Lesson 4a
Covalent Bonds
When certain atoms combine to form a solid material, they arrange themselves in a fixed pattern
called a crystal. The atom within the crystal structure are held together by covalent bonds, which
are created by the interaction of the valence electron of atoms. Silicon is a crystalline material.
The figure below shows how each silicon atom position itself with four adjacent silicon atom to form
a silicon crystal. A silicon atom with its four valence electrons shares an electron with each of its
four neighbors. This effectively creates eight valence electron for each atom and produces a state
of chemical stability. Also, this sharing of valence electron is attracted equally by two adjacent
atoms which share it. Covalent bonding in an intrinsic silicon crystal is shown in the figure below.
An intrinsic crystal is one that has no impurities. Covalent bonding for germanium is similar
because it also has four valence electrons.
9
Conduction in Semiconductors:
How a material conducts electrical current is important in understanding how electronic devices
operate. You can’t really understand the operation of a device such as a diode or transistor without
knowing something about the basic current phenomenon.
Conduction Electrons and Holes:
An intrinsic (pure) silicon crystal at room temperature derives heat (thermal) energy from the
surrounding air, causing some valence electron to gain sufficient energy to jump the gap from the
valence band into the conduction band, becoming free electrons not bound to any one atom but
free to drift. Free electrons are also called conduction electron.
10
When an electron jump to the conduction band, a vacancy is left in the valence band. This vacancy
is called a hole. For every electron raised to the conduction band by external energy, there is one
hole left in the valence band, creating what is called an electron-hole pair. Recombination occurs
when a conduction-band electron loses energy and fall back into a hole in the valence band.
To summarize, a piece of intrinsic silicon at room temperature has, at any instant, a number of
conduction-band (free) electrons that are unattached to any and are essentially drifting randomly
throughout the material. There is also an equal number of holes in the valence band created when
these electron jump into the conduction band.
Electron and Hole Current:
When a voltage is applied across a piece of intrinsic silicon, as shown in the figure, the thermally
generated free electrons in the conduction band, which are free to move randomly in the crystal
structure, are now easily attracted toward the positive end. This movement of free electrons is one
type of current in a semiconductor material and is called electron current.
Another type of current occurs at the valence level, where the holes created by the free electrons
exist. Electrons remaining in the valence band still attached to their atoms and are not free to move
randomly in the crystal structure as are the free electron. However, a valence electron can move
into a nearby hole, with a little change in its energy level, thus leaving another hole where it came
from. Effectively the hole has moved from one place to another in the crystal structure, as shown in
the figure below. This is called hole current.
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N-type and P-type Semiconductors:
Semiconductor materials do not conduct current well and are of little value in their intrinsic state.
This is because of the limited number of free electrons in the conduction bond and holes in the
valence band. Intrinsic silicon (or germanium) must be modified by increasing the free electrons
and holes to increase its conductivity and make it useful in electronic devices. This is done by
adding impurities to the intrinsic material as you will learn in this section. Two types of extrinsic
(impure) semiconductive material, n-type and p-type, are the key building blocks for all types of
electronic devices.
Doping:
The conductivity of silicon and germanium can be drastically increased by the controlled addition of
impurities to the intrinsic (pure) semiconductive material. This process, called doping, increased
the number of current carriers (electron or holes). The two categories of impurities are n-type and
p-type.
N-type Semiconductor:
To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity
atoms are added. These are atoms with five valence electrons such as arsenic (As), phosphorus
(P), bismuth (Bi), and antimony (Sb).
As illustrated in each pentavalent atom (antimony, in this case) forms covalent bonds with four
adjacent silicon atoms. Four of the antimony atom’s valence electrons are used to form the
covalent bonds with silicon atoms, leaving one extra electron. This extra electron becomes a
conduction electron because it is not attached to any atom. Because the pentavalent atom gives up
an electron, it is often called a donor atom.
Majority and Minority Carriers: Since most of the current carriers are electrons, silicon (or
germanium) doped with pentavalent atoms is an n-type semiconductive material (the n stands for
the negative charge on an electron). The electrons are called the majority carriers in n-type
material. Although the majority of current carriers in n-type material are electrons, there are also a
few holes that are created when electron-hole pair is thermally generated. These holes are not
produced by the addition of the pentavalent impurity atoms. Holes in an n-type material are called
minority carriers.
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P-Type Semiconductor:
To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are
atoms with three valence electron such as aluminum (Al), boron (B), indium (In), and gallium (Ga).
As illustrated in figure, each trivalent atom (boron, in this case) forms covalent bonds with four
adjacent silicon atoms. All three of the boron atom’s valence electrons are used in the covalent
bonds; and, since four electrons are required, a hole results when each trivalent atom is added.
Because the trivalent atom can take an electron, it is often referred to as an acceptor atom. The
number of holes can be carefully controlled by the number of trivalent impurity atoms added to the
silicon. A hole created by this doping process is not accompanied by a conduction (free) electron.
Majority and Minority Carriers: Since most of the current carriers are holes, silicon (or
germanium) doped with trivalent atoms is called a p-type semiconductive material. Holes can be
thought of as positive charges because the absence of an electron leaves a net positive charge on
the atom. The holes are the majority carriers in p-type material. Although the majority of current
carriers in p-type material are holes, there are also few free electrons that are created when
electron-hole pair are thermally generated. These free electrons are not produced by the addition
of the trivalent impurity atoms. Electrons in p-type material are the minority carriers.
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Lesson 5a
The PN Junction
If you take a block of silicon and dope half of it with a trivalent impurity and the other half with a
pentavalent impurity, a boundary called the pn junction is formed between the resulting p-type and
n-type portions. The pn junction is the feature that allows diodes, transistors, and other devices to
work.
A p-type material consists of silicon atoms and trivalent impurity atoms such as boron. The boron
atom adds a hole when it bonds with the silicon atoms. However, since the number of protons and
the number of electrons are equal throughout the material, there is no net charge in the material
and so it is neutral.
An n-type silicon material consists of silicon atoms and pentavalent impurity toms such as
antimony. As you have seen, an impurity atom releases an electron when it bound with four silicon
atoms. Since there is still an equal number of protons and electrons (including the free electrons)
throughout the material, there is no net charge in the material and so it is neutral.
If a piece of intrinsic silicon is doped so that half is n-type and the other half is p-type, a pn junction
forms between the two regions as indicated in the figure below. The p-region has many holes
(majority carriers) from the impurity atoms and only a few thermally generated free electrons
(minority carriers). The n-region has many free electrons (majority carriers) from the impurity atoms
and only a few thermally generated holes (minority carriers)
Formation of the Depletion Region:
As you have seen, the free electrons in the n-regions are randomly drifting in all directions. At the
instant of the pn junction formation, the free electrons near the junction in the n-region begin to
diffuse across the junction into the p-region where they combine with holes near the junction, as
shown in the figure.
Before the pn junction is formed, recall that there are as many electrons as protons in the n-type
material making the material neutral in terms of net charge. The same is true for the p-type
material.
When the pn junction is formed, the n region loses free electrons as they diffuse across the
junction. This creates a layer of positive charges (pentavalent ions) near the junction. As the
electrons moves across the junction, the p region loses holes as the electrons and holes combine.
This creates a layer of negative charges (trivalent ions) near the junction. These two layers of
positive and negative charges form the depletion region, as shown in the figure b. The term
depletion refers to the fact that the region near the pn junction is depleted of charge carriers
(electrons and holes) due to the diffusion across the junction.
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After the initial surge of free electrons across the pn junction, the depletion region has expanded to
a point where equilibrium is established and there is no further diffusion of electrons across the
junction.
Keep in mind that the depletion region is formed very quickly and is very thin compared to the n
and p region. The width of the depletion region in the figure above is exaggerated for purpose of
illustration.
Barrier Potential
Any time there is a positive charge and a negative charge near each other, there is a force acting
on the charges as described by Coulomb’s law. In the depletion region there are many positive
charges and many negative charges on opposite sides of the pn junction. The forces between the
opposite charges form a ‘field of forces’ called an electric field. as illustrated in figure b above by
the arrows between the positive charges and the negative charges. This electric field is a barrier to
the free electrons in the n-region, and energy must be expended to move an electron through the
electric field. That is, external energy must be applied to get the electrons to move across the
barrier of the electric field in the depletion region.
The potential difference of the electric field across the depletion region is the amount of energy
required to move electrons through the electric field. This potential difference is called the barrier
potential and is expressed in volts. Stated another way, a certain amount of voltage equal to the
barrier potential and with the proper polarity must be applied across a pn junction before electrons
will begin to flow across the junction.
The barrier potential of a pn junction depends on several factors, including the type of
semiconductor material, the amount of doping, and the temperature. The typical barrier potentials
are:
0.7V for silicon at 25C
0.3V for germanium at 25C
Coulombs Law : The attraction or repulsion between two electric charges is proportional to the
product of their magnitudes and inversely proportional to the square of the distance between them.
This is also called the Law of Electrostatic Attraction. Coulomb is a unit of electric charge.
Biasing The PN Junction:
As you have learned, no electrons move through the pn junction at equilibrium; and since the flow
of electrons is electrical current, there is no electrical current through the pn junction. In electronics,
the term bias refers to the use of a DC voltage to establish certain operating conditions for an
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electronic device. Either of these bias conditions is established by connecting a sufficient DC
voltage of the proper polarity across the pn junction.
Forward Bias:
To bias a pn junction, you apply an external dc voltage across it. Forward bias is the condition
that allows current thought a pn junction. The figure below shows a dc voltage source connected
by conductive material (contacts and wire) across a pn junction in the direction to produce forward
bias. This external bias voltage is designated as VBIAS. The resistor, R, limits the current to a value
that will not damage the pn structure.
Notice that the negative side of VBIAS is connected to the n region of the pn junction and the positive
side is connected to the p-region. This is one requirement for forward bias. A second requirement
is that the bias voltage, VBIAS, must be greater than the barrier potential. A fundamental picture of
what happens when a pn junction is forward-biased is shown in the figure below. Because like
charges repel, the negative side of the bias-voltage source ‘pushes’ the free electrons, which are
the majority carriers in the n-region, toward the pn junction. This flow of free electrons is called the
flow of electrons. The negative side of the source also produces a continuous flow of electron
through the external connection (conductor) and into the n-region as shown.
The bias-voltage source imparts sufficient energy to the free electrons for them to overcome the
barrier potential of the depletion region and move on through into the p-region. Once in the pregion, these conduction electrons have lost enough energy to immediately combine with holes in
the valence band.
Now the bias-voltage source imparts sufficient energy to the free electrons for them to overcome
the barrier potential of the depletion region and move on through into the p-region. Since unlike
charges attracts, the positive side of the bias–voltage source attracts the valence electrons toward
the left end of the p-region. The holes in the p-region provide the medium or ‘pathway’ for these
electrons to move through the p-region. The electrons move from on hole to the next toward the
left. The holes, which are the majority carriers in the p-region, effectively (not actually) move to the
right toward the junction, as you can see in the figure above. This effective flow of holes is called
the hole current. You can also view the hole current as the flow of valence electrons through the pregion, with the holes providing the only means for these electrons to flow.
The effect of Forward Bias on the Depletion Region:
As more electrons flow into the depletion region, the number of positive ions is reduced. As more
holes effectively flow into the depletion region on the other side of the pn junction, the number of
negative ions is reduced. This reduction in positive and negative ions during forward bias causes
the depletion region to narrow as indicated in the figure below.
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The Effect of the Barrier Potential During Forward Bias:
Recall that the electric field between the positive and negative ions in the depletion region on either
side of the junction creates an ‘energy hill’ that prevents free electrons from diffusing across the
junction at equilibrium. This is known as the barrier potential.
Reverse Bias:
Reverse bias is the condition that prevents current through the pn junction. The figure below shows
a dc voltage source connected across a pn junction in the direction to produce reverse bias. This
external bias voltage is designed as VBIAS just as it was for forward bias. Notice that the positive
side of VBIAS is connected to the n region of the pn junction and the negative side is connected to
the p region. Also note that the depletion region is shown much wider than in forward bias or
equilibrium.
An illustration of what happens when a pn junction is reverse-bias is shown in the figure below.
Because unlike charges attracts, the positive side of the bias voltage source ‘pulls’ the free
electrons, which are the majority carriers in the n region, away from the pn junction. As the
electrons flow toward the positive side of the voltage source, additional positive ions are created.
This results in a widening of the depletion region and a depletion of majority carriers.
In the p-region, electrons from the negative side of the voltage source enter as valence electron
and move from hole to hole toward the depletion region where they create additional negative ions.
This results in a widening of the depletion region and a depletion of majority carriers. The flow of
valence electron can be viewed as holes being ‘pulled’ toward the positive side.
The initial flow of charge carriers is transitional and last for only a very short time after the reversebias voltage is applied. As the depletion region widens, the availability of majority carriers
decreases. As more of the n and p regions become depleted of majority of carriers, the electric
field between the positive and negative ions increases in strength until the potential across the
depletion region equals the bias voltage, VBIAS.
Reverse Current:
The extreme small current that exists in reverse bias after the transition current dies out is caused
by the minority carriers in the n and p regions that are produced by thermally generated electronhole pairs. The small number of free minority electrons in the p region are ‘pushed’ toward the pn
junction by the negative bias voltage. When these electrons reach the wide depletion region, they
‘fall down the energy hill’ and combine with the minority holes in the n region as valence electrons
and flow toward the positive bias voltage, creating a small hole current.
The conduction band in the p region is at a higher energy level than the conduction band in the n
region. Therefore, the minority electrons easily pass through the depletion region because they
require no additional energy. Reverse current is illustrated in the figure below.
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Reverse breakdown:
Normally, the reverse-current is so small that it can be neglected. However, if the external reverse
bias voltage is increased to a value called the breakdown voltage, the reverse current will
drastically increase.
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Current-Voltage Characteristic of a PN Junction:
VF
+ volts -
p
IF
+ amp -
n
VBIAS
RL
+ volts -
-
+
VBIAS = 0
a) No bias voltage. PN junction is at equilibrium
VF
+ volts -
p
IF
+ amp -
n
VBIAS
RL
+ volts -
+
-
VBIAS = small
b) Small forward-bias voltage (VF < 0.7V), very small forward current.
VF
+ volts -
p
IF
n
+ amp -
VBIAS
RL
+ volts -
+
-
VBIAS = enough
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c) Forward voltage reaches and remains at approximately 0.7V. Forward current continues
to increase as the bias voltage is increased.
As you have learned, forward bias produces current through a pn junction and reverse bias prevent
current except for a negligible reverse current. Reverse bias prevents current as long as the
reverse-bias voltage does not equal of exceed the breakdown voltage of the junction.
I-V Characteristic for Forward Bias:
As you have learned, when a forward bias voltage is applied across a silicon pn junction, there is
current through the junction. This current is called the forward current and is designated I F. The
figure above illustrates what happens as the forward-bias voltage is increased positively from 0 V.
The resistor is used to limit the forward-bias current to a value that will not overheat the pn junction
and cause damage.
With 0 V across the pn junction, there is not forward current, as indicated in the figure a above. As
you gradually increase the bias voltage, the forward current and the voltage across the pn junction
gradually increase, as shown in part b. A portion of the applied bias voltage is dropped across the
limiting resistor. When the applied bias voltage is increased to a value where the voltage across
the pn junction reaches approximately 0.7V (barrier potential), the forward current begins to
increase rapidly.
As you continue to increase the bias voltage, the current continues to increase very rapidly, but the
voltage across the pn junction increases very gradually above 0.7V, as illustrated in figure c. This
small increase in the pn junction voltage above the barrier potential is due to the voltage drop
across the dynamic resistance of the semiconductive material.
Graphing the I-V Curve:
If you plot the result of the type of measurement shown in the figure above on a graph, you get the
I-V characteristic curve for a forward-biased pn junction, as shown in the figure a below. The
forward current (IF) increases upward along the vertical axis and the pn junction forward voltage
(VF) increases to the right along the horizontal axis.
As you can see in the figure a below, the forward current increases very little until the forward
voltage across the junction voltage remains at approximately 0.7 V, but IF increases rapidly. As
previously mentioned, there is a slight increase in VF above 0.7 V as the current increases due
mainly to the voltage drop across the dynamic resistance. Normal operation for a forward-biased
pn junction is above the knee of the curve. The IF scale is typically in mA, as indicated.
The three points A, B and C shown on the curve in the figure a below can be related to the
measurements in figure above. Point A corresponds to figure a, which is a zero-bias condition.
Point B corresponds to figure b where the forward voltage is less than the barrier potential of 0.7 V.
Point C corresponds to figure c where the forward voltage V F approximately equals the barrier
potential. As the external bias voltage and forward current continue to increase above the knee, the
forward voltage will increase slightly above 0.7 V. In reality, the forward voltage can be as much as
0.90 V, depending on the forward current.
Dynamic Resistance:
Unlike the linear resistance, the resistance of the forward-biased pn material is not constant over
the entire curve. Because the resistance changes as you move along the I-V curve, it is called
dynamic or ac resistance. Internal resistances of electronic devices are usually designate4d by
lowercase italic with a prime, instead of the standard R. The dynamic resistance of a diode is
designated rd’.
Below the knee of the curve the resistance is greatest because the current increases very little for
a given change in voltage (rd’ = VF/IF). The resistance begins to decrease in a region of the
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knee of the curve and becomes smallest above the knee where there is a large change in current
for a given change in voltage. The characteristic is illustrated in figure b below for equal changes in
VF (VF) on a magnified segment of the I-V curve below and above the knee.
IF(mA)
IF(mA)
C
IF
IF
A
0
B
VF
0.7 V
0
a) I-V characteristic curve
(forward bias)
VF
VF
VF
b) dynamic resistance decreases as you move
up the curve (rd’ = VF/IF)
I_V characteristic for Reverse Bias:
When a reverse-bias voltage is applied across a pn junction, there is only an extremely small
reverse current (IR) through the junction. The figure below shows what happened as the reversebias voltage is increased negatively from 0 V.
With 0 V across the pn junction, there is no reverse current. As you gradually increase the reversebias voltage there is a very small reverse current and the voltage across the pn junction increases,
as shown in the figure. When the applied bias voltage is increased to a value where the reverse
voltage across the pn junction (VR) reaches the breakdown value (VBR), the reverse current begins
to increase rapidly.
As you continue to increase the bias voltage, the current continues to increase very rapidly, but the
voltage across the pn junction increases very little above VBR as shown in the figure below.
breakdown
- volts +
p
n
- amp +
RL
- volts +
-
+
VBIAS
a) A reverse-bias voltage less than breakdown value produces an extremely small reverse
current.
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breakdown
- volts +
p
- amp +
n
RL
- volts +
-
+
VBIAS
b) When the bias voltage exceeds the breakdown value, the reverse current rapidly
increases and damage from excessive heat can destroy the pn material.
Graphing the I-V Curve:
If you plot the result of the type of measurements shown in the figure above on a graph, you get
the I-V characteristic curve for a reverse –biased pn junction. A typical curve is shown below. the
reverse current (IR) increases downward along the vertical axis and the pn junction reverse voltage
(VR) increases to the left along the horizontal axis. Point A on the curve corresponds to the
measurements in figure A above, and point B corresponds to the measurements in figure b above.
VR
0
VBR
A
Knee
B
IR(A)
I-V characteristic curve for reverse bias
As you can see, there is very little reverse current (usually A or nA) until reverse voltage across
the pn junction reaches approximately the breakdown value VBR. After this point, the reverse
voltage remains approximately VBR but IR increases very rapidly resulting in overheating and
possible damage
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IF(mA)
VR
0
VBR
0
0.7 V
VF
Knee
IR(A)
Temperature Effects on the I-V Characteristic
For a forward-biased pn junction, as temperature is increased, the forward current increases for a
given value of forward voltage. Also for a given value of forward current the forward voltage
decreases.
The DIODE
Diode Structure and Symbol
The general-purpose or rectifier diode is a single pn junction device with conductive contacts and
wire leads connected to each region. One half of the diode is an n-type semiconductor and the
other half is a p-type semiconductor.
The schematic symbol for a general-purpose or rectifier diode is shown in the figure below. The nregion is called the cathode and the p-region is called the anode. The ‘arrow’ in the symbol points
the direction of conventional current (opposite to electron flow).
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Semiconductor pn junction structure:
p
Metal contacts and wire leads
n
a) Basic diode structure
Cathode(K)
Anode(A)
b) Schematic symbol
Diode structure and schematic symbol
Forward-Bias Connection:
A diode is forward-biased when a voltage is connected in the figure below. The positive terminal of
the source is connected to anode through s current-limiting resistor. The negative terminal of the
source is connected to the cathode. The forward current (I F) is from cathode to anode. The forward
voltage drop (VF) is due to the barrier potential and is from positive at the anode and negative at
the cathode.
+
VF
-
IF
R
VBIAS
-
+
a) Forward Bias
-
VF
+
IF = 0
R
VBIAS
-
+
b) Reverse Bias
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Reverse-Bias Connection
A diode is reverse-biased when a voltage source is connected as shown in the figure b above. The
negative terminal is connected to the anode and the positive terminal is connected to the cathode.
A resistor is not needed for reverse bias but is shown for circuit consistency. The current is zero
(neglecting small reverse current). Noticed that the entire bias voltage (V BIAS) appears across the
diode.
Ideal diode model
K
A
Ideal diode model
K
A
IF
IF = 0
R
R
VBIAS
VBIAS
+
-
-
+
a) Forward Bias
b) Reverse Bias
IF
VR
VF
IR
c) Ideal Characteristic Curve
The Ideal Model of a Diode:
Ideal diode model
K
A
Ideal diode model
K
A
VF
+
IF
IF = 0
R
R
VBIAS
+
a) Forward Bias
VBIAS
+
b) Reverse Bias
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IF
VR
0.7V
VF
IR
c) Characteristic Curve (Silicon)
The Practical Model of a Diode:
VF = 0.7V (silicon)
VF = 0.3V (germanium)
The forward current is determined with the following formula:
IF =
VBIAS - VF
R
Example:
+
VF
-
IF
R1
2.2K
VBIAS = 8V
+
-
For the series diode configuration of the figure above, find VF, VR1, and IF.
Answer:
VF = 0.7V
VR1 = 7.3V
IF = 3.32mA
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Example:
Repeat the figure above with the diode reversed
Answer:
VF = 8V
VR1 = 0V
IF = 0V
Problem No. 1:
+
VF(ge)
-
+
VF(si)
VO
IF
+
R1
VBIAS = 12V
2.2K
-
Find VO, IF, VF(ge), VF(si)
Problem No. 2:
+
VF(si1)
-
+
VF(si2)
VO
IF
+
VBIAS = 10V
R1
10K
-
Find VO, IF, VF(si1), VF(si2)
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