Other Two-Terminal Devices: Schottky, Varactor, Power Diodes, and More PDF

Other Two-Terminal Devices Chapter 16: Electronic Devices and Circuit Theory by Boylestad Other Two-Terminal Devices Schottky diode Solar cells Varactor diode Thermistors Power diodes Tunnel diode Photodiode Photoconductive cells I...

Other Two-Terminal Devices Chapter 16: Electronic Devices and Circuit Theory by Boylestad Other Two-Terminal Devices Schottky diode Solar cells Varactor diode Thermistors Power diodes Tunnel diode Photodiode Photoconductive cells IR emitters Liquid crystal displays Schottky Diode Also called Schottky-barrier, surface-barrier, or hot-carrier diode. Characteristics Compared with General-Purpose Diodes Lower forward voltage drop (0.2- 0.63V) Higher forward current (up to 75A) Significantly lower peak inverse voltage (PIV) Higher reverse current Faster switching rate Schottky Diode Applications High frequency switching applications Low-voltage high-current applications AC-to-DC converters Communication equipment Instrumentation circuits Varactor Diode Also called a varicap, VVC (voltage variable capacitance), or tuning diode. It basically acts like a variable capacitor. A reverse-biased varactor acts like a capacitor. Furthermore, the amount of reverse bias voltage determines the capacitance. As VR increases the capacitance decreases. Varactor Diode Under reverse-bias conditions, there is a region of uncovered charge on either side of the junction that together make up the depletion region and define the depletion width Wd. The transition capacitance CT established by the isolated uncovered charges is determined by: where ϵ is the permittivity of the semiconductor materials, A is the p-n junction area, and W d is the depletion width. Varactor Diode In terms of the applied reverse bias, the transition capacitance is given approximately by: where K = constant determined by the semiconductor material and construction technique; VT = knee potential; VR = magnitude of the applied reverse-bias potential; n = ½ for alloy junctions and 1/3 for diffused junctions Varactor Diode In terms of the capacitance at the zero-bias condition C(0), the capacitance as a function of VR is given by: Varactor Diode The capacitance temperature coefficient is defined by: where ∆C is the change in capacitance due to the temperature change T1 - T0 and C0 is the capacitance at T0 for a particular reverse-bias potential. Varactor Diode Applications FM modulator Automatic-frequency-control devices Adjustable bandpass filters Parametric amplifiers Power Diodes Power diodes used in high-power and high-temperature applications, such as power rectifier circuits, must be rated for power Power diodes are sometimes referred to as rectifiers They have the same symbol and operation as a general-purpose diode purpose Power diodes are physically larger than general-purpose diodes, and they require heat sinking. Tunnel Diodes A tunnel diode has a negative resistance region, which means its current decreases as the forward-bias voltage increases. Tunnel Diode Operation The characteristics of the tunnel diode indicate the negative resistance region. Note that this is only a small region of the characteristic curve. If the forward bias voltage is beyond the negative resistance region, the tunnel diode acts like a general-purpose diode. If the forward bias voltage is in the negative resistance region then the diode can be used as an oscillator. Tunnel Diode Applications High frequency circuits Oscillators Switching networks Pulse generators Amplifiers Photodiodes A photodiode conducts when light is applied to the junction. Operation The photodiode is operated in reverse bias. When light of a particular wavelength strikes the junction it conducts. The higher the intensity of light (measured in foot-candles), the higher the conduction through the photodiode. Photodiodes Applications Instrumentation circuits as a sensor Alarm system sensor Detection of objects on a conveyor belt (measured in foot- candles), the higher the conduction through the photodiode. Photoconductive Cells Operation A photoconductive cell varies resistance with intensity of light. Like a common resistor, a photoconductive cell has no polarity and can be placed into the circuit in either direction. Applications Light/darkness detection Controlling intensity of lighting systems IR Emitters These are diodes that emit IR (infrared radiation) Operation IR emitter produce infrared radiation when forward biased. The higher the forward bias current, the greater the intensity of infrared radiation. The radiation pattern can vary from widely dispersed to a very narrow, focused beam. IR Emitters Applications Card readers Shaft encoders Intrusion alarms IR Transmitters Liquid Crystal Displays (LCDs) There are two varieties—those with a light background and dark display or those with a dark background and light display. Operation The background is either light or dark, when a voltage is applied to a segment then the alphanumeric display is visible. The amount of voltage necessary for display varies depending on the type of display, from 2 to 20V. Low power LCDs require less power than LEDs. But LEDs have faster response times and longer life. Liquid Crystal Displays (LCDs) Application Digital clocks Digital thermometers Odometers Solar Cells Solar cells produce a voltage when subjected to light energy. The greater the light intensity, the greater amount of voltage produced. Thermistors Thermistors are resistors whose value changes with temperature. Operation Thermistors are negative- coefficient devices—their resistance decreases as the temperature increases. Applications Sensors in instrumentation circuits Temperature correction circuitry Seatwork #1 Answer the following questions. Seatwork (15 points) 1. Describe in your own words the mode of operation of the hot-carrier diode. (3 points) 2. In your own words, describe the basic operation of an LCD. (3 points) 3. What are the essential differences between a semiconductor junction diode and a tunnel diode? (3 points) 4. Give one application of solar cells. (1 point)  Assignment Solve problems 3, 7, 11, 23 and 33 of Chapter 16 of Boylestad.