Point Contact diode, Schottky diode and Performance Chrs. of Tubes

 

 

Performance Characteristics of Klystrons:

 

1. Frequency                  : 250 MHz to 100 GHz (60 GHZ nominal)

2. Power                         : 10 kW – 500 kW (Continuous Wave – CW) 30 MHz (pulsed)

3. Power gain                 : 15 dB – 70 dB (60 dB nominal)

4. Bandwidths               : Limited (because cavity resonators are being used) 10-60 MHz –  generally used in fixed frequency applications.

5. Noise figure                : 15 – 20 dB (Sometimes greater than 25 dB)

6. Theoretical efficiency  : 58 % (30% - 40 % nominal)

 

Performance Characteristics of Reflex Klystron:

 

1. Frequency range        : 4 to 200 GHz

2. Output power             : 1.0 mW to 2.5 W

3. Theoretical efficiency  : 22.78%

4. Practical efficiency      : 10% to 20%

5. Tuning range             : 5 GHz at 2 watts to 30 GHz at 10mW

 

Performance Characteristics of TWT:

 

1. Frequency of operation        : 0.5 GHz to 95 GHz

2. Power outputs                      : 5 mW (10-40GHz) (Low power TWT)

250 kW (Continuous Wave) at 3GHz (High Power TWT)

                                                   10 MW (pulsed) at 3 GHz

3. Efficiency                             : 5 % to 20 % (30% with depressed collector)

(A depressed collector is a set of collector rings maintained at successively higher potentials to effectively slow down the electron beam that still possesses a considerable KE)

4. Noise figure                          : 4 – 6 dB (Low Power TWT 0.5 to 16 GHz)

                                                  25 dB (High Power TWT at 40 GHz)

 

Types of Magnetrons

 

1. Negative Resistance type: make use of negative resistance between two anode segments but have low efficiency and are useful only at low frequencies (<500 MHz)

2. Cyclotron frequency magnetrons: depend upon synchronism between an alternating component of electric and periodic oscillation of electrons in a direction parallel to this field. These are useful only for frequencies greater than 100 MHz.

3. Cavity magnetrons: depend on the interaction of electrons with a rotating electromagnetic field of constant angular velocity. These provide oscillations of very high peak power and hence are very useful in radar applications.

 

Performance Characteristics of Magnetrons:

 

1. Power output    : in excess of 250 kW (Pulsed mode)

                               10mW (UHF band) 2mW (X band)

2. Frequency         : 500 MHz to 12 GHz

3. Duty Cycle        : 0.1 %

4. Efficiency           : 40 % to 70%

 

Point Contact Diodes

 

Consists of a brass base on which a small pellet of silicon, germanium, gallium arsenide or indium phosphide is mounted. A fine gold-plated tungsten wire, with a diameter of 80 to 400 μm and a sharp point, makes contact with the polished top of the semiconductor pellet and is pressed down on it slightly for spring contact. This “cat’s whisker” is connected to the top brass contact, which is the cathode of the device. The semiconductor and the cat’s whisker are surrounded by wax to exclude moisture and are located in metal-ceramic housing as shown in figure.

 

 

 

Such diodes can be fitted into coaxial or waveguide mounts (See figure 10-45) and are available at frequencies in excess of 100 GHz. Point contact diodes are used as microwave mixers or detectors.

 

 

Schottky-Barrier diode

 

Schottky diodes are metal-semiconductor barrier diodes as shown in figure. The diode is constructed on a thin silicon (n+ type) substrate by growing epitaxially on n-type active layer of about 2 micron thickness. A thin SiO2 layer is grown thermally over this layer. Metal-semiconductor junction is formed by depositing metal over SiO2. Schottky diodes also exhibits a square law characteristic and have a higher burnout rating, lower noise and better reliability than point-contact diodes. When the device is forward biased, the majority carriers (electrons) can be easily injected from the highly doped n-semiconductor material into the metal. When it is reverse biased, the barrier height becomes too high for the electrons to cross and no conduction takes place. 

 

 

Schottky barrier diode is an extension of point-contact diode. Here the metal-semiconductor interface is a surface-the schottky barrier- rather than a point contact. It shares the advantage of the point contact diode in that there are no minority carriers in the reverse-biased condition; i.e. there is no significant current from the metal to the semiconductor with back bias. Thus the delay present in junction diodes, due to hole-electron recombination time, is absent here. However because of a larger contact area between the metal and semiconductor than in the point contact diode, the forward resistance is lower and hence the noise.

  • Si or GaAs can be used for the construction.
    • GaAs has lower noise and higher operating frequency
    • Silicon is easier to fabricate
  • Device is also called ESBAR (acronym for Epitaxial Schottky-barrier) or hot-electron diode.
  • Schottky barrier diodes are available for microwave frequencies upto atleast 100 GHz.
  • Used as detectors and mixers

 

Microwave Transistors and Integrated Circuits

 

Refer Text Book: Electronic Communication Systems by KENNEDY – Page Nos. 422-427