TRAPATT Diode Tutorial

- the TRAPATT diode is related to the IMPATT diode, but offers a higher level of efficiency than an IMPATT.

The TRAPATT or TRApped, Plasma Avalanche Triggered Transit diode belongs to the same basic family as the IMPATT diode.

However the TRAPATT has a number of advantages and as a result it is used in a number of applications.

Essentially the TRAPATT normally used as a microwave oscillator, but has the advantage of a greater level of efficiency - typically the DC to RF signal conversion efficiency may be in the region of . 20 to 60%.


TRAPATT basics

The TRAPATT diode is based around the initial concept of the IMPATT. However for the TRAPATT, the doping level between the junction and the anode.

Typically the construction of the device consists of a p+ n n+, although where for higher power levels an n+ p p+ structure is better.

For operation the TRAPATT is excited using a current pulse which causes the electric field to increase to a critical value where avalanche multiplication occurs. At this point the field collapses locally due to the generated plasma.

The separation and drift of the electrons and holes are then driven by a very much smaller field. It virtually appears that they have been 'trapped' behind with a velocity smaller than the saturation velocity. After the plasma spreads across the whole active region, the holes and electrons begin to drift to the opposite terminals and then the electric field begins to rise again.

Basic TRAPATT diode structure
Diagrammatic TRAPATT diode structure

The criterion for operation in TRAPATT operation is that the avalanche front advances fasrer than the saturation velocity of the carriers. In general it exceeds the saturation value by a factor of around three.

The TRAPATT mode does not depend upon the injection phase delay.

Although the TRAPATT diode provides a much higher level of efficiency than the IMPATT, its major disadvantage is that the noise levels on the signal are even higher than they are when using an IMPATT. A balance needs to be made according to the application required.

By Ian Poole


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