What Is a Varactor Diode?
aractor diodes, also called varicaps, are semiconductor devices that behave like variable capacitors. When reverse-biased, they have a
Significance
Varactor diodes are commonly found in communication equipment where electronic tuning is a necessity. They are an important component of radio frequency or RF applications.
Identification
Operation
The capacitance of a varactor decreases when the reverse-voltage gets larger. They are usually placed in parallel with an inductor in order to form a resonant frequency circuit. When the reverse voltage changes, so does the resonant frequency, which is why varactors may be substituted for mechanically tuned capacitors.
Function
Varactor diodes are found in radios, FM receivers, televisions, and microwaves.
Expert Insight
The variable-voltage capacitance effect occurs in all diodes, but varactors are specifically made for this purpose. For a reverse-biased diode, the
Tunnel Diode
Discovery
Leo Esaki, a PhD student in Japan, discovered the tunnel diode in 1958 while studying germanium junctions in transistors.
Definition
The tunnel diode is a diode that causes oscillation at high frequencies,
Operation
Electrical currents flow through the diode and create a "tunneling" effect. This occurs when electrons tunnel through a barrier. This tunneling creates a sharp increase in voltage that couldn't occur in a normal diode.
Advantages
The tunnel diode operates quickly, which increases the speed of any device that uses it. It also handles high frequencies well.
Uses
Tunnel diodes are primarily used in oscillators, amplifiers, frequency
The Uses of Tunnel Diodes
Advantages and disadvantages
The tunnel diode is not as widely used these days as it was oat one time. With the improvement in performance of other forms of semiconductor technology, they have often become the preferred option. Nevertheless it is still worth looking at a tunnel diode, considering its advantages and disadvantages to discover whether it is a viable option.
Advantages
Very high speed: The high speed of operation means that the tunnel diode can be used for microwave RF applications.
Longevity: Studies have been undertaken of the tunnel diode and its performance has been shown to remain stable over long periods of time, where other
semiconductor devices may have degraded.
Disadvantages
Reproducibility: It has not been possible to make the tunnel diode with as reproducible performance to the levels often needed.
Low peak to valley current ratio: The negative resistance region and the peak to valley current is not as high as is often be required to produce the levels of
Gunn diodes
Gunn diodes consist of a piece of doped semiconductor with two electrical contacts on either end. They are made from materials that exhibit a region of negative differential resistance. The resistance created by the diode negates the real and positive resistance of an actual load, therefore
Construction
Most diodes combine positively and negatively doped semiconductors to create a p-n junction. Semiconductors are doped by intentionally adding impurities that have more free electrons (p-type) or fewer (n-type). Doping changes the semiconductor's electrical properties.
Gunn diodes are different because they use a single piece of n-type semiconductor, but the interior is more heavily doped than the exterior,
creating a similar effect. Because there is no p-n junction, it can be argued that a Gunn diode is not a true diode.
Function
When there is a voltage across a Gunn diode, most of the potential difference is across the center, which is more heavily doped. Once the voltage reaches a certain level, a pulse of current is formed and travels across the interior of the diode. This pulse reduces the potential difference across the interior,
Tuning
Since Gunn diodes allow pulses to cross one at a time, instead of allowing a steady current, the frequency of the pulses is important. The frequency of a Gunn diode is mostly determined by the thickness of the interior, heavily doped region. Since this cannot be changed after construction, Gunn diodes are difficult to tune. Connecting the Gunn diode to a varactor or placing it in a YIG sphere allows for some tuning.
Precautions
Gunn diodes generate power with an efficiency of about 2 to 5 percent, meaning they give off a lot of heat. Although they are built to withstand high temperatures, it is important to have the Gunn diode attached to or embedded in a heat sink to
