EXPERIMENTS USING SEMICONDUCTOR DIODES

Transcription

1 EXPERIMENT 9 EXPERIMENTS USING SEMICONDUCTOR DIODES Semiconductor Diodes Structure 91 Introduction Objectives 92 Basics of Semiconductors Revisited 93 A p-n Junction Operation of a p-n Junction A Forward and Reverse Biased p-n Junction Identifying a Diode I-V Characteristics of a p-n Junction 94 Zener Diode Working of a Zener Diode I-V Characteristics of a Zener Diode 95 Some Applications of Semiconductor Diodes Rectification of ac Zener Diode as Voltage Regulator 31

2 Experiments with Electrical and Electronic Circuits 91 INTRODUCTION From your 10+2 physics course, you will recall that materials can be broadly classified into conductors, insulators and semiconductors on the basis of their resistivity The resistivity of a conductor is of the order of 10 7 Ωm and that of an insulator is of the order of Ωm The resistivity of a semiconductor lies in-between the resistivities of a conductor and an insulator Germanium (Ge) and Silicon (Si) are the most commonly used semiconductors At absolute zero, the semiconductor also acts as a near perfect insulator But with increase in temperature, the conductivity of the semiconductor increases This change in conductivity with temperature is different for different semiconducting materials The conductivity of a semiconductor can also be influenced by doping it with some impurity elements, called dopants like boron, phosphorus, arsenic etc Depending on the type of carrier added by a dopant, the semiconductor is classified as p-type (hole carriers) or n-type (electron carriers) The p-type impurity is acceptor type, whereas the n-type impurity is donor type A p-n junction is usually formed by doping a part of a pure semiconductor with acceptor impurities and the remainder with donor impurities Semiconductors have very useful properties (small size, light weight and efficient operation) and are being extensively used in electronic equipments (These have completely replaced vacuum tubes used earlier in electronic circuits) You may recall that microelectronic chips are the cores of computers and these are also made using semiconductor junctions Now this limit has been extended to nano-electronic devices When the anode is connected to +ve terminal of battery and the cathode is connected to ve terminal of the battery, the device is said to be forward-biased and vice versa A p-n junction is said to be forward biased when p-type region is connected to +ve terminal of the battery and n-type region is connected to ve terminal of the battery A p-n junction is also called a diode There are various types of diodes In this experiment, you will draw the I-V characteristic curves of a p-n junction and a zener diode You will discover that the function of a device can be influenced and determined by external conditions While a p-n junction works as rectifying diode, a zener diode acts as voltage regulator, depending on biasing conditions Objectives After performing this experiment, you should be able to: draw current-voltage (I-V) characteristic curves of a p-n junction and a zener diode in forward and reverse bias conditions; determine the material of a diode from its I-V characteristic curves; devise a zener voltage regulator circuit and determine the range of constancy; and measure the effects of variation in input voltage and load on the output of a zener diode regulator 32

3 92 BASICS OF SEMICONDUCTORS REVISITED Semiconductor Diodes You have learnt about semiconductors in your school physics You have also read about p-type and n-type semiconductors However, for brevity, we recapitulate the important characteristics of semiconductors Semiconductors are of two types: intrinsic and extrinsic A pure semiconductor is said to be an intrinsic semiconductor But in practical applications, intrinsic semiconductors are of little use due to their high resistivity or low conductivity In an electronic circuit, it is both necessary and desirable to tailor their conductivity by doping an impurity Doped semiconductors are termed as extrinsic semiconductors The most commonly used semiconducting materials are crystalline silicon and germanium In recent years, compound semiconductors, amorphous semiconductors, and semiconducting polymers have also been developed In this experiment, we will confine ourselves only to devices made of elemental semiconductors Doping is a process of adding small quantities of other elements, called impurity, in a pure semiconductor in order to modify its electrical conductivity From the electronic configuration of Si ( 1s 2s 2p 3s 3p ), you will recall that in all 14 electrons are bound to the nucleus and revolve around it Of these, four electrons revolve in the outermost orbit In an intrinsic silicon semiconductor, the Si atom attains stability by sharing one outermost electron each with four neighbouring Si atoms (This is called covalent bonding) The same holds true for germanium whose electronic configuration is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 2 When silicon (or germanium) is doped with a pentavalent (five electrons in the outermost orbit) atom like phosphorus, arsenic or antimony, four electrons form covalent bonds with the four neighbouring silicon atoms, but the fifth (valence) electron remains unbonded and is available for conduction, as shown in Fig 91 Thus, when a silicon (or germanium) crystal is doped with a pentavalent element, it develops excess free electrons and is said to be an n-type semiconductor Such impurities are known as donor impurities Fig 91: Covalent bonding in an n- type semiconductor 33

4 Experiments with Electrical and Electronic Circuits If silicon (or germanium) is doped with a trivalent (three electrons in the outermost shell) atom like boron, aluminium, gallium or indium, three valence electrons form covalent bonds with three silicon atoms and deficiency of one electron is created This deficiency (of an electron) is referred to as a hole It is shown in Fig 92 Such a semiconductor is said to be a p-type semiconductor and the impurities are known as acceptor impurities Fig 92: Covalent bonding in a p-type semiconductor Let us now discuss the formation of a p-n junction 93 A p-n JUNCTION The most useful form of semiconductor devices is obtained when p- and n- type semiconductors form a junction This is achieved by introducing donor impurities into one side and acceptor impurities into the other side of a single semiconducting crystal, as shown in Fig 93 Let us now understand how charge carriers behave in such a situation Fig 93: A p-n junction with depletion region 931 Operation of a p-n Junction 34 You now know that there is greater concentration of electrons in the n-region of the crystal and of holes in the p-region Because of this, electrons tend to diffuse to the p-region and holes to the n-region You may think that this process will continue indefinitely But it is not so The movement of electrons and holes creates (leaves behind) positively and negatively charged ions near the junction in n- and p-regions, respectively Due to accumulation of charges

5 near the junction, an electric field is established This gives rise to electrostatic potential, known as barrier potential This barrier has polarities, as shown in Fig 94 When there is no external electric field, this barrier prevents the movement of charge carriers across the junction and a narrow region near the junction is depleted in mobile charge carriers It is about 05 µm thick and is called the depletion region or space-charge region Semiconductor Diodes Fig 94: Barrier potential due to depletion region The barrier potential is characteristic of the semiconductor material It is about 03 ev for Ge and about 07 ev for Si The junction acts as a diode It is symbolically represented as shown in Fig 95 Here A corresponds to p-region and acts as an anode in a diode Similarly, K indicates n-region and corresponds to a cathode in a diode Fig 95: Symbol of a p-n junction (diode) 932 A Forward and Reverse Biased p-n Junction When an external electric field is applied to a p-n junction, as shown in Fig 96a, the p-end becomes positively biased and the n-end becomes negatively biased The junction is then said to be forward biased When the bias exceeds barrier potential, holes cross the junction from the p-region to the n-region Similarly, electrons cross the junction in the reverse direction This sets in a forward current in the diode The current increases with voltage and is of the order of a few milliampere Under the forward bias condition, the junction offers low resistance to flow of current The value of junction resistance, called forward resistance, is in the range 10 Ω to 30 Ω Fig 96: a) Forward biased; and b) reverse biased p-n junction When the terminals of the battery are reversed, ie p- and n-ends are connected to negative and positive terminals of the battery respectively as 35

6 Experiments with Electrical and Electronic Circuits To test a p-n junction using a multimeter, set the multimeter on resistance measurement mode Connect the junction in forward bias with the multimeter probes and measure its resistance Next, reverse the multimeter probes to measure the resistance of the junction in the other direction You will observe a large difference between these values shown in Fig 96b, the junction is said to be reverse biased In this case, holes in the p-region and electrons in the n-region move away from the junction Does it mean that no current shall flow in the circuit? No, a small current flows because a few electron-hole pairs are generated due to thermal excitations This small current caused by the minority carriers is called reverse saturation current or leakage current In most commercially available diodes, the reverse current is almost constant and independent of the applied reverse bias Its magnitude is of the order of a few nanoamperes to microamperes A p-n junction offers low resistance when forward biased, and high resistance when reverse biased You can easily test it using a multimeter This property of p-n junction is used for ac rectification 933 Identifying a Diode Semiconductor diodes are designated by two letters followed by a serial number The first letter indicates the material: A is used for material with a band gap of 06 ev to 10 ev such as germanium B is used for material with a band gap of 10 ev to 13 ev, such as silicon The second letter indicates the main application: A signifies detection diode, B denotes a variable capacitance diode, E for tunnel diode, Y for rectifying diode and Z denotes zener diode The serial numbers specify the diodes with particular values of power rating, peak reverse voltage, maximum current rating etc For example, BY127 and BZ148 respectively denote a silicon rectifier diode and a silicon zener diode You have to refer to manufacturer s catalogue to know exact details To make visual identification of anode and cathode, the diode manufacturers employ one of the following ways (typically shown in Fig 97): the symbol is painted on the body of the diode; red and blue marks are used on the body of the diode Red mark denotes anode, whereas blue indicates the cathode; Fig97: Identification of a diode (Printed with permission from M/s Power Technology, New Zealand) a small ring is printed at one end of the body of the diode that corresponds to the cathode Always work within the specified range of diode ratings to avoid damages to the device You are now ready to perform the first part of the experiment, ie, to draw the static characteristic curves of a p-n junction You will need the following apparatus Apparatus A general purpose p-n junction diode, a variable power supply with voltage range 0-10V, a voltmeter, a milliammeter (0-50mA), a resistance box, a microammeter (0-50µA), and a multimeter 36

7 934 I-V Characteristics of a p-n Junction Semiconductor Diodes First check that the junction is working properly using a multimeter Next make a circuit as shown in Fig 98 for forward bias I-V characteristics V s is a variable power supply Keep the voltage control in the minimum position and switch on the power supply Increase the voltage in steps of 01V and note the corresponding values of current, until an appreciable deflection is observed You will note that current in the circuit is small as long as the applied voltage is less than the barrier potential Once this potential is crossed, the current will increase rapidly with small increase in voltage The forward voltage required to get the junction in conduction mode is called knee voltage Beyond knee voltage, current increases rapidly Record your readings in Observation Table 91 Note that in no case, you should exceed the maximum forward current rating of the diode in the forward bias condition Fig 98: Circuit diagram for I-V characteristics of a p-n junction in forward bias Next decrease the voltage in same steps and note down the corresponding current values Record these also in Observation Table 91 Are the values of current same in both cases? Calculate the mean value of current for each value of V Observation Table 91: Forward biased junction characteristics SNo Forward voltage (V) Forward current (ma) Increasing voltage Decreasing voltage Mean forward current (ma) To study the reverse bias characteristics, the circuit is made as shown in Fig 99 In this case, the connections of the p-n junction have been reversed Also, the milliammeter is replaced by a microammeter since the reverse current is expected to be small 37

8 Experiments with Electrical and Electronic Circuits Fig 99: Circuit diagram for I-V characteristics of a p-n junction in reverse bias Vary the voltage (from minimum) in steps of one volt and record the corresponding current values in Observation Table 92 Here, you should not exceed the peak inverse voltage rating of the junction Observation Table 92: Reverse biased junction characteristics SNo Reverse voltage (V) Reverse current (µa) Increasing voltage Decreasing voltage Mean reverse current (µa) Draw the characteristic curves for both forward and reverse biased conditions by plotting voltage along x-axis and current along y-axis, as shown in Fig 910 From this graph, you can calculate the forward and reverse resistances as well as knee voltage Extrapolate the linear part of the forward bias characteristic curve to meet the x-axis The intercept on the x-axis gives the value of knee-voltage 38 Fig 910: I-V characteristics of a p-n junction diode

9 Calculations: From your plot of I-V characteristics, you can easily calculate forward resistance and reverse resistance using the following relations: and Result: Vf Rf = If Vr Rr = I r Forward resistance =Ω Reverse resistance =Ω Knee-voltage =V Semiconductor Diodes A conventional solid state diode does not allow flow of significant current if reverse bias is below its reverse break down voltage Once voltage across p-n junction exceeds reverse bias breakdown voltage, it is subject to high current flow due to Avalanche breakdown and can be permanently damaged You may now like to answer the following SAQ SAQ 1 : Diode characteristics Spend 4 min a) You are given a resistor and a p-n junction How would you identify these? b) How will you determine whether a p-n junction is made of silicon or germanium? What was the material of the junction you characterised in this experiment? Let us now learn about a special kind of diode, called zener diode 94 ZENER DIODE Zener diode allows current to flow not only in the forward direction like a rectifying diode, but also in the reverse direction, when the voltage is more than the breakdown voltage This voltage is also called zener voltage 941 Working of a Zener Diode The p- and n- regions in a zener diode are heavily doped These result in a thin depletion layer, due to availability of a large number of carriers for recombination near the junction However, the minority carriers present in the diode as a result of thermal excitations cannot cross the junction due to its barrier potential When a reverse bias is applied, a large electric field is established across the junction This field (i) accelerates the already available minority carriers, which, in turn, collide with the atoms of the semiconductor material and eject more electrons through energy transfer (avalanche effect), and (ii) breaks covalent bonds resulting in creation of additional electron-hole pairs in the junction region (zener effect) Both these processes give rise to large reverse current even for a small increase in reverse bias voltage This process is termed as zener breakdown However, since the (magnitude of) n p Fig 911: Symbol of zener diode 39

10 Experiments with Electrical and Electronic Circuits reverse voltage is small, the junction is not damaged In silicon diodes, zener effect dominates up to about 56V, and beyond this, avalanche effect prevails The symbol of zener diode is shown in Fig 911 A typical I-V characteristic plot of a zener diode is shown in Fig 912 The reverse breakdown voltage is indicated by V z Fig 912: I-V characteristics of a zener diode The zener breakdown voltage (V z ) is of great significance in the operation of zener diode as a voltage regulator You will learn it in the later part of this experiment 942 I-V Characteristics of a Zener Diode You now know that zener diode can sustain a constant voltage across it in reverse breakdown condition For this reason, it is always used as voltage reference in reverse bias Since resistance in breakdown region is very small, the current through the diode has to be limited by varying the resistance in the circuit The value of resistor is chosen in such a way that the product of zener breakdown voltage and reverse current through the zener, ie the power dissipated across the junction, is within the power handling capability of the diode If this limit is exceeded, a large current may damage the diode We now list the apparatus with which you will work, in this part of the experiment Apparatus Zener diode (with breakdown voltage in the range of 3 to 10V), variable voltage supply, voltmeter, milliameter and a resistor 40 The circuit to study forward I-V characteristics of zener diode is shown in Fig913a In this circuit, the value of resistor R is determined by the power rating of the zener diode The maximum current flowing through R should be less than the diode current rating I Z

11 Semiconductor Diodes Fig 913: Circuit diagram to determine I-V characteristics of zener diode in a) forward bias; and b) reverse bias Take a variable dc voltage supply V s in the range 0-15V If zener breakdown voltage (V z ) is 10V and maximum current rating (I z ), is 100 ma, the value of R is given by V = max V R z I z = ( 15 10) 100 ma V = 50Ω First connect zener diode in forward bias (anode to positive end and cathode to negative end) Take observations using the procedure outlined for p-n junction diode rectifier and record the readings in Observation Table 93 Observation Table 93: Forward bias characteristics of zener diode S No Forward voltage (V) Forward current (ma) With increasing voltage With decreasing voltage Mean forward current (ma) Now reverse the zener diode bias by connecting the cathode to the positiveend and the anode to the negative-end of supply This configuration is shown in Fig 913b Note that here also, you have to use a milliammeter Start the power supply from zero volt and increase voltage in steps of 1V Note down the voltage across the zener diode and the corresponding current flowing through the circuit Record your readings in Observation Table 94 Plot forward and reverse bias I-V characteristic curves of zener diode Do your curves resemble the I-V characteristics shown in Fig 912? 41

12 Experiments with Electrical and Electronic Circuits Observation Table 94: Reverse bias I-V characteristic of a zener diode S No Reverse voltage (V) When increasing voltage Reverse current (ma) When decreasing voltage Mean reverse current (ma) Result: Knee voltage =V Forward resistance =Ω Breakdown voltage =V Reverse resistance =Ω You may now like to answer the following SAQ Spend 4 min SAQ 2 : Zener diode characteristics Compare your results with those obtained in the previous part of the experiment and discuss the physics of differences 95 SOME APPLICATIONS OF SEMICONDUCTOR DIODES 951 Rectification of ac Fig 914: a) ac signal, b) half-wave rectification, c) full- wave rectification The general purpose p-n junction is used as a rectifier diode From your school physics classes, you may recall that conversion of ac voltage into dc voltage is known as rectification As you know, the ac voltage is sinusoidal (Fig 914a) When we place a diode in a circuit, it allows unidirectional current in the circuit As a result, negative half-cycle is eliminated and we obtain pulsating dc (Fig 914b) That is, the original signal has been modified (rectified) to the extent that only one-half part of the input is being used here Obviously it is not only of little use but inefficient also Therefore, in actual practice, we use two diodes in such a way that the negative half cycle is also made available in the circuit, as shown in Fig 914c Such an arrangement constitutes a full-wave rectifier circuit (Fig 915) 42

13 Semiconductor Diodes Fig 915: Full wave rectifier circuits: a) centre tapped and b) bridge rectifier In the circuit shown in Fig 915a, diode D 1 conducts in the positive half cycle, whereas diode D 2 conducts in the negative half cycle You must have observed that here we need a centre-tapped transformer, which is fairly costly However, the same action can be achieved with a normal transformer in a bridge circuit, which consists of four diodes (Fig 915b) Diodes D 1 and D 3 conduct in the positive half cycle, while D 2 and D 4 conduct in negative half cycle You will note that the circuit output exhibits fluctuations and can not be put to any practical use To minimise fluctuations, we use a pi-filter (π-filter) which consists of an inductor and two capacitors While a capacitor filters out ac component, an inductor allows maintenance of dc level For details you should refresh your knowledge by reading your 12 th standard physics book To understand the rectification action, you should build the circuits shown in Fig 915 and observe the input and output waveforms using a cathode ray oscilloscope (CRO) 952 Zener Diode as Voltage Regulator While studying the I-V characteristics of zener diode, you must have noted that in reverse bias condition, the voltage across zener diode remains constant at V z, and, independent of input voltage value when input is more than V z This characteristic of zener diode gives rise to a very interesting application in that it can be used as a constant voltage source If we connect a load across the zener diode, a constant voltage becomes available across it The circuit of zener regulated voltage supply is shown in Fig 916 R L is the load across which the stabilised voltage is obtained In this circuit, the excess voltage in dissipated across resistor R 43

14 Experiments with Electrical and Electronic Circuits a) Line Regulation Fig 916: Zener diode as a voltage regulator Fig 917: Line regulation by a zener diode Line regulation is a measure of regulation against any change in input voltage To study line regulation, the load is maintained at a fixed value (say 1kΩ) The input voltage (V i ) to the regulator is varied in steps of about 20V with the help of multiple tappings in the secondary of the transformer Note the corresponding output voltages (V o ) and record these in Observation Table 95 (You can also use a variable ac voltage supply in the form of a dimmerstat in the primary of the transformer to vary the input supply to the regulator) Take at least 12 readings, starting from zero volt Now draw a graph by plotting input ac voltage (V i ) along x-axis and the corresponding output voltage along y-axis We expect you to obtain a curve similar to that shown in Fig 917 The percentage change in the output voltage per unit change in the input ac voltage in the linear region of the graph gives line regulation Observations Table 95: Line regulation by a zener diode Load resistance, R L = 1kΩ SNo Input voltage, V i (V) Regulated voltage, V o (V) Result: Line Regulation =%

15 b) Load Regulation Load regulation is a measure of regulation against change in the load resistance, ie the current drawn from the regulator circuit To determine load regulation, we begin with no load in the circuit At constant input voltage, measure output voltage V NL and record the value in Observation Table 96 Next, connect a variable load resistance R L and decrease it from 1 kω to 100 Ω in steps of 100 Ω Note the output voltage V L in each case Calculate the load regulations using the relation VNL VR Load regulation = 100 VNL Semiconductor Diodes Fig 918: Load regulation by a zener diode Next, plot a graph by taking R L along the x-axis and the corresponding output voltage V R along the y-axis as shown in Fig 918 The minimum load resistance for the regulated output would be that value of R L for which V R begins to drop significantly Observation Table 96: Load Regulation Voltage without Load, V NL = V SNo R L (Ω) Voltage V R (V) Percentage regulation Result: Load regulation at maximum loading condition (at the minimum value of R L ) = % 45

16 Experiments with Electrical and Electronic Circuits Spend 5 min SAQ 3 : Zener voltage regulator In the circuit shown in Fig 916, suppose that V i = 124V, R L = 500 Ω, V Z = 6 V and R = 100 Ω Calculate i) Current through R ii) Current through zener diode iii) Power dissipation in the diode and R L 46