Special-Purpose Diodes

Objective of Lecture What is Diode element ? Ideal Vs Practical Diodes Special Diodes Application of Diodes

Diode Element A diode has two leads connected to the external circuit. Since a diode behaves differently depending upon forward or reverse bias, it is critical to be able to distinguish the leads. The anode connects to the p-type material, the cathode to the n-type material of the diode.

Chapter 9 - Diodes and Diode Circuits Ideal Diode In an ideal diode, current flow freely through the device when forward biased, having no resistance. In an ideal diode, there would be no voltage drop across it when forward biased. All of the source voltage would be dropped across circuit resistors. In an ideal diode, when reverse biased, it would have infinite resistance, causing zero current flow. Chapter 9 - Diodes and Diode Circuits

Practical Diodes A practical diode does offer some resistance to current flow when forward biased. Since there is some resistance, there will be some power dissipated when current flows through a forward biased diode. Therefore, there is a practical limit to the amount of current a diode can conduct without damage. A reverse biased diode has very high resistance. Excessive reverse bias can cause the diode to conduct.

Special Diodes Zener Diode LEDs Photo Diode Varactor Diode Tunnel Diode Schottky Diode Schokley Diode

Zener Diode A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property It is heavily doped in comparison with normal diode to reduce the breakdown voltage. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts.

Zener Diodes – Operating Range 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. This typical characteristic curve illustrates the operating range for a zener. Note that its forward characteristics are just like a normal diode. Fig 3-2 b zener curve Operating range

Zener Diodes – Regulation Ranges The zener diode’s breakdown characteristics are determined by the doping process. Low voltage zeners (>5V), operate in the zener breakdown range. Those designed to operate <5 V operate mostly in avalanche breakdown range. Zeners are available with voltage breakdowns of 1.8 V to 200 V. Zener zone Diode zone Fig 3-3 Avalanche zone 5V. This curve illustrates the minimum and maximum ranges of current operation that the zener can effectively maintain its voltage.

Zener Diodes – Equivalent Circuit Ideal Zener exhibits a constant voltage, regardless of current draw. Ideal Zener exhibits no resistance characteristics.

Introduction 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. RLoad Fig. 3-9 Simple zener diode reg. circuit & label zener terminals This particular zener circuit will work to maintain 10 V across the load.

Zener Diodes – Equivalent Circuit Zener exhibits a near constant voltage, varied by current draw through the series resistance ZZ. As Iz increases, Vz also increases.

Zener Diode - Applications Regulation In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages. Zener current will increase or decrease directly with voltage input changes. The zener current, Iz, will vary to maintain a constant Vz. Note: The zener has a finite range of current operation. Fig. 3-12 Zener Reg. VZener remains constant

Zener Diode - Applications Regulation In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage. Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation. Fig. 3-12 Zener Reg. VZener remains constant See Ex. 3-5

Zener Diode - Applications Vin IZ Vout Fig. 3-12 Zener Reg. Vin IZ Vout

Zener Diode - Applications ** Zener Diode - Applications 1N4740 PDMAX = 1W. VZ = 10V. IZK = 0.25mA to IZM = 100mA VRmin = IZKR= .25mA x 220 = 55mV VRmax = IZM = 100mA x 220 = 22V. Calculate VZRegulate: (pg.118) VinMIN = VR + VZ = 55mV + 10V = 10.055V. VR = IZR = (100mA)(220) = 22V. Vin(max) = 22V + 10V = 32V VReg is ≈10V to 32V. Fig. 3-12 Zener Reg. See Ex. 3-5 See Ex. 3-6 See Ex. 3-7

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. Fig 3-16a,b,&c See Ex.3-8

Varactor Diode A junction diode which act as a variable capacitor under changing reverse bias is known as a varactor diode.

Varactor Diodes A varactor diode is best explained as a variable capacitor. Think of the depletion region as a variable dielectric. The diode is placed in reverse bias. The dielectric is “adjusted” by reverse bias voltage changes. Fig 3-20 depletion region

Varactor Diodes The varactor diode can be useful in filter circuits as the adjustable component for resonance frequency selection. Fig 3-23 Varactor Bias adjust

Resonant Band-pass Filter w/ Varactor Diode Parallel Resonant Tank Series Resonant Tank 31.6V Varactor VR = 2.85 to 28.7V VBIAS = 2.9V to 29V CVaractor = 17pF to 55pF Resonant Frequency Range: fr = 679kHz to 1.22MHz. Varactor Bias See Related Problem pg.130

Semiconductors bring quality What is LED? Semiconductors bring quality to light! LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The qaunta of light energy released is approximately proportional to the band gap of the semiconductor.

How does it work? Recombination produces light!! Electrical Contacts P-n junction Electrical Contacts A typical LED needs a p-n junction There are a lot of electrons and holes at the junction due to excitations Electrons from n need to be injected to p to promote recombination Recombination produces light!! Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen

Optical Diodes Electroluminescence, the process of emitting photons from a parent material (substrate), is the basis for LEDs. Colors result from the choice of substrate material and the resulting wavelength; Todays LEDs (green,red, yellow) are based on indium gallium aluminum phosphide Blue uses silicon carbide or gallium nitride IR (infrared) – GaAs (gallium arsenide) LED Biasing: 1.2V to 3.2V is typical. Note: Some newer LED’s run at higher voltages and emit immense light energy. Applications: Traffic signals Outdoor video screens Runway markers Fig 3-27 A strong +bias encourages conduction-band electrons in the N-material to leap the junction and recombine with available holes releasing light and heat.

Excitation E Electron (excited by the biased forward voltage) is in the conduction band k Normally the recombination takes place between transition of electrons between the bottom of the conduction band and the top of the valance band (band exterma). The emission of light is therefore; hc/ = Ec-Ev = Eg(only direct band gap allows radiative transition) Hole is in valance band

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. Fig 3-27

Getting to know LED Advantages of Light Emitting Diodes (LEDs) Longevity: The light emitting element in a diode is a small conductor chip rather than a filament which greatly extends the diode’s life in comparison to an incandescent bulb (10 000 hours life time compared to ~1000 hours for incandescence light bulb) Efficiency: Diodes emit almost no heat and run at very low amperes. Greater Light Intensity: Since each diode emits its own light Cost: Not too bad Robustness: Solid state component, not as fragile as incandescence light bulb

Applications of LEDs See “Light Emitting Diodes.pdf” The seven segment display is an example of LEDs use for display of decimal digits. Fig 3-32 See “Light Emitting Diodes.pdf”

Applications of LEDs

Visible LED Violet ~ 3.17eV Blue ~ 2.73eV Green ~ 2.52eV Definition: LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = 390- 770nm. This coincides with the energy value of 3.18eV- 1.61eV which corresponds to colours as stated below: The band gap, Eg that the semiconductor must posses to emit each light Violet ~ 3.17eV Blue ~ 2.73eV Green ~ 2.52eV Yellow ~ 2.15eV Orange ~ 2.08eV Red ~ 1.62eV Colour of an LED should emits

PHOTODIODES Photodiodes - a semiconductor p-n junction device whose region of operation is limited to the reversed bias region. Converts non-electrical energy such as light to electrical energy. Photons - energy transmitted as discrete packages,has a level directly related to the frequency of the travelling wave. This energy associated with incident light waves is directly related to the frequency of the travelling wave. Dark current - current that will exist with no applied illumination. Conductivity of semi-conductor is increased. Current flow in the semi-conductor is induced. BAE 5413

Photodiodes Unlike LED’s, photodiodes receive light rather than produce light. The photodiode varies it’s current in response to the amount of light that strikes it. It is placed in the circuit in reverse bias. As with most diodes, 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 (irradiance). Note: Photodiodes all exhibit a “reverse leakage current” which appears as an inverse variable resistance. Irradiance causes the device to exhibit a reduction in the variable resistance characteristic. Fig 3-35

Photodiodes You have this one in your kit.

photon absorption in the depletion region induces current flow Photodiode fundamentals Based on PN or PIN junction diode photon absorption in the depletion region induces current flow Spectral sensitivity Material Band gap (eV) Spectral sensitivity silicon (Si) 1.12 250 to 1100 nm indium arsenide (InGaAs) ~0.35 1000 to 2200 nm Germanium (Ge) .67 900 to 1600 nm

Schottky Diode The Schottky diode’s (hot-carrier diodes) 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 lightly-doped n-material and heavily-doped (conduction-band electrons) metal bounded together. Response is very quick…high speed digital communications. Fig 3-39 Schottky schem. & Fig 3-40. Heavily doped (conduction-band electrons) Lightly doped

Tunnel Diode The tunnel diode exhibits 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 create a very thin depletion region which permits electrons to “tunnel” thru the barrier region. Tank circuits oscillate but “die out” due to the internal resistance. A tunnel diode will provide “negative resistance” that overcomes the loses and maintains the oscillations. Germanium or Gallium

Tunnel Diodes Tank circuits oscillate but “die out” due to the internal resistance. A tunnel diode will provide “negative resistance” that overcomes the loses and maintains the oscillations.

Other Diode Types The laser diode (light amplification by stimulated emission of radiation) produces a monochromatic (single color) “coherent” light. Laser diodes in conjunction with photodiodes are used to retrieve data from compact discs. Fig 3-47 a, b, & c Forward bias the diode and electrons move thru the junction, recombination occurs (as ordinary). Recombinations result in photon release, causing a chain reaction of releases and avalanching photons which form an intense laser beam.

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. Fig 3-48b A properly functioning zener will work to maintain the output voltage within certain limits despite changes in load.

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. Fig 3-49a&b zener reg

Troubleshooting With excessive zener impedance the voltage would be higher than normal but less than the full unregulated output. Fig 3-50 zener reg

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).

Summary Light emitting diodes (LED) emit either infrared or visible light when forward-biased. Photodiodes exhibit an increase in reverse current with light intensity. The laser diode emits a monochromatic light