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Saturday, 19 October 2019

Formation of PN Junction Diode

Volt Ampere Characteristics of Diode:

A P-N junction is called a diode. In the forward biased the current increases exponentially except for a small range near the origin. When the diode is reversed biased the reverse current is constant and is independent of applied reverse biased. Hence Io is referred as reverse saturated current. The volt - amp character is shown in Figure.



The current in forward bias is plotted in milli amperes and in the reverse bias in micro amperes. The dotted line indicates that diode exhibits an abrupt behaviour.

The voltage at which this takes place is called breakdown voltage. It is shown in the Figure by Vz. Another important voltage that exists is the voltage at which the current starts increasing in the forward biased state. This voltage is also called cut in, offset, break point or threshold voltage Vr. The current below this voltage is very small. Vr is 0.2 V for Germanium and 0.6 V for Silicon.



Formation of Diode :

It is an electron device using semi conductors to convert an alternating current to undirection current or voltage. Initially P-N junction diodes were made by forming point contacts to material such as Germanium, Silicon, or Galena. Though these are in use more reproducible diodes are made by other techniques like alloying. For example, starting with single crystal germanium devices are formed by alloying with a dot of some material such as lead or tin doped with a suitable Group V (n-type) or Group III (p-type) impurity. In this 1 - in - 10 mil diameters alloy dot is heated so that it melts and dissolves a small amount of germanium. On cooling the germanium is the first to solidify from the solution and regrows epitaxially on top of the unmelted germanium. During the regrowth a small amount of impurity of about 1% is incorporated into the regrown region forming a P-N junction. In the case of silicon, a diffusion process was developed to form the diode. The diffusion process is carried out by incorporating a suitable liquid or gaseous group Ill or group V compound such as boric oxide as a source and is either painted on silicon surface or deposited during diffusion cycle. When the boron is diffused into arsenic doped silicon, the boron concentration will exceed the initial lighter arsenic dopant and a p-n junction is formed. The diffused planar diode is formed in an epitaxially deposited slice as shown in Figure.
PN Junction Diode

Symbolic Representation
When a diode is connected to an alternating current (a-c), during the forward biased condition current flows through the diode and during the reversed bias condition no current flows through the diode. Due to which the output of the diode rectifier will have only positive cycle of the alternating current. The output is not a steady d-c but only a pulsating d.c wave having a ripple frequency to that of the input frequency. By using filter circuits a steady d.c can be obtained.

COMPARISON OF GERMANIUM AND SILICON DIODES:

Properties
Germanium
Silicon
1. Availability
Found in the earth 0.001% of the earth's crest
Abundantly
2. Cost
Cheap
Costly
3. Specific weight
5.3 gm / cm3
2.5 gm / cm3
4. Permittivity
16
12.5
5. Melting point
935°C
1414°C
6. Heat capacity
0.08 Cal / gm / °C
0.17 Cal / gm/°C
7. Themal conductivity
0.55 W / cm °C
0.8 W / cm °C
8. Thermal co-efficient of expansion
6 x 10-6/ degree
42 x 10-8 / degree
9. Total electrons in Atom
32
14
10. Valence electrons
4
4
11. Energy required to break covalent bond
0.75 eV
1.12 ev
12. Maximum operation voltage
Usually 32V can be increased to 250 volts.
Usually 60V may be increased 500 volts.
13. Highest operating temperature
85 - 100°C
200°C




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