Other diodes: Diode types
The TRAPATT or TRApped, Plasma Avalanche Triggered Transit diode belongs to the same basic family as the IMPATT diode but it provides a number of advantages in some applications.
The TRAPATT diode is normally used as a microwave oscillator. It has the advantage of a greater level of efficiency when compared to an IMPATT microwave diode. Typically the DC to RF signal conversion efficiency may be in the region of 20% to 60% which is particularly high..
TRAPATT diode basics
The TRAPATT diode is based around the initial concept of the IMPATT but it has been enhanced by increasing 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. Silicon is also typically used in the fabrication of these devices.
The TRAPATT is excited using a current pulse. This 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.
The criterion for operation in TRAPATT operation is that the avalanche front advances faster 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.
As the TRAPATT diode is biased beyond its breakdown point, the current density is larger than that for an IMPATT. This decreases the field in the space charge region and increases the transit time. As a result the frequency of operation is typically below about 10 GHz.
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. It also has very high levels of harmonics as a result of the short current pulses that are used. A balance needs to be made between the different options according to the particular application.
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