Electronics¶
p-n Junction & its Characteristic¶
A p-n junction is formed when a crystal of germanium or silicon is grown in such a way that if one half of crystal is doped with a trivalent impurity and the other half with pentavalent impurity
- Its n-region contains free electrons as majority charge carriers and p-region contains holes as majority charge carriers.
- In N-region the minority charge carriers are holes while electrons are minority charge carriers in case of P-region.
- Just after the formation of the junction, the free electrons in the n-region because of their random motion diffuse into p-region. As a result, a charge less region is formed around the junction in which charge Carriers are not present. This region is known as depletion region.
Info
An extrinsic semi conductor ie. semi conductor after doping still remains electrically neutral whether it is p-type or n-type.
Forward Biased p-n Junction¶
When external potential difference is applied across a p-n junction such as p-side is connected with positive and n-side with negative terminal of the battery
- The external potential difference supplies energy to free electrons and holes in the n-region and p-region respectively. When this energy is sufficient to overcome the potential barrier a current of a few mili-amperes begins to floe across the p-n junction. In this state, p-n junction. is said to be forward biased.
- If forward bias voltage is increased by \(\Delta V\), the current increases by \(\Delta I\), The ratio \({\Delta V}/{\Delta I}\) is known as forward resistance of the p-n junction. i.e. $$ r=\frac{\Delta V}{\Delta I} $$
Info
The forward biased barrier potential for silicon is \(0.7V\) while for Germanium is \(0.3V\).
Revered Biased p-n Junction¶
When the external source of voltage is applied across a p-n junction such that its positive terminal is connected to n-region and its negative terminal to p-region
- In reverse biased situation, no current flows due to the majority charge carriers. However a very small current, of the order of few micro amperes, flows across the junction due to flow of minority charge carriers. known as reverse current or leakage current.
- Leakage current increases with increase in voltage.
- At a certain high voltage the reverse current increases sharply. This voltage is called break down voltage.
Info
The leakage current for \(Ge\) is in amperes while for silicon it is in microamperes.
Rectification¶
Conversion of alternating current into direct current is called rectification.
- There are two very common types of rectification.
Half-wave rectification (HWR)¶
- One diode is used.
- For upper half or lower half is rectified.
- Can be used for charging battery.
Draw Backs¶
- More pulsations in the output wave form.
- Average value of the output DC signal drops
Full wave Rectification (FWR)¶
Can be carried- out in two ways: * Center tap transformer with two diodes. * Ordinary transformer with bridge rectifier circuit employing four diodes.
Bridge Rectifier¶
- Four diodes are used.
- Two diodes remains ON (FWD Biased) in each half of the input cycle while other two remains off (Reverse Biased) in the same half o the cycle.
Summary¶
If the input wave has "T" Time period then
| Period | \(D_1\) & \(D_3\) | \(D_2\) & \(D_4\) |
|---|---|---|
| \(0-t/2\) | OFF | ON |
| \(T/2-T\) | ON | OFF |
Advantages¶
- Very inexpensive method of rectification.
- Average values do not drop significantly.
- Lesser pulsations are there in the signal compared to HWR.
Info
The frequency of the FWR wave is two times the source frequency
Specially Designed p-n Junctions¶
Light Emitting Diode¶
- Light emitting diodes (LED) are made from special semi-conductors such as gallium arsenide and gallium arsenide phosphide.
- When an electron combines with a hole during forward bias conduction, a photon of Visible light is emitted.
- Convert Electric energy in to light
Photo Diode¶
- Photo diode is used for detection of light.
- A photo diode can turn its current ON and OFF in nano seconds.
- Photo diode is fastest photo detection device,
Info
The photo diode is operated in reverse biased region.