Page 20 - Electrician - TT (Volume 2)
P. 20

ELECTRICIAN - CITS






               Fig 6





















           The intensity of the field is known as ‘barrier height’ or ‘potential’ hill’.  The internal voltage set up due to positive
           and negative ions at the junction is called barrier potential.  If any more electrons have to go over from the N-side
           to
           P-side, they have to over come this barrier potential. This means, only when the electrons on the N-side are
           supplied with energy to overcome the barrier potential they can go over to the P-side.
           In order to cancel  the barrier potential and the electrons to cross over a potential difference of 0.7 V is required for
           a silicon diode and 0.3 V for a germanium diode. The barrier voltage is more for silicon because its lower atomic
           number allows more stability in the covalent bonds. The barrier potential decreases at higher temperatures.
           Old system : Some earlier semiconductor diodes and transistors have type numbers, consisting of two or three
           letters followed by group of one, two or three figures. The first letter is always ‘O’, indicating a semi-conductor
           device.
           The second (and third) letter(s) indicate the general class of the device.
           A  –   diode or rectifier

           AP –   photo-diode
           AZ –   voltage regulator diode
           C  –   transistor
           CP –   phototransistor

           The group of figures in a serial number indicating a particular design or development.
           Present system : This system consists of two letters followed by a serial number. The serial number may consists
           of three figures of one letter and two figures depending on the main application of the device.
           The first letter indicates the semiconductor material used.

           A  Germanium
           B  Silicon
           C  Compound materials such as gallium arsenide
           R  Compound materials such as cadmium sulphide
           The second letter indicates the general function of the device.

           A  detection diode, high speed diode, mixer diode
           B  variable capacitance diode
           C  transistor for I.F. applications (not power types)
           D  power transistor for A.F. applications (not power types)

           E  tunnel diode

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                                    CITS : Power - Electrician & Wireman - Lesson 60-69
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