IMPATT and TRAPATT

IMPATT diode

• IMPATT Stands for IMPact ionization Avalanche Transit-Time

IMPATT DIODE Operation

Figure 1: Impatt Diode Operation

• The diode is operated in reverse bias near breakdown, and both the N and N- regions are completely depleted

• Because of the difference in doping between the "drift region" and "avalanche region", the electric field is highly peaked in the avalanche region and nearly flat in drift region.

• In operation, avalanche breakdown occurs at the point of highest electric field, and this generates a large number of hole-electron pairs by impact ionization.

• The holes are swept into the cathode, but the electrons travel across the drift region toward anode.

V and I characteristics

Resonant frequency of IMPATT diode is given asf = Vd/2L

Vd—carrier drift velocity, L– Length of the drift space

Construction

1.Operate at frequencies between about 3 and 100 GHz or more.2. High power capability.

ADVANTAGES OF IMPATT DIODE:

DISADVANTAGES OF IMPATT DIODE :

1. Major drawback of using IMPATT diodes is the high level of phase noise they generate. 

2. The noise figure for IMPATT is 30dB .3. Tuning range is not as good as Gunn diodes.

APPLICATION OF IMPATT DIODE:

1. MICROWAVE GENERATOR

2. MODULATED OUTPUT OSCILLATOR

3. RECEIVER LOCAL OSCILLATOR 4. PARAMETRIC AMPLIFIER (par amps)

5. INTRUSION ALARM NETWORK

6. FM TELECOMMMUNICATION TRANSMITTERS

7. CW DOPPLER RADAR TRANSMITTER.

TRAPATT DIODE

• TRApped-Plasma Avalanche Trigged Transit Diode .

Construction of TRAPATT Diode:

• It is derived from IMPATT Diode.

• Silicon or Gallium Arsenide is used for fabricating TRAPATT Diode.

• TRAPATT Diode can be constructed either by p+ -n-n+ or n+-p-p+.

TRAPATT DIODE WAVEFORM:

AB – Charging of junction capacitanceBC – Electron and hole plasma formation by depressed fieldDE – Plasma extraction EF – Residual extraction FG – Charging of diode GA – Constant voltage after full charging

SALIENT FEATURES OF TRPATT DIODE

1.It is a high efficiency diode oscillator .

2. Its oscillations depend on delay in current caused by avalanche process.

3.The diode diameter is about 50 mm for CW operations and is about 750 mm at lower frequency for high peak power application.

4. It can be operated over a range of 400MHz to 12GHz.

5. Its has an efficiency of 20% - 40%.

ADVANTAGES OF TRAPATT DIODE:

1. Its efficiency is relatively high.2. It can be used over a frequency range of a few hundred MHz

to several GHz.

DISAVANTAGE OF TRAPATT DIODE:

1. It has high noise figure.2. Its use at upper microwave frequencies is limited .3. It generate strong harmonics due to the short duration of

current pulse.

• TED’s are semiconductor devices with no junctions and gates.

• They are fabricated from compound semiconductors like GaAs, InP, CdTe etc.

Transferred Electron Devices (TED)

Gunn Diode

•Such type of semiconductor device which have only N type doped (semiconductor) material, is called “Gunn Diode.” Symbols and Circuit Diagram:

•Most Common Materials :Gallium Arsenide (GaAs) and Indium Phosphide (InP).

Gunn Effect:

• Above some critical voltage (Corresponding to Electric field of 2k-4k V/cm) the current passing through n-type GaAs becomes a periodic fluctuating function of time.

• Frequency of oscillation is determined mainly by the specimen, not by the external circuit.

• Period of oscillation is inversely proportional to the specimen length and is equal to the transit time of electrons between the electrodes

• The current waveform was produced by applying a voltage pulse of 16V and 10ns duration to an n-type GaAs of 2.5 x 10-3 cm length. The oscillation frequency was 4.5Ghz.

Applications

• Anti-lock brakes

• Sensors for monitoring the flow of traffic

• Pedestrian safety systems

• Distance traveled" recorders

• Traffic signal controllers

• Automatic traffic gates