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< P-N Junction Diodes Diode EquationBias of PN Junctions
Semiconductor devices have three modes of operation:
1. Thermal Equilibrium
At thermal equilibrium there are no external inputs such as
light or
applied voltage. The currents balance each other out so there is
no
netcurrent within the device.
2. Steady State
Under steady state there are external inputs such as light
or
applied voltage, but the conditions do not change with time.
Devices
typically operate in steady state and are either in forward
or
reverse bias.
3. Transient
If the applied voltage changes rapidly, there will be a short
delay
before the solar cell responds. As solar cells are not used for
high
speed operation there are few extra transient effects that need
to
be taken into account.
Diodes under Forward Bias
Forward bias refers to the application of voltage across the
device
such that the electric field at the junction is reduced. By
applying a
positive voltage to the p-type material and a negative voltage
to
the n-type material, an electric field with opposite direction
to that
in the depletion region is applied across the device. Since
the
resistivity of the depletion region is much higher than that in
the
remainder of the device (due to the limited number of carriers
in
the depletion region), nearly all of the applied electric field
is dropped across the depletion region. The net electr
field is the difference between the existing field in the
depletion region and the applied field (for realistic device
the built-in field is always larger than the applied field),
thus reducing the net electric field in the depletion regio
Reducing the electric field disturbs the equilibrium existing at
the junction, reducing the barrier to the diffusion o
carriers from one side of the junction to the other and
increasing the diffusion current. While the diffusion curren
increases, the drift current remains essentially unchanged since
it depends on the number of carriers generate
within a diffusion length of the depletion region or in the
depletion region itself. Since the depletion region is onreduced in
width by a minor amount, the number of minority carriers swept
across the junction is essentiall
unchanged.
Christiana Honsberg
and Stuart Bowden
Instructions
1. Introduction
2. Properties of Sunlight
3. PN Junction
Introduction
BasicsSemiconductor MaterialsSemiconductor StructureConduction
in SemiconductorsBand GapIntrinsic Carrier
ConcentrationDopingEquilibrium CarrierConcentration
Generation
Absorption of LightAbsorption CoefficientAbsorption
DepthGeneration Rate
RecombinationTypes of RecombinationLifetimeDiffusion
LengthSurface Recombination
Carrier TransportMovement of Carriers
inSemiconductorsDiffusionDrift
P-n JunctionsFormation of a PN-JunctionP-N Junction DiodesBias
of PN JunctionsDiode Equation
Diode Equations for PVIdeal Diode Equation DerivationBasic
EquationsApplying the Basic Equations toa PN JunctionSolving for
Depletion RegionSolving for Quasi NeutralRegionsFinding Total
Current
Eg1: Wide Base DiodeEg2: Narrow Base DiodeSummaryChapter 3
Quiz
4. Solar Cell Operation
5. Design of Silicon Cells
6. Manufacturing Si Cells
7. Modules and Arrays
8. Characterization
9. Material Properties
Appendicies
Korean VersionPV Workshop
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Overview
1. Forward bias occurs when a
voltage is applied across the the
solar cell such that the electric field
formed by the P-N junction is
decreased. It eases carrier diffusion
across the depletion region, and
leads to increased diffusion current.
2. In the presence of an external
circuit that continually provides
majority carriers, recombination
increases which constantly depletes
the influx of carriers into the solar
cell. This increases diffusion and
ultimately increases current across
the depletion region.
3. Reverse bias occurs when a voltage
is applied across the solar cell such
that the electric field formed by the
P-N junction is increased. Diffusion
current decreases.
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< P-N Junction Diodes Diode Equation
Carrier Injection and Forward Bias Current Flow
The increased diffusion from one side of the junction to the
other causes minority carrier injection at the edge o
the depletion region. These carriers move away from the junction
due to diffusion and will eventually recombin
with a majority carrier. The majority carrier is supplied from
the external circuit and hence a net current flow
under forward bias. In the absence of recombination, the
minority carrier concentration would reach a new, highe
equilibrium concentration and the diffusion of carriers from one
side of the junction to the other would cease, muc
the same as when two different gasses are introduced. Initially,
gas molecules have a net movement from the hig
carrier concentration to the low carrier concentration region,
but when a uniform concentration is reached, there
no longer a net gas molecule movement. In a semiconductor
however, the injected minority carriers recombine an
thus more carriers can diffuse across the junction.
Consequently, the diffusion current which flows in forward bia
is a recombination current. The higher the rate of recombination
events, the greater the current which flows acrothe junction.
The "dark saturation current" (I0) is an extremely important
parameter which differentiates one diode from
another. I0 is a measure of the recombination in a device. A
diode with a larger recombination will have a larger I
Reverse Bias
In reverse bias a voltage is applied across the device such that
the electric field at the junction increases. Th
higher electric field in the depletion region decreases the
probability that carriers can diffuse from one side of th
junction to the other, hence the diffusion current decreases. As
in forward bias, the drift current is limited by th
number of minority carriers on either side of the p-n junction
and is relatively unchanged by the increased electr
field. A small increase in the drift current is experienced due
to the small increase in the width of the depletio
region, but this is essentially a second-order effect in silicon
solar cells. In many thin film solar cells where th
depletion region is around half the thickness of the solar cell
the change in depletion region width with voltage has
large impact on cell operation.
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