SYED AMMAL ENGINEERING COLLEGE (Approved by the AICTE, New Delhi, Govt. of Tamilnadu and Affiliated to Anna University, Chennai) Established in 1998 - An ISO 9001:2008 Certified Institution Dr. E.M.Abdullah Campus, Lanthai, Ramanathapuram 623 502. Phone: 304000, 304222, 304101 (04567) Web: www.syedengg.ac.in, E.mail:saec@syedengg.ac.in DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING 1. What is Semiconductor? EC6201- ELECTRONIC DEVICES 2 MARKS QUESTIONS UNIT-I SEMICONDUCTOR DIODE The materials, whose electrical properties lie between those of conductors and insulators are known as Semiconductor. 2. What is energy level diagram? The each orbit has fixed amount of energy associated with energy levels. The electrons moving in a particular orbit possess the energy of the orbit. The convenient way of representing the energy of different orbit. It is known as Energy Level Diagram. 3. What are the classification of Semiconductors? The Semiconductors are classified into two types, i) Intrinsic Semiconductor ii) Extrinsic Semiconductor 4. What is doping? Doping. The process of adding impurity atoms to the intrinsic Semiconductor is called 5. What are the classification of Extrinsic Semiconductor? Extrinsic Semiconductors are classified into two types, i. N - type Semiconductor, ii. P - type Semiconductor. 6. What is N - type Semiconductor? A small amount of pentavalent impurity is added to a pure semiconductor is known as N - type Semiconductor.
7. What is P - type Semiconductor? A small amount of trivalent impurities is added to a pure Semiconductor is known as P - type Semiconductor. 8. Define Mass Action Law? The product of the number of holes and the number of electrons is constant and is independent of the amount of donor and accepts impurity doping under thermal equilibrium. This relation is known as Mass Action Law. Where, n. p = n i 2 n = Free electron Concentration. p = hole Concentration. n i = Intrinsic Concentration. 9. Define Drift Current? When an electric field is applied across the semiconductor material, the charge carriers attain drift velocity v d, which is equal to the product of the mobility of the charge carriers and the applied electric field intensity E. The holes moves towards the negative terminal of the battery and the electrons move towards the positive terminal of the battery. This combined effect of the movement of the charge carriers constitutes a current known as Drift Current. 10. Define Diffusion Current? An electric current can flow in a Semiconductor even in the absence of the applied voltage provided a concentration gradient exists in the material. A concentration gradient is said to exist if the number of either electrons or holes is greater in one region of the semiconductor as compared to the rest of the region. I a semiconductor material, the charge carriers have the tendency to move from the region of higher concentration to that of lower concentration of the same type of charge carriers. This movement of charge carriers takes places resulting in a current called Diffusion Current. 11. Write down the Einstein relationship equation? The equation which relates mobility µ and diffusion coefficient D is called Einstein relationship. (D p / µ p ) = (D n / µ n ) = V T = (T / 11,600) Where, K = Boltzmann's constant.
T = Temperature in o K. q = Magnitude of electric charge. D p = Diffusion coefficient for P type material. µ p = Mobility for P type material. 12. Define diffusion length The average distance that a charge carrier can diffuse during its lifetime is called Diffusion length L. L = D τ Where, τ is called Carrier Life Time. 13. Define Carrier Life Time The time for which, on an average, a charge carriers will exist before recombination with a carrier of opposite charge is called Carrier Life Time. 14. Define P - N junction When a P - type Semiconductor is joined to a N - type Semiconductor the contact surface is called PN junction or PN diode. 15. Write down the diode current equation? The Diode current equation relating the voltage V and current I is given by I = I 0 [e (V / ηv T ) - 1) 16. What is Dynamic Resistance? The Dynamic Resistance of a diode is defined as the reciprocal of the slope of the current - voltage characteristics. It is given by, r = Change in voltage / Resulting change in current r = ( V) / ( I) 17. What is Reverse resistance? The reverse resistance offered by the PN junction diode under reverse bias condition. It is very large compared to the forward resistance. Reverse resistance is in the
range of MΩ. 18. Define Transition Capacitance? An applied reverse bias causes the majority carriers to move away from the junction, thereby uncovering the immobile charges. As this reverse voltage is increased, the thickness of the space - charge region at the junction increases. This further increases the number of uncovered charge and result in capacitive effects. This effect is defined by an incremental capacitance, C T, given by. This capacitance which is due depletion layer is known as Depletion Capacitance (or) Transition Capacitance can be expressed as, C T = dq / dv 19. Define Diffusion Capacitance? The diode is biased in the forward direction, the potential barrier at the junction is lowered and diffusion of charge carriers take place. Hence the electrons which enter the p - side from n- side and holes which enter the n - side from the p - side constitute minority carriers and this process is called Minority carrier injection. Hence, we introduce an incremental capacitance which is defined as the rate of charge of injected charge with applied voltage. This capacitance is called Diffusion (or) Storage Capacitance C D. 20. What are the PN diode applications? Rectifiers in d.c power supplier. Clamping circuits used as d.c restorer in TV receivers and voltage multipliers. PN diode in demodulation (detector) circuits. Switch in digital logic circuits used in computers. Clipping circuits used as wave shaping circuits used in TV receivers, radio, computers and radars. Zener diode in voltage regulators. Varactor diodes in tuning sections of radio and TV receivers. Detectors in optical communication circuits.
Tunnel diodes as a relaxation oscillator at microwave frequency. Light emitting diodes in digital displays. UNIT II BIPOLAR JUNCTION 21. What is Transistor? Transistor consists of two junctions formed by sandwiching either P - type or N - type Semiconductor between a pair of opposite types. 22. What are the three terminals in a BJT? Emitter (E) Collector (C) Base (B) 23. What is Biasing? Applying external voltage to a transistor is called Biasing. In order to operate transistor properly as an amplifier, it is necessary to correctly bias the two pn junction with external voltages. 24. What are the types of transistor configuration? There are three types of configurations for the operation of a transistor i. Common Base (CB) Configuration ii. Common Emitter (CE) Configuration iii. Common Collector (CC) Configuration 25. What is Early Effect? (Refer page NO. 56 of chapter3) 26. Write the current amplification factor for a CB transistor? (Refer page NO.58 of chapter3) 27. Write the relationship between α and β? α = β / (1+β) (or) β = α / (1-α) 28. Compare Transistor configurations. (Refer page NO. 60 of chapter3)
29. Which is the most commonly used transistor configuration? Why? The CE Configuration is most commonly used. The reasons are i. High current gain ii. iii. iv. High voltage gain High power gain Moderate input to output ratio 30. Explain about Reach Through or Punch Through. (Refer page NO. of chapter3) 31. Define Delay Time (t d ) The time delayed for the collector current to flow from the application of the input pulse is termed as delay time t d. 32. Define Rise Time (t r ) The time taken to reach from 10% to 90% of the maximum level is termed as rise time t r. 33. Define turn - on time (t on ) The turn - on time t on is the addition of t r and t d. t on = t r + t d 34. Define Storage time (t s ) The time taken for the current to rise from 10% to 90% of I CS is called Rise time (t r ). Therefore, total turn - on time t on = t d +t r. Thereafter, the current increases and remains a constant a maximum value of i c (i.e) I CS. The input waveform now returns to its initial state at t = T, but the current i c fails to respond immediately. The interval which elapses between the transition of the input waveform and the time when i c has dropped to 90% of I CS is called Storage time (t s ). 35. Describe how Switching are achieved by a BJT? Cut - Off region and saturation region of output characteristics are used for
switching in BJT. When it is saturated, it acts as a closed switch and BJT is operating in Cut Off region, it acts as a on open switch. 36. What is FET? UNIT-III FIELD EFFECT TRANSISTORS The Field Effect Transistor (FET) is a Semiconductor device which depends for its operation on the control of current by an Electric field. The output characteristics of FET are controlled by Input voltage and not by the Input current, so it is also known as Voltage - Controlled Device. 37. What are the features of FET? The FET has several advantages over the Conventional Transistor. 38. What are the types of FET? Its operation depends upon the flow of majority carrier only. So it is called Unipolar Device. It is relatively immune to radiation. It exhibits a high input resistance, typically many mega ohms. It is less noisy than a tube of a Bipolar Transistor. It exhibits no offset voltage at Zero Drain Current. It has thermal stability. There are two basic types of FET 39. Define Pinch Off region. i. Junction Field Effect Transistor (JFET) ii. Metal Oxide Semiconductor FET (MOSFET) (or) Insulated Gate Field Effect Transistor (IGFET). When the gate to source voltage (V GS ) is increased, further, a stage is reached at which two depletion regions touch each other. It is called as "Pinch Off Region". This reduces the drain current to zero. 40. Define Pinch Off Voltage.
At the drain to source voltage corresponding to point B, the channel width is reduced to a minimum value and is known as "Pinch - Off". The drain to source voltage at which the channel pinch off occurs is known as Pinch Off Voltage (V P ). 41. List the characteristics parameters of JFET. The parameters of JFET are i. Transconductance, ii. iii. iv. Drain resistance, Drain conductance, Amplification factor. 42. Define Transconductance. Transconductance is also called as Forward Transconductance (or) Forward Transmittance. If both the gate and drain voltages are varied, the change in drain current is given approximately. 43. Define Amplification factor. g m = ( i D ) / ( V GS ) = (i d ) / (V gs ), Where V DS constant. It is given by the ratio of small change in drain to source voltage ( V DS ) to the corresponding change in gate to source voltage ( V GS ) for a constant drain current (I D ). 44. Compare FET and BJT. (Refer page NO. of chapter4) 45. What are the application of JFET? (Refer page NO. of chapter4) 46. What is MOSFET? µ = ( V DS ) / ( V GS ), Where I D = Constant MOSFET is a three Terminal Device that terminal are source, gate and drain. The gate of a MOSFET is insulated from the channel. Because of this, the MOSFET is also known as an IGFET (Insulated gate FET). 47. What are the types of MOSFET? There are two types of MOSFET. a. Depletion - type MOSFET.
b. Enhance - type MOSFET. 48. What are the two modes of MOSFET? The two modes of MOSFET are i. Depletion mode, ii. Enhancement mode 49. What is the other name for MOSFET? Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is also called as Insulated Gate Field Effect Transistor(IGFET). 50. Compare JFET and MOSFET S.NO JFET (JUNCTION FIELD EFFECT TRANSISTOR) MOSFET (METAL OXIDE SEMICONDUCTOR FET) 1. JFET's can only be operated in depletion mode. 2. Even though the gate is reverse biased, the gate current is forge. MOSFET's can be operated in depletion or in enhancement mode. The gate current is very small. 3. JFET's are difficult to manufacture. They are very easy to manufacture. 4. The input resistance is large. The input resistance is very high that JFETS (10 15 ). 5. The drain resistance is high. The drain resistance is low. 6. It is not easy to fabricate. It is easy to fabricate. 7. Due to manufacture problems they are less widely used in IC's. They are most widely used in IC's. 51. Compare N - channel and P - channel MOSFETs. S.NO N CHANNEL MOSFET P CHANNEL MOSFET 1. N - channel devices are the electrons, which have a mobility of 1300 cm 2 / vs. P - channel devices are the holes which have a mobility of 500 cm 2 / vs. 2. Larger size. Smaller size.
3. It is not easy to fabricate. It is easy to fabricate. 4. ON resistance is Low. ON resistance is High. 5. The Electric current in N - channel MOSFET is Larger. The Electric current in P - channel MOSFET is smaller. 6. It has lower packing density. It has higher packing density. 7. Higher switching speed. Lower switching speed. 52. Write the use of JFET as a voltage variable resistor? One of the application of voltage variable resistor (JFET) is to vary the gain of a multi stage amplifier, as the signal level is increased. This action is called Automatic Gain Control (AGC). 53. What is Zener Breakdown? UNIT-IV SPECIAL SEMICONDUCTOR DEVICES The Breakdown using heavily doped P and N regions direct rupture of covalent bonds takes place. Because of the strong electric field at the junction of PN diode. The new electron - hole pairs so created increases the reverse current in a reverse biased PN diode. 54. What is Avalanche Breakdown? The Normally doped ON diode transition region is thick. This type of PN diode for reverse bias in excess of 6V. The avalanche breakdown is process of avalanche multiplication. In reverse biased diode, a thermally generated carriers falls down the junction barrier and acquires energy from the applied voltage. The sufficient kinetic energy to distrupt a covalent bond, at this carrier collides with crystal. This is known as Impact Ionization. 55. What is Zener Diode? A Zener Diode is a special purpose diode that is operated in reverse - biased condition. Its operation depends on the Zener Breakdown Phenomenon. 56. What are the applications of Zener Diode? Voltage regulator.
Fixing reference voltage in electronic circuits such as power supplies and transistor biasing. Clippers in wave - shaping circuits. Square wave generation. 57. What are the breakdown in Zener junction diode? The diode breakdown may occur in two methods: i. Zener Breakdown, ii. Avalanche Breakdown. 58. What is varactor diode? A varactor diode is a specially manufactures reverse biased PN junction diode with a suitable impurity concentration profile. It is also called as varicap or voltacap. It is used as a variable reactance capacitance. 59. What are the applications of varactor diode? The main application of this diode is as a tuning devices in receivers. It is used in high frequency. It is used in adjustable band - pass filter. It is used in FM modulation. It is used in automatic frequency control devices. It is used in parametric amplifier. 60. What are the application of Tunnel diode? Tunnel diode is used as, Ultra - High speed switch. In relaxation oscillator. As an amplifier. As logic memory storage device. As microwave oscillator. 61. Compare Tunnel diode and P - N junction diode.
S.NO TUNNEL DIODE p - n JUNCTION DIODE 1. Doping levels at p and n sides are very high. 2. Tunnelling current consists of majority carriers - electrons from n-side to the p-side. 3. Majority carriers current responds much faster to voltage changes - suitable to microwaves. 4. At a small values of reverse voltage a large current flows due to considerable overlap between conduction band and valence band useful as frequency converter. 5. Shows negative resistance characteristics - useful for reflection amplifiers and oscillators. 6. Preferred semiconductor are Ge and GaAs. Doping is normal in both p and n sides. Current consists of minority carriersholes from p-side to the n-side. Majority carrier current does not responds so fast to voltage changes - suitable for low frequency application only. Current is extremely small (leakage current) up to considerable reverse bias voltage and then increases abruptly to extremely high at a particular voltage called Breakdown voltage. Does not shows negative resistance - useful for detector and mixers. Preferred semiconductor are Ge and Si. 7. It is Low noise device. Moderate noise characteristics. 8. Low power device. Lower power device. 62. What is LASER? LASER is similar to that of a SCR except the light triggering. It has a window and lens which focuses light on the gate junction area. 63. Give the applications of LASER. Optical light control. Phase control. In relays. Motor control.
64. What is Photodiode? UNIT-V POWER DEVICES AND DISPLAY DEVICES It is a light sensitive device used to convert light signal into electrical signal. It is also called Photodetector. 65. What is Photo transistor? The Photo transistor is a light detector, it combines a photo diode and photo transistor. The photo transistor cannot be directly used in control application. 66. What is Photovoltaic sensor? It is a light - sensitive semiconductor device and it produces a voltage, when the voltage increases and the intensity of light falling on the semiconductor junction of this photovoltaic cell increases. 67. What are the types of LED? a. Surface - emitting LEED. b. Edge - emitting LED. 68. What are the applications of LED? Used for numeric display in pocket calculators. Used in the field of optical communication. Used in burglar - alarm system. Used for applying input power to lasers. Used for entering information into optical computer memories. Used for solid video displays. Used in image sensing circuits. 69. What are the applications of LCD? Used as numerical counters for counting production items. Analog quantities like voltage, current etc, can also be displayed as a number on a suitable device. (e.g) Digital multimeter. Used for solid state video displays. Used for image sensing circuits.
70. Compare LCD and LED. Used for numerical display in pocket calculators. S.NO LCD LED 1. LCD is a liquid unit which modulates the light to have display. LED is a light emitted diode which emits light during forward biasing. 2. LCD is a passive display device. LED is an active display device. 3. Cost is very Low. Cost is High. 4. Suitable for large area display. Not suitable for large area display. 5. Low power consumption (microwatts). 6. Operating temperature is 10 o C to 47 o C. 7. Response time is in microseconds (10-16 sec). 8. Intensity of light cannot be controlled. 9. Colour displays will not be available at low. 10. Requires an external or internal light sources. High consumption power (milliwatts). Operating temperature is 0 o C to 70 o C. Response time is in Nanoseconds (10-9 sec). Intensity of light can be controlled. Different colour displays are available at low cost. Capable of generating its own light. 11. Life time comparatively. More life time. 12. In displays turn on and turn off time is slow. In displays turn on and turn off time is faster than LCD. 71. Describe the basic structure of SCR? SCR consists of four semiconductor layers forming a PNPN structure. It has three PN junctions namely J 1,J 2 and J 3. There are three terminals called anode (A), cathode (C) and the gate (G). 72. What is Forward Breakover Voltage? SCR is forward bias with a small voltage, it is in 'OFF' and no current flows
through the SCR. The applied forward voltage is increased, a certain critical voltage called Forward Breakover Voltage (V BO ). 73. Define Holding Current? Holding Current is the current below which the SCR switches from the conduction state (ON state) to the Forward blocking state. 74. Give the applications of SCR. application include: Main applications of an SCR is as a power control device. Common areas of As over light detector. Relay control. Regulated power supplies. Static switches. Motor control. Battery charges. Heater controls. Phase controls. For speed control of DC shunt motor. 75. What are the advantages of SCR? 76. What is TRIAC? SCR controls large current in the load by means of a small gate current. SCR size is very compact. Switching speed is High. TRIAC is a three terminal semiconductor switching device which can conduct in either forward or reverse direction. The TRIAC is the combination of two SCR's connected in parallel but in opposite direction. 77. What is the application of TRIAC? Heater control.
Phase control. Light dimming control. Static switch to turn a.c power ON and OFF. Speed control of motor. 78. Compare SCR and TRIAC. S.NO SCR TRIAC 1. Unidirectional device. Bidirectional device. 2. Fast turn OFF time. So it can be used as switch. 3. The gate current can be only in direction to turn ON SCR. 4. Unijunction transistor (UJT) is used for triggering. 5. The application are Phase control, protection of power supplies etc. Slow turn OFF time. For both the direction of gate current, TRIAC conducts. The DIAC is used for triggering. The applications are Phase control, light dimmer, motor control. 79. What is DIAC? A DIAC is two terminal semiconductor device and three layer bidirectional device, which can be switched from of its OFF to ON state for either negative or positive polarity of applied voltage. 80. What is the applications of DIAC? The DIAC is used as a triggering device, it is not a control device. It is used in, Temperature control, Triggering of TRIAC, Light dimming circuits, Motor speed control. 81. What is UJT?
Unijunction transistor is a three terminal semiconductor device consisting of only one PN junction. It differ from ordinary PN diode in the sense that it has three terminals namely Emitter, Base 1, Base 2. 82. What is intrinsic stand OFF ratio of UJT? The intrinsic stand OFF ratio (η) is defined as the ratio between the internal dynamic resistance (R B1 ) and the inner base resistance (R BB ). Where, η = R B1 / R BB. R BB = R B1 + R B2 R B1 = Internal dynamic resistance. R BB = Interbase resistance. 83. What are the applications of UJT? Non sinusoidal oscillator, Timing circuits, Sawtooth generators, Triggering device for SCR and TRIAC, Switching circuits, Voltage regulated supplies.