NEET Physics Notes Modern Physics-Electronic Devices-Semiconductor Diode
Semiconductor Diode
A p-n junction is obtained by joining a small p-type crystal with a small n-type crystal without employing any other binding material in between them.
Whenever a p-n junction is formed, electrons from n-region diffuse through the junction into p-region and the holes from p-region diffuse into n-region.
As a result of which neutrality of both n and p-regions is disturbed, and a thin layer of immobile negative charged ions appear near the junction in the p-crystal and a lpyer of positive ions appear near the junction in n-crystal.
This layer containing immobile ions is called depletion layer. The thickness of depletion layer is approximately of the order of 106 m.
The potential difference developed across the p-n junction due to diffusion of electrons and holes is called the potential barrier Vb (or emf of fictitious battery). For germanium diode barrier potential is 0.3 V but for Si diode its value is 0.7 V. The barrier electric field developed due to it, is of the order of 105 Vm-1.
Mobility of Charge Carriers
The mobility of a charge carrier is defined as the velocity gained by its per unit electric field, i.e.
p = Vd/E.
I-VCharacteristics in Forward and Reverse Bias
When we join an external potential source, such that p-side of p-n junction is joined to positve of voltage source and n-side to negative of voltage source, the junction is said to be forward biased and applied electric field E opposes the barrier electric field Eb. As a result width of depletion layer is reduced and on applying a voltage V > Vb, a forward current begins to flow. Resistance offered by p-n junction in forward bias is small.
If connections of potential source are reversed [Fig (b)] the junction is said to be reverse biased and in this case E and Eb, being in same direction, are added up. So, the depletion layer broadens and potential barrier is fortified. Consequently, an extremely small leakage current flows across the junction due to minority charge carriers and junction resistance is extremely high 
. For a sufficiently high reverse bias voltage (25 V .or even more) the reverse current suddenly increases. This voltage is called . Zener voltage or breakdown voltage or avalanche voltage.
Diode as a Rectifier
Rectifier is a device use to convert AC voltage into DC voltage.
Types of Rectifier diode
There are two types of diode.
Half Wave Rectifier
- A rectifier, which rectifies only one-half of each AC input supply cycle, is called a half wave rectifier.
- A half wave rectifier gives discontinuous and pulsating DC output.
- As no output is obtained corresponding to alternate half cycles of the AC input supply, its efficiency is quite low.
Full Wave Rectifier
- A rectifier, which rectifies both halves of each AC input cycle is called a full wave rectifier.
- The output of a full wave rectifier is continuous but pulsating in nature. However, it can be made smooth by using a filter circuit.
- As output is obtained corresponding to both the half cycles of the AC input supply, its efficiency is more than, that of half wave rectifier.
Special Purpose Diodes
The special purpose diode that perform many different functions, e.g. diodes are used to regulate voltage (zener diodes), to produce light, Light Emitting Diode (LED) are given below:
Zener Diode
It is a highly doped p-n junction diode which is not damaged by high reverse current. It is always used in reverse bias in break down voltage region and is chiefly used as a voltage regulator.
Zener Diode as Voltage Regulator
The following circuit is used for stabilising voltage across a load RL. The circuit consists of a series voltage-dropping resistance R and a Zener diode in parallel with the load RL
The Zener diode is selected with Zener voltage Vz equal to the voltage desired across the load. The fluctuating DC input voltage may be the DC output of a rectifier. Whenever the input voltage increases, the excess voltage appears as increased voltage across the resistance R.
This causes an increase in the input current I. This increase is taken away by the Zener diode while the current through the load and hence, the voltage across it remains constant at Vz.
Likewise, a decrease in the input voltage causes a decrease in the input current I. The current through the diode decreases correspondingly, again maintaining the load current.