SPECIAL-PURPOSE DIODES 1
CONTENTS 3-1 zener diodes 3-2 zener diodes applications 3-3 varactor diodes 3-4 optical diodes 3-5 other types of diodes 3-6 trouble shooting 2
OBJECTIVES Discuss the basic characteristics of the current regulator diode, the pin diode, the step-recovery diode, the tunnel diode, and the laser diode. Discuss the operation and characteristics of LEDs and photodiodes Describe the varactor diode and its variable capacitance characteristics Explain how a zener is used in voltage regulation and limiting Describe the characteristics of a zener diode and analyze its operation 3
INTRODUCTION 3-1 ZENER DIODE The basic function of zener diode is to maintain a specific voltage across its terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power supplies and other equipment. This particular zener circuit will work to maintain 10 V across the load. 4
ZENER DIODES A zener diode is much like a normal diode, the exception being is that it is placed in the circuit in reverse bias and operates in reverse breakdown. The breakdown voltage of a zener diode is set carefully by controlling the doping level during manufacture. This typical characteristic curve illustrates the operating range for a zener. Note that its forward characteristics are just like a normal diode. 5
Figure 3 2 General diode V-I characteristic. 6
ZENER BREAKDOWN The zener diode s breakdown characteristics are determined by the doping process. Low voltage zeners less than 5V operate in the zener breakdown range. Those designed to operate more than 5 V operate mostly in avalanche breakdown range. Zeners are available with voltage breakdowns of 1.8 V to 200 V. 7
BREAKDOWN CHARACTERISTICS Figure 3 3 Reverse characteristic of a zener diode. V Z is usually specified at the zener test current, I ZT, and is designated V ZT. 8
ZENER BREAKDOWN CHARACTERISTICS: Referring to the Zener Diode characteristics, it can be observed that as the reverse voltage is increased, the reverse current remains extremely small upto the knee voltage of the curve. The current at the knee voltage is called zener knee current I ZR. At this point the break down begins, the zener impedance begins to decrease and the reverse current increases rapidly. From the bottom of the knee, the zener breakdown voltage V Z remains essentially constant although it increases slightly as the Zener current I Z increases. 9
ZENER REGULATION: The ability to keep the reverse voltage across the terminals essentially constant is the key factor of the zener diode. A zener voltage acting in the breakdown acts as a voltage regulator because it maintains a nearly constant voltage across its terminals over a specified range of reverse current values. There is a maximum current I ZM, above which the diode may be damaged due to excessive power dissipation. A Zener diode maintains a constant voltage across its terminals for values of reverse currents ranging from I ZR to I ZM. V ZT will be specified in the data sheet at a reverse current called Zener test current I ZT 10
Figure 3 4 Zener diode equivalent circuit models and the characteristic curve illustrating Z Z. 11
ZENER EQUIVALENT CIRCUIT The ideal model of a zener diode under reverse bias is shown in the Figure 3.4 (a). It has a constant voltage drop equal to the nominal Zener voltage V ZT. This constant voltage drop is represented by a dc source. The practical model of a zener diode is shown in Figure 3.4 (b), where the zener impedance Z Z is included. Since the actual voltage curve is not ideally vertical, a change in zener current ΔI Z produces a small current in zener voltage ΔV Z. Applying Ohm s law, Z Z = ΔV Z / ΔI Z 12
ZENER DIODE APPLICATIONS ZENER REGULATION WITH A VARYING INPUT VOLTAGE The Figure illustrates how a zener diode can be used to regulate a varying dc voltage. As the input voltage varies (within limits), the zener diode maintains nearly a constant output voltage across its terminals. As V IN changes, I Z also changes proportionally so that the limitations on the input voltage variation are set by the minimum and maximum current (I ZK and I ZM ) with which the zener can operate. 13
Figure 3 8 Zener regulation of a varying input voltage. 14
ZENER REGULATION WITH A VARIABLE LOAD: The zener diode maintains a nearly constant voltage across R L as long as the zener current is greater than I ZK and less than I ZM. When the output terminals of the zener regulator are open, (R L = infinite), the load current is zero and all the current flows through the zener. When a load resistor R L is connected, part of the total current is through the zener and part through R L. The total current through R remains essentially constant as long as zener is regulating. As R L is decreased, the load current, I L, increases and I Z decreases. The zener diode continues to regulate the voltage until I Z reaches its minimum value, I ZK. At this point the load current is maximum, and a full load condition exists. 15
Figure 3 12 Zener regulation with a variable load. 16
ZENER LIMITING Zener diodes can used for limiting just as normal diodes. Recall in previous chapter studies about limiters. The difference to consider for a zener limiter is its zener breakdown characteristics. 17
VARACTOR DIODES A varactor diode is best explained as a variable capacitor. Think of the depletion region a variable dielectric. The diode is placed in reverse bias. The dielectric is adjusted by bias changes. Figure 3 19 capacitor. The reverse-biased varactor diode acts as a variable 18
A varactor diode always operates in reverse bias and is doped to maximize the inherent capacitance of the depletion layer. During reverse bias, the depletion layer is widened and acts as a dielectric material. The p and n regions act as the capacitor plates. As the reverse bias is increased, the depletion layer widens, increasing the plate separation and the dielectric thickness. Subsequently decreasing the capacitance value. (Refer Figure) As the reverse bias voltage increases, the net capacitance value decreases. Varactor diodes are typically available in the range of few picofarads to several hundred picofarads. 19
Figure 3 20 Varactor diode capacitance varies with reverse voltage. 20
VARACTOR DIODES The varactor diode can be useful in filter circuits as the adjustable component. 21
A typical tuner circuit is shown in the Figure. In this circuit, the varactor diode acts as a variable capacitor, thus allowing the resonant frequency to be adjusted by a variable voltage. C1, C2, C3 and C4 are coupling capacitors used to prevent the dc bias circuit from being loaded by the filter circuit. Resistors R2, R3, R5 and potentiometer R4 form a variable dc voltage divider for biasing the varactor. The parallel resonance frequency is fc = 1/ (2*phi*sqrt(L*C)) 22
3-4 OPTICAL DIODES The light-emitting diode (LED) emits photons as visible light. Its purpose is for indication and other intelligible displays. Various impurities are added during the doping process to vary the color output. 23
When LED is forward biased, electrons cross the pn junction from the n type material and recombine with holes in the p type material. These free electrons are in the conduction band and at a higher energy than the holes in the valence band. When recombination takes place, the recombining electrons release energy in the form of heat and light. A large exposed surface area of one layer of the semi conductor material permit photons to be emitted as visible light. This process is called as electroluminescence. Various types of impurities are added during doping process to establish the wavelength of the emitted light. The wave length determines the colour of light. 24
Figure 3 27 Basic operation of an LED. 25
Figure 3 28 Examples of typical spectral output curves for LEDs. 26
Figure 3 30 Typical LEDs. 27
THE SEVEN SEGMENT LED DISPLAY The seven segment display is an example of LEDs use for display of decimal digits. 28
PHOTO DIODE The photodiode is used to vary current by the amount of light that strikes it. It is placed in the circuit in reverse bias. As with most diodes when in reverse bias, no current flows when in reverse bias, but when light strikes the exposed junction through a tiny window, reverse current increases proportional to light intensity. 29
3-5 OTHER DIODE TYPES Current regulator diodes keeps a constant current value over a specified range of forward voltages ranging from about 1.5 V to 6 V. 30
THE SCHOTTKY DIODE S The Schottky diode s significant characteristic is its fast switching speed. This is useful for high frequencies and digital applications. It is not a typical diode in that it does not have a p-n junction. Instead, it consists of a heavily-doped n-material and metal bound together. 31
THE PIN DIODE The pin diode is also used in mostly microwave frequency applications. Its variable forward series resistance characteristic is used for attenuation, modulation, and switching. In reverse bias it exhibits a nearly constant capacitance. 32
THE STEP-RECOVERY DIODE The step-recovery diode is also used for fast switching applications. This is achieved by reduced doping at the junction. 33
THE TUNNEL DIODE The tunnel diode has negative resistance. It will actually conduct well with low forward bias. With further increases in bias it reaches the negative resistance range where current will actually go down. This is achieved by heavily-doped p and n materials that creates a very thin depletion region. 34
THE LASER DIODE The laser diode (light amplification by stimulated emission of radiation) produces a monochromatic (single color) light. Laser diodes in conjunction with photodiodes are used to retrieve data from compact discs. 35
3-6 TROUBLESHOOTING Although precise power supplies typically use IC type regulators, zener diodes can be used alone as a voltage regulator. As with all troubleshooting techniques we must know what is normal. A properly functioning zener will work to maintain the output voltage within certain limits despite changes in load. 36
TROUBLESHOOTING With an open zener diode, the full unregulated voltage will be present at the output without a load. In some cases with full or partial loading an open zener could remain undetected. 37
TROUBLESHOOTING With excessive zener impedance the voltage would be higher than normal but less than the full unregulated output. 38
SUMMARY The zener diode operates in reverse breakdown. A zener diode maintains a nearly constant voltage across its terminals over a specified range of currents. Line regulation is the maintenance of a specific voltage with changing input voltages. Load regulation is the maintenance of a specific voltage for different loads. There are other diode types used for specific RF purposes such as varactor diodes (variable capacitance), Schottky diodes (high speed switching), and PIN diodes (microwave attenuation and switching). 39
SUMMARY The laser diode emits a monochromatic light Photodiodes exhibit an increase in reverse current with light intensity. Light emitting diodes (LED) emit either infrared or visible light when forward-biased. 40