excess of electrons (N-type). It is a non-linear device.
Structure
In a p-type material, the hole is the majority carrier and the electron is the minority carrier. In an n-type material the electron is the majority carrier and the hole is the minority carrier.
At the instant the p –type and n-type are joined, the electrons and holes in the region of the junction will combine, resulting in a lack of carriers in the region near the junction.
Depletion layer and barrier potential.
Each pair of positive and negative ions at the junction is called a dipole. As the number of dipoles builds up, the region near the junction is emptied of carriers. This charge-empty region is the depletion layer. The electric field between the ions is equivalent to a difference of potential called the barrier potential.
At 25°C, the barrier potential = 0.3 V for germanium diodes
= 0.7 V for silicon diodes.
Operation
Forward-Bias Condition (VD >0 V)
A forward-bias or on condition is established by applying the positive potential to the p-type material and the negative potential to the n-type material. The application of a forward-bias potential VD will pressure electrons in the n-type material and holes in the p-type material to recombine with the ions near the boundary and reduce the width of the depletion region. As the applied bias increases in magnitude, the depletion region will continue to decrease in width until a flood of electrons can pass through the junction, re-sulting in an exponential rise in current.
Cut in (or) threshold (or) knee voltage
If forward voltage is less than cut in voltage, the forward current is very small. It is 0.3 V for germanium and 0.7 V for silicon diodes.
Forward biased PN junction
Reverse-Bias Condition (VD < 0 V)
Reverse biased PN junction
If an external potential of V volts is applied across the p-n junction such that the positive terminal is connected to the n-type material and the negative terminal is connected to the p-type material. The large number of free electrons are drawn to the positive potential of the applied voltage. The number of uncovered positive ions in the depletion region of the n-type material will increase. The number of uncovered negative ions will increase in the p-type material. The net effect is a widening of the depletion region. This widening of the depletion region will establish a great barrier for the majority carriers current flow.
Characteristics
Schokley equation
VT= volt-equivalent of temperature
Reverse saturation current
The minority carriers, electrons in the P-region and holes in the N-region
Breakdown
As the voltage across the diode increases in the reverse-bias region, the velocity of the minority carriers increase. Their velocity and associated kinetic energy will be sufficient to release additional carriers through collisions with stable atoms. Ionization process will result and valence electrons absorb sufficient energy to leave the parent atom. These additional carriers can aid the ionization process so that high avalanche current causes avalanche breakdown.
Breakdown voltage
The reverse voltage at which the junction breakdown occurs is breakdown voltage VBD
Peak Inverse Voltage PIV
It is the maximum reverse voltage that can be applied to the PN junction. If the voltage across the junction exceeds PIV, Under reverse bias condition , the junction gets damaged
Application
1.Rectifiers in power supplies
2. Switch in digital circuits used in computers
3.clampling circuit in TV receivers (optional)
4.clipping ciruit in computer (optional)
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