PN Junction Semiconductor:
Two blocks of PN Junction Semiconductor material are represented in Fig. 1-1;
one block is p-type material, and the other is n-type. The small circles in the p-type
material represent holes, which are the majority charge carriers in p-type. The dots
in the n-type material represent the majority charge carrier free electrons within
that material. Normally, the holes are uniformly distributed throughout the volume
of the p-type semiconductor, and the electrons are uniformly distributed in the n-
type.
Figure 1-1 P-type and N-type semiconductor materials
In Figure 1-12, shows Semiconductor materials are shown representing a PN
Junction. Holes and electrons are close together at the junction, so some free
electrons from the n-side are attracted across the junction to fill adjacent holes on
the p-side. They are said to diffuse across the junction from a region of high carrier
concentration to one of low concentration. The free electrons crossing the junction
create negative ions on the p-side by giving some atoms one more electron than
their total number of protons. The electrons also leave positive ions (atoms with
one fewer electron than the number of protons) behind them on the n-side.
Figure 1-2 At a pn-junction, electrons cross from the n-side to fill holes in a layer of the p-side close
to the junction
Barrier Voltage in PN Junction:
The n-type and p-type materials are both electrically neutral before the charge
carriers diffuse across the junction. When negative ions are created on the p-side,
,the portion of the p-side close to the junction acquires a negative voltage, (see Fig.
1-2). Similarly, the positive ions created on the n-side gives the n-side a positive
voltage close to the junction. The negative voltage on the p-side tends to repel
additional electrons crossing from the n-side. Also, (thinking of the holes as
positive particles) the positive voltage on the n-side tends to repel any hole
movement from the p-side. Thus, the initial diffusion of charge carriers creates a
barrier voltage in pn junction, which is negative on the p-side and positive on the
n-side. The transfer of charge carriers and the resultant creation of the barrier
voltage occur when the PN Junction Semiconductor are formed during the
manufacturing
The magnitude of the barrier voltage in pn junction Semiconductor can be
calculated from a knowledge of the doping densities, electronic charge, and
junction temperature. Typical barrier voltages at 25°C are 0.3 V for germanium
junctions and 0.7 V for silicon.
Figure 1-3 The barrier voltage at a pn junction assists the flow of minority charge carriers and opposes the flow of majority
carriers
The barrier voltage in pn junction opposes both the flow of electrons from the n-
side and the flow of holes from the p-side. Because electrons are the majority
charge carriers in the n-type material, and holes are the majority charge carriers in
the p-type, it is seen that the barrier voltage opposes the flow of majority carriers
across the PN Junction Semiconductor, (see Fig. 1-3). Any free electrons generated
by thermal energy on the p-side of the junction are attracted across the positive
barrier to the n-side. Similarly thermally generated holes on the n-side are attracted
to the p-side through the negative barrier presented to them at the
junction. Electrons on the p-side and holes on the n-side are minority charge
carriers. Therefore, the barrier voltage assists the flow of minority carriers across
the junction, (Fig. 1-3).
Depletion Region in PN Junction:
The movement of charge carriers across the junction leaves a layer on each side
which is depleted of charge carriers. This is the Depletion Region in PN
Junction shown in Fig. 1-20(a). On the n-side, the depletion region consists of
donor impurity atoms which, having lost the free electron associated with them,
have become positively charged.
, Figure 1-4 Charge carrier diffusion across a pn –junction creates a region depleted of charge carriers which penetrate
deepest into the most lightly doped side.
The depletion region on the p-side is made up of acceptor impurity atoms which
have become negatively charged by losing the hole associated with them. (The
hole has been filled by an electron.)
On each side of the junction, equal numbers of impurity atoms are involved in the
depletion region. If the two blocks of PN Junction Semiconductor material have
equal doping densities, the depletion layers on each side have equal widths, (Fig 1-
4(a)). If the p-side is more heavily doped than the n-side, as illustrated in Fig. 1-
4(b), the depletion region penetrates more deeply into the n-side in order to include
an equal number of impurity atoms on each side of the junction. Conversely, when
the n-side is most heavily doped, the depletion region penetrates deepest into the p-
type material.
PN Junction Forward Bias:
Consider the effect of an external bias voltage applied with the polarity of PN
Junction Forward Bias shown in Fig. 1-5; positive on the p-side, negative on the n-
side. The holes on the p-side, being positively charged particles, are repelled from
the positive terminal and driven toward the junction. Similarly, the electrons on the
n-side are repelled from the negative terminal toward the junction. The result is
that the depletion region width and the barrier potential are both reduced.
Two blocks of PN Junction Semiconductor material are represented in Fig. 1-1;
one block is p-type material, and the other is n-type. The small circles in the p-type
material represent holes, which are the majority charge carriers in p-type. The dots
in the n-type material represent the majority charge carrier free electrons within
that material. Normally, the holes are uniformly distributed throughout the volume
of the p-type semiconductor, and the electrons are uniformly distributed in the n-
type.
Figure 1-1 P-type and N-type semiconductor materials
In Figure 1-12, shows Semiconductor materials are shown representing a PN
Junction. Holes and electrons are close together at the junction, so some free
electrons from the n-side are attracted across the junction to fill adjacent holes on
the p-side. They are said to diffuse across the junction from a region of high carrier
concentration to one of low concentration. The free electrons crossing the junction
create negative ions on the p-side by giving some atoms one more electron than
their total number of protons. The electrons also leave positive ions (atoms with
one fewer electron than the number of protons) behind them on the n-side.
Figure 1-2 At a pn-junction, electrons cross from the n-side to fill holes in a layer of the p-side close
to the junction
Barrier Voltage in PN Junction:
The n-type and p-type materials are both electrically neutral before the charge
carriers diffuse across the junction. When negative ions are created on the p-side,
,the portion of the p-side close to the junction acquires a negative voltage, (see Fig.
1-2). Similarly, the positive ions created on the n-side gives the n-side a positive
voltage close to the junction. The negative voltage on the p-side tends to repel
additional electrons crossing from the n-side. Also, (thinking of the holes as
positive particles) the positive voltage on the n-side tends to repel any hole
movement from the p-side. Thus, the initial diffusion of charge carriers creates a
barrier voltage in pn junction, which is negative on the p-side and positive on the
n-side. The transfer of charge carriers and the resultant creation of the barrier
voltage occur when the PN Junction Semiconductor are formed during the
manufacturing
The magnitude of the barrier voltage in pn junction Semiconductor can be
calculated from a knowledge of the doping densities, electronic charge, and
junction temperature. Typical barrier voltages at 25°C are 0.3 V for germanium
junctions and 0.7 V for silicon.
Figure 1-3 The barrier voltage at a pn junction assists the flow of minority charge carriers and opposes the flow of majority
carriers
The barrier voltage in pn junction opposes both the flow of electrons from the n-
side and the flow of holes from the p-side. Because electrons are the majority
charge carriers in the n-type material, and holes are the majority charge carriers in
the p-type, it is seen that the barrier voltage opposes the flow of majority carriers
across the PN Junction Semiconductor, (see Fig. 1-3). Any free electrons generated
by thermal energy on the p-side of the junction are attracted across the positive
barrier to the n-side. Similarly thermally generated holes on the n-side are attracted
to the p-side through the negative barrier presented to them at the
junction. Electrons on the p-side and holes on the n-side are minority charge
carriers. Therefore, the barrier voltage assists the flow of minority carriers across
the junction, (Fig. 1-3).
Depletion Region in PN Junction:
The movement of charge carriers across the junction leaves a layer on each side
which is depleted of charge carriers. This is the Depletion Region in PN
Junction shown in Fig. 1-20(a). On the n-side, the depletion region consists of
donor impurity atoms which, having lost the free electron associated with them,
have become positively charged.
, Figure 1-4 Charge carrier diffusion across a pn –junction creates a region depleted of charge carriers which penetrate
deepest into the most lightly doped side.
The depletion region on the p-side is made up of acceptor impurity atoms which
have become negatively charged by losing the hole associated with them. (The
hole has been filled by an electron.)
On each side of the junction, equal numbers of impurity atoms are involved in the
depletion region. If the two blocks of PN Junction Semiconductor material have
equal doping densities, the depletion layers on each side have equal widths, (Fig 1-
4(a)). If the p-side is more heavily doped than the n-side, as illustrated in Fig. 1-
4(b), the depletion region penetrates more deeply into the n-side in order to include
an equal number of impurity atoms on each side of the junction. Conversely, when
the n-side is most heavily doped, the depletion region penetrates deepest into the p-
type material.
PN Junction Forward Bias:
Consider the effect of an external bias voltage applied with the polarity of PN
Junction Forward Bias shown in Fig. 1-5; positive on the p-side, negative on the n-
side. The holes on the p-side, being positively charged particles, are repelled from
the positive terminal and driven toward the junction. Similarly, the electrons on the
n-side are repelled from the negative terminal toward the junction. The result is
that the depletion region width and the barrier potential are both reduced.
