Sunday, May 20, 2012

The P-N Junction Diode

      The p-n junction diode forms a popular semiconductor device called p-n junction diode. The p-n junction has two terminals called electrodes, one each from p-region and n-region. Due to the two electrodes it is called diode i.e. di + electrode.
       To connect the n and p-regions to the external terminals, a metal is applied to the heavily doped n and p type semiconductor regions. Such a contact between a metal and a heavily doped semiconductor is called ohmic contact. Such an ohmic contact has two important properties,
1. It conducts current equally in both the directions.
2. The drops across the contact is very small, which do not affect the performance of the device.
       Thus ohmic contacts are used to connect n and p type regions to the electrodes.
       The Fig. 1(a) shows schematic arrangement of p-n junction diode while the Fig. 1(b) shows the symbol of p-n junction diode. The p-region acts as anode while the n-region acts as cathode. The arrowhead in the symbol indicates the direction of the conventional current, which can flow when an external voltage is connected in a specific manner across the diode.
Fig. 1

1.1 Biasing of P-N Junction Diode
       Applying external d.c. voltage to any electronic device is called biasing. As seen, there is no current in the unbiased p-n junction at equilibrium.
Note : The usefulness of p-n junction lies in the fact that it allows current flow only in one direction, under biased condition.
       Depending upon the polarity of the d,c, voltage externally applied to it, the biasing is classified as Forward biasing and Reverse biasing.
1.2 Types of Diodes
       When forward current flows under forward biasing, diode gets heated. Hence forward current should not exceed the particular maximum value. Similarly the diode can be damaged due to large reverse voltage applied to it during reverse biasing. This voltage also must be maintained below the particular maximum value. These maximum values are specified in the manufacturer's datasheet.
Note : Practically the diodes which can carry large forward current and handle large reverse voltage are physically large in size.
       The diodes which are small in size can carry low forward current and can handle low reverse voltage. The Fig. 2 shows the typed of diode based on forward current carrying and reverse voltage withstanding capacity.
Fig. 2  Types of diodes


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