History of semiconductor device development

Transcription

1 History of semiconductor device development 1900s Semiconductors had been used in the electronics field for some time before the invention of the transistor. Around the turn of the 20th century they were quite common as detectors in radios, used in a device called a "cat's whisker". These detectors were somewhat troublesome, however, requiring the operator to move a small tungsten filament (the whisker) around the surface of a galena (lead sulfide) or carborundum (silicon carbide) crystal until it suddenly started working. Then, over a period of a few hours or days, the cat's whisker would slowly stop working and the process would have to be repeated. At the time their operation was completely mysterious. After the introduction of the more reliable and amplified vacuum tube based radios, the cat's whisker systems quickly disappeared. The "cat's whisker" is a primitive example of a special type of diode still popular today, called a Schottky diode. World War II During World War II, radar research quickly pushed radar receivers to operate at ever higher frequencies and the traditional tube based radio receivers no longer worked well. The introduction of the cavity magnetron from Britain to the United States in 1940 during the Tizard Mission resulted in a pressing need for a practical high-frequency amplifier. On a whim, Russell Ohl of Bell Laboratories decided to try a cat's whisker. By this point they had not been in use for a number of years, and no one at the labs had one. After hunting one down at a used radio store in Manhattan, he found that it worked much better than tube-based systems. Ohl investigated why the cat's whisker functioned so well. He spent most of 1939 trying to grow more pure versions of the crystals. He soon found that with higher quality crystals their finicky behaviour went away, but so did their ability to operate as a radio detector. One day he found one of his purest crystals nevertheless worked well, and interestingly, it had a clearly visible crack near the middle. However as he moved about the room trying to test it, the detector would mysteriously work, and then stop again. After some study he found that the behaviour was controlled by the light in the room more light caused more conductance in the crystal. He invited several other people to see this crystal, and Walter Brattain immediately realized there was some sort of junction at the crack. Further research cleared up the remaining mystery. The crystal had cracked because either side contained very slightly different amounts of the impurities Ohl could not remove about 0.2%. One side of the crystal had impurities that added extra electrons (the carriers of electrical current) and made it a "conductor". The other had impurities that

2 wanted to bind to these electrons, making it (what he called) an "insulator". Because the two parts of the crystal were in contact with each other, the electrons could be pushed out of the conductive side which had extra electrons (soon to be known as the emitter) and replaced by new ones being provided (from a battery, for instance) where they would flow into the insulating portion and be collected by the whisker filament (named the collector). However, when the voltage was reversed the electrons being pushed into the collector would quickly fill up the "holes" (the electron-needy impurities), and conduction would stop almost instantly. This junction of the two crystals (or parts of one crystal) created a solid-state diode, and the concept soon became known as semiconduction. The mechanism of action when the diode is off has to do with the separation of charge carriers around the junction. This is called a "depletion region". Semiconductor device fundamentals The main reason semiconductor materials are so useful is that the behaviour of a semiconductor can be easily manipulated by the addition of impurities, known as doping. Semiconductor conductivity can be controlled by introduction of an electric field, by exposure to light, and even pressure and heat; thus, semiconductors can make excellent sensors. Current conduction in a semiconductor occurs via mobile or "free" electrons and holes, collectively known as charge carriers. Doping a semiconductor such as silicon with a small amount of impurity atoms, such as phosphorus or boron, greatly increases the number of free electrons or holes within the semiconductor. When a doped semiconductor contains excess holes it is called "p-type", and when it contains excess free electrons it is known as "n-type", where p (positive for holes) or n (negative for electrons) is the sign of the charge of the majority mobile charge carriers. The junctions which form where n-type and p-type semiconductors join together are called p-n junctions. SEMICONDUCTOR : Definition Semiconductors are materials which are neither conductors or insulators, having conductivities intermediate to those of conductors like copper and insulators like wood or plastic. Common semiconductors are Silicon and Germanium. The reason semiconductors are important is that with some engineering they can sometimes both conduct and insulate depending on their connections. Thus they serve as the basis for switching and amplification, the fundamental actions of computer elements.

3 Semiconductor device fabrication NASA's Glenn Research Center cleanroom. Semiconductor device fabrication is the process used to create chips, the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is the most commonly used semiconductor material today, along with various compound semiconductors. The entire manufacturing process from start to packaged chips ready for shipment takes six to eight weeks and is performed in highly specialized facilities referred to as fabs. Semiconductor device applications All transistor types can be used as the building blocks of logic gates, which are fundamental in the design of digital circuits. In digital circuits like microprocessors, transistors act as on-off switches; in the MOSFET, for instance, the voltage applied to the gate determines whether the switch is on or off. Transistors used for analog circuits do not act as on-off switches; rather, they respond to a continuous range of inputs with a continuous range of outputs. Common analog circuits include amplifiers and oscillators. Circuits that interface or translate between digital circuits and analog circuits are known as mixed-signal circuits. Power semiconductor devices are discrete devices or integrated circuits intended for high current or high voltage applications. Power integrated circuits combine IC technology

4 with power semiconductor technology, these are sometimes referred to as "smart" power devices. Several companies specialize in manufacturing power semiconductors. Semiconductor device materials By far, silicon (Si) is the most widely used material in semiconductor devices. Its combination of low raw material cost, relatively simple processing, and a useful temperature range make it currently the best compromise among the various competing materials. Germanium (Ge) was a widely used early semiconductor material but its thermal sensitivity makes it less useful than silicon. Today, germanium is often alloyed with silicon for use in very-high-speed SiGe devices; IBM is a major producer of such devices. Gallium arsenide (GaAs) is also widely used in high-speed devices but so far, it has been difficult to form large-diameter boules of this material, limiting the wafer diameter to sizes significantly smaller than silicon wafers thus making mass production of GaAs devices significantly more expensive than silicon. Other less common materials are also in use or under investigation. Silicon carbide (SiC) has found some application as the raw material for blue lightemitting diodes (LEDs) and is being investigated for use in semiconductor devices that could withstand very high operating temperatures and environments with the presence of significant levels of ionizing radiation. IMPATT diodes have also been fabricated from SiC. Various indium compounds (indium arsenide, indium antimonide, and indium phosphide) are also being used in LEDs and solid state laser diodes. Selenium sulfide is being studied in the manufacture of photovoltaic solar cells. List of common semiconductor devices Two-terminal devices: Avalanche diode (avalanche breakdown diode) DIAC Diode (rectifier diode) Gunn diode IMPATT diode Laser diode Light-emitting diode (LED)

5 Photocell PIN diode Schottky diode Solar cell Tunnel diode VCSEL VECSEL Zener diode Three-terminal devices: Bipolar transistor Darlington transistor Field effect transistor IGBT (Insulated Gate Bipolar Transistor) SCR (Silicon Controlled Rectifier) Thyristor Triac Unijunction transistor Four-terminal devices: Hall effect sensor (magnetic field sensor) Multi-terminal devices: Charge-coupled device (CCD) Microprocessor Random Access Memory (RAM) Read-only memory (ROM) DOPING: Doping refers to the addition of impurities to a semiconductor. The addition of impurities adds charge carrying elements to the semiconductor. The two classes of doping are p-type and n-type which refer to the introduction of positive and negative charge carriers. For instance if one introduces a Phosphorus atom into a silicon lattice, the phosphous atom would prefer to shed one of the electrons in its outer shell in order to fit in with the silicon lattice. This electron is then available to slide through the material, carrying current. This is an example of n-type doping. By doping the same lattice with Boron, the Boron site wishes to suck an electron out of the silicon lattice to fit neatly into the structure. The site which is now

6 missing an electron represents a positive charge, and therefore the doping is p- type. Movement of this site constitutes a current. Doping becomes important when p-doped and n-doped materials are connected. Diodes Diodes are electric components which force current to flow in only one direction. They are formed by connecting p-type and n-type semiconductors. When current flows from the p-type to the n-type material, the positive holes and the negative electrons are forced into close contact at the boundary. At the boundary, the electrons fill the holes across the boundary while the terminals supply new holes and electrons. Thus, in the forward bias case a continual current flows. In the reverse bias case, the charge carriers are pulled apart. There is no longer an easy way for electrons to tunnel through the barrier as there are no longer many empty holes waiting on the opposite side. The circuit-diagram representation of a diode is representing the direction current is allowed to flow. with the arrow

7 Exposing a semiconductor to light can generate electron hole pairs, which increases the number of free carriers and its conductivity. P-N Energy Bands For a p-n junction at equilibrium, the fermi levels match on the two sides of the junctions. Electrons and holes reach an equilibrium at the junction and form a depletion region. The upward direction in the diagram represents increasing electron energy. That implies that you would have to supply enery to get an electron to go up on the diagram, and supply energy to get a hole to go down. Index Semiconductor concepts Semiconductors for electronics HyperPhysics***** Condensed Matter R Nave Go Back

8 P-N Energy Bands To reverse-bias the p-n junction, the p side is made more negative, making it "uphill" for electrons moving across the junction. The conduction direction for electrons in the diagram is right to left, and the upward direction represents increasing electron energy. Index Semiconductor concepts Semiconductors for electronics HyperPhysics***** Condensed Matter R Nave Go Back

9 P-N Energy Bands To forward bias the p-n junction, the p side is made more positive, so that it is "downhill" for electron motion across the junction. An electron can move across the junction and fill a vacancy or "hole" near the junction. It can then move from vacancy to vacancy leftward toward the positive terminal, which could be described as the hole moving right. The conduction direction for electrons in the diagram is right to left, and the upward direction represents increasing electron energy. Index Semiconductor concepts Semiconductors for electronics Show more detail on conduction under forward bias. HyperPhysics***** Condensed Matter R Nave Go Back

10 Forward Biased Conduction When the p-n junction is forward biased, the electrons in the n-type material which have been elevated to the conduction band and which have diffused across the junction find themselves at a higher energy than the holes in the p-type material. They readily combine with those holes, making possible a continuous forward current through the junction. Index Semiconductor concepts Semiconductors for electronics Show more detail about charge carriers. HyperPhysics***** Condensed Matter R Nave Go Back

11 Forward Biased Conduction The forward current in a p-n junction when it is forward-biased (illustrated below) involves electrons from the n-type material moving leftward across the junction and combining with holes in the p-type material. Electrons can then proceed further leftward by jumping from hole to hole, so the holes can be said to be moving to the right in this process. Index Semiconductor concepts Semiconductors for electronics HyperPhysics***** Condensed Matter R Nave Go Back

12 Reverse Biased P-N Junction The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction. When the potential formed by the widened depletion layer equals the applied voltage, the current will cease except for the small thermal current.

13 To understand how a pn-junction diode works, begin by imagining two separate bits of semiconductor, one n-type, the other p-type.

14 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'.

15 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 depletion zone. 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.

16 Usually, we represent this barrier by 'bending' the conduction and valence bands as they 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.

17 Elements of a Power Supply Introduction When dealing with electronic circuits, we have to meet the basic requirement of providing electrical power for them to work. Without that power, your circuit is no more useful or meaningful than a single raindrop in a hurricane. The basic purpose of a power supply is to provide one or more fixed voltages to the working circuit, with sufficient current-handling capacity to maintain the operating conditions of the circuit. The power source doesn't have to be fancy; the typical hand-held transistor radio uses a 9-volt battery as its power source. A flashlight uses cells that are physically much larger, but provide a lower voltage. Major electronic appliances such as television sets, VCRs, and microwave ovens have electronic circuits built in that take power from a wall socket and convert it to the form and voltages required by the other internal circuits of the appliance. Although each power supply has its own individual specifications and characteristics, all power supplies have certain characteristics in common. The Main Sections A basic power supply consists of three main sections, as shown in the block diagram.

18 Ripple factor Transformer. In general, the ac line voltage present in your house wiring is not suitable for electronic circuits. Most circuits require a considerably lower voltage, while a few require higher voltages. The transformer serves to convert the ac line voltage to a voltage level more appropriate to the needs of the circuit to be powered. At the same time, the transformer provides electrical isolation between the ac line and the circuit being powered, which is an important safety consideration. However, a line transformer is generally large and heavy, and is rather expensive. Therefore, some power supplies (notably for PCs) are deliberately designed to operate directly from the ac line without a line transformer. The output of the transformer is still an ac voltage, but now of an appropriate magnitude for the circuit to be powered. Rectifier. The next step is to force current to flow in one direction only, preventing the alternations that occur in the transformer and the ac line. This process is known as rectification, and the circuit that accomplishes the task is the rectifier. There are many different rectifier configurations that may be used according to the requirements of the circuit. The output of the rectifier is a pulsating dc, which still has some of the variations from the ac line and transformer. Filter. The pulsating dc from the rectifier is generally still not suitable to power the actual load circuit. The pulsations typically vary from 0 volts to the peak output voltage of the transformer. Therefore, we insert a circuit to store energy during each voltage peak, and then release it to the load when the rectifier output voltage drops. This circuit is called a filter, and its job is to reduce the pulses from the rectifier to a much smaller ripple voltage. No filter configuration can be absolutely perfect, but a properly designed filter will provide a dc output voltage with only a small ac ripple. To measure the effectiveness of each circuit, we compare the magnitude of the remaining ac component, or ripple, with the dc component of the total voltage appearing at the output of that section. The ratio of ac voltage to dc voltage is known as the ripple factor. The goal of any power supply design is to reduce the ripple factor as much as possible, or at least to the point where the load circuit will not be adversely affected by the remaining ac ripple voltage.

19 Rectifier A rectifier is an electrical device that converts alternating current to direct current, a process known as rectification. Rectifiers are used as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. A circuit which performs the opposite function (converting DC to AC) is known as an inverter. When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". In gas heating systems flame rectification can be used to detect a flame. Two metal electrodes in the outer layer of the flame provide a current path and rectification of an applied alternating voltage, but only while the flame is present. Half-wave rectification A half wave rectifier is a special case of a clipper. In half wave rectification, either the positive or negative half of the AC wave is passed easily, while the other half is blocked, depending on the polarity of the rectifier. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one phase supply. Image:Halfwave.rectifier.en.png Image

20 Full-wave rectification Full-wave rectification converts both polarities of the input waveform to DC(direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. This is due to each output polarity requiring two rectifiers each, for example, one for when AC terminal 'X' is positive and one for when AC terminal 'Y' is positive. The other DC output requires exactly the same, resulting in four individual junctions (See semiconductors, diode). Four rectifiers arranged this way are called a diode bridge or bridge rectifier: A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output by reversing the negative (or positive) portions of the alternating current waveform. The positive (or negative) portions thus combine with the reversed negative (or positive) portions to produce an entirely positive (or negative) voltage/current waveform. For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) form a full-wave rectifier Image:Fullwave.rectifier.en.png Fullwave.rectifier.en.png ( pixels, file size: 16 KB, MIME type: image/png) Image:Gratz.rectifier.en.png Gratz.rectifier.en.png ( pixels, file size: 17 KB, MIME type: image/png)

21 Schematic diagram for Rectifiers: Half wave Rectifier without filter Half wave Rectifier with filter

22 VOLTAGE MULTIPLIER A Full-Wave Voltage Doubler Consider the dual-output half-wave rectifier circuit shown to the left. If we simply add two filter capacitors to the two outputs, we'll have two output voltages: one negative, and one positive. Each output will have a significant amount of ripple as soon as load current is drawn from it, but the peak output voltage will be equal to the peak voltage of the whole transformer secondary winding. Now, suppose we make one change, as shown in the circuit diagram to the right. This change is simply to use the negative output as our ground reference, and take the positive output voltage as our only output from the power supply. Since each capacitor charges to the peak secondary voltage, the output voltage from this circuit will be the sum of the two capacitor voltages, or twice the peak voltage of the secondary winding. This circuit, then, operates in such a way as to produce an output voltage that is twice the transformer secondary voltage. Therefore, it is known as a voltage doubler. More accurately, it is a full-wave voltage doubler, because it uses both half-cycles of the incoming ac wave. Of course, doubling the output voltage comes at a price. Each capacitor is charged individually from its rectifier, but they appear in series to the output. Therefore, the available output current is only half the current that would be available from a half-wave rectifier by itself. The other factor is the ripple voltage. Each capacitor is recharged while the other is discharging, so there is some cancellation of the ripple voltages. Nevertheless, the output

23 ripple of this circuit is significant, and will normally require either additional filtering or regulation to be usable by most electronic circuits. A Voltage Tripler And Beyond One very useful feature of the modified voltage doubler circuit above is that it is expandable. In the circuit to the right we have re-drawn the diodes and capacitors, and added a third section. The result here is a voltage tripler the output voltage is triple the transformer secondary voltage. Theoretical limit With ideal components, there is no theoretical limit to how far this circuit can be extended. In the real world, however, there are always practical limits. Between diode voltage drops and charge lost in imperfect capacitors, any attempt to extend this circuit beyond about 10 sections will fail to provide any useful voltage increase. Also, the presence of any load will drastically reduce the output voltage of a high-order voltage multiplier. Practical applications Nevertheless, there are practical uses for such circuits. Consider a typical electronic flashgun for photography. It is powered by a battery composed of two to four AA or AAA-sized cells, at 1.5 volts each. When you turn it on, you hear a high-pitched whine for several seconds, and then the circuit is ready to fire the actual flash tube. What you hear is an oscillator circuit, which generates an ac output when powered from a dc source. The whine is a harmless side effect. The ac output is applied to a high-order voltage multiplier which builds up enough voltage to operate the flash tube. When the flash is triggered, the capacitors all discharge through it, thus providing one momentary burst of light from the flash tube. Then the voltage multiplier must recharge for several seconds before the circuit is ready to fire the flash tube again.

24 Any application that calls for a brief application of high voltage at infrequent intervals is a good candidate for a high-order voltage multiplier circuit. Zener Regulator The constant reverse voltage of the zener diode makes it a valuable component for the regulation of the output voltage against both variations in the input voltage from an unregulated power supply or variations in the load resistance. The current through the zener will change to keep the voltage at within the limits of the threshold of zener action and the maximum power it can dissipate. Zener Diode Voltage Regulator Circuit

25 Pictured above is a very simple voltage regulator circuit requiring just one zener diode and one resistor. As long as the input voltage is a few volts more than the desired output voltage, the voltage across the zener diode will be stable. As the input voltage increases the current through the Zener diode increases but the voltage drop remains constant - a feature of zener diodes. Therefore since the current in the circuit has increased the voltage drop across the resistor increases by an amount equal to the difference between the input voltage and the zener voltage of the diode.