2018-01-08
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Start here
1. Read the text below. Match the descriptions 1- 3 with the diagrams a-c in Fig. 2.5. Put the descriptions 1-3 in logical order. Indicate the n-type and p-type regions, holes and electrons, reverse and forward bias.
a)
b)
c)
Fig. 2.5. How a p-n junction works.
A diode is a two-terminal device, having two active electrodes, between which it allows the transfer of current in one direction only. Diodes are known for their unidirectional current property, wherein, the electric current is allowed to flow in one direction.
1) Suppose you connect a battery to this little p-type/n-type junction. What will happen? It depends which way the battery is connected. If you put it so that the battery's negative terminal joins the n-type silicon, and the battery's positive terminal joins the p-type silicon, the depletion zone shrinks drastically. Electrons and holes move across the junction in opposite directions and current flows. This is called forward-bias.
2) However, if you reverse the current, all that happens is that the depletion zone gets wider. All the holes push up toward one end, all the electrons push up to the other end, and no current flows at all. This is called reverse-bias.
3) Interesting things happen when you start putting p-type and n-type silicon together. Suppose you join a piece of n-type silicon (with slightly too many electrons) to a piece of p-type silicon (with slightly too few). What will happen? Some of the extra electrons in the n-type will nip across the join (which is called a junction) into the holes in the p-type so, either side of the junction, we'll get normal silicon forming again with neither too many nor too few electrons in it. Since ordinary silicon doesn't conduct electricity, nor does this junction. Effectively it becomes a barrier between the n-type and p-type silicon and we call it a depletion zone because it contains no free electrons or holes.
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That's how an ordinary diode works and why it allows an electric current to flow through it only one way. Think of a diode as an electrical one-way street.
Grammar
2. Study the sentences below and pay attention to the underlined verb forms in italics.
| I am an English teacher. I am teaching English now. |
| What are you? What are you doing now? |
| You are students. You are studying English now. You are listening to me. |
| Aren’t you? |
| This is Andrew. He is a future electronics engineer. He is learning about diodes at this moment. |
| That is Helen. She is learning English Grammar. |
| What are we doing? What are they doing? |
Answer the following questions:
1) What are these verb forms?
2) am, is, are. What verb has such forms?
3) teaching, doing, studying, listening, learning. What do these verb forms have in common?
4) What grammatical notion is it? Is it a tense? What tense is it?
It is the Present Continuous or Present Progressive Tense.
3. How do we form sentences in the Present Continuous? Complete the table.
| Positive | Negative | Interrogative |
| Subject + am, is, are+ Verb + ing. | Subject + am, is, are+ not Verb + ing. | Am, is, are + Subject+Verb + ing? |
| I am teaching. You are studying. He She It We They | I am not studying. You are not teaching. | Am I studying? Are you teaching? |
4. So, now we can form the Present Continuous. But when do we use the Present Continuous? Study the following table and give your own examples.
| Actions are happening… | Examples |
now
right now
at the moment of speaking
at this very moment
| · I am teaching English now. · You are not learning German right now, because this is an English class. · At this very moment you are studying the Present Continuous. |
around now
around the moment of speaking
currently,
for longer actions in progress
| · He is studying to become an electronics engineer. · Are you working on any special projects at work? · Elizabeth is currently writing a children’s book. · They are working hard to earn money. |
| this week, month, year these days, for temporary actions | · I’ m riding a bike to get to work because my car is broken. · Helen is studying really hard for her exams this month. · We aren't working hard these days. |
tonight
tomorrow
next week, month, year,
for future arrangements and plans
| · I am going to the theatre tonight. · Polly is coming for dinner tomorrow. · They are eating at a restaurant next week. · He is flying to Rome in September. · We are meeting our friends in the evening. |
always
constatly
continually,
for irritating repeated actions
| · She is continually complaining about everything! · John is always asking stupid questions! · You are constantly coming to class late! |
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5. Make a presentation on the topic: “Types of diodes”. You may present all types of diodes, or two of them to compare, or just the only one. Use the information in the table to help you.
| How do I start? | · You could grab your audience's attention by starting with a question or a challenging statement. Use pictures or objects. So, how much do you know about...? Have you ever asked yourself why...? What I'm going to tell you about today will change the way you think about... Pass around the picture/object. What do you think it is? |
| How do I organize the presentation? | · Make it short. Write down the points you want to make, edit them, then decide which order you are going to make them in. Introduce each point with an expression from the list below. The first/key thing to say about _________ is... The main point to make about _________ is… What you really need to know about__________ is... Now let's look at... Let's turn to/move on to... Another interesting thing to say about__________ is... Finally, I'd like to say a few words about... |
| What do I say? | · After introducing the point, add information briefly in two, three, or, at the most, four sentences. Use markers like the ones below to construct long, well-balanced sentences. Anyway...; Naturally...; Of course... Similarly...; Surprisingly …; Remarkably... Despite...; However...; Although...; Whereas... Consequently...; In addition...; Moreover...; Furthermore... Incidentally...; By the way...; It's worth noting that... |
| How do I finish? | · Conclude the presentation by briefly summarizing what you have said, or the points you have made. You could end by asking for comments or questions. In conclusion...; To sum up... So, remember that……..is all about ……., and... So, there are three things to remember about… Does anybody have any questions? |
TYPES OF DIODES
Public Speaking
1. Deliver your presentations on different types of diodes.
Test yourself
2. Match the definitions 1 – 9 with the types of diodes in the Fig. 2.6.
Fig. 2.6.Types of diodes.
1) ……………. is also called snap-off diode or charge-storage diode or memory varactor that has the ability to generate extremely short pulses and is used in microwave electronics as pulse generator or parametric amplifier.
2) ……………. is also known as a transferred electron device (TED) and used in high-frequency electronics. Its internal construction is unlike other diodes, it consists only of n-doped semiconductor material.
3) ……………. is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage.
4) …………… uses the barrier formed between a specially prepared semiconductor surface and a metal point to produce the rectifying action.
5) …………… allow a current through them to rise to a certain value, and then level off at a specific value. Unlike Zener diodes, these diodes keep the current constant instead of the voltage constant. These devices keep the current flowing through them unchanged when the voltage changes.
6) …………… is used to protect sensitive electronics from voltage spikes induced on connected wires. It is a clamping device, suppressing all overvoltages above its breakdown voltage.
7) …………… incorporate the low forward voltage drop of the Schottky diode with the surge-handling capability and low reverse leakage current of a normal p–n junction diode.
8) ……………. is capable of very fast operation, well into the microwave frequency region, by using the quantum mechanical effect called tunneling.
9) ……………. emits invisible light when an electric current passes through it.
3. Do this multiple choice test on types of diodes.
1) Light-emitting diodes are:
a) used as indicator lamps in many devices and are increasingly used for other lighting;
b) capable of converting light into either current or voltage, depending upon the mode of operation.
2) Laser diode:
a) acts as a heat pump which can cool or warm when current is passed through it;
b) produces coherent radiation in the visible or infrared (IR) spectrum when current passes through it.
3) Varactor diode is
a) a type of diode whose capacitance varies as a function of the voltage applied across its terminals;
b) a specialized diode that has a layer of intrinsic semiconductor material between the P and N materials.
4) Schottky diode is:
a) designed to pass very small currents, and have several applications in signal processing;
b) also known as hot carrier diode with a low forward voltage drop and a very fast switching action.
5) A Shockley diode is:
a) identical to a thyristor with its gate left disconnected;
b) is a two-electrode vacuum tube.
6) A Silicon-Controlled Rectifier (SCR) is:
a) designed to go through avalanche breakdown at a specified reverse bias voltage;
b) a four-layer solid state current controlling device.
4. Match the types of diodes with their electronic symbols. The symbol used for a semiconductor diode in a circuit diagram specifies the type of diode.
| Tunnel diode Varicap Schottky diode Transient voltage suppression (TVS) diode Light - emitting diode (LED) Diode Zener diode Photodiode |
DIODE APPLICATIONS
Start here
1. Work in small groups. Study the diagrams in Fig. 2.7 and discuss the questions below.
a)
b)
c)
Fig. 2. 7. Diode applications.
1) What diode applications are shown in the diagrams?
2) What is the difference between a modulated signal and a demodulated one?
Listening
2. Listen to the diagram descriptions. With your group, match the descriptions 1 – 3 with the diagrams a – c in Fig. 2.7. Come up with the titles for the diagrams a – c.
1) A diode can be used as a temperature measuring device, since the forward voltage drop across the diode depends on temperature, as in a silicon band gap temperature sensor.
2) The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. AM signals consist of alternating positive and negative peaks of a radio carrier wave, whose amplitude or envelope is proportional to the original audio signal. The diode (originally a crystal diode) rectifies the AM radio frequency signal, leaving only the positive peaks of the carrier wave. The audio is then ex tracted from the rectified carrier wave using a simple filter and fed into an audio amplifier or transducer, which generates sound waves.
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3) Diode logic (DL) or diode-resistor logic constructs Boolean logic gates from diodes acting as electrically operated switches. While diode logic has the advantage of simplicity, the lack of an amplifying stage in each gate limits its application. Not all logical functions can be implemented in diode logic. Only the non-inverting logical AND and logical OR functions can be realized by diode gates. If several diode logic gates are cascaded, the voltage levels at each stage are significantly changed, so one-stage applications are used. Diodes can be combined with other components to construct AND and OR logic gates.
Reading
3. Read the text and make your own questions to it.
Power conversion
Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Automotive alternators are a common example, where the diode, which rectifies the AC into DC, provides better performance than the commutator or earlier, dynamo.
