symbol for an N-channel and a P-channel depletion MOSFET are shown below
- 58. The drain D at a positive potential with respect to the source, the electrons (majority carriers) flow through the N-channel from S to D. The introduction of the positive charge causes depletion of mobile electrons in the channel. Thus a depletion region is produced in the channel.
- 59. The depletion MOSFET may also be operated in an enhancement mode. It is only necessary to apply a positive gate voltage so that negative charges are induced into the N-type channel. As the depletion MOSFET can be operated with bipolar input signals irrespective of doping of the channel, it is also called as dual mode MOSFET.
- 61. Circuit Symbol
- 62. Equivalent circuit of OP-Amp
- 63. Characteristics of Ideal OP-Amp
- 64. Block Diagram of Operational Amplifier
- 65. Operational Amplifier IC 741
- 69. Non - Inverting Amplifier
- 70. Digital to Analog Converters
- 71. Digital to Analog Converters
- 72. Binary Weighted Resistor DAC
- 73. Binary Weighted Resistor DAC
- 74. R – 2R Ladder
- 75. R – 2R Ladder
- 76. R – 2R Ladder
- 77. Equivalent circuit of practical op-Amp