# Misan University College of Engineering Electrical Engineering Department. Exam: Final semester Date: 17/6/2017

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1 Misan University College of Engineering Electrical Engineering Department Subject: Electronic I Class: 1 st stage Exam: Final semester Date: 17/6/2017 Examiner: Dr. Baqer. O. TH. Time: 3 hr. Note: Answer five question only of each question 12 mark Q A 1 : Explain with each of the following 1. Problems with the Bohr Model The Bohr model applies only to one electron atoms. The Bohr model doesn t account for the observed spectra of multielectron elements or ions. The movement of electrons in atoms is much less clearly defined than Bohr allowed. 2. The uncertainty principle The Heisenberg s uncertainty principle says that you cannot determine the position and momentum of an electron at the same time. only probability of finding an electron with a given energy a given space. ( x)(δp) ħ 2 ħ = h 2π ( x)(δp) h 4π 3. Lyman Series.. All these lines are in the ultra-violet region of the emission spectrum. They are called the Lyman Series. Electrons from states with n i > 2 can return, initially, to the first-excited state (n f = 2), emitting one photon and then to the ground state emitting a second photon, with E ph = 10.2 ev, which is part of the Lyman series 1

2 4. Wave-Particle Duality Of Matter Based on the idea that light and all other electromagnetic Radiation may be considered a particle or a wave nature, Louis de Broglie suggested that the same kind of duality must be applicable to matter If electromagnetic radiation behaves as a particle, de Broglie reasoned, why couldn t a particle in motion, such as an electron, behave as a wave? : 5. Ionic Bonding forms ionic compounds transfer of e - electrons are transferred between valence shells of atoms ionic compounds are made of ions ionic compounds are called Salts or Crystals Always formed between metals and non-metals [metals ] + [non-metals ] - - Lost e - grain e 2

3 The electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged particles or ions. 6. primitive unit cell is a volume of space that when translated through all the vectors in a Bravais lattice just fills all of space without either overlapping itself or leaving voids. A primitive cell must contain precisely one lattice point 3

4 Q B 1 : Calculate the energy required to remove the electron from a hydrogen atom in its ground state. Removing the electron from a hydrogen atom in its ground state corresponds to taking the electron from n initial = 1 to n final =. Thus, E 1 = x J(1 2 /1 2 ) = x J E 2 = x J(1 2 / ) = 0 E = x J = x J The energy required to remove the electron from a hydrogen atom in its ground state is x J. Q C 1 : 1. Compute the Miller Indices for a plane intersecting at Z= ¼, y=1, and x=1/2, 2. Graph the plane and determine the axis intersects of a surface with the Miller Index (013). 4

5 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Q A 2 : Calculate the drift current density for a given semiconductor. Consider silicon at T = 300 K doped with arsenic atoms at a concentration of N d = cm 3. Assume the applied electric field is 100 V/cm Q B 2 What is the resistivity of a Si sample with /cm 3 Boron ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 5

6 Q A 3 : count only the following items: 1. Semiconductor Properties Generation Creation of an electron (e - ) and hole (h + ) pairh + is simply a missing electron, which leaves an excess positive charge (due to an extra proton) Recombination if an e - and an h + come in contact, they annihilate each other Electrons and holes are called carriers because they are charged particles when they move, they carry current Therefore, semiconductors can conduct electricity for T > 0K but not much current (at room temperature (300K), pure silicon has only 1 free electron per 3 trillion atoms) 2. Electronic Properties of Si Silicon is a semiconductor material. Pure Si has a relatively high electrical resistivity at room temperature. There are 2 types of mobile charge-carriers in Si: Conduction electrons are negatively charged; Holes are positively charged. The concentration (#/cm 3 ) of conduction electrons & holes in a semiconductor can be modulated in several ways: 1. by adding special impurity atoms ( dopants ) 2. by applying an electric field 3. by changing the temperature 4. by irradiation 3. Doping methods n-type semiconductor Pentavalent impurities(5 valence e-) are added to Si or Ge, the result is an increase the free electrons Extra electrons becomes a conduction electrons because it is not attached to any atom 6

7 p-type semiconductor Trivalent impurities(3 valence e-) are added to Si or Ge to create a deficiency of electrons or hole charges Q B 3 : A Si sample is doped with As Atom/cm 3. What is the equilibrium hole concentration p 0 at 300 K? Where is E F relative to E i? ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Q A 4 : calculate the space charge capacitance and the diffusion capacitance of a Si diode if the width of the depletion layer is 0.33 m and the cross-sectional area is 0.02 mm 2. Let us the operating point of I=1 ma if =100 ns! 7

8 8 A C Sp F S pf 3 I CD F 3. 85nF U T Q B 4 : Explain three operating regions in figure below and find diode current of each region of the Shockley Equation i D = I s [exp ( v D ηv T ) 1] Forward bias region Plot the result of measurement in Figure below, you get the V-I characteristic curve for a forward bias diode V F Increase to the right I F increase upward. i D = I s [exp ( v D ηv T ) 1] = I s exp ( v D ηv T ) 8

9 Reverse bias region Breakdown voltage not a normal operation of pn junction devices the value can be vary for typical Si Reverse current= I s Breakdown region i D = I s exp ( v D ηv T ) I s = I s The breakdown region is entered when the magnitude of the reverse voltage exceed a threshold called breakdown voltage or the zener knee voltage. In the breakdown region the current increase rapidly with a very small increase in the associated voltage. There is an avalanche of electrons flowing across the junction with the result that the diode overheat. Provided that the power dissipated in the diode is limited by external circuitry to a safe level (typically specified in the data sheets) breakdown won t be destructive. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Q A 5 : Consider the Zener diode regulator shown in figure below. Find the load voltage v L and the source current I s if V ss =24V, R=1.2kW and R L =6kW 9

10 Q B 5 : 10

11 Q C 5 : In the bridge type circuit shown in Fig. below, the diodes are assumed to be ideal. Find :(i) d.c. output voltage (ii) peak inverse voltage (iii) output frequency. Assume primary to secondary turns to be 4 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 11

12 Q 6 : Explain with each of the following 1. Why is LED not made of silicon or germanium? 2. How does photo-diode differ from an ordinary diode? A photo-diode differs from a rectifier diode in that when its pn junction is ex-posed to light, the reverse current increases with the increase in light intensity and viceversa. 3. What is dark resistance of photo-diode? When no light is incident on the pn junction of photo-diode, the reverse current Ir is extremely small. This is called dark current. The resistance of photo-diode with no incident light is called dark resistance ( 4. Explain the V-I characteristics of a tunnel diode.v-i Characteristic. Fig (i) shows the V-I characteristic of a typical tunnel diode i) As the forward voltage across the tunnel diode is increased from zero, electrons ( from the n-region tunnel through the potential barrier to the p-region. As the forward voltage increases, the diode current also increases until the peak-point P is reached. The diode current has now reached peak current IP (= 2.2 ma) at about peak-point voltage VP (= 0.07 V). Until now the diode has.exhibited positive resistance ii) As the voltage is increased beyond VP, the tunneling action starts decreasing and ( the diode 12

13 current decreases as the forward voltage is increased until valley-point V is reached at valley-point voltage VV (= 0.7V). In the region between peak-point and valley-point (i.e., between points P and (iii) When forward bias is increased beyond valley-point voltage VV (= 0.7 V), the tunnel diode behaves as a normal diode. In other words, from point V onwards, the diode current increases with the increase in forward voltage i.e., the diode exhibits positive resistance once again. Fig (ii) shows the symbol of tunnel diode. It may be noted that a tunnel diode has a high reverse current but operation under this condition is not generally used 5. How does the width of depletion layer change the capacitance of a varactor When a pn junction is formed, depletion layer is created in the junction area. Since there are no charge carriers within the depletion zone, the zone acts as an insulator. The p-type material with holes (considered positive) as majority carriers and n-type material with electrons ( ve charge) as majority carriers act as charged plates. 13

14 Q A 6 : Determine the state of diode for the circuit shown in Fig. below and find I D and V D. Assume simplified model for the Practical diode. q = 1.6 X c m e = 0.26 m 0 m h = 0.39 m 0 μ e = 1350 cm 2 V 1 s 1, μ h = 450 cm 2 V 1 s 1 m 0 = Kg h = J. s 14

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