A p-n junction is formed by combining N-type and P-type semiconductors
together in very close contact. The term junction refers to the region where the
two types of semiconductor meet. It can be thought of as the border region
between the P-type and N-type blocks as shown in the following diagram:
A
silicon p-n junction with no applied voltage. The p-n junction possesses some
interesting properties which have useful applications in modern electronics.
P-doped semiconductor is relatively conductive. The same is true of N-doped
semiconductor, but the junction between them is a nonconductor. This
nonconducting layer, called the depletion zone, occurs because the electrical
charge carriers in doped n-type and p-type silicon (electrons and holes,
respectively) attract and eliminate each other in a process called
recombination. By manipulating this nonconductive layer, p-n junctions are
commonly used as diodes: electrical switches that allow a flow of electricity in
one direction but not in the other (opposite) direction. This property is
explained in terms of the forward-bias and reverse-bias effects, where the term
bias refers to an application of electric voltage to the p-n junction.
A common type of transistor, the bipolar junction transistor, consists of two
p-n junctions in series, for example in the form n-p-n; no current can flow
through it unless a separate small voltage is applied to the middle layer. The
most common type of solar cell is basically a large p-n junction; the free
carrier pairs created by light energy are separated by the junction and
contribute to current.
The invention of the p-n junction is usually attributed to Russell Ohl, Bell
Laboratories.
Equilibrium (zero bias)
In a p-n junction, without an external applied voltage, an equilibrium condition
is reached in which a potential difference is formed across the junction. This
potential difference is called built-in potential .
In an equilibrium PN junction, electrons near the PN interface tend to diffuse
into the p region. As electrons diffuse, they leave positively charged ions
(donors) on the n region. Similarly holes near the PN interface begin to diffuse
in the n-type region leaving fixed ions (acceptors) with negative charge. The
regions nearby the PN interfaces lose their neutrality and become charged,
forming the space charge region or depletion layer (see figure A).
Enlarge picture
Figure A. A p-n junction in thermal equilibrium with zero bias voltage applied.
Electrons and holes concentration are reported respectively with blue and red
lines. Gray regions are charge neutral. Light red zone is positively charged.
Light blue zone is negatively charged. The electric field is shown on the
bottom, the electrostatic force on electrons and holes and the direction in
which the diffusion tends to move electrons and holes.
The electric field created by the space charge region opposes the diffusion
process for both electrons and holes. There are two concurrent phenomena: the
diffusion process that tends to generate more space charge, and the electric
field generated by the space charge that tends to counteract the diffusion. The
carrier concentration profile at equilibrium is shown in figure A with blue and
red lines. Also shown are the two counterbalancing phenomena that establish
equilibrium.
Enlarge picture
Figure B. A PN junction in thermal equilibrium with zero bias voltage applied.
Under the junction, plots for the charge density, the electric field and the
voltage are reported.
The space charge region is a zone with a net charge provided by the fixed ions
(donors or acceptors) that have been left uncovered by majority carrier
diffusion. When equilibrium is reached, the charge density is approximated by
the displayed step function. In fact, the region is completely depleted of
majority carriers (leaving a charge density equal to the net doping level), and
the edge between the space charge region and the neutral region is quite sharp
(see figure B). The space charge region has the same charge on both sides of the
PN interfaces, thus it extends farther on the less doped side (the n side in
figures A and B).
Forward-bias
Forward-bias occurs when the P-type block is connected to the positive terminal
of a battery and the N-type block is connected to the negative terminal, as
shown below.
A silicon p-n junction in Forward-bias.
With this set-up, the 'holes' in the P-type region and the electrons in the
N-type region are pushed towards the junction. This reduces the width of the
depletion zone. The positive charge applied to the P-type block repels the
holes, while the negative charge applied to the N-type block repels the
electrons. As electrons and holes are pushed towards the junction, the distance
between them decreases. This lowers the barrier in potential. With increasing
bias voltage, eventually the nonconducting depletion zone becomes so thin that
the charge carriers can tunnel across the barrier, and the electrical resistance
falls to a low value. The electrons which pass the junction barrier enter the
P-type region (moving leftwards from one hole to the next, with reference to the
above diagram).
This makes an electric current possible. An electron starts flowing around from
the negative terminal to the positive terminal of the battery. It starts at the
negative terminal, moving towards the N-type block. Having reached the N-type
region it enters the block and makes its way towards the p-n junction. The
junction barrier can no longer keep the electron in the N-type region due to the
forward-bias effect (in other words, the thin depletion zone produces very
little electrical resistance against the flow of electrons). The electron will
therefore cross the junction and move ahead into the P-type block. Once inside
the P-type region, the electron, being thermally free (from bonding)—or
mobile—will move through the rest of the crystal, making its way to the positive
terminal of the power supply. Please note that the electron does not jump from
one hole to the next in the p-region. This actually qualifies as electron-hole
recombination which immobilises both hole and electron. The electron can move
freely through the crystal without needing to jump into holes which is what
happens when electrons do cross the depletion layer. This process will be
repeated over and over again, producing a complete circuit path through the
junction.
The Shockley diode equation models the operation of a p-n junction outside the
avalanche region.
Reverse-bias
Connecting the P-type region to the negative terminal of the battery and the
N-type region to the positive terminal, produces the reverse-bias effect. The
connections are illustrated in the following diagram:
A silicon p-n junction in Reverse-bias.
Because the P-type region is now connected to the negative terminal of the power
supply, the 'holes' in the P-type region are pulled away from the junction,
causing the width of the nonconducting depletion zone to increase. Similarly,
because the N-type region is connected to the positive terminal, the electrons
will also be pulled away from the junction.
This effectively increases the potential barrier and greatly increases the
electrical resistance against the flow of charge carriers. For this reason there
will be minimal electric current across the junction.
At the middle of the junction of the p-n material, the depletion region widens
with increasing reverse bias. The electric field grows as the reverse voltage
increases. When the electric field increases beyond a critical level, the
junction breaks down and current begins to flow, usually by either the Zener or
avalanche breakdown processes. Both of these breakdown processes are
non-destructive and reversible so long as current density does not exceed levels
that could cause thermal damage.
Summary
The forward-bias and reverse-bias properties of the p-n junction imply that it
can be used as a diode. A p-n junction diode allows electric charges to flow in
one direction, but not in the opposite direction; negative charges (electrons)
can easily flow through the junction from n to p but not from p to n and the
reverse is true for holes. When the p-n junction is forward-biased, electric
charge flows freely due to reduced resistance of the p-n junction. When the p-n
junction is reverse-biased, however, the junction barrier (and therefore
resistance) becomes greater and charge flow is minimal.
Non-rectifying junctions
In the above diagrams, contact between the metal wires and the semiconductor
material also creates metal-semiconductor junctions called Schottky diodes. In a
simplified ideal situation a semiconductor diode would never function, since it
would be composed of several diodes connected back-to-front in series. But in
practice, surface impurities within the part of the semiconductor which touches
the metal terminals will greatly reduce the width of those depletion layers to
such an extent that the metal-semiconductor junctions do not act as diodes.
These "nonrectifying junctions" behave as ohmic contacts regardless of applied
voltage polarity.
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