Thursday, February 1, 2018

Trapatt Diodes

      TRAPATT Diode means Trapped-Plasma Avalanche Trigged Transit diode.
      It has p+ n n+ or n+ p p+ structure. It is useful to produce high microwave power.
1. Salient Feature of TRAPATT diode
. Its structure is similar to IMPATT diode.
. Its has p+ n n+ or n+ p p+ structure.
. It is a high efficiency diode oscillator.
. Its oscillation depend on the delay in the current caused by avalanche process.
. The diode diameter is about 50 mm for CW operation and is about 750 mm at lower frequency for high peak power application.
. It can be operated over a range of 400 MHz to 12 GHz.
. It has an efficiency of 20% - 40%.
. It is used in phased array radar systems, proximity  fuse sources, altimeters, instrument handing systems and IFF transmitters.
. At low frequencies, peak power and efficiency are high.
. At high frequencies, peak power and efficiency are low.
2. Principle of Operation of TRAPATT Diode
       A typical structure of TRAPATT diode is shown in Fig. 1.

       It is operated in the reverse bias. During the application of high reverse voltage, avalanche phenomena takes place. The depletion layer is filled with a plasma of  electrons and holes. These are trapped in low field region after propagating in high field region. A voltage pulse is produced which oppose the applied d.c. voltage. The decreased voltage reduces the current pulse. The decreased current with increased voltage creates a negative resistance region. Basically, TRAPATT oscillators depend on the delay in the current caused by the avalanche process.
       The avalanche zone velocity, vaz is given by

Here,       J = Current density, A/m2,
               e = Electron charge = 1.6 x 10-19 C.
               NA = Doping concentration.
       The transit time of charge carries is given by

Here,           l = Drift region length.
       vs = Saturated carrier drift velocity.
       The voltage waveform for TRAPATT diode is shown in Fig. 2.

       The current waveform is shown in Fig. 3.

       The frequency of oscillation of a TRAPATT diode is given by

w = Depletion layer width.
vf = Front velocity.
vp = Plasma velocity
vs = Saturated drift velocity.
       The discharge time of plasma is greater than the transit time, Ԏ1 of the diode at high field and TRAPATT diode can be operated at low frequencies. In this diode, the time delay charge carries during the transit is used to produce the required current phase shift which generates oscillations.
3. Applications of TRAPATT Diode
       It is used
. in lower power Doppler radars.
. as local oscillators in different radars, phased array radars etc.
. in ratio altimeters.
. in microwave beacons and landing systems.
4.Advantages of TRAPATT Diode
. Its efficiency is relatively high.
. It can be used over a frequency range of a few hundred MHz to several GHz.
5. Disadvantages of TRAPATT Diode
. It has high noise figure..Its use at upper microwave frequencies is limited.
. It generates strong harmonic due to the short duration of current pulse.

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