MICROWAVE DIODES—IMPATT diodes

—TRAPATT diodes—BARIT diodes

RF and Microwave Diodes

Diodes which finds application in RF and Microwave are classified under this category. They are : Varactor Diodes Pin Diodes Step Recovery Diodes Mixer Diodes Detector Diodes Avalanche Transit Time devices like IMPATT,

TRAPATT, BARITT ,QWITT diodes etc. Tunnel Diodes

Avalanche Transit-Time Devices

Avalanche Transit-Time Devices

Avalanche Transit-Time Devices

Ref: Sze

Diode Configuration

IMPATT(impact avalanche and transit time) Mode Diodes

Structure of the Diode

Operation

A d.c biasing voltage is applied just enough to start avalanche multiplication. When an AC voltage is applied over the DC, during the negative half cycle, the diode gets more reverse biased, and avalanche multiplication starts. For the entire half cycle, it develops. When the AC voltage decreases to zero, ( i.e, voltage across the diode equals DC reverse voltage ) avalanche current becomes maximum ( at the junction ). Because of the positive voltage at the cathode, electrons are attracted towards cathode. During this time applied AC voltage decreases to zero. While constructing the diode, the length and area of the semiconductor is selected such that, the electrons reaches the cathode when the applied voltage is reaching the negative peak. This is indicated in the figure:

figure

IMPATT modes Diodes

• Advantage:—one of the most powerful solid state sources-highest continuous wave power o/p.• Disadvantage:—noisy due to avalanche breakdown.

BARrier Injection Transit Time Devices (BARITTs)

BARrier Injection Transit Time Devices (BARITTs)

The injected carrier density increases with the ac voltage. Then the carriers will traverse the drift region.The injected hole pulse at 90o and the corresponding induced current which travels 3/4s of a cycle to reach the negative terminal. Or w/vs = ¾ (1/f)Note that for /2 t, both the ac voltage and external current are positive therefore ac power is dissipated in the device. Consequently, the BARITT diodes have low power capabilities and low efficiencies but they also have low noise (avoiding the avalanche phenomena).

Structure

When no bias voltage is applied, the electric field profile of this structure determined by the built-in field region of the p⁺n and np⁺ junctions; this is shown in fig. a.

When bias voltage is applied , the conditions change drastically: one of the junction becomes forward biased and other reverse biased as shown in fig. b.

Performance

The possibilities offered by BARITT diode is quite restricted to around 8GHz as compared to IMPATT diode which offers 100GHz.

But compare to TED performance of BARITT diode is better as it has higher power efficiency.

One of the advantage is that of its quite simpler fabrication using a less sophisticated material.

They also have low noise.

TRApped Plasma Avalanche Triggered Transit Time Devices (TRAPATTs)

Operation

Continued……

In avalanche electrons and hole plasma completely fill up the depletion region and bring about sudden drop in voltage across the terminals(fig. a, period 1) as well of the field E and increase in current up to saturation level(fig. b, period 1).

Since the voltage and field E are recovering in parallel with the extraction of plasma the velocity will increase simultaneously and with that the rate of rise of recovery voltage and field will be increased(fig. a, period 2) but the current will stay close to its maximum value and at the end of period it will drop(fig. b, period 2).

Finally in period 3 of fig. a & b current will remain low and voltage stays at high level, slightly below the breakdown value Vb.

At the end of this period situation returns exactly to that found at start of period 1.

Advantages

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