Rob's web

The tunnel diode, a new semiconductor device

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - The tunnel diode, a new semiconductor device

Diodes, transistors, and spacistors are some of the semiconductor devices that have appeared on the scene over the last few years. The latest, the "tunnel" diode, seems to have just as many exciting properties as the others do.

The tunnel diode's characteristics allow it to be used in a wide variety of applications - as amplifier, oscillator or switching device. Although only a two-terminal device and still in the experimental stage, the tunnel diode is already capable of operating as an r.f. oscillator to 2000 Mc - yet it has a minimum power requirement of only one millionth of a watt (compare this to one thousandth of a watt for transistors). One of the big advantages the tunnel diode has over the other amplifying devices is its relatively low noise when operating as an amplifier. The tunnel diode compares to the klystron and, and although it is not as "quiet" as the parametric amplifier or maser, it does not require any external r.f. power, supercooling, or magnetic bias. It is far superior to the transistor and the common vacuum tube when it comes to low-noise amplification at u.h.f.

Pic 1
The two-terminal tunnel diode looks like a conventional crystal diode but has the ability to amplify.

Although similar to a transistor in some respects, the tunnel diode operates on a different principle. Conventional amplifying devices such as tubes and transistors depend on emitting electrodes, collecting electrodes, and a third signal-controlling electrode. The speed at which these three-terminal devices operate depends on how much time it takes the carriers to cross between elements. In transistors, and even in vacuum tubes, this time is relatively long as compared with the time it would take a signal to travel an equivalent distance along a metallic conductor. This is because the signal moves down the conductor by charge transfer process rather than by actual travel of a specific group of electrons.

The tunnel diode uses somewhat the same principle that the conductor does - carrying the signal by way of the fast charge transfer process. However, unlike the conductor, the tunnel diode can amplify because under certain conditions it can exhibit negative resistance; that is, an increase in voltage can result in a decrease in the current.

The name tunnel diode is derived from the "tunnel effect," discovered about a year ago by a Japanese scientist. This term is used to describe the rather complicated manner in which electrical charges move through the device under certain conditions. More information on the tunnel diode can be found in a technical information sheet on tunnel diodes published by the General Electric Research Laboratory, Schenectady, New York.

The tunnel diode is still in the experimental stage and not yet available commercially.