Discuss the difference between conductors,insulators and semi conductors. Understand the P-N junction and explain the origin of the depletion region

Transcription

1 UNIT III SEMICONDUCTOR DIODES Review of intrinsic & externsic semiconductors Theory of PN junction diode Energy band structure current equation space charge and diffusion capacitances effect of temperature and breakdown mechanism Zener diode and its characteristics. OBJECTIVES You will be able to Discuss the difference between conductors,insulators and semi conductors Understand the P-N junction and explain the origin of the depletion region Draw diagrams to show the effects of forward biasing and reverse biasing the P-N junction Discuss temperature effects on p-n junction Calculate current and voltage levels at a P-n junction INTRODUCTION In order to understand the concept and working of the PN junction diode, it is necessary to know what goes on at the atomic level of a semiconductor so the characteristics of the semiconductor can be understood. In many cases a detailed explanation of why some of the phenomena occur is not required or supplied. Just knowing that certain phenomena occur allows us to understand why semiconductors behave the way they do. Semiconductors Semiconductors are atoms that contain 4 valence electrons. A good conductor has 1 valence electron and an insulator has eight valence electrons. The semiconductor has 4 valence electrons. It is neither a good conductor nor a good insulator. Three of the most commonly used semiconductor materials are silicon(si), germanium(ge), and carbon.

2 These atoms are shown in Figure 5. It is noted that all of them have 4 valence electrons. The Doping of Semiconductors The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors. Pentavalent impurities Impurity atoms with 5 valence electrons produce n-type semiconductors by contributing extra electrons. Trivalent impurities Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a "hole" or electron deficiency.

3 P- and N- Type Semiconductors N-Type Semiconductor The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).

4 P-Type Semiconductor The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,called "holes". It is typical to use B 2 H 6 diborane gas to diffuse boron into the silicon material. Bands for Doped Semiconductors The application of band theory to n-type and p-type semiconductors shows that extra levels have been added by the impurities. In n-type material there are electron energy levels near the top of the band gap so that they can be easily excited into the conduction band. In p-type material, extra holes in the band gap allow excitation of valence band electrons, leaving mobile holes in the valence band.

5 P-N Junction A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams such as Figure below. The term diode is customarily reserved for small signal devices, I 1 A. The term rectifier is used for power devices, I > 1 A. Semiconductor diode schematic symbol: Arrows indicate the direction of electron current flow. When placed in a simple battery-lamp circuit, the diode will either allow or prevent current through the lamp, depending on the polarity of the applied voltage. (Figure below) Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased. When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is backward and the diode blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch: closed when forward-biased and open when reversebiased.

6 Oddly enough, the direction of the diode symbol's arrowhead points against the direction of electron flow. This is because the diode symbol was invented by engineers, who predominantly use conventional flow notation in their schematics, showing current as a flow of charge from the positive (+) side of the voltage source to the negative (-). This convention holds true for all semiconductor symbols possessing arrowheads: the arrow points in the permitted direction of conventional flow, and against the permitted direction of electron flow. Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valve allows fluid flow through it in only one direction as in Figure below. Hydraulic check valve analogy: (a) Electron current flow permitted. (b) Current flow prohibited. Check valves are essentially pressure-operated devices: they open and allow flow if the pressure across them is of the correct polarity to open the gate (in the analogy shown, greater fluid pressure on the right than on the left). If the pressure is of the opposite polarity, the pressure difference across the check valve will close and hold the gate so that no flow occurs. Like check valves, diodes are essentially pressure- operated (voltage-operated) devices. The essential difference between forward-bias and reverse-bias is the polarity of the voltage dropped across the diode. Let's take a closer look at the simple battery-diodelamp circuit shown earlier, this time investigating voltage drops across the various components in Figure below.

7 Diode circuit voltage measurements: (a) Forward biased. (b) Reverse biased. A forward-biased diode conducts current and drops a small voltage across it, leaving most of the battery voltage dropped across the lamp. If the battery's polarity is reversed, the diode becomes reverse-biased, and drops all of the battery's voltage leaving none for the lamp. If we consider the diode to be a self-actuating switch (closed in the forward-bias mode and open in the reverse-bias mode), this behavior makes sense. The most substantial difference is that the diode drops a lot more voltage when conducting than the average mechanical switch (0.7 volts versus tens of millivolts). This forward-bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage. If no voltage applied is across a semiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current flow. (Figure below (a)) The depletion region is almost devoid of available charge carriers, and acts as an insulator: Diode representations: PN-junction model, schematic symbol, physical part.

8 The schematic symbol of the diode is shown in Figure above (b) such that the anode (pointing end) corresponds to the P-type semiconductor at (a). The cathode bar, nonpointing end, at (b) corresponds to the N-type material at (a). Also note that the cathode stripe on the physical part (c) corresponds to the cathode on the symbol. If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it. (Figure below) Depletion region expands with reverse bias. Conversely, if a forward-biasing voltage is applied across the P-N junction, the depletion region collapses becoming thinner. The diode becomes less resistive to current through it. In order for a sustained current to go through the diode; though, the depletion region must be fully collapsed by the applied voltage. This takes a certain minimum voltage to accomplish, called the forward voltage as illustrated in Figure below Increasing forward bias from (a) to (b) decreases depletion region thickness. For silicon diodes, the typical forward voltage is 0.7 volts, nominal. For germanium diodes, the forward voltage is only 0.3 volts. The chemical constituency of the P-N junction comprising the diode accounts for its nominal forward voltage figure, which is why silicon and germanium diodes have such different forward voltages. Forward voltage drop remains approximately constant for a wide range of diode currents, meaning that

9 diode voltage drop is not like that of a resistor or even a normal (closed) switch. For most simplified circuit analysis, the voltage drop across a conducting diode may be considered constant at the nominal figure and not related to the amount of current. (Actually, forward voltage drop is more complex. An equation describes the exact current through a diode, given the voltage dropped across the junction, the temperature of the junction, and several physical constants. It is commonly known as the diode equation) One of the crucial keys to solid state electronics is the nature of the P-N junction. When p- type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of material alone. Specifically, current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode. This non-reversing behavior arises from the nature of the charge transport process in the two types of materials. The open circles on the left side of the junction above represent "holes" or deficiencies of electrons in the lattice which can act like positive charge carriers. The solid circles on the right of the junction represent the available electrons from the n-type dopant. Near the junction, electrons diffuse across to combine with holes, creating a "depletion region". The energy level sketch above right is a way to visualize the equilibrium condition of the P-N junction. The upward direction in the diagram represents increasing electron Depletion Region When a p-n junction is formed, some of the free electrons in the n-region diffuse across the junction and combine with holes to form negative ions. In so doing they leave behind positive ions at the donor impurity sites.

10 Depletion Region Details In the p-type region there are holes from the acceptor impurities and in the n-type region there are extra electrons When a p-n junction is formed, some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes Filling a hole makes a negative ion and leaves behind a positive ion on the n-side. A space charge builds up, creating a depletion region which inhibits any further electron transfer unless it is helped by putting a forward bias on the junction

11 Bias effect on electrons in depletion zone Equilibrium of junction Coulomb force from ions prevents further migration across the p-n junction. The electrons which had migrated across from the N to the P region in the forming of the depletion layer have now reached equilibrium. Other electrons from the N region cannot migrate because they are repelled by the negative ions in the P region and attracted by the positive ions in the N region. Reverse bias An applied voltage with the indicated polarity further impedes the flow of electrons across the junction. For conduction in the device, electrons from the N region must move to the junction and combine with holes in the P region. A reverse voltage drives the electrons away from the junction, preventing conduction. Forward-bias

12 An applied voltage in the forward direction as indicated assists electrons in overcoming the coulomb barrier of the space charge in depletion region. Electrons will flow with very small resistance in the forward direction. Forward-bias occurs when the P-type semiconductor material is connected to the positive terminal of a battery and the N-type semiconductor material is connected to the negative terminal, as shown below. This usually makes the p n junction conduct. A silicon p n junction in Forward-bias. With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed towards the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type material repels the holes, while the negative charge applied to the N-type material repels the electrons. As electrons and holes are pushed towards the junction, the distance between them decreases. This lowers the barrier in potential. With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field can't counteract charge carrier motion across the p n junction, consequently reducing electrical resistance. The electrons which cross the p n junction into the P-type material (or holes which cross into the N-type material) will diffuse in the near-neutral region. Therefore, the amount of minority diffusion in the near-neutral zones determines the amount of current that may flow through the diode. Only majority carriers (electrons in N-type material or holes in P-type) can flow through a semiconductor for a macroscopic length. With this in mind, consider the flow of electrons across the junction. The forward bias causes a force on the electrons pushing them from the N side toward the P side. With forward bias, the depletion region is narrow enough that electrons can cross the junction and inject into the P-type material. However, they do not continue to flow through the P-type material indefinitely, because it is energetically favorable for them to recombine with holes. The average length an electron travels through the P-type material before recombining is called the diffusion length, and it is typically on the order of microns. [1] Although the electrons penetrate only a short distance into the P-type material, the electric current continues uninterrupted, because holes (the majority carriers) begin to flow in the opposite direction. The total current (the sum of the electron and hole currents) is constant in space, because any variation would cause charge buildup over time (this is Kirchhoff's current law). The flow of holes from the P-type region into the N-type region is exactly analogous to the flow of electrons from N to P (electrons and holes swap roles and the signs of all currents and voltages are reversed).

13 Therefore, the macroscopic picture of the current flow through the diode involves electrons flowing through the N-type region toward the junction, holes flowing through the P-type region in the opposite direction toward the junction, and the two species of carriers constantly recombining in the vicinity of the junction. The electrons and holes travel in opposite directions, but they also have opposite charges, so the overall current is in the same direction on both sides of the diode, as required. The Shockley diode equation models the forward-bias operational characteristics of a p n junction outside the avalanche (reverse-biased conducting) region. Reverse bias An applied voltage with the indicated polarity further impedes the flow of electrons across the junction. For conduction in the device, electrons from the N region must move to the junction and combine with holes in the P region. A reverse voltage drives the electrons away from the junction, preventing conduction. Reverse-biased usually refers to how a diode is used in a circuit. If a diode is reverse biased, the voltage at the cathode is higher than that at the anode. Therefore, no current will flow until the diode breaks down. Connecting the P-type region to the negative terminal of the battery and the N-type region to the positive terminal, produces the reverse-bias effect. The connections are illustrated in the following diagram: Because the P-type material is now connected to the negative terminal of the power supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the depletion zone to increase. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore the depletion region widens, and does so increasingly with increasing reversebias voltage. This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing minimal electric current to cross the p n junction. The strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p n

14 junction depletion zone breaks-down and current begins to flow, usually by either the Zener or avalanche breakdown processes. Both of these breakdown processes are nondestructive and are reversible, so long as the amount of current flowing does not reach levels that cause the semiconductor material to overheat and cause thermal damage. This effect is used to one's advantage in zener diode regulator circuits. Zener diodes have a certain - low - breakdown voltage. A standard value for breakdown voltage is for instance 5.6V. This means that the voltage at the cathode can never be more than 5.6V higher than the voltage at the anode, because the diode will break down - and therefore conduct - if the voltage gets any higher. This effectively regulates the voltage over the diode. Here is how a simple zener regulation circuit would look: Another application where reverse biased diodes are used is in Varicap diodes. The width of the depletion zone of any diode changes with voltage applied. This varies the capacitance of the diode. For more information, refer to the Varicap article. To understand how a pn-junction diode works, begin by imagining two separate bits of semiconductor, one n-type, the other p-type.

15 Bring them together and join them to make one piece of semiconductor which is doped differently either side of the junction. Free electrons on the n-side and free holes on the p-side can initially wander across the junction. When a free electron meets a free hole it can 'drop into it'. So far as charge movements are concerned this means the hole and electron cancel each other and vanish. As a result, the free electrons and holes near the junction tend to eat each other, producing a region depleted of any moving charges. This creates what is called the depletionzone.

16 Now, any free charge which wanders into the depletion zone finds itself in a region with no other free charges. Locally it sees a lot of positive charges (the donor atoms) on the n- type side and a lot of negative charges (the acceptor atoms) on the p-type side. These exert a force on the free charge, driving it back to its 'own side' of the junction away from the depletion zone. The acceptor and donor atoms are 'nailed down' in the solid and cannot move around. However, the negative charge of the acceptor's extra electron and the positive charge of the donor's extra proton (exposed by it's missing electron) tend to keep the depletion zone swept clean of free charges once the zone has formed. A free charge now requires some extra energy to overcome the forces from the donor/acceptor atoms to be able to cross the zone. The junction therefore acts like a barrier, blocking any charge flow (current) across the barrier. Usually, we represent this barrier by 'bending' the conduction and valence bands as they

17 cross the depletion zone. Now we can imagine the electrons having to 'get uphill' to move from the n-type side to the p-type side. For simplicity we tend to not bother with drawing the actual donor and acceptor atoms which are causing this effect! The holes behave a bit like balloons bobbing up against a ceiling. On this kind of diagram you require energy to 'pull them down' before they can move from the p-type side to the n-type side. The energy required by the free holes and electrons can be supplied by a suitable voltage applied between the two ends of the pn-junction diode. Notice that this voltage must be supplied the correct way around, this pushes the charges over the barrier. However, applying the voltage the 'wrong' way around makes things worse by pulling what free charges there are away from the junction This is why diodes conduct in one direction but not the other. P-N Junction - V-I characteristics Voltage-Current relationship for a p-n junction (diode)

18 Current-Voltage Characteristics THE IDEAL DIODE PRACTICAL DIODE Diode curve: showing knee at 0.7 V forward bias for Si, and reverse breakdown. Typically, the PIV rating of a generic rectifier diode is at least 50 volts at room temperature. Diodes with PIV ratings in the many thousands of volts are available for modest prices.

19 Temperature Effects The PN Junction diode conductivity is directly proportional to the temperature of the diode Whenever the temperature increases the conductivity of the diode also increases In case of voltage there is a slight drop whenever the temperature increases. This indicates it have negative temperature co-efficient Whenever the temperature increases change in breakdown voltage also occurs Drift and Diffusion Currents Current Flow: Drift: charged particle motion in response to an electric field. Diffusion: Particles tend to spread out or redistribute from areas of high concentration to areas of lower concentration Recombination: Local annihilation of electron-hole pairs Generation: Local creation of electron-hole pairs REVIEW: A diode is an electrical component acting as a one-way valve for current. When voltage is applied across a diode in such a way that the diode allows current, the diode is said to be forward-biased. When voltage is applied across a diode in such a way that the diode prohibits current, the diode is said to be reverse-biased. The voltage dropped across a conducting, forward-biased diode is called the forward voltage. Forward voltage for a diode varies only slightly for changes in forward current and temperature, and is fixed by the chemical composition of the P-N junction.

20 Silicon diodes have a forward voltage of approximately 0.7 volts. Germanium diodes have a forward voltage of approximately 0.3 volts. The maximum reverse-bias voltage that a diode can withstand without breaking down is called the Peak Inverse Voltage, or PIV rating. Positive voltage yields finite current Negative voltage yields zero current Zener Diode What is a Zener Diode Find out more about the Zener diode Diodes are electronic components which will let current flow in just one direction. They are used for example in PV Solar Panel installations to ensure that current can flow into the battery bank when it is sunny, but not escape through the solar panel when it is very cloudy or at night. What is a Zener Diode A Zener Diode is a special kind of diode which permits current to flow in the forward direction as normal, but will also allow it to flow in the reverse direction when the voltage is above a certain value - the breakdown voltage known as the Zener voltage. The Zener voltage of a standard diode is high, but if a reverse current above that value is allowed to pass through it, the diode is permanently damaged. Zener diodes are designed so that their zener voltage is much lower - for example just 2.4 Volts. When a reverse current above the Zener voltage passes through a Zener diode, there is a controlled breakdown which does not damage the diode. The voltage drop across the Zener diode is equal to the Zener voltage of that diode no matter how high the reverse bias voltage is above the Zener voltage.

21 The illustration above shows this phenomenon in a Current vs. Voltage graph. With a zener diode connected in the forward direction, it behaves exactly the same as a standard diode - i.e. a small voltage drop of 0.3 to 0.7V with current flowing through pretty much unrestricted. In the reverse direction however there is a very small leakage current between 0V and the Zener voltage - i.e. just a tiny amount of current is able to flow. Then, when the voltage reaches the breakdown voltage (Vz), suddenly current can flow freely through it. ZENER DIODE The diodes designed to work in breakdown region are called zener diode. The power handling capacity of these diodes is better. The power dissipation of a zener diode equals the product of its voltage and current. PZ =VZ IZ. When zener is forward biased it works as a diode and drop across it is 0.7V when it works in breakdown region the voltage across it is constant Vz and the current through it is decided by external resistance. Zener diode is used for regulating the the dc voltage. It maintains the output voltage constant even through the current through changes.

22 To operate the zener in Breakdown region Vs should always be greater then Vz. Rs is used to limit the current. If the Vs voltage changes operating point also changes simultaneously but voltage across zener is almost constant. Fig. 37 The first approximation zener diode is a voltage source of Vz magnitude and second approximation includes the resistance also.

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24 (Fig.38) The resistance produces more I*R drop as the current increases. The voltage at Q1 is V1 = Rz +Vz At Q2 = I2 +R2 +Vz Change in voltage is If zener is used to regulate the voltage across a load resistance. The zener is will work in the breakdown region only if the thevenin voltage across zener is more than V Z. If zener is operating in breakdown region, the series current will be given by

25 Zener Drop Output point : For a zener regulator to hold the output voltage constant, zener diode must remain in the breakdown region under all operating conditions, i.e. there must be zener current for all source voltage load currents. The worst case occurs for minimum source voltage and maximum load current. The critical point occurs when maximum load current equals minimum series current When the zener diode operates in breakdown region, the voltage Vz across it remains fairly constant even though the current Iz through it vary considerably. If the load IL should increase, the current Iz should decrease by the same percentage in order to maintain load current constant Is. This keeps the voltage drop across Rs constant and hence the output voltage. If the input voltage should increase, the zener diode passes a larger current, that extra voltage is dropped across the resistance Rs. If input voltage falls, the current Iz falls such that Vz is constant. Uses of Zener Diodes Since the voltage dropped across a Zener Diode is a known and fixed value, Zener diodes are typically used to regulate the voltage in electric circuits. Using a resistor to ensure that the current passing through the Zener diode is at least 5mA (0.005 Amps), the circuit designer knows that the voltage drop across the diode is exactly equal to the Zener voltage of the diode. SERIES AND SHUNT VOLTAGE REGULATORS The schematic for a typical series voltage regulator is shown in figure Notice that this regulator has a transistor (Q1) in the place of the variable resistor found in figure Because the total load current passes through this transistor, it is sometimes called a "pass transistor." Other components which make up the circuit are the current limiting resistor (R1) and the Zener diode (CR1).

26 Figure - Series voltage regulator. Recall that a Zener diode is a diode that block current until a specified voltage is applied. Remember also that the applied voltage is called the breakdown, or Zener voltage. Zener diodes are available with different Zener voltages. When the Zener voltage is reached, the Zener diode conducts from its anode to its cathode (with the direction of the arrow). In this voltage regulator, Q1 has a constant voltage applied to its base. This voltage is often called the reference voltage. As changes in the circuit output voltage occur, they are sensed at the emitter of Q1 producing a corresponding change in the forward bias of the transistor. In other words, Q1 compensates by increasing or decreasing its resistance in order to change the circuit voltage division. Now, study figure Voltages are shown to help you understand how the regulator operates. The Zener used in this regulator is a 15-volt Zener. In this instance the Zener or breakdown voltage is 15 volts. The Zener establishes the value of the base voltage for Q1. The output voltage will equal the Zener voltage minus a 0.7-volt drop across the forward biased base-emitter junction of Q1, or 14.3 volts. Because the output voltage is 14.3 volts, the voltage drop across Q1 must be 5.7 volts.

27 Figure - Series voltage regulator (with voltages). Study figure 4-36, view A, in order to understand what happens when the input voltage exceeds 20 volts. Notice the input and output voltages of 20.1 and 14.4 volts, respectively. The 14.4 output voltage is a momentary deviation, or variation, from the required regulated output voltage of 14.3 and is the result of a rise in the input voltage to 20.1 volts. Since the base voltage of Q1 is held at 15 volts by CR1, the forward bias of Q1 changes to 0.6 volt. Because this bias voltage is less than the normal 0.7 volt, the resistance of Q1 increases, thereby increasing the voltage drop across the transistor to 5.8 volts. This voltage drop restores the output voltage to 14.3 volts. The entire cycle takes only a fraction of a second and, therefore, the change is not visible on an oscilloscope or readily measurable with other standard test equipment. Figure - Series voltage regulator. INCREASE IN OUTPUT View B is a schematic diagram for the same series voltage regulator with one significant difference. The output voltage is shown as 14.2 volts instead of the desired 14.3 volts. In this case, the load has increased causing a lowered voltage drop across R L to 14.2 volts.

28 When the output decreases, the forward bias of Q1 increases to 0.8 volt because Zener diode CR1 maintains the base voltage of Q1 at 15 volts. This 0.8 volt is the difference between the Zener reference voltage of 15 volts and the momentary output voltage. (15 V V = 0.8 V). At this point, the larger forward bias on Q1 causes the resistance of Q1 to decrease, thereby causing the voltage drop across Q1 to return to 5.7 volts. This then causes the output voltage to return to 14.3 volts. Figure - Series voltage regulator. DECREASE IN OUTPUT The schematic shown in figure 4-37 is that of a shunt voltage regulator. Notice that Q1 is in parallel with the load. Components of this circuit are identical with those of the series voltage regulator except for the addition of fixed resistor R S. As you study the schematic, you will see that this resistor is connected in series with the output load resistance. The current limiting resistor (R1) and Zener diode (CR1) provide a constant reference voltage for the base-collector junction of Q1. Notice that the bias of Q1 is determined by the voltage drop across R S and R1. As you should know, the amount of forward bias across a transistor affects its total resistance. In this case, the voltage drop across R S is the key to the total circuit operation. Figure - Shunt voltage regulator.

29 Figure 4-38 is the schematic for a typical shunt-type regulator. Notice that the schematic is identical to the schematic shown in figure 4-37 except that voltages are shown to help you understand the functions of the various components. In the circuit shown, the voltage drop across the Zener diode (CR1) remains constant at 5.6 volts. This means that with a 20-volt input voltage, the voltage drop across R1 is 14.4 volts. With a base-emitter voltage of 0.7 volt, the output voltage is equal to the sum of the voltages across CR1 and the voltage at the base-emitter junction of Q1. In this example, with an output voltage of 6.3 volts and a 20-volt input voltage, the voltage drop across R S equals 13.7 volts. Study the schematic to understand fully how these voltages are developed. Pay close attention to the voltages shown. Figure - Shunt voltage regulator (with voltages). Now, refer to view A of figure This figure shows the schematic diagram of the same shunt voltage regulator as that shown in figure 4-38 with an increased input voltage of 20.1 volts. This increases the forward bias on Q1 to 0.8 volt. Recall that the voltage drop across CR1 remains constant at 5.6 volts. Since the output voltage is composed of the Zener voltage and the base-emitter voltage, the output voltage momentarily increases to 6.4 volts. At this time, the increase in the forward bias of Q1 lowers the resistance of the transistor allowing more current to flow through it. Since this current must also pass through R S, there is also an increase in the voltage drop across this resistor. The voltage drop across R S is now 13.8 volts and therefore the output voltage is reduced to 6.3 volts. Remember, this change takes place in a fraction of a second.

30 Figure - Shunt voltage regulator. INCREASE IN OUTPUT VOLTAGE Study the schematic shown in view B. Although this schematic is identical to the other shunt voltage schematics previously illustrated and discussed, the output voltage is different. The load current has increased causing a momentary drop in voltage output to 6.2 volts. Recall that the circuit was designed to ensure a constant output voltage of 6.3 volts. Since the output voltage is less than that required, changes occur in the regulator to restore the output to 6.3 volts. Because of the 0.1 volt drop in the output voltage, the forward bias of Q1 is now 0.6 volt. This decrease in the forward bias increases the resistance of the transistor, thereby reducing the current flow through Q1 by the same amount that the load current increased. The current flow through R S returns to its normal value and restores the output voltage to 6.3 volts. Figure - Shunt voltage regulator. DECREASE IN OUTPUT VOLTAGE

31 OBJECTIVE TYPE QUESTIONS 1. The potential barrier across a p-n P N junction is due to a. Negative and positive charge carriers on the same side b. Immobile donor and positive acceptor ions c. Negative and positive charge carriers on the opposite side 2. Depletion voltage is a. More for Ge b. More for Si c. Equal in Si and Ge 3. Depletion voltage increases with a. Forward bias b. Reverse bias c. Without forward and reverse biases 4. Depletion with has a. Negative charge carriers b. Positive charge carriers c. No charge carriers 5. Depletion width with forward bias a. Increases b. Decreases c. Remains constant 6. PN junction capacitance with increasing reverse bias a. Increases b. Decreases

32 c. Remains constant 7. Forward bias across p-n pnjunction means a. Only positive terminal connected to p-type b. Positive terminal connected to p and negative to n c. Positive terminal connected to n and negative to p 8. In an unbiased p-n pnjunction current does not flow because a. Carriers do not cross the pnjunction b. Equal and opposite charge carriers cross the pnjunction c. Same type of charge carrier cross the pnjunction in opposite direction 9. Diffusion current is due to a. Different concentrations of the two types to charge carriers in the same region b. Different concentrations of same types to charge carriers in Different region C.Same concentrations in two regions 10.Total current through any p-n pnjunction is only due to a. Drift of charge carriers b. Diffusion of charge carriers c. Both type of carriers 11.The forward current in p-n pnjunction increases rapidly a. Form zero onwards b. Only after the value of potential barrier c. When the depletion area becomes equal to space charge area 12.Zener breakdown refers to

33 a. Forward bias region b. Reverse bias region c. No bias region 13. Avalanche breakdown voltage is a. Lower then zener breakdown voltage b. Higher then zener breakdown voltage C.Equal to zener breakdown voltage D.None of the above 14. Zener breakdown depends on A. Electric field created across the depletion region B. Velocity of the carriers C. No of donor iorns D. No.of acceptor ions 15.Both avalanche and zener breakdown are commonly know as a. Zener breakdown b. Avalanche breakdown c. Current breakdown 16. Zener diodes are used as a. Reference voltage elements b. Reference current elements c. Reference resistance. 17. The reverse saturation current with increasing reverse bias a. Increases b. Decreases c. Remains constant 18. The magnitude of reverse saturation current is

34 a. Less than forward current b. Larger than forward current c. Equal to forward current 19. With rise in temperature reverse saturation current a. Increase linearly b.. Increase exponentially c. decreases linearly 20. With increasing temperature the pnjunction voltage a. Increase b.. Decrease exponentially c. Remains constant 21. Potential barrier for Ge p-n pnjunction a. 0.2 V b. 0.02V c.0.7v 22. Potential barrier across Si diode is a. 0.2 V b.0.7 V c. 1 V 23. The voltage drop across an ideal diode is a. 0.2 V b. 0.7 V c. 0.V 24. Resistance of an ideal diode is

35 a. Very large b. Zero c. Small 25.The current flow in a diode is a. Unidirectional b. Bi directional c.none of these 26. Diode is a a.polar sensitive Device b. Non polar sensitive device c.bipolar sensitive device 27. Diodes can beused as a.amplifier b. Rectifier c. Filter 28. V-1 characteristics of diode can result in a. Static resistance only b. Dynamic resistance only c. Forward resistance 29. Diffusion current in a p-n pnjunction is influenced a. By concentration gradient of carriers b. Applied voltage c. concentration of carriers 30. Dift current is influenced by

36 a. Magnitude of voltage b. Concentration gradient of carriers c. Concentration of carriers 31. Increasing reverse bias a. Decreases the pn juction capacitor b. Increases the pn juction capacitor c. Has no effect on its capacitor 32.Reverse break down in p-n pnjunction at high temperature occurs a. At higher reverse bias b. At lower reverse bias c. At forward bias 33. The reverse saturation current 1 co fo Si diode varies a. T 2. b. T 3 c. T1/2 d. T3/2 Answers : 1-b,2-b,3-b,4-c,5-b,6-a,7-b,8-a,9-b,10-c,11-b,12-b,13-b,14-a,15-a 16-a,17-a,18- a,19-b,20-b,2 1-a,22-b,23-c,24-b,25-a,26-a,27-b,28-a,29-a,30-a,31-b,32-c,33-d DESCRIPTIVE QUESTIONS 2 marks 1. Define electron volt? 2. What is doping? 3. What is intrinsic and extrinsic semiconductor? 4. What are acceptor & donor?

37 5. List out the common diode applications? descriptive 6. Define avalanche breakdown? 7. Define zener breakdown? 8. What are the current components of a diode? 9. Define forward recovery time and reverse recovery time? 10. Define PIV of the diode 11. Define drift and diffusion current 12. What is mean by reverse recovery time 12-MARKS 1. How a p type and n-type semiconductor can be obtained? 2. Explain insulator, semiconductor& conductor with help of energy band structure? 3. Explain the forward and reverse bias operation and vi characteristics of a pn junction diode. 4. Derive the diode current equation? 5. Discuss the current components of pn junction diode? 6. Explain any two applications of diode with neat diagram. 7. Explain the characteristics and applications of zener diode? 8. Explain the mechanism of avalanche and zener break down? 9. Explain the switching characteristics of pn junction diode. 10. Explain the concept of diffusion and drift current. 11. explain the working principle of zener diode with i-v characteristics