PN junction:One
of the crucial keys to solid state electronics is the nature of the P-N
junction. When p-type and n-type materials are placed in
contact with each other, the junction behaves very differently than either type
of material alone. Specifically, current will flow readily in one direction
(forward biased) but not in the other (reverse biased), creating the
basic diode. This non-reversing behavior arises from the nature of the
charge transport process in the two types of materials.
The open circles on the left side of the
junction above represent "holes" or deficiencies of electrons in the
lattice which can act like positive charge carriers. The solid circles on the
right of the junction represent the available electrons from the n-type dopant.
Near the junction, electrons diffuse across to combine with holes, creating a
depletion region. The energy level sketch above right is a way of visualize the
equilibrium condition of the pn junction. The upward direction in the diagram
represents increasing electron energy.
Depletion region: When a p-n junction is formed, some of the free
electrons in the n-region diffuse across the junction and combine with holes to
form negative ions. In so doing they leave behind positive ions at the donor
impurity sides.In the p-type region there are holes from the acceptor
impurities and in the n-type region there are extra electrons.
When a
p-n junction is formed, some of the electrons from the n-region which have
reached the conduction band are free to diffuse across the junction and combine
with holes. Filling a hole makes a negative ion and leaves behind a positive
ion on the n-side. A space charge builds up, creating a depletion region which
inhibits any further electron transfer unless it is helped by putting a forward
bias on the junction.
Equilibrium of junction: Coulomb force from
ions prevents further migration across the p-n junction. The electrons which
had migrated across from the N to the P region in the forming of the depletion
layer have now reached equilibrium. Other electrons from the N region cannot
migrate because they are repelled by the negative ions in the P region and
attracted by the positive ions in the N region.
Forward bias: An applied voltage in
the forward direction as indicated assists electrons in overcoming the coulomb
barrier of the space charge in depletion region. Electrons will flow with very
small resistance in the forward direction.
Reverse bias: An applied voltage
with the indicated polarity further impedes the flow of electrons across the
junction. For conduction in the device, electrons from the N region must move
to the junction and combine with holes in the P region. A reverse voltage
drives the electrons away from the junction, preventing conduction.
The relationship between
voltage and current for a PN junction is described by this
equation, sometimes referred to as the "diode equation," or
"Shockley's diode equation" after its discoverer:
ID = IS (e [(qVD)/NkT] −
1)
Where,
ID = Current through the PN junction, in amps
IS = PN junction saturation current, in amps (typically 1 Pico amp)
e = Euler's number ≈ 2.718281828
q = Electron unit charge, 1.6 ×10−19 coulombs
VD = Voltage across the PN junction, in volts
N = Nonideality coefficient, or emission coefficient (typically between 1 and 2)
k = Boltzmann's constant, 1.38 ×10−23
T = Junction temperature, degrees Kelvin
At first this equation may seem very daunting, until you realize that there are really only three variables in it: ID, VD, and T. All the other terms are constants. Since in most cases we assume temperature is fairly constant as well, we are really only dealing with two variables: diode current and diode voltage. Based on this realization, re-write the equation as proportionality rather than equality, showing how the two variables of diode current and voltage relate:
ID = Current through the PN junction, in amps
IS = PN junction saturation current, in amps (typically 1 Pico amp)
e = Euler's number ≈ 2.718281828
q = Electron unit charge, 1.6 ×10−19 coulombs
VD = Voltage across the PN junction, in volts
N = Nonideality coefficient, or emission coefficient (typically between 1 and 2)
k = Boltzmann's constant, 1.38 ×10−23
T = Junction temperature, degrees Kelvin
At first this equation may seem very daunting, until you realize that there are really only three variables in it: ID, VD, and T. All the other terms are constants. Since in most cases we assume temperature is fairly constant as well, we are really only dealing with two variables: diode current and diode voltage. Based on this realization, re-write the equation as proportionality rather than equality, showing how the two variables of diode current and voltage relate:
ID∝eVD
The graph described by
the "diode formula" is a standard exponential curve, rising sharply
as the independent variable (VD, in this case) increases. The corresponding
graph for a resistor, of course, is linear.
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