Diodes have two terminals and conduct current in only one direction, from the cathode to the anode. By manipulating the characteristics of the semiconductor material, manufacturers can make diodes useful in a wide variety of applications.
Take, for example, the Zener diode. Zener diodes actually allow current to flow in both directions, but only when the reverse bias voltage reaches a specific voltage, called the Zener voltage. When the voltage across the Zener diode reaches that point, it begins to conduct current in the reverse direction and maintains a constant voltage drop across the diode. This makes it useful in voltage regulator circuits.
QUESTION: What is the most useful characteristic of a Zener diode? (E6B01)
ANSWER: A constant voltage drop under conditions of varying current
Another example is the varactor diode. The capacitance of a varactor diodes changes as you change the voltage across it. This allows you to use it as a voltage-controlled capacitor in tuning circuits.
QUESTION: What type of semiconductor device is designed for use as a voltage-controlled capacitor? (E6B04)
ANSWER: Varactor diode
PIN diodes are diodes that operate as a variable resistor at RF and microwave frequencies. One common use for PIN diodes is as an RF switch. One characteristic of a PIN diode that makes it useful as an RF switch or attenuator is low junction capacitance. The forward DC bias current level is used to control the attenuation of RF signals by a PIN diode.
QUESTION: What characteristic of a PIN diode makes it useful as an RF switch? (E6B05)
ANSWER: Low junction capacitance
QUESTION: What is used to control the attenuation of RF signals by a PIN diode? (E6B11)
ANSWER: Forward DC bias current
Schottky diodes have a metal-semiconductor junction and have less forward voltage drop than other types of diodes. For this reason, are often used as a power supply rectifier. When used in digital ICs, the lower forward voltage drop allow the ICs to switch faster, and they are also often used as a VHF/UHF mixer or detector.
QUESTION: What is an important characteristic of a Schottky diode as compared to an ordinary silicon diode when used as a power supply rectifier? (E6B02)
ANSWER: Less forward voltage drop
QUESTION: Which of the following is a Schottky barrier diode? (E6B08)
ANSWER: Metal-semiconductor junction
QUESTION: Which of the following is a common use of a Schottky diode? (E6B06)
ANSWER: As a VHF/UHF mixer or detector
Point-contact diodes are also commonly used as RF detectors.
QUESTION: What is a common use for point-contact diodes? (E6B09)
ANSWER: As an RF detector
Light-emitting diodes, or LEDs, are widely used in amateur radio as visual indicators. When you forward bias an LED, current flows through the diode and it emits light. The schematic symbol for an LED is similar to the symbol for a signal diode, but has two arrows pointing away from the diode that are meant to show that it is emitting light.
QUESTION: What type of bias is required for an LED to emit light? (E6B03)
ANSWER: Forward bias
QUESTION: In Figure E6-2, what is the schematic symbol for a light-emitting diode? (E6B10)
ANSWER: 5
No matter what type of diode you are using, it’s very important to not exceed the forward current specification. Doing so, will cause the junction temperature to increase, and ultimately cause the diode to fail.
QUESTION: What is the failure mechanism when a junction diode fails due to excessive current? (E6B07)
ANSWER: Excessive junction temperature
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