Photosensitive Diodes

Virtually all P-N junctions exhibit conductivity that varies with exposure to radiant electromagnetic energy such as IR, visible light, and UV. The reason that conventional diodes are not affected by these rays is that they are enclosed in opaque packages. Some photosensitive diodes have variable dc resistance that depends on the intensity of the electromagnetic rays. Other types of diodes produce their own dc in the presence of radiant energy.

Silicon Photodiodes

A silicon diode, housed in a transparent case and constructed in such a way that visible light can strike the barrier between the P-type and N-type materials, forms a silicon photodiode. A reverse-bias voltage is applied to the device. When radiant energy strikes the junction, current flows. The current is proportional to the intensity of the radiant energy, within certain limits.
 
Silicon photodiodes are more sensitive at some wavelengths than at others. The greatest sensitivity is in the near infrared part of the spectrum, at wavelengths just a little bit longer than the wavelength of visible red light. When radiant energy of variable intensity strikes the P-N junction of a reverse-biased silicon photodiode, the output current follows the light-intensity variations. This makes silicon photodiodes useful for receiving modulated-light signals of the kind used in fiber-optic communications systems.

The Optoisolator

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An optoisolator has an LED or IRED at the input and a photodiode at the output.
An LED or IRED and a photodiode can be combined in a single package to get a component called an optoisolator. This device, the schematic symbol for which is shown in above figure, creates a modulated-light signal and sends it over a small, clear gap to a receptor. An LED or IRED converts an electrical signal to visible light or IR; a photodiode changes the visible light or infrared back into an electrical signal.
 
When a signal is electrically coupled from one circuit to another, the two stages interact. The input impedance of a given stage, such as an amplifier, can affect the behavior of the circuits that feed power to it. This can lead to various sorts of trouble. Optoisolators overcome this effect, because the coupling is not done electrically. If the input impedance of the second circuit changes, the impedance that the first circuit sees is not affected, because it is simply the impedance of the LED or IRED. That is where the “isolator” in “optoisolator” comes from. The circuits can be electronically coupled, and yet at the same time remain electrically isolated.

Photovoltaic Cells

A silicon diode, with no bias voltage applied, can generate dc all by itself if enough electromagnetic radiation hits its P-N junction. This is known as the photovoltaic effect. It is the principle by which solar cells work.
 
Photovoltaic cells are specially manufactured to have the greatest possible P-N junction surface area. This maximizes the amount of light that strikes the junction. A single silicon photovoltaic cell can produce about 0.6 V of dc electricity. The amount of current that it can deliver, and thus the amount of power it can provide, depends on the surface area of the junction.
 
Photovoltaic cells can be connected in series-parallel combinations to provide power for solidstate electronic devices such as portable radios. These arrays can also be used to charge batteries, allowing for use of the electronic devices when radiant energy is not available (for example, at night!).
 
A large assembly of solar cells, connected in series-parallel, is called a solar panel. The power produced by a solar panel depends on the intensity of the light that strikes it, the sum total of the surface areas of all the cells, and the angle at which the light strikes the cells. Some solar panels can produce several kilowatts of electrical power in direct sunlight that shines in such a way that the sun’s rays arrive perpendicular to the surfaces of all the cells.

Energy Emission

Some semiconductor diodes emit radiant energy when a current passes through the P-N junction in a forward direction. This phenomenon occurs as electrons fall from higher to lower energy states within atoms.

LEDs and IREDs

Depending on the exact mixture of semiconductors used in manufacture, visible light of almost any color can be produced by diodes when bias is applied to them in the forward direction. Infraredemitting devices also exist. The most common color for a light-emitting diode (LED) is bright red. An infrared-emitting diode (IRED) produces energy at wavelengths slightly longer than those of visible red light.
 
The intensity of the radiant energy from an LED or IRED depends to some extent on the forward current. As the current rises, the brightness increases, but only up to a certain point. If the current continues to rise, no further increase in brilliance takes place. The LED or IRED is then said to be in a state of saturation.

Digital Displays

Because LEDs can be made in various different shapes and sizes, they are ideal for use in digital displays. You’ve seen digital clock radios that use them. They are common in car radios. They make good indicators for “on/off,” “a.m./p.m.,” “battery low,” and other conditions. In recent years, LED displays have been largely replaced by liquid crystal displays (LCDs). The LCD technology has advantages over LED technology, including lower power consumption and better visibility in direct sunlight. However, LCDs require backlighting when the ambient illumination is low.

Communications

Both LEDs and IREDs are useful in communications because their intensity can be modulated to carry information. When the current through the device is sufficient to produce output, but not enough to cause saturation, the LED or IRED output follows along with rapid current changes. Analog and digital signals can be conveyed over light beams in this way. Some modern telephone systems make use of modulated light, transmitted through clear fibers. This is known as fiber-optic technology.
 
Special LEDs and IREDs produce coherent radiation. These are called laser diodes. The rays from these diodes aren’t the intense, parallel beams that most people imagine when they think about lasers. A laser LED or IRED generates a cone-shaped beam of low intensity. But it can be focused into a parallel beam, and the resulting rays have some of the same advantages found in larger lasers, including the ability to travel long distances with little decrease in their intensity.

Oscillation and Amplification

Under certain conditions, diodes can be made to produce microwave RF signals. Three types of diodes that can do this are Gunn diodes, IMPATT diodes, and tunnel diodes.

Gunn Diodes

A Gunn diode can produce up to 1 W of RF power output, but more commonly it works at levels of about 0.1 W. Gunn diodes are usually made from gallium arsenide. A Gunn diode oscillates because of the Gunn effect, named after J. Gunn of International Business Machines (IBM), who first observed it in the 1960s. A Gunn diode doesn’t work like a rectifier, detector, or mixer. Instead, the oscillation takes place as a result of a quirk called negative resistance.
 
Gunn-diode oscillators are often tuned using varactor diodes. A Gunn-diode oscillator, connected directly to a microwave horn antenna, is known as a Gunnplexer. These devices are popular with amateur-radio experimenters at frequencies of 10 GHz and above.

IMPATT Diodes

The acronym IMPATT comes from the words impact avalanche transit time. This, like negative resistance, is a rather esoteric phenomenon. An IMPATT diode is a microwave oscillating device like a Gunn diode, except that it uses silicon rather than gallium arsenide. An IMPATT diode can be used as an amplifier for a microwave transmitter that employs a Gunn-diode oscillator. As an oscillator, an IMPATT diode produces about the same amount of output power, at comparable frequencies, as a Gunn diode.

Tunnel Diodes

Another type of diode that will oscillate at microwave frequencies is the tunnel diode, also known as the Esaki diode. It produces enough power so it can be used as a local oscillator in a microwave radio receiver, but not much more. Tunnel diodes work well as amplifiers in microwave receivers, because they generate very little unwanted noise. This is especially true of gallium arsenide devices.

Frequency Control

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Connection of a varactor diode in a tuned circuit.
When a diode is reverse-biased, there is a region at the P-N junction with dielectric (insulating) properties. As you know from previous sections, this is called the depletion region, because it has a shortage of majority charge carriers. The width of this zone depends on several things, including the reverse-bias voltage.
 
As long as the reverse bias is less than the avalanche voltage, varying the bias affects the width of the depletion region. This in turn varies the junction capacitance. This capacitance, which is always small (on the order of picofarads), varies inversely with the square root of the reverse-bias voltage, as long as the reverse bias remains less than the avalanche voltage. Thus, for example, if the reverse-bias voltage is quadrupled, the junction capacitance drops to one-half; if the reverse-bias voltage is decreased by a factor of 9, then the junction capacitance increases by a factor of 3.
 
Some diodes are manufactured especially for use as variable capacitors. Such a device is known as varactor diode, as you learned in previous sections. Varactors are used in a special type of circuit called a voltage-controlled oscillator (VCO). Above figure is a simple example of the LC circuit in a VCO, using a coil, a fixed capacitor, and a varactor. This is a parallel-tuned circuit. The fixed capacitor, whose value is large compared with that of the varactor, serves to keep the coil from short-circuiting the control voltage across the varactor. Notice that the symbol for the varactor has two lines on the cathode side.

Amplitude Limiting

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At A, connection of two diodes to act as an ac limiter. At B, illustration of sinewave peaks cut off by the action of the diodes in an ac limiter.
In previous sections you learned that a diode will not conduct until the forward-bias voltage is at least as great as the forward breakover voltage. There’s a corollary to this: a diode will always conduct when the forward-bias voltage reaches or exceeds the forward breakover voltage, when the device is conducting current in the forward direction. In the case of silicon diodes this is approximately 0.6 V. For germanium diodes it is about 0.3 V, and for selenium diodes it is about 1 V.
 
This phenomenon can be used to advantage when it is necessary to limit the amplitude of a signal, as shown in above figure. By connecting two identical diodes back-to-back in parallel with the signal path (A), the maximum peak amplitude is limited, or clipped, to the forward breakover voltage of the diodes. The input and output waveforms of a clipped signal are illustrated at B. This scheme is sometimes used in radio receivers to prevent “blasting” when a strong signal comes in.
 
The downside of the diode limiter circuit, such as the one shown in above figure, is the fact that it introduces distortion when clipping occurs. This might not be a problem for reception of digital signals, for frequency-modulated signals, or for analog signals that rarely reach the limiting voltage. But for amplitude-modulated signals with peaks that rise past the limiting voltage, it can cause trouble.

Voltage Regulation

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Current through a Zener diode as a function of the bias voltage.
Most diodes have an avalanche breakdown voltage that is much higher than the reverse bias ever gets. The value of the avalanche voltage depends on how a diode is manufactured. Zener diodes are specially made so they exhibit well-defined, constant avalanche voltages.
 
Suppose a certain Zener diode has an avalanche voltage, also called the Zener voltage, of 50 V. If reverse bias is applied to the P-N junction, the diode acts as an open circuit as long as the bias is less than 50 V. But if the reverse-bias voltage reaches 50 V—even for a brief instant of time—the diode conducts. This effectively prevents the reverse-bias voltage from exceeding 50 V.
 
The current through a Zener diode, as a function of the voltage, is shown in above figure. The Zener voltage is indicated by the abrupt rise in reverse current as the reverse-bias voltage increases. A simple Zener-diode voltage-limiting circuit is shown in following figure. Note the polarity of the diode: the cathode is connected to the positive pole, and the anode is connected to the negative pole.
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Connection of a Zener diode for voltage regulation.

Switching

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The PIN diode has a layer of intrinsic (I type) semiconductor material at the P-N junction.
The ability of diodes to conduct with forward bias, and to insulate with reverse bias, makes them useful for switching in some electronic applications. Diodes can perform switching operations much faster than any mechanical device.
 
One type of diode, made for use as an RF switch, has a special semiconductor layer sandwiched in between the P-type and N-type material. The material in this layer is called an intrinsic (or I-type) semiconductor. The intrinsic layer (or I layer) reduces the capacitance of the diode, so that it can work at higher frequencies than an ordinary diode. A diode with an I-type semiconductor layer sandwiched in between the P- and N-type layers is called a PIN diode (above figure).
 
Direct-current bias, applied to one or more PIN diodes, allows RF currents to be effectively channeled without using relays and cables. A PIN diode also makes a good RF detector, especially at very high frequencies.

Signal Mixing

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Spectral (frequency domain) illustration of signal mixing.
When two waves having different frequencies are combined in a nonlinear circuit, new waves are produced at frequencies equal to the sum and difference of the frequencies of the input waves. Diodes can provide this nonlinearity.
 
Suppose there are two signals with frequencies f1 and f2. For mathematical convenience, let’s assign f2 to the wave with the higher frequency, and f1 to the wave with the lower frequency. If these signals are combined in a nonlinear circuit, new waves result. One of them has a frequency of f2 + f1, and the other has a frequency of f2 − f1. These sum and difference frequencies are known as beat frequencies. The signals themselves are called mixing products or heterodynes (above figure).
 
Above figure, incidentally, is an illustration of a frequency domain display. The amplitude (on the vertical scale or axis) is shown as a function of the frequency (on the horizontal scale or axis). This sort of display is what engineers see when they look at the screen of a lab instrument known as a spectrum analyzer. In contrast, an ordinary oscilloscope displays amplitude (on the vertical scale or axis) as a function of time (on the horizontal scale or axis). The oscilloscope provides a time domain display.

Frequency Multiplication

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When current passes through a diode, half of the cycle is cut off, as shown in above figure B. This occurs no matter what the frequency, from 60-Hz utility current through RF, as long as the diode capacitance is not too great.
 
The output wave from the diode looks much different than the input wave. This condition is known as nonlinearity. Whenever there is nonlinearity of any kind in a circuit—that is, whenever the output waveform is shaped differently from the input waveform—there are harmonics in the output. These are waves at integer multiples of the input frequency.
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A frequency-multiplier circuit using a semiconductor diode.
Often, nonlinearity is undesirable. Then engineers strive to make the circuit linear, so the output waveform has exactly the same shape as the input waveform. But sometimes harmonics are desired. Then nonlinearity is introduced deliberately to produce frequency multiplication. Diodes are ideal for this purpose. A simple frequency-multiplier circuit is shown in above figure. The output LC circuit is tuned to the desired nth harmonic frequency, nfoo.
 
For a diode to work as a frequency multiplier, it must be of a type that would also work well as a detector at the same frequencies. This means that the component should act like a rectifier, but not like a capacitor.

Detection

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Schematic diagram of a crystal-set radio receiver.
One of the earliest diodes, existing even before vacuum tubes, was actually a primitive semiconductor device. Known as a cat whisker, it consisted of a fine piece of wire in contact with a small piece of the mineral galena. This strange-looking contraption had the ability to act as a rectifier for extremely weak RF currents. When the cat whisker was connected in a circuit such as the one shown in above figure, the result was a device capable of picking up amplitude-modulated (AM) radio signals and producing audio output that could be heard in the headset.
 
The galena, sometimes called a “crystal,” gave rise to the nickname crystal set for this primitive radio receiver. You can still build a crystal set today, using a simple RF diode, a coil, a tuning capacitor, a headset, and a long-wire antenna. Notice that there’s no battery! The audio is provided by the received signal alone.
 
The diode in above figure acts to recover the audio from the radio signal. This process is called detection; the circuit is called a detector or demodulator. If the detector is to be effective, the diode must be of the proper type. It must have low junction capacitance, so that it can work as a rectifier (and not as a capacitor) at radio frequencies. Some modern RF diodes are microscopic versions of the old cat whisker, enclosed in a glass case with axial leads.

Rectification

The hallmark of a rectifier diode is that it passes current in only one direction. This makes it useful for changing ac to dc. Generally speaking, when the cathode is negative with respect to the anode, current flows; when the cathode is positive relative to the anode, there is no current. The constraints on this behavior are the forward breakover and avalanche voltages, as you learned about in previous topics.
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At A, a half-wave rectifier circuit. At B, the output of the circuit shown at A when an ac sine wave is applied to the input.
Examine the circuit shown at A in above figure. Suppose a 60-Hz ac sine wave is applied to the input. During half the cycle, the diode conducts, and during the other half, it doesn’t. This cuts off half of every cycle. Depending on which way the diode is hooked up, either the positive half or the negative half of the ac cycle will be removed. Drawing B in above figure shows a graph of the output of the circuit at A. Remember that electrons flow from negative to positive, against the arrow in the diode symbol.

The circuit and wave diagram of above figure show a half-wave rectifier circuit. This is the simplest possible rectifier. That’s its chief advantage over other, more complicated rectifier circuits. You’ll learn about the various types of rectifier diodes and circuits in previous topics.