BARITT Diode Tutorial

- the BARITT diode or Barrier Injection Transit Time diode has many similarities with the IMPATT and is used in specialist microwave RF signal applications but with lower noise.

The BARITT diode or Barrier Injection Transit Time diode, bears many similarities to the more widely used IMPATT diode.

Like the more familiar IMPATT diode, the BARITT is used in microwave signal generation, often in applications including burglar alarms and the like, where it can easily produce a simple microwave signal with a relatively low noise level.

BARITT basics

The BARITT is very similar, in many respects to the IMPATT, but the main difference is that the BARITT diode uses thermionic emission rather than avalanche multiplication.

One of the advantages of using this form of emission is that the process is far less noisy and as a result the BARITT does not suffer from the same noise levels as does the IMPATT.

Essentially the BARITT diode consists of two back to back diodes. When a potential is applied across the device, most of the potential drop occurs across the reverse biased diode.

If the voltage is then increased until the edges of the depletion region meet, then a condition known as punch through occurs.

A plot showing the characteristic or a BARRITT diode with the forward and reverse turn on characteristics
Graphical representation of the BARITT diode IV characteristic

It can be seen within the diagram that the punch through voltages, Vpt are different for the two directions. This difference results from asymmetry in the two junctions and can be controlled during the manufacture stages of the diode. They can be made to be different or almost the same.

A diagram shwoing how the current through a BARRITT diode occurs and negative and positive resistance areas exist
Graphical representation of the BARITT diode voltage and current waveforms

After a charge is injected, it travels to the substrate with the saturation velocity.

As seen from the diagram, it can be seen that the injection current is in phase with the RF voltage waveform. This results in a non-ideal current waveform situation which flows in the positive resistance region and therefore losses are higher in the BARITT than in an IMPATT.

The terminal current pulse width is determined by the transit time which is L/vsat (Where the electrodes are spaced L apart and vsat is the saturation velocity). This constitutes around three quarters of the cycle.

In view of the physical restraints of the BARITT diode, the power capability decreases approximately as the square of the frequency because higher frequencies require a smaller separation between the electrodes and this in turn limits the voltages that can be used.Also the efficiency falls away with increasing frequency. For low frequency operation it may be around 5% or a little more.

By Ian Poole

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