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)
