Self-study guidelines for the discipline “English for Specific purposes”, Module “Basics of Electronics” Goals and objectives The purpose of the individual student work is formation of competences, special knowledge and skills. This work teaches students to work individually and in a team, to show the skills required for professional and personal development. Self-study organization All students are divided into groups of 2 or 3 students in each. Each group studies the materials on one type of diode. Using the materials students compose mind map on given type of diode. Each group presents the mind map using whiteboard or flipchart. All other groups prepare and ask the questions on the topic. Example questions: 1. What is the normal operating region for a Zener diode? 2. Draw the symbols of a photodiode. Is it true that a photodiode is used in a reverse-bias position, and it will increase conduction as the light intensity increases? 3. Draw the Zener diode characteristic (V/A). Mark the Zener voltage VZ. 4. Why the Schottky diodes are used in very fast-switching circuits? 5. What do we call the process of emitting photons from a semiconductive material? 6. What diode is used in seven-segment displays: Zener, LED or Schottky? 7. Draw the symbol of a bidirectional Zener diode. Where can we use this device? Special-Purpose Diodes Diodes are known for their unidirectional current property. Basically, diodes are used for rectifying waveforms, and can be used within power supplies or within radio detectors. They can also be used in circuits where ‘one way’ effect of diode is required. Diodes transmit electric currents in one direction, however, the manner in which they do so can vary. Several types of diodes are available for the use in the electronics design. Some of diode types are: Zener Diode: This type of the diode provides a stable reference voltage. It is a very useful type and has a wide application. The diode runs in reverse bias, and breaks down at a certain voltage. A stable voltage is produced, if the current flowing through the resistor is limited. Schottky Diodes: These diodes feature a lower forward voltage drop if compared to the ordinary silicon p-n junction diodes. The voltage drop may be in the range from 0.15 to 0.4 volts at a low current, if compared to the 0.6 volts for a silicon diode. Photodiode: Photodiodes are usually used to detect light. Generally, these diodes operate in reverse bias, wherein even a small current flow, resulting from the light, can be detected. Photodiodes can be used to generate electricity, used as solar cells and even in photometry. Light Emitting Diode (LED): It is one of the most popular types of diodes. When this diode permits the transfer of electric current between the electrodes, light is produced. The color of light depends on the energy gap of the semiconductor. Avalanche Diode: This type of a diode operates in the reverse bias, and uses avalanche effect. The avalanche breakdown takes place across the entire p-n junction, when the voltage drop is constant and is independent of the current. Generally, the avalanche diode is used for photo-detection, wherein high levels of sensitivity can be obtained by the avalanche process. Laser Diode: This type of a diode is different from the LED type, as it produces coherent light. These diodes are used in DVD and CD drives, laser pointers, etc. Laser diodes are more expensive than LEDs. However, they are cheaper than other laser generators. Varicap Diode or Varactor Diode: This type of a diode uses a reverse bias placed upon it, which varies the width of the depletion layer depending on the voltage applied to the diode. This diode acts as a capacitor. By altering the bias on the diode, the width of the depletion region changes, thereby varying the capacitance. Rectifier Diode: These diodes are used to rectify alternating power inputs in power supplies. They can rectify current levels that range from an amp upwards. If low voltage drops are required, Schottky diodes can be used, however, generally they are p-n junction diodes. (a) (b) (c) (d) (e) (f) (g) (h) Figure 1. Diode symbols: (a) diode; (b) Zener diode; (c) bidirectional Zener diode; (d) tunnel diode; (e) Schottky diode; (f) varicap diode; (g) photodiode; (h) light emitting diode Diodes are used widely in electronics, from design to production. Besides the above mentioned types, other diodes are PIN diodes, point contact diodes, signal diodes, step recovery diodes, tunnel diodes and gold doped diodes. The diode type to transfer the electric current depends on the type and amount of the transmission, as well as on specific applications. Zener Diode A Zener diode is a special kind of a diode which allows the current to flow in the forward direction in the same manner as an ideal diode, but it also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, ‘Zener knee voltage’ or ‘Zener voltage’. The device was named after Clarence Zener, who discovered this electrical property. A conventional solid-state diode does not allow the significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to the high current due to the avalanche breakdown. Unless this current is limited by the circuitry, the diode will be permanently damaged due to the overheating. In the case of a large forward bias, the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and doping concentrations. A Zener diode exhibits almost the same properties, except the device is specially designed to have a greatly reduced breakdown voltage, the so-called Zener voltage. The volt-ampere characteristic of a Zener diode showing the breakdown region is shown in Figure 4.6. By contrast with the conventional device, a reverse-biased Zener diode exhibits a controlled breakdown and allows the current to keep the voltage across the Zener diode close to the Zener breakdown voltage. For example, a diode with a Zener breakdown voltage of 4.7 V exhibits a voltage drop of very nearly 4.7 V across a wide range of reverse currents. The Zener diode is therefore ideal for various applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications. +IF Cathode Anode Forward K A Current Forward Bias Region Zener -VZ Reverse Bias Voltage +V -VR F I Forward Bias Z(min) VF “Zener” Breakdown Region IZ(max) Reverse Current -IR Figure 2. Zener diode symbol and I-V characteristics The Zener diode’s operation depends on the heavy doping of its p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states. This occurs as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts. Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. These two types of the diode are in fact constructed in the same way and both effects are present in diodes of this type. In silicon diodes up to 5.6 volts, the Zener effect is predominant and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient. In a 5.6 V diode, the two effects occur together and their temperature coefficients nearly cancel each other out, thus a 5.6 V diode is the component of choice in temperature-critical applications. Modern manufacturing techniques have produced devices with the voltage lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode. All such diodes are usually marketed under the term of ‘Zener diode’. The Zener Diode Regulator Zener diodes can be used to produce a stabilized voltage output with low ripple under varying load current conditions (Fig. 4.7). A small current passes through the diode from a voltage source via a suitable current limiting resistor (RS). Then the Zener diode will conduct sufficient current to maintain a voltage drop of Vout. We remember from the previous chapters that the d.c. output voltage from the half or full-wave rectifiers contains ripple superimposed onto the d.c. voltage. By connecting a simple zener stabilizer circuit as shown below across the output of the rectifier, a more stable output voltage can be produced. The resistor, RS is connected in series with the Zener diode to limit the current flow through the diode with the voltage source, VS being connected across the combination. The stabilised output voltage Vout is taken from across the Zener diode. The Zener diode is connected with its cathode terminal connected to the positive rail of the d.c.