MODULE NO. : 10
TITLE
: SEMICONDUCTOR DIODES
In this Module:
ILO10.1 Recognizes p and n type materials and explains its characteristics
ILO10.2 Discuss how PN junction is created,
ILO10.3 Explains ideal diode, equivalent circuit and DC characteristics of diode.
ILO10.4 Recognizes half-wave, full-wave and bridge rectifier circuits and explains the operation of
these circuit.
ILO10.5 Test and determine the electrodes of a diode by its body marking and with the VOM.
INTRODUCTION:
Electronic components play an important role in the construction of many electronic circuits.
Diode is a semiconductor component mainly used in most electronic devices. An analogous to
check valve which is mainly used to carry out ON and OFF operation [1,2].
PRESENTATION:
A semiconductor material has an electrical conductivity value falling between that of a metal,
like copper, gold, etc. and an insulator, such as glass [3] . Their resistance decreases as their
temperature increases, which is behavior opposite to that of a metal.
The behavior of charge carriers which include electrons, ions and electron holes at these
junctions is the basis of diodes, transistors and all modern electronics. Some examples of
semiconductors are silicon, germanium, and gallium arsenide. After silicon, gallium arsenide is
the second most common semiconductor used in laser diodes, solar cells, microwave frequency
integrated circuits, and others. Silicon is a critical element for fabricating most electronic
circuits.
Diode - a 2-electrode active device Semiconductor
Parts:
1. anode
2. cathode
* letter symbol is D
Figure 1.1 Types of Diode and Its Corresponding Symbols
Unbiased Rectifier Diode
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When no voltage supplied to a rectifier diode then it is called
as an Unbiased Diode, N-side will have a majority number of
electrons, and very few numbers of holes (due to thermal
excitation) whereas the P- side will have a majority charge
carriers holes and very few numbers of electrons.
In this process, free electrons from N-side will diffuse
(spread) into the P side and recombine takes place in holes
present there, leaving +ve immobile (not moveable) ions in
N- side and creating -ve immobile ions in the P side of the
diode.
The immobile in the n-type side near the junction edge. Similarly, the immobile ions in the ptype side near the junction edge. Due to this, numbers of positive ions and negative ions will
accumulate at the junction. This region so formed is called as depletion region.
At this region, a static electric field called as Barrier Potential is created across the PN junction
of the diode.
It opposes the further migration of holes and electrons across the junction.
Unbiased Diode (No Voltage Applied)
Forward Biased Diode
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Forward Biasing: In a PN junction diode, the positive terminal of a voltage source is connected
to the p-type side, and the negative terminal is connected to the n-type side, the diode is said
to be in forwarding bias qcondition.
The electrons get repelled by the negative terminal of DC
voltage supply and drift towards the positive terminal.
So, under the influence of applied voltage, this electron
drift causes current to flow in a semiconductor. This
current is termed as “Drift current”. As majority carriers
are electrons, current in n-type is the electron current.
As holes are majority carriers in p-type, these get repelled
by positive terminal of DC supply and move across the
junction towards the negative terminal. So, the current in
p-type is the hole current.
So, the overall current due to majority carriers creates a
Forward current.
The direction of conventional current flows from positive
to negative of battery in the direction of conventional current is opposite to the electrons flow.
Reverse Biased Diode
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Reverse Biased condition: if the diode is the positive
terminal of the source voltage is connected to the n-type end,
and the negative terminal of the source is connected to the
p-type end of the diode, there will be no current through the
diode except reverse saturation current.
This is because at the reverse biased condition the depletion
layer of the junction becomes broader with increasing
reverse biased voltage.
Although there is a small current flowing from the n-type to
p-type end in the diode due to minority carriers. This current
is called Reverse Saturation Current.
Minority carriers are mainly thermally generated electrons/
holes in p-type semiconductor and n-type semiconductor
respectively.
Now if reverse applied voltage across the diode is continually increased, then after certain
voltage the depletion layer will destroy which will cause a huge reverse current to flow through
the diode.
If this current is not externally limited and it reaches beyond the safe value, the diode may be
permanently destroyed.
These fast moving electrons collide with the other atoms in the device to knock off some more
electrons from them. The electrons, so released further release much more electrons from the
atoms by breaking the covalent bonds.
This process is called as carrier multiplication and leads to a considerable increase in the flow
of current through the p-n junction. The associated phenomenon is called Avalanche
Breakdown.
What is an Ideal Diode?
An ideal diode is a diode that acts like a perfect
conductor when voltage is applied forward biased
and like a perfect insulator when voltage is applied
reverse biased.
So when positive voltage is applied across the anode to the cathode, the diode conducts forward
current instantly. When voltage is applied in reverse, the diode conducts no current at all.
Below is a graph of the I-V characteristics curve of an ideal diode:
Ideal Diode Characteristics Curve
You can see that when the diode receives forward voltage, it
instantly conducts an infinite amount of current which it can
supply to a circuit. When reverse voltage is fed to the diode,
it conducts no current at all, no matter how great this reverse
voltage is.
These, again, are ideal circumstances. Diodes actually do
not act like this.
The I-V characteristics curve of a conventional diode would look like the following shown
below.
Conventional Diode Characteristics Curve
Table 10.1 Below is a chart showing the differences between, ideal and conventional diodes
Characteristics
Ideal Diode
Coventional Diode
Ideal diodes do not have a
threshold voltage. Once any
Threshold Voltage forward voltage is applied across
the diode, it will conduct current
instantly across its junctions.
Conventional diodes do have a threshold
voltage. This is the voltage which must be
supplied to the diode for it to conduct any
considerable forward current. For silicon
diodes, the threshold voltage is
approximately 0.7V. For germanium diodes,
it is approximately 0.3V. This voltage is
needed so that the electrons of the n junction
can have enough push to break the barrier in
between and cross over into the p junction.
This is how current flows in a diode. Any
voltage below the threshold voltage will not
be sufficient to push the electrons through
from one barrier to the next. You can see this
marked on the diode characteristics curve.
Before the threshold point, very little current
flows across the diode. However, after the
diode receives a voltage above this, it
produces considerable current flow.
Forward Current
Ideal diodes have infinite forward
Conventional diodes conduct a large current
current when any forward voltage
when forward voltage above its threshold
is applied across their terminals.
voltage is supplied to the diode, but it is still
This is because in the ideal
a finite amount of current. Conventional
condition, the internal resistance of
diodes, even though small, still have internal
the diode would be 0. The diode
resistance. It is impossible to create any
would have no internal resistance
physical component that does not have some
at all. Since current, I=V/R, an
internal resistance. The resistance ensures
infinite amount of current would
that the current will be finite in nature and
be conducted and supplied to a
cannot be infinite.
circuit with an ideal diode.
Breakdown
Voltage
Conventional diodes do have a breakdown
voltage. This is the reverse voltage that when
Ideal diodes do not have a
applied to the diode will cause the diode
breakdown voltage. This is
junctions to break down and conduct a large
because ideal diodes have infinite
amount of current, even though the voltage
resistance to reverse voltage. It
is applied with incorrect polarity. A diode
will not conduct any current at all
should not conduct current when voltage is
when voltage is applied in reverse,
applied the wrong way. However, after a
no matter how great the voltage is.
certain point, called the breakdown point, it
will give way and conduct.
Reverse (leakage)
Current
Since an ideal diode does not have
a breakdown point, it never
conducts any reverse current,
called leakage current. It is a
perfect insulator when voltage is
applied in reverse.
A conventional diode will conduct some
leakage current even when reverse voltage is
applied, even when the reverse voltage hasn't
reach the breakdown point. After the
breakdown voltage is reached, it will
conduct a large amount of current, called
avalanche current, in reverse. This is because
a conventional diode can never be a perfect
insulator and will give way if enough reverse
voltage is supplied to it.
Like stated above, an ideal diode is impossible to produce. Diodes have a junction barrier they
must overcome, so they need a threshold voltage. Diodes cannot conduct infinite current because
they will always have some internal resistance. And diodes cannot be perfect insulators when
reverse biased, so they will conduct some leakage current in reverse.The closest real-world item
that an ideal diode can be compared to or explained in terms of is a simple SPST switch.
When forward biased, the ideal diode acts like a closed switch:
When reverse biased, the ideal diode acts like an open switch.
The ideal diode, therefore, is a semiconductor device that is not a switch, but functions as a
perfect, intelligent switch which knows when to shut off and turn on based on the voltage applied
to its junctions. So why even study ideal diodes or any ideal components, since they do not exist?
The answer is, it helps us learn more about how diodes function if they had perfect conditions.
Therefore, it helps us to better understand the characteristics of a diode in the simplest approach.
How would a diode act if it didn't have any internal resistance? Therefore, when we know add
these elements, we better understand the actual diode curve. So in a way, it provides an excellent
starting point for understanding diodes in their simplest (ideal) state. We then can move on to
actual real-life characteristics with a better understanding.
Diode Approximations
Diode approximations are a way to analyze diodes in circuits. In all approximations, the diode
acts as a switch. There are 3 approximations that are used for diodes.
First Approximation
The first approximation is viewing the diode as
an ideal diode. With this first approximation, the
diode acts as a perfect switch that doesn't
consume any voltage and doesn't have any
internal resistance.
It is not used for real-life situations but just as
general approximations when preciseness isn't
needed.
For certain circuits where the forward voltage needed to turn on diode is seen as trivial, the first
approximation can be adopted and used.
In the second approximation, the diode is seen
as as a diode that needs voltage in order to
turn on.
For a silicon diode, the diode needs about
0.7V in order to turn on. When the voltage fed
into the diode forward biased is 0.7V or
greater, the diode switches on. When the
voltage is less than 0.7V, the diode turns off.
In the third approximation, the diode is seen as a diode that consumes 0.7V (for silicon diode)
and voltage across the internal bulk resistance of the diode.
This is most real-world form of a diode and is usually used when a circuit is in design, especially
an intricate circuit. For basic circuits, the second approximation may be used; this is especially
true when the load resistance is high, because the bulk resistance is usually very low, such as less
than 1Ω. The third approximation is crucial if the load resistance is very low.
The bulk resistance, RB, of a diode is the approximate
resistance across the terminals of the diode when a
forward voltage and current are applied across the
diode. The bulk resistance represents the resistance of
the p and n materials of the p-n junction of the diode.
The bulk resistance is not a fixed resistance but a
dynamic one. It changes according to the amount of
forward voltage and current going through the diode at
any particular time, according to the formula, RB=
∆VF/∆IF. RB is a very small resistance, usually less
than 1Ω and almost always less than 10Ω.
Let's now take 2 examples at both sides of the extremes of how the bulk resistance can affect a
circuit, depending on the load resistance of the circuit.
Let's say the bulk resistance for both circuits is 1Ω and the power source powering the circuits is
5V.
The first circuit has a load, which has a resistance of 1KΩ.
According to this circuit, the (silicon) diode will consumes its normal 0.7V to operate and
0.004995V. This is because if 1Ω/(1Ω + 1KΩ)= 0.000999 and 4.3(0.000999)= 0.0042957V.
Even if the output resistance is 75Ω, the diode will only consume 0.0658V more on top of 0.7V.
This is because 1Ω/(1Ω + 75Ω)= 0.0131589 and 4.3V(0.0131589)= 0.05658.
Now let's look at a circuit that has a load with a very low resistance, let's say 1Ω. The situation
will be much different. So the diode will consume its necessary 0.7V and an additional 2.15V.
This is because voltage division between 1Ω (bulk resistance of the diode) and 1Ω (load
resistance of the circuit) gives 1Ω(1Ω + 1Ω)= 0.5 and 4.3V(0.5)= 2.15V. Therefore, the voltage
divides up by dropping 2.85V across the diode and 2.15V across the (1Ω) load. Therefore, you
can see that the voltage the diode now consumes is great because the load resistance is so small.
So when the resistance is so small, the bulk resistance has to be taken into effect. If the load
resistance is fairly large (several ohms or greater), then the bulk resistance is not as important.
So this is a basic summary of the approximations of diodes.
Conventional Rectifier
* allow current to flow in 1 direction when forward bias
* Allow current to flow into a circuit, but block its return
Use: to rectify AC
Application: Power Supply
Figure 1.2 shows the different types of rectifies, while Figure 1.3 depitcs the rectifier – schottky
voltage comparison
Figure 10.5 Different type of rectifier
Figure 10.6 Rectifier – Schottky voltage comparison
Advantages Over Tube Diodes:
1. efficient
2. no warm up time
3. rugged
4. Lightweight
5. cheaper
Biasing
Basic specifications of a diode
1. (Vf): indicates fwd voltage drop when current flows from p to n terminal is .7v
2. (If): is the maximum fwd current a diode can handle (20ma for lead)
3. (Vr): is the reverse break down voltage when current flows from n to p terminal is 50v
4. (Ir): The amount of current that flows when a diode is reverse biased
Forward Bias
(a) Connection
* positive of bias source connected to the P type material (Anode).
* negative of bias source connected to the N Type
material (Cathode).
APPLICATIONS OF RECTIFIER DIODES
Rectifier Circuits: Unregulated power supply
1. Half Wave Rectifier
Features and
Characteristics:
* uses 1 diode
*
does not use center tap of
HVwinding
* utilizes only 1/2 of input cycle
* less power developed for a given
size transformer
Figure 10.7 Halfwave Rectifier circuit and signal.
Application:
* used in small current drain circuits
* with + at the top of the secondary winding and - at the bottom, diode conducts
* conduction path is from the bottom of the winding, through the RL, through the diode,
then to the top of the winding
* voltage is developed across the RL, + at the top, producing a + going output
* with - at the top of the winding and + at the bottom, diode does not conduct
* no current flows through RL, no voltage developed, hence no output
Stages of Rectifier Circuit
In order to power any circuit, a power supply is needed; and if you want to power electronic
devices from an AC supply, a rectifier is needed.
Figure 1.1 illustrates a schematic diagram of a DC power supply. There is a 120 V (rms), 60 Hz
AC line that feeds the power supply, which delivers a voltage VO to the electronic circuit (load
block). VO must be a stable DC voltage to ensure that the electronic circuitry functions correctly.
Figure 10.8
Looking at the diagram, first we see the transformer. This transformer is a step-down transformer
that “steps down” the high AC input voltage to a lower AC voltage to be inputted into the
rectifier. This transformer consists of two separate coil windings (primary and secondary
windings) that have a different number of turns, N 1 for the primary and N2 for the secondary.
Thus, the AC voltage vS can be written as 120(N2/N1) V (rms) and is measured between the two
terminals of the secondary winding.
Next, the diode rectifier converts the AC voltage vS to a DC voltage. This voltage will exhibit
large variations and thus will not be suitable for electronic circuitry. A filter is used to smooth
out these variations.
Even after filtering, though, the voltage will exhibit small variations known as ripple.
Consequently, a voltage regulator is used to greatly reduce the ripple and establish a reliable DC
supply rail.
How the Half-Wave Rectifier Circuit
The half-wave rectifier eliminates the negative portions of the input sinusoid. In Figure 1.2 (A),
the half-wave rectifier is illustrated. In this article, we will use the constant voltage drop (CVD)
model of a diode owing to its simplicity. From this model, we are provided with
v0=0v0=0 when vS<VDvS<VD
Equation 1.1 (A)
v0=vS−VDv0=vS−VD when vS≥VDvS≥VD
Equation 1.1 (B)
where VD ≈ 0.7 V. The above equations lead to the transfer characteristic illustrated in Figure 1.2
(B). Figure 1.2 (C) illustrates the voltage output that is provided when the input voltage vS is
sinusoidal.
Figure 10.9 (A) The half-wave rectifier
Figure 10.9 (B) Transfer characteristics of the rectifier circuit
Figure 10.9 (C) Input and output waveforms
When determining which diodes to use in a rectifier circuit, there are two things to take into
consideration: 1) the diode's ability to handle current, which must be chosen based on the largest
current that is expected to be conducted by the diode, and 2) the peak inverse voltage (PIV),
which is the highest reverse voltage to which the diode will be subjected; the diode must be able
to withstand the PIV. Looking at Figure 1.2 (A), we can observe that when the
voltage vS is negative, the diode will be cut off and the voltage vO will have a value of zero,
leading to a reverse voltage across the diode of magnitude vS. Thus, the PIV is the peak of vS:
PIV = VS
where VS (with an uppercase V) represents the peak amplitude of the input sinusoid.
Equation 1.2
One thing worth noting is that the circuit clearly will not operate effectively when the input
sinusoid's peak amplitude is not significantly higher than VD. For example, a sinusoidal input
with peak amplitude of 200 mV will not be rectified at all because the diode will never "turn on,"
i.e., it will never conduct significant amounts of current.
2. Full Wave Rectifier
Figure 10.10 Fullwave Rectifier Circuit
Features and Characteristics
* uses 2 diodes
* uses center top of HV winding
Application:
* widely used in radio transmitters and receivers
Full-Wave Rectifier Circuit
Unlike the half-wave rectifier, the full-wave rectifier can utilize both the negative and the
positive portion of the AC input voltage. In order to achieve a unipolar output, the negative
portion of the sinusoidal waveform must be inverted. This can be accomplished by using the
circuit shown in Figure 1.3 (A).
FIGURE 10.11 (A) Full-wave rectifier circuit; the transformer has a center-tapped secondary winding
In this configuration, the step-down transformer's secondary winding is what is called "centertapped." A center tap, or CT, is an electrical contact made halfway along the winding. This CT is
used to provide two equal voltages, vS, across the two halves of the transformer's secondary
winding. When the input voltage is positive, both vS signals will also be positive, and when the
input voltage becomes greater than VD, diode D1 will be conducting and diode D2 will be
reverse-biased. The current that flows into diode D1 will also flow through resistor R and then
back to the CT. The circuit behaves just like the half-wave rectifier during the positive half-cycle
of an input sinusoid.
During the negative half-cycle, both vS voltages will be negative. Now, diode D1 is reversebiased and diode D2 is conducting. The current that flows through D2 will then flow through
resistor R and back to the CT.
Thus, current flows during both half-cycles, and furthermore the current through the resistor will
always flow in the same direction. The result is a unipolar output voltage, as shown in Figure
1.3 (C).
FIGURE 10.12 (B) Transfer characteristic for the full-wave rectifier
If we consider the circuit's operation during a positive half-cycle, the voltage at the cathode
of D2 is (vS - VD) and the voltage at the anode of D2 is -vS. Thus, the PIV is (VS - VD) - (-VS):
PIV = 2VS - VD
Equation 1.3
Note that this PIV is roughly double that of the half-wave rectifier.
Figure 10.13 (C) Input and output waveforms
3. Bridge Rectifier
Figure 10.14 Full wave Bridge Rectifier Circuit
Features and Characteristics
* uses 4 diodes
* does not use CT
* operates as Full Wave Rectifier
Application:
* used in high current drain circuits
4. Split Type Bridge Rectifier
Figure 10.15 Fullwave Split Type Bridge Rectifier Circuit
Features and Characteristics
* practically the same components as the Bridge Rectifier
* uses center tap of the secondary winding
* load consists of 2 equal value resistors in series
Application:
* used for Op Amps
Filter Circuits
* Low Pass types
Components:
1. Capacitor (usually electrolytic type)
2. Coil (usually iron core)
Types:
1. Capacitor-Input
* Capacitor is in shunt with load
* Provides higher voltage output
* Poor voltage regulation
* Used in low current drain circuits
Additional Information
* Ripple frequency for Half Wave rectifier is the same as the line frequency
* Ripple frequency for both the Full Wave and Bridge rectifiers is twice the line
frequency
* PRV (peak reverse voltage) - maximum reverse voltage that can be applied
across the diode without destroying it
* Ripple - variation of the rectifier output
REGULATED POWER SUPPLY
With the help of a rectifier, it converts AC supply into DC. Its function is to supply a
stable voltage (or less often current), to a circuit or device that must be operated within certain
power supply limits.
Figure 10.16Block diagram of Regulated Power Supply
Additional Information
* Silicon and germanium are the most commonly used materials in the manufacture of
solid state devices including the diodes.
* Potential barrier of germanium junction is around 0.2 volt.
* Potential barrier of silicon junction is around 0.6 volt.
* Germanium has higher conduction than silicon and is used in low and medium
power diodes and transistors.
* Silicon has higher resistance and can be operated at higher temperature than
germanium.
* Silicon is suitable for higher power applications than germanium
* Solid state devices are very sensitive to heat.
* Heat sink is used in soldering solid state devices and during operation of power solid
state devices.
* Heat sink - any material or device (generally metal) used to dissipate heat away
from solid state
device.
Determining Anode and Cathode Leads with a Volt-Ohmmeter Multi-meter (VOM)
* Cathode or anode marking are displayed on the body of a diode
* Sometimes, markings are improperly stamped or worn out.
* When testing solid state devices with a VOM, the battery of the VOM is used to
apply forward or reverse bias.
* It is therefore a "MUST" to know the polarity of the battery.
* In Asian VOMs (Sanwa, Standard, etc.), the negative terminal of the battery is actually
connected to the + (POs) Jack
* The positive terminal of the battery is connected to the - (Com) jack
Diode testing using an analog instrument is shown in Figure 1.15.
Procedures to follow when using VOM
1. Set VOM at R X 1 range.
2. Connect the VOM prods to the diode leads in order to produce a low resistance
reading.
* This low resistance reading is called forward resistance
* The diode led to where the positive terminal of the battery is connected is the Anode.
3. Reverse the connection to the diode.
* For a good diode, the meter reads high resistance or infinity
* This high resistance reading is called back or reverse resistance.
Figure 10.17 Good Diode testing using an analog instrument
Troubles and Tests
(a) Open
* VOM indicates infinity in all ranges.
* Reversing leads does not Alter meter indication.
(b) Shorted
* VOM reads zero in all ranges
* Reversing leads does not alter meter indication
(c) Leaky
* VOM indicates resistance reading
* Reversing leads does not alter much resistance reading.
(D) GOOD
* Connection indicates low resistance reading (forward resistance)
* Reversing leads reads very high resistance or infinity (reverse or back resistance)
Peak inverse voltage (PIV) or peak reverse voltage (PRV)
The peak inverse voltage is either the specified maximum voltage that a diode rectifier can block,
or, alternatively, the maximum voltage that a rectifier needs to block in a given circuit [4,5]. It is
the maximum voltage that a diode can withstand in the reverse direction without breaking down.
If this voltage is exceeded the diode may be destroyed. Diodes must have a peak inverse voltage
rating that is higher
than the maximum
Type of Diode
Repetitive Peak
voltage that will be
applied to them in a
Reverse
Voltage
given application.
1N 4001
1N 4002
1N 4003
1N 4004
1N 4005
1N 4007
50 V
100 V
200 V
400 V
600 V
1000 V
Table 10.2 peak inverse voltage(PIV)