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1 Exercise 1 The Diode EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the operation of a diode. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Description of a diode Operating principles of a diode Characteristic E-I curve of a silicon diode Semiconductors DISCUSSION Description of a diode A diode is a two-terminal semiconductor device that acts similar to a switch which has no movable parts. The two terminals (or electrodes) are called the anode (A) and the cathode (K). The schematic symbol for a diode and its physical representation are shown in Figure 4. The electrode at the end of the diode case marked with a band is the cathode while the electrode at the other end is the anode. The arrowhead in the diode symbol points toward the cathode. The direction of the diode symbol s arrowhead points against the direction of electron flow. Scientists of the 17 th century arbitrarily decided that current flows from the positive terminal to the negative terminal. This so-called conventional current direction is still used today, and is the accepted direction of current flow, but it is worth noting that the actual direction of electron flow is opposite to the conventional current direction. Anode Cathode Schematic symbol Physical representation Band Figure 4. Schematic symbol and physical representation of a diode. The most common kind of diodes is the semiconductor diode. Semiconductors are materials that have a certain resistance at room temperature. Therefore, they are neither good conductors nor good insulators. However, applying a voltage to a semiconductor greatly varies its resistance. With certain voltage values, the resistance decreases dramatically and the semiconductor becomes a good conductor. Conversely, with certain other voltage values, the resistance increases dramatically and the semiconductor becomes an excellent insulator. This explains why these materials are referred to as semiconductors. Silicon and germanium are the two most commonly used semiconductor materials, silicon being the prevalent material used in modern electronics components. Festo Didactic

2 Operating principles of a diode Although it has no moving parts, a diode acts like a high-speed switch whose contacts open and close according to the following rules: When no voltage is applied across a diode, it acts as an open switch, and no current flows between A and K. See Figure 5. The letter V can also be used to represent the voltage. 0 Figure 5. When no voltage is applied across a diode, it acts as an open switch. When a reverse voltage is applied across the diode, so that the anode is negative with respect to the cathode, the diode continues to act as an open switch. In this case, the diode is said to be reverse biased. See Figure 6. Figure 6. When the anode is negative with respect to the cathode, the diode acts as an open switch. When a forward voltage is applied across the diode, so that the anode is positive with respect to the cathode, the terminals (or electrodes) become short-circuited. The diode acts as a closed switch and a current immediately flows from the anode to the cathode. In this case, the diode is said to be forward biased. See Figure 7. Figure 7. When the anode is positive with respect to the cathode, the diode acts as a closed switch. As long as current flows between the anode and the cathode, the diode acts as a closed switch. When current stops flowing, the diode returns to its original open state. See Figure 8. 0 Figure 8. As long as a current flows between the anode and the cathode, the diode acts as a closed switch. In summary, a diode turns on only when a forward voltage is applied. It remains on, until the current stops flowing. 4 Festo Didactic

3 Characteristic E-I curve of a silicon diode The voltage-current - characteristic curve of a silicon diode is shown in Figure 9. The curve shows that virtually no current flows when the diode is reverse biased, but that the current increases very rapidly when it is forward biased. The curve also shows that a voltage drop occurs at the diode terminals when it is forward biased. The voltage drop increases so little with current that it is often considered constant and equal to about 0.7 V in the case of silicon diodes. 0.7 V Figure 9. Characteristic - curve of a silicon diode. Semiconductors a This section is given as information. It will help you understand how solar cells work, but it is not required to perform the exercises. The properties of any solid material, including semiconductors, depend upon the nature of the constituent atoms and upon the way in which the atoms are grouped together. In other words, these properties are a function of both the atomic structure of the atoms and the crystal structure of the solid. An atom consists of a positively charged nucleus (proton) surrounded by electrons located in discrete orbits. Electrons can exist in stable orbits near the nucleus only for certain discrete values of energy called energy levels of the atom. When atoms come close together to form a solid crystal, electrons in upper levels of adjacent atoms interact to bind the atoms together. Because of the interaction between these outer, or valence, electrons, the upper energy levels can be drastically altered. Elements of the periodic table can be classified according to the number of electrons in their outer orbital (valence electrons). Generally, elements located on the same column of the periodic table share the same amount of valence electrons. Thus, carbon, silicon, and germanium all have four valence electrons. Because of this, these three elements can form crystals that are similar in Festo Didactic

4 appearance to metals. This happens when their four valence electrons each form a perfect covalent bond with a valence electron from a neighboring atom, thus resulting in the lattice shown in Figure 10. Figure 10. Lattice formed by silicon atoms. As the figure shows, all valence electrons of silicon crystals are engaged in perfect covalent bonds and thus cannot be exchanged between atoms. Because of this, in its pure form, a silicon crystal severely impedes the flow of electrical current, thereby acting as an insulator. This is contrary to other types of elements such as metals, for example, whose outer orbital usually contains one or more valence electrons, thereby allowing the flow of electrical current. Doping However, a crystal of silicon or germanium can be rendered conductive of electrical current by mixing a small amount of impurity into the crystal. This process is named doping. Two types of impurities can be introduced into the crystal: N-type and P-type. The name of these impurities is related to the type of charge they add to the crystal. N-type doping adds an electron, resulting in a negative (N) charge, while P-type doping removes an electron, resulting in a positive (P) charge. N-type doping is achieved by inserting minute amounts of phosphorus or arsenic in the crystal, as shown in Figure 11. These two elements are used because their atoms have five valence electrons (one more than silicon and germanium, which have four). This means that, when these doping elements are added to the crystal lattice, they have an extra valence electron that can be exchanged between atoms, as it is not bonded to any neighbouring atom. This extra valence electron allows electrical current to flow through the crystal, thus transforming it from an insulator to a good conductor. Note that only small amounts of N-type impurities are required to provide enough valence electrons for the flow of electrical current. 6 Festo Didactic

These two elements are used because their atoms have three valence electrons (one less than silicon and germanium, which have four).

5 Extra valence electron Phosphorus or arsenic atom Figure 11. Silicon doped with N-type impurities. P-type doping is achieved by inserting minute amounts of boron or gallium in the crystal, as shown in Figure 12. These two elements are used because their atoms have three valence electrons (one less than silicon and germanium, which have four). This means that, when these doping elements are added to the crystal lattice, they liberate one valence electron of an atom from its covalent bond and create a hole in the lattice. The unbonded valence electron is then free to form a covalent bond with a valence electron from a neighbouring atom. This displaces the hole, thereby allowing electrical current to flow through the crystal and transforming it from an insulator to a good conductor. Note that only small amounts of P-type impurities are required to provide enough missing valence electrons for the flow of electrical current. Boron or gallium atom Missing valence electron (hole) Figure 12. Silicon doped with P-type impurities. The P-N junction It is possible to create a crystal with new properties by combining P-type crystal on one side and N-type crystal on the other side, as shown in Figure 13. The resulting crystal is designated as a semiconductor crystal. As explained in the previous section, P-type crystal contains positive charges (in the form of missing valence electrons or holes), while N-type crystal contains negative charges (in the form of extra valence electrons). At or near the junction between the two materials, the extra valence electrons contained in the N-type crystal bond with the free valence electrons of the P-type crystal, thus taking the place of the missing valence electrons and filling the holes. Due to these transfers of electrons, the N-type crystal acquires a positive charge (due to the outflow of electrons), while the P-type crystal acquires a negative charge (due to the influx of electrons). In the process, the area of crystal at or near the junction loses most of its charge carriers (i.e., the negative extra valence electrons and the positive missing valence electrons or holes). This area is known as the depletion region. Festo Didactic

Due to the absence of most charge carriers, the depletion region prevents the flow of electrical current and acts as an insulator, just as non-doped crystal.

The creation of the depletion region and the separation of electric charges along the P-N junction prevent the semiconductor crystal from conducting electrical current.

6 Due to the absence of most charge carriers, the depletion region prevents the flow of electrical current and acts as an insulator, just as non-doped crystal. Electrons N-type P-type Holes Depletion region Figure 13. Semiconductor built using P-type crystal on one side and N-type crystal on the other side. The creation of the depletion region and the separation of electric charges along the P-N junction prevent the semiconductor crystal from conducting electrical current. In order to make the semiconductor crystal conduct, it is necessary to overcome the potential barrier of the crystal (i.e., to overcome the force preventing the flow of electrons through the semiconductor crystal). This is achieved by applying to the crystal a voltage of a sufficient magnitude. The magnitude of this voltage (and thus the magnitude of the potential barrier of the semiconductor crystal) depends on the elements composing the crystal. For example, silicon semiconductor crystals have a higher potential barrier and thus necessitate a higher voltage to conduct electrical current than germanium semiconductor crystals. Consider the circuit in Figure 14 showing a voltage source whose negative terminal is connected to the N-type crystal of a semiconductor crystal, while its positive terminal is connected to the P-type crystal. Since electrons flow from the negative terminal of the voltage source to the positive terminal, they flow toward the P-N junction by passing through the N-type crystal. Meanwhile, electrons flow from the P-type crystal toward the positive terminal of the voltage source, thus creating more missing valence electrons or holes, which migrate toward the P-N junction. As the charge carriers in both types of crystal flow toward the P-N junction, the width of the depletion region decreases more and more. If the voltage applied to the semiconductor crystal is high enough and the potential barrier is overcome, the depletion region becomes too small to prevent the passage of electrons and electrical current can flow across the semiconductor crystal. When the terminals of the voltage source are connected in this way to the semiconductor crystal (i.e., when electrical current is allowed to flow), it is said to be forward biased. N-type P-type Electrons flow in conductors Figure 14. In a forward-biased semiconductor crystal, the charge carriers migrate toward the P-N junction. 8 Festo Didactic

This prevents the passage of electrons and thus the flow of electrical current across the semiconductor crystal.

7 Consider now that the polarity of the voltage source is reversed, as shown in the circuit of Figure 15. This causes both the extra valence electrons and the missing valence electrons to migrate away from the P-N junction, thus increasing the width of the depletion region. This prevents the passage of electrons and thus the flow of electrical current across the semiconductor crystal. When the terminals of the voltage source are connected in this way to the semiconductor crystal (i.e., when electrical current is prevented from flowing), it is said to be reverse biased. N-type P-type Depletion region Negligible current Figure 15. In a reverse-biased semiconductor crystal, the charge carriers migrate away from the P-N junction. A P-N junction made of silicon crystal is in fact a silicon diode. The cathode of a diode symbol corresponds to the N-type semiconductor. The anode corresponds to the P-type semiconductor. See Figure 16. N-type Extra valence electrons P-type Missing valence electrons (holes) Cathode Anode Figure 16. Schematic symbol and P-N junction representation of a diode. In a forward-biased diode, any increase in the voltage applied to it causes the amount of current flowing through the diode to increase slightly. When the voltage applied to a diode approaches the voltage necessary to overcome the potential barrier of the semiconductor crystal composing the diode (0.6 V in the case of silicon crystal), the amount of current flowing through the diode is small but measurable. However, as soon as the potential barrier of the semiconductor crystal is overcome, the amount of current flowing in the diode increases significantly. Applying a too high voltage to a diode can seriously damage it. The voltage necessary to overcome the potential barrier of a diode is called the forward voltage and its magnitude depends on the type of crystal used in the diode. The forward voltage of silicon ranges from 0.6 V to 0.7 V, while it ranges from 0.2 V to 0.3 V for germanium. The nominal value of the forward current of a diode can range from a few milliamperes to thousands of amperes. Festo Didactic

8 Figure 17. Quality inspection of a solar cell during manufacturing (photo courtesy of [[User:]]). 10 Festo Didactic

9 Exercise 1 The Diode Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Observation of the diodes on the Solar Panel Test Bench Characteristic E-I curve of a diode Forward-biased diode in a simple circuit Reverse-biased diode in a simple circuit PROCEDURE Observation of the diodes on the Solar Panel Test Bench In this part of the exercise, you will locate the diodes on the Solar Panel Test Bench and compare the position of the band on the diode case and the symbol silkscreened on the front panel to the theory presented in the discussion. 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise. 2. On the Solar Panel Test Bench, observe that there are three diode symbols silkscreened on the front panel. Locate the diodes on the back side of the front panel and observe the position of the band on the case of the diodes. 3. Does the position of the band correspond with the position of the cathode on the diode symbol silkscreened on the front panel as indicated in the discussion? Yes No Characteristic E-I curve of a diode In this part of the exercise, you will plot the characteristic - curve of a diode and observe how the current varies when the diode is forward biased and reverse biased. 4. Set up the circuit shown in Figure 18. Use one of the batteries in the Lead-Acid Batteries module as the voltage source, and the potentiometer and a diode on the Solar Panel Test Bench. Potentiometer Figure 18. Circuit used to determine the characteristic - curve of a diode. Festo Didactic

10 Exercise 1 The Diode Procedure 5. Using the potentiometer, set the voltage at the diode terminals to the values shown in Table 1. For each voltage setting, record the corresponding current shown by ammeter I. a The battery connections shown in Figure 18 must be reversed to apply a negative voltage to the diode. Table 1. Characteristic - curve of a diode. Voltage (V) Current (ma) Voltage (V) Current (ma) (1) 0.20 (1) Set the voltage so that the current does not exceed 100 ma 6. Using the values in Table 1, plot the characteristic - curve of the diode in Figure Current (ma) Voltage (V) Figure 19. Characteristic - curve of a diode. 7. Describe the operation of the diode when it is reverse biased. 12 Festo Didactic

11 Exercise 1 The Diode Procedure 8. Describe how the current varies when the voltage at the diode terminals varies from 0 V to 0.8 V. Forward-biased diode in a simple circuit In this part of the exercise, you will supply an electrical load (variable resistor) via a forward-biased diode and observe the circuit current and load voltage. 9. Modify your circuit as shown in Figure 20 to use the potentiometer in the Solar Panel Test Bench as an electrical load (variable resistor). Figure 20. Forward-biased diode in a simple circuit. 10. Turn the potentiometer knob fully counterclockwise, then turn it five turns in the clockwise direction. Does current flow in the circuit? Explain why. 11. Measure the source voltage and load voltage. Source voltage V Load voltage V Festo Didactic

12 Exercise 1 The Diode Conclusion 12. Considering that the diode acts as a closed switch, explain why the source voltage and the load voltage are not equal. Reverse-biased diode in a simple circuit In this part of the exercise, you will supply a load via a reverse-biased diode and observe the circuit current and load voltage. 13. Reverse the connections at the diode in your circuit. This reverses the diode orientation in your circuit, and thus, applies a reverse-bias voltage to the diode. Does current flow in the circuit? Explain why. 14. Measure the source voltage and the load voltage. Source voltage V Load voltage V 15. Explain why the load voltage is null. 16. Confirm your explanation by measuring the anode-cathode voltage. Anode-cathode voltage V CONCLUSION In this exercise, you learned how a diode operates. You saw that when the anode is positive with respect to the cathode, the diode acts as a closed switch and current flows from the anode to the cathode. Conversely, you saw that when the anode is negative with respect to the cathode, the diode acts as an open switch and no current flows from the anode to the cathode. You also learned that there is a small voltage drop at the diode terminals when it is forward biased. REVIEW QUESTIONS 1. Complete the following sentence: The electrode marked with a band at the end of the diode case refers to the cathode. anode. 14 Festo Didactic

13 Exercise 1 The Diode Review Questions 2. Explain why there is a voltage drop across the diode when it is forward biased. 3. Name the most commonly used semiconductor materials. 4. The arrowhead of the diode symbol points toward the anode. True False 5. At certain voltage values, the resistance of a diode increases dramatically and the diode becomes an excellent insulator. True False Festo Didactic

Exercise 12. Semiconductors EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION. Introduction to semiconductors. The diode

Exercise 12. Semiconductors EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION. Introduction to semiconductors. The diode Exercise 12 Semiconductors EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the operation of a diode. You will learn how to use a diode to rectify ac voltage to produce