Electronics Components Quiz

• Other two-terminal devices discussed are Schottky diodes, varactor diodes, power diodes, tunnel diodes, photodiodes, photoconductive cells, IR emitters, liquid crystal displays, solar cells, and thermistors.

Schottky Diode

• These are also called Schottky-barrier, surface-barrier, or hot-carrier diodes.
• Compared to general-purpose diodes, Schottky diodes have a lower forward voltage drop (0.2-0.63V), higher forward current (up to 75A), significantly lower peak inverse voltage (PIV), higher reverse current, and a faster switching rate.
• Applications include high-frequency switching, low-voltage/high-current scenarios, AC-to-DC converters, communication equipment, and instrumentation circuits.

Varactor Diode

• These are also called varicap, VVC (voltage variable capacitance), or tuning diodes and act as a variable capacitor.
• A reverse-biased varactor acts like a capacitor, where the amount of reverse bias voltage determines the capacitance; increasing reverse bias decreases capacitance.
• Under reverse-bias conditions forms a depletion region with width Wd, the transition capacitance C established by uncovered charges is:
  • C = є * (A / Wd)
  • where e is semiconductor permittivity, A is the p-n junction area, and Wd is the depletion width.
• The transition capacitance is approximated by:
  • C = K / (V + V)"
  • where K is a material constant, V is the knee potential, and V is the magnitude of reverse-bias potential.
  • n = 1/2 for alloy junctions and 1/3 for diffused junctions.
• In terms of the capacitance at zero-bias C(0):
  • CT (VR) = C(0) / (1 + |VR / VT|)^n
• Capacitance temperature coefficient is defined by:
  • TC = (ΔC / C0(T1 - T0)) * 100%
  • where ΔC is capacitance change due to temperature change (T1 - T0); C0 is the capacitance at T0.
• Applications for varactor diodes include FM modulators, automatic-frequency-control devices, adjustable bandpass filters, and parametric amplifiers.

Power Diodes

• Used in high-power and high-temperature applications like power rectifier circuits, power diodes must be rated for power.
• These diodes are sometimes generally referred to as rectifiers.
• Power diodes have the same symbol and operation as general-purpose diodes.
• Power diodes are physically larger and require heat sinking.

Tunnel Diodes

• A tunnel diode has a negative resistance region, meaning its current decreases as the forward-bias voltage increases.
• The characteristics of the tunnel diode indicate a negative resistance region which is a small portion of the characteristic curve.
• The tunnel diode acts like a general-purpose diode if the forward bias voltage is beyond the negative resistance region.
• Can be used as an oscillator if the forward bias voltage is in the negative resistance region.
• Applications for tunnel diodes include high frequency circuits, oscillators, switching networks, pulse generators, and amplifiers.

Photodiodes

• A photodiode conducts when light is applied to the junction.
• Under reverse bias Photodiodes conduct if light of a particular wavelength strikes the junction.
• Higher light intensity (measured in foot-candles) yields more conduction.
• Photodiode applications include instrumentation circuits and alarm systems, and detecting objects on conveyor belts.

Photoconductive Cells

• A photoconductive cell's resistance varies with the intensity of light
• Like a common resistor, has no polarity
• Applications include light/darkness detection and lighting system intensity control.

IR Emitters

• These are diodes that emit infrared radiation.
• IR emitters produce infrared radiation when forward biased; higher forward bias current produce greater intensity.
• Radiation patterns can vary from widely dispersed to narrowly focused.
• Common applications include card readers, shaft encoders, intrusion alarms, and IR transmitters.

Liquid Crystal Displays (LCDs)

• Come in two varieties: light backgrounds with dark displays, or dark backgrounds with light displays.
• The alphanumeric display is visible when voltage is applied to a segment.
• Voltage necessary for display varies, from 2 to 20V, depending on display type.
• Low-power LCDs use less power than LEDs, but LEDs have faster response times with shorter lives.
• Common applications include digital clocks, thermometers, and odometers.

Solar Cells

• Solar cells produce voltage when subjected to light energy.
• The greater the light intensity, the greater the voltage produced

Thermistors

• Thermistors are resistors whose value changes with temperature.
• Thermistors are negative-coefficient devices (their resistance decreases as temperature increases).
• Applications include instrumentation sensors and temperature correction circuitry.