Story of Electronics

Hi I'm Jhonel and I'm a License Avionics :) The goal of my blog is to introduce a reader to the basics of electronics. This blog is recommended for those who want to study electronics. You may get your first knowledge of electronic concepts and basic components.

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Transistor

The word “transistor” was coined to describe the operation of a “transfer resistor”. First, a point-contact transistor was produced. It included two diodes placed very closely together such that the current in either diode had an important effect upon the current in the other diode. By the proper biasing the diodes, it was possible to obtain power amplification of electric signals between the diode common layer, which lead was called a base, and other layers. One of the leads of this device was designated as an emitter, the corresponding diode was biased in the forward direction, the other was a collector and its diode was biased in the reverse direction. Power amplification was obtained by virtue of the fact that the few variations in the base current caused a large variation in the emitter-collector current. The point-contact transistor had certain drawbacks:

-high sensitivity to temperature, either ambient or self-generated;                         -production problems, i.e., a difficulty to reproduce the same electrical qualities in close;                                                                                                                    -low amplification, especially at high frequencies.

Intensive research has been done to diminish or remove these drawbacks. As a result, developers have produced semiconductor materials that are not so sensitive to temperature, inexpensive, operate at high frequencies, have low power dissipation, and internal noise of the transistor. A device, which is more stable both mechanically and electrically, has been constructed by forming junctions rather than point contacts. General classes of transistors that are used in electronics today are as follows:

-bipolar junction transistors (BIT); -junction field-effect transistors (IFET); -metal-oxide semiconductor field-effect transistors (MOSFET) up to some kilowatts, hundreds amperes, and tenths gigahertz; -insulated-gate bipolar transistors (IGBT) up to thousands of kilowatts, some kiloamperes, and hundreds kilohertz.

More powerful devices have been built on the thyristors though IGBTs have the potential to replace them.

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Summary ( Special-purpose of diodes )

Summary. Special-purpose diodes successfully operate in the breakdown region, high-frequency applications, and other ad hoc conditions. The most widespread of them are Zener and Schottky diodes used in low-signal and middle-power applications, as well as optoelectronic devices for signal circuits and control systems.

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More diodes

Besides the special- purpose diodes discussed so far, there are a few more. A constant-current diode works in a way exactly opposite to the Zener diodes. Instead of holding the voltage constant, this diode holds the current constant when the voltage changes.

A step-recovery diode has an unusual doping profile because the density of carriers decreases near the junction. This phenomenon is called a reverse snap-of. During the positive half cycle, the diode conducts like any rectifier diode. Nevertheless, during the negative half cycle, the reverse current exists for a while because of the stored charges, and then suddenly drops to zero. This phenomenon is useful in frequency multipliers.

Zener diodes normally have breakdown voltages greater than -2 V. By increasing the doping level, a manufacturer achieves the Zener effect to occur near zero (approximately -0,1 V). A diode like this is called a back diode because it conducts better in the reverse than in the forward direction. Back diodes are occasionally used to rectify weak signals.

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Optoelectronics

Figure above,a displays a light-emitting diode (LED). This diode emits visible and invisible light rays when forward current through it exceeds the turn-on current. In the forward-biased LED, free electrons cross the junction and fall into holes. As these electrons fall from the higher to a lower energy level, they radiate energy. In rectifier diodes, this energy goes off in the form of heat. However, in a LED the energy is radiated as light. LEDs have replaced incandescent lamps in many applications because of their low voltage, long life, and fast on-off switching. LEDs are constructed of gallium arsenide or gallium arsenide phosphide. While their efficiency can be obtained when conducting as little as 2 mA of current, the usual design goal is in the vicinity of 10 mA. During conduction, a voltage drop on the diode is about 2 to 3 V that is twice more than the rectified diode.

Until the low-power liquid-crystal displays were developed, LED displays were common, despite high current demands in battery-powered instruments, calculators and watches. They are still commonly used as on-board enunciators, displays, and solid-state indicator lamps. Manufacturers produce LEDs that radiate green, yellow, blue, orange, or infrared (invisible) rays.

The same principle is used in photoelectric cells. When light energy bombards a pn junction, it can dislodge valence electrons. The more light striking the junction, the larger is the reverse current in a diode. Among the photoelectric cells that use this phenomenon, the most popular optoelectronic device is a photodiode. A photodiode is the one that has been optimized for its sensitivity to light. In this diode, a window lets light pass through the package to the junction. The incoming light produces free electrons and holes. The stronger the light, the greater the number of minority carriers and the larger the reverse current. In the figure , b shows of reverse biasing of the photodiode, where light becomes brighter and the reverse current increases.

The sensitivity zone of a photodiode spectrum is between 0,45 and 0,95 um, which corresponds to the interval from blue to infrared light. A human eye perceives waves in the range of 0,45 to 0,65 um therefore the photodiode can operate in the invisible rays.

In a sense, the photodiode is similar to a photoresistor also known as a light-dependent resistor (LDR) or a photovoltaic cell (FVC).

Another optoelectronic device is an optocoupler also called optoisolator that combines a LED and a photodiode in a single package. Figure above illustrates the optocoupler that has a LED on the input side and a photodiode on the output side. The left source voltage and the series resistor set up a current through the LED. Then the light from the LED hits the photodiode, and this sets up a reverse current in the output circuit. This reverse current produces a voltage across the output resistor. The output voltage then equals the output supply voltage minus the voltage across the resistors. When the input voltage is varying, the amount of light is fluctuating and the output voltage is varying in step with the input voltage. In this way, the device can couple an input signal to the output circuit.

The key benefit of the optocoupler is electrical isolation between the input and output circuits as the only contact between the input and the output is a beam of light. Because of this, it is possible to have an insulation resistance between the two circuits in the thousands of megohms. Power optoelectronic modules can operate on 2 kV and 0,5 kA.

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Tunnel diode

Diodes with a breakdown level equal to zero are called tunnel diodes, or Shockley diodes. The tunnel diode is a heavily doped diode that is used in high-frequency communication circuits for such applications as amplifiers, oscillators, modulators, and demodulators. The unique operating curve of the tunnel diode is a result of the heavy doping used in the manufacturing of the diode. The tunnel diode is doped about one thousand times as heavily as a standard pn-junction diode. This type of a diode exhibits a negative resistance. This means that a decrease in voltage produces an increase in current (Figure below). The negative resistance is useful in high-frequency circuits called oscillators, which create the sinusoidal signals.

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Schottky diode

As the frequency increases, the ordinary diode reaches a point where it cannot turn off fast enough to prevent noticeable current during the reverse half cycle. A special-purpose high frequency diode with no depletion layer, no pn junction, and extremely short reverse recovery time is called a Schottky diode or reverse diode (Figure below).

The Schottky diodes are much faster than the rectifier diodes, but their breakdown voltage is relatively low. The operation of the Schottky diode is based on the concept that electrons in different materials have different absolute potential energies and potential energy of electrons in materials is lower than the potential energy of the free electrons. If an n-type semiconductor is in contact with a metal the electrons of which have a lower potential energy than the electrons in the semiconductor, the flux of electrons from the semiconductor into the metal will be much larger than the opposite flux because of the higher potential energy of electrons in the semiconductor. As a result, the metal will become negatively charged and the semiconductor will be charged positively. By that way, a metal-semiconductor junction is formed (ms junction), where the metal replaces the p-type side of the pn-junction. Compared with the pn-junction bipolar devices with a minority carrier current flow, in the Schottky diodes only the flow of majority carrier occurs.

The on-state voltage drop of the Schottky diode is approximately 0,3 V that is much less than the voltage drop of a rectifier diode (0,7. . .1 V). This will lead to smaller energy losses. The main advantage of the Schottky diodes over rectifier diodes is their very fast switching process near zero voltage with very small junction capacitance. They can operate at frequencies up to 20 GHz. These devices have a limited blocking voltage capability of 50 to 100 V (some series up to 1200 V) and sufficiently high current rating available is well below 100 A. The most important application area of the Schottky diodes belongs to computers the speed of which depends on how fast their electronic devices can turn on and turn off.

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Bi-directional breakdown diode

Lightning, power-line faults, etc. can pollute the line voltage by superimposing dips, spikes, and other transients on the normal voltage. Dips are severe voltage drops lasting microseconds or less. Spikes are short overvoltages of 500 or more than 2000 V. One of the devices used for line filtering is a set of two reverse-parallel-connected Zener diodes with a high breakdown voltage in both directions known as a transient suppressor or voltage suppressor (Figure below). It contains a pair of Zener diodes that are connected back-to-back, making the voltage suppressor bi-directional. This characteristic enables it to operate in either direction to monitor under voltage dips and over-voltage spikes of the ac input. It is used as a filtering device to protect voltage-sensitive electronic devices from high-energy voltage transients. The voltage suppressor is connected across a primary winding of transformers to clip voltage dips and spikes and protect the equipment. The voltage suppressor must have extremely high power dissipation ratings because most of surges in ac power line contain a relatively high amount of power, in the hundreds of watts or higher. It must also be able to turn on rapidly to prevent damage to the power supply. In dc applications, a single unidirectional voltage suppressor can be used instead of a bi-directional voltage suppressor. It is shunt-connected with the dc input and reverse biased (cathode to positive dc). Often, a varistor (nonlinear voltage-dependent resistor) is used instead of the breakdown diode.

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Zener diode.

A Zener diode sometimes called breakdown diode or stabilitrone, is designed to operate in the reverse breakdown, or Zener, region, beyond the peak inverse voltage rating of normal diodes. This reverse breakdown voltage is called the Zener, or reference voltage, which can range between -2,4 V and -200 V (Figure below). The Zener effect causes a “soft” breakdown whereas the avalanche effect causes a sharper turnover. Both effects are used in the Zener diode. The manufacturer predetermines the Zener and avalanche voltages.

A significant parameter of the Zener diode is the temperature coefficient that is the breakdown voltage deviation during the temperature rise or fall. The temperature coefficient of the Zener diode changes from negative to positive near -6 V. Because of this, by selecting the current value the designer may minimize the instability of the device. In all types of devices, the output levels are affected by variations in the load. Lower percentage values, approaching 0%, indicate better regulation. The Zener diode is the backbone of voltage regulators, circuits that hold the load voltage constant despite the large changes in line voltage and load resistance. When used as a voltage regulator, the Zener diode is reverse biased so that it will operate in the breakdown region with highly stable Zener voltage. In this region, changes in current through the diode have no effect on the voltage across it. The Zener diode establishes a constant voltage across the load within a range of output voltages and currents. Out of this range, the voltage drop remains constant and the current flow through the diode will vary to compensate the changes in load resistance.

A power Zener diode is called an avalanche diode. It can withstand kilovolts voltages and currents of some thousands of amperes.

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Varactor

All the junction diodes have a measurable capacitance between anode and cathode when the junction is reverse biased, and this capacitance varies with the value of the reverse voltage, being least when the reverse voltage is high. In a varactor (Figure below) also called voltage-variable capacitance, varicap or tuning diode, the width of the depletion layer increases with the reverse voltage. Since the depletion layer gets wider with more reverse voltage, the capacitance becomes smaller. This is why the reverse voltage can control the capacitance of the varactor. This phenomenon is used in remote tuning of radio and television sets.

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Special-Purpose of Diodes

Rectifier diodes are used in the circuits of 50 Hz to 50 kHz frequencies. They are never intentionally operated in the breakdown region because this may damage them. They cannot operate properly under abnormal conditions and high frequency. Devices of other types have been developed for such kind of operations.

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Summary ( Power of diode )

Power diode is adapted for switching power applications. In addition to bulk resistance, it has high ohmic resistance. To withstand the essential losses that appear when the diode goes from the off state to the on state and backward, cooling is very important. To obtain a higher value of breakdown voltage, some measures are usually taken, such as lightly doped junctions, sufficiently wide drift layer, and the boundary control of the depletion layer. These measures result in a more complex design of power diodes but shorten the reverse recovery time and increase their lifetime.

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Switching

For power devices, switching process is the most common operation mode. A power diode requires a finite time interval to switch over from the off state to the on state and backwards. During there transitions, current and voltage in a circuit vary in a wide range. This process is accompanied with energy conversion in the circuit components. A power circuit contains many components that can store energy (reactors, capacitors, electric motors, etc.). Their energy level cannot vary instantaneously because the power used is restricted. Therefore, switching properties of power devices are analyzed at a given rate of current change, as shown transients in Figure below.

The most essential data of power switching are the forward voltage overshoot UF MAX when a diode turns on and the reverse current peak value IR MAX when a diode turns off. 

During the process, when the space charge is removed from the depletion region, the ohmic and inductive resistances cause a forward voltage overshoot of tens volts. The duration of the turn-on process of the power diode is the sum of two time intervals - the current growing time t1 up to the steady state value IF of the diode and the time t2 up to stabilizing the forward on-state voltage. With high-voltage diodes (some kilovolts), the first time interval is approximately some hundreds of nanoseconds and the second about one microsecond, whereas usual diodes have these values tenfold less. Commonly, a shorter turn-on transients and lower on-state losses cannot be achieved simultaneously. The turn-off current and voltage transient process duration is the sum of three time intervals - the decreasing time t3 of the forward current, the rise time t4 of the reverse current, and the stabilizing time t5 of the reverse voltage. The maximum value of the reverse current IR max is fixed at the end of the second time interval and then the current value drops quickly. After the diode turns off, the current drops almost to zero with only small leakage current flows. A decrease in the diode reverse current raises the reverse voltage UR max, the maximum value of which reaches UR mm. The sum of t4 and t5 is called a reverse recovery time.

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Reverse biasing

In the case of reverse-biased voltage, only the small leakage current flows through the diode. This current is independent of the reverse voltage until the breakdown voltage is reached. After that, the diode voltage remains essentially constant while the current increases dramatically. Only the resistance of the external circuit limits the maximum value of current. Large current at the breakdown voltage operation leads to excessive power dissipation that should quickly destroy the diode. Therefore, the breakdown operation of the diode must be avoided.

To obtain a higher value of breakdown voltage, the three measures could be taken. First, to grow the breakdown voltage, lightly doped junctions are required because the breakdown voltage is inversely proportional to the doping density. Second, the drift layer of high voltage diodes must be sufficiently wide. It is possible to have a shorter drift region (at the same breakdown voltage) if the depletion layer is elongated. In this case, the diode is called a punch-through diode. The third way to obtain higher breakdown voltage is the boundary control of the depletion layer. All of these technological measures will result in the more complex design of power diodes.

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Forward biasing.

Most power is dissipated in a diode in the forward-biased on-state operation. For small-signal diodes, power dissipation is approximately proportional to the forward current of the diode. For power diodes, this formula is true only with small currents. For large currents, the effect of ohmic resistance must be added. In a high frequency switching operation, significant switching losses will appear when the diode goes from the off-state to the on-state, or vice versa. Real operation currents and voltages of power diodes are essentially restricted due to power losses and the thermal effect of power dissipation. Therefore, in power devices cooling is very important.

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Characteristics

In a diode, large currents cause a significant voltage drop. Instead of the conventional exponential output relationship for small-signal diodes, the forward bias characteristic of the power diode is approximately linear. This means the voltage drop is proportional both to the current and to ohmic resistance. The maximum current in the forward bias is a function of the area of the pn junction. Today, the rated currents of power diodes are thousands of amperes and the area of the pn junction may be tens of square centimeters.

The structure and the method of biasing of a power diode are displayed in figure above. The anode is connected to the p layer and the cathode to the substrate layer n. In the case of power diode, an additional n¯  layer exists between these two layers. This layer termed as a drift region can be quite wide for the diode. The wide lightly doped region adds significant ohmic resistance to the forward-biased diode and causes larger power dissipation in the diode when it is conducting current.

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Power of Diode

A power diode is more complicated in structure and operational characteristics than the small-signal diode. It is a two-terminal semiconductor device with a relatively large single pn junction, which consists of a two-layer silicon wafer attached to a substantial copper base. The base acts as a heat sink, a support for the enclosure and one of electrical leads of the device. The extra complexity arises from the modifications made to the small-signal device to be adapted for power applications. These features are common for all types of power semiconductor devices.