Unit 9a kho tài liệu bách khoa

VERY SHORT AND SHORT-ANSWERS QUESTIONS 10 11 12 13 14 15 16 17 18 19 What happens to the resistance of a semiconductor on heating? What are the charge carriers in semiconductors? Distingusih between intrinsic and extrinsic semiconductors? What is Fermi energy? What are the energy gaps for Ge and Si? What is the value of forbidden energy gap for a conductor? What are p-type semiconductors? What are n-type semiconductors? Which type of impurity should be added to Ge to have a p-type semiconductor? Which type of impurity should be added to Ge to have an n-type semiconductor? Name the minority carriers in an n-type Ge crystal Name the minority carriers in p-type Ge crystal What type of charge carriers are there in an n-type semiconductor? (A.I.S.S.C.E 1992) What you mean by doping of a semiconductor? What is a hole? A n-type semiconductor has a large number of free electrons, still it is neutral Why? What is a junction diode? What you mean by forward and reverse biasing of a junction diode? In which of the two biasings, forward or reverse, does the junction diode offer low resistance to the flow of current? 20 Why is a semiconductor damaged by a large current? S Chand & Company Limited 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 What is potential barrier in a pn junction? What is the magnitude of potential barrier for (a) Ge junction (b) Si junction? What is characterstics curve of a p-n junction? What is a rectifier? What is a Zener diode? Draw the circuit diagram of a junction diode used as a half wave rectifier? Draw the circuit diagram of a junction diode used as a full wave rectifier What is an intrinsic semiconductor? Why does its conductivity increase with rise of temperature? Which process causes depletion region in a pn junction (A.I.S.S.C.E Delhi 1992) Explain the formation of potential barrier in a p-n junction? (A.I.S.S.C.E Delhi 1991) What is depletion region in a p-n junction? (A.I.S.S.C.E Delhi 1992) What type of charge carriers are there in a p-type semiconductor? (A.I.S.S.C.E Delhi 1992) Distinguish between n-type and p-type semiconductors on the basis of energy band diagram (A.I.S.S.C.E 1997) Draw the graph showing the variation of current with voltage for a p-n junction (A.I.S.S.C.E 1997) Which type of biasing results in very high resistance of a pn junction? (A.I.S.S.C.E 1996, C.B.S.E Sample Paper) What are npn and pnp transistors? Name the different components of a transistor In a transistor, base is made thin and is doped very lightly Why? Why is a common-emitter amplifier generally preferred over a common base amplifier? (A.I.S.S.C.E 1997) S Chand & Company Limited 39 Draw the circuit of a common-emitter amplifier 40 Why is the current gain of a common emitter configuration higher than that of a common base configuration? 41 Define transconductance of a transistor 42 When does a diode work as an open switch? 43 Why is the emitter of a transistor forward biased and the collector reverse biased? 44 Define a.c voltage gain of an amplifier 45 Give the relation between power gain, voltage gain and current gain for a transistor amplifier 46 Give the phase relationship between the input and output voltages of a (i) common-base amplifier, D R (ii) common-emitter – 10 V amplifier 47 Is the diode D forward or reversed biased in this diagram? (A.I.S.S.C.E Delhi 1997) Fig 9.1 48 Which type of extrinsic semiconductor is obtained by doping of silicon with indium? 49 How is a p-n-p transistor represented symbolically? 50 Define a.c current gain α ac for a transistor in common-base configuration ANSWERS The resistance of a semiconductor decreases with rise in temperature In semiconductors the charge carriers are electrons and holes A pure semiconductor is called an intrinsic semiconductor A semiconductor doped with a small quantity of trivalent or a pentavalent impurity is called an extrinsic semiconductor S Chand & Company Limited The highest energy level occupied by the electrons in a solid at absolute zero is called the Fermi level and the corresponding energy is called the Fermi energy Ge : 0.7 eV; Si : 1.1 eV Zero If a trivalent impurity is added to a pure semiconductor, the resulting extrinsic semiconductor has excess of holes over electrons It is called a p-type semiconductor If a pentavalent impurity is added to a pure semiconductor, the resulting extrinsic semiconductor has excess of electrons over holes It is called an n-type semiconductor A trivalent impurity, such as aluminium 10 A pentavalent impurity, such as phosphorous 11 Holes 12 Electrons 13 The majority charge carriers in an n-type semiconductors are electrons and the minority carriers are holes 14 The process of adding a measured quantity of a trivalent or a pentavalent impurity to a pure semiconductor is called doping 15 Hole is a vacancy created in the valence band due to the transfer of an electron to the conduction band It effectively behaves as a positively charged particle 16 The total number of electrons in an n-type semiconductor is equal to the total number of protons in it Therefore, the semiconductor as a whole is neutral 17 When a p-type semiconductor is in contact with an n-type semiconductor, the device so obtained is called a junction diode In actual practice the two regions are created side by side in the same piece of semiconductor S Chand & Company Limited 18 In forward biasing the p-region is connected to the positive terminal of the battery and the n-region to the negative terminal (Fig.9 3a) In reverse biasing the p-region is connected to the negative terminal of the battery and the n-region to the positive terminal (Fig 9.3b) Fig 9.3(a) Fig 9.3(b) 19 In forward bias the junction diode offers low resistance to the flow of current 20 When a large current is passed through a semiconductor, the covalent bonds are broken and a large number of free electrons are obtained Thus the semiconductor behaves like a conductor 21 The potential difference developed across the pn junction due to the migration of electrons into the p-region and holes into the n-region is called potential barrier 22 For Ge junction diode the value of the potential barrier is about 0.3 volt, and for Si diode its value is 0.7 volt 23 A graph showing the variation of junction current with bias voltage is called a characteristic curve 24 A device which is used to convert alternating current into direct current is called a rectifier 25 A junctions diode which can operate in the reverse breakdown voltage region is called a Zener diode S Chand & Company Limited RL Fig 9.4 27 Full wave rectifier circuit is shown in Fig 9.5 D1 RL A C Input Output Fig 9.5 D2 S Chand & Company Limited Output A C Input 26 Circuit diagram for a half wave rectifier is shown in Fig 9.4 28 Pure semiconductors, i.e., those in which no impurity is mixed, are called intrinsic semiconductors Effect of temperature on the conductivity of a semiconductor : In a semiconductor the forbidden gap is relatively small As the temperature is increased, electrons from the top of the valence band acquire enough energy to jump into the conduction band They leave behind an equal number of holes Thus the carrier concentration and hence the conductivity increases 29 Diffusion of electrons and holes across the junction, and their consequent recombination, causes depletion 30 When a p-type semiconductor is in contact with an n-type semiconductor, then electrons from the n-type and holes from the p-type migrate across the junction As a result, the nregion becomes positive and the p-region becomes negative This establishes an electric field across the junction directed from the n-region to the p-region This gives rise to a potential barrier (VB) which prevents further diffusion of carriers into opposite regions 31 When a pn junction is formed, the electron and holes diffuse across the junction Due to electron hole recombination, a thin layer around the junction becomes devoid of free charge carriers This is called the depletion region Its thickness is of the order of a few microns 32 In a p-type semiconductor, the majority charge carriers are holes and the minority carriers are electorns S Chand & Company Limited 33 C.B C.B Donor level Acceptor level V.B V.B In an n-type semiconductor, the impurity is pentavalent Only four of the five valence electrons of the impurity atoms participate in forming the covalent bonds, the fifth one becomes almost “free” The energy level of this electron lies very close to the bottom of the conduction band It is called the donor level (Fig 9.6(a)) Thus, this type of semiconductor has excess of electrons over holes The conductivity can be varied by changing the concentration of impurity atoms In a p-type semiconductor,the impurity is trivalent The three valence electrons of an impurity atom form covalent bonds with the three neighbouring atoms of the semiconductor, but one electron is lacking for the formation of the bond with the fourth atom, leaving a hole in this bond The energy level of such holes, called the acceptor level, lies only a little above the top of the valence band (Fig 9.6 (b)) Thus, this type of semiconductor has excess of holes over electrons S Chand & Company Limited 34 Zener Voltage V (volt) I (µ A) I (mA) O V (volt) Fig 9.7 (a) : Forward Bias Fig 9.7 (b) : Reverse Bias 35 Reverse biasing 36 An npn transistor consists of a thin p-type layer sandwitched between two thick n-type layers A pnp transistor consists of a thin n-type layer sandwitched between two thick p-type layers The three layers are called the emitter, the base and the collector 37 Base is made very thin and is doped lightly so that most of the carriers are attracted straight into the collector and very few combine in the base 38 The current gain of a common-emitter amplifier is much more than that of a common-base amplifier Therefore the voltage amplification is larger in the former S Chand & Company Limited 39 Common-Emitter Amplifier : Common-Emitter Amplifier IC RL Output (V0) Input Signal (V1) IB VCE IE VBE Fig 9.8 40 Current gain of common-base configuration I α= C IE Current gain of common-emitter configuration, β = IC Since IB < IE, we have β > α IB S Chand & Company Limited 41 The ratio of the change in collector current to the change in base-emitter voltage in called transconductance It is denoted by gm : δI C δVBE 42 In reverse bias, a diode works as an open switch 43 The forward biased emitter sends the majority carriers into the base regions and the reverse biased collector pulls these majority carriers from the base region 44 The ratio of the change in output voltage to the corresponding change in input voltage in called the a.c voltage gain of an amplifier Let ∆VO & ∆Vi be the changes in output and input voltages, gm = respectively Then the a.c voltage gain, AV = ∆VO ∆Vi 45 Power gain = voltage gain × current gain 46 (i) In a common base amplifier, the output voltage is in phase with the input voltage (ii) In a common emitter amplifier, the output voltage lags behind the input voltage by a phase difference of π 47 Reverse biased as the p-region is at a lower potential than the n-region 48 p-type semiconductor 49 The p-n-p transistor is represented as shown in Fig 9.9 50 In common base configuration, the a.c current gain is defined as the ratio of small change in collector current to the corresponding change in emitter current, i.e., α ac = δI C δI E S Chand & Company Limited