| MadSci Network: Engineering |
A conventional diode conducts in the forward direction when forward biased in excess of the junction voltage (around 0.7 V for a silicon diode, around 0.3 V for a germanium diode). A diode will also conduct in the reverse direction when the reverse voltage exceeds the reverse breakdown voltage. (Diodes in which this reverse breakdown voltage is carefully controlled to allow them to act as voltage references are called Zener diodes).
Conventional diodes have a nonlinear I-V characteristic, but the slope of the curve is always positive. That is to say, the local slope R= dI/dV is positive. This a "positive resistance" characteristic. A tunnel diode is a specially built diode that features a "negative resistance" characteristic at very low forward bias voltages. That means that for some range of voltages, the current decreases with increasing voltage. The "tunneling" is a quantum mechanical effect whereby electrons "tunnel" through a very thin depletion region.
Illustration of this principle may be found at this tutorial on the tunnel diode.
The tunnel diode is used in extremely high frequency (multi-GHz) and high speed applications such as amplifiers for satellite communications, as described in this more comprehensive discussion describing the history and application of the tunnel diode in satellite applications.
The tunnel diode has not found widespread use in general applications because circuit design involving the tunnel diode is a bit tricky. In fact, until I did a little Web searching, I did not even realize that these devices were still in use. The reason for their lack of popularity is that the tunnel diode is a "one-port" device, making isolation between input and output difficult. Furthermore, the performance of conventional silicon transistors have caught up for very high frequency applications, and the use of the tunnel diode is on the decline.
Steve Czarnecki
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