BASIC DIODE ELECTRONICS
INTRODUCTION TO DIODES
The p-n Junction
The p-n junction is a homojunction between a p-type and an n-type semiconductor. It acts as a
diode, which can serve in electronics as a rectifier, logic gate, voltage regulator (Zener diode),
switching or tuner (varactor diode); and in optoelectronics as a light-emitting diode (LED), laser
diode, photodetector, or solar cell.
In a relatively simplified view of semiconductor materials, we can envision a semiconductor as
having two types of charge carriers-holes and free electrons which travel in opposite directions
when the semiconductor is subject to an external electric field, giving rise to a net flow of current in
the direction of the electric field. Figure 1 illustrates the concept.
A p-n junction consists of a p-type and n-type section of the same semiconductor materials in
metallurgical contact. The p-type region has an abundance of holes (majority carriers) and a few
mobile electrons (minority carriers); the n-type region has an abundance of mobile electrons and a
few holes (Fig. 2). Both charge carriers are in continuous random thermal motion in all directions.
Fig. 2. Energy levels and carrier concentrations for
a p-type and n-type semiconductor before
contact.
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When a section of p-type material and a section of n-type material are brought in contact to
form a pn junction, a number of interesting properties arise. The pn junction forms the basis of the
semiconductor diode.
ƒ Electrons and holes diffuse from areas of high concentration toward areas of low concentration.
Thus, electrons diffuse from the n-region to the p-region., leaving behind positively charged
ionized donor atoms. In the p-region the electrons recombine with the abundant holes. Similarly,
holes diffuse from the p-region into the n-region, leaving behind negatively charged ionized
acceptor atoms. In the n-region the holes recombine with the abundant mobile electrons. This
diffusion process does not continue indefinitely, however, because it causes a disruption of the
charge balance in the two regions.
ƒ As a result, a narrow region on both sides of the junction becomes nearly depleted of the mobile
charge carriers. This region is called the depletion layer. It contains only the fixed charges
(positive ions on the n-side and negative ions on the p-side). The thickness of the depletion layer
in each region is inversely proportional to the concentration of dopants in the region. The net
effect is that, the depletion region sees a separation of charge, giving rise to an electric field
pointing from the n side to the p side.
ƒ The fixed charges create an electric field in the depletion layer that points from the n-side towards
the p-side of the junction. The charge separation therefore causes a contact potential (also known
as built-in potential) to exist at the junction. This built-in field obstructs the diffusion of further
mobile carriers through the junction region.
ƒ An equilibrium condition is established that results in a net contact potential difference Vo
between the two sides of the depletion layer, with the n-side exhibiting a higher potential than the
p-side. This contact potential is typically on the order of a few tenths of a volt and depends on the
material (about 0.5 to 0.7 V for silicon).
ƒ The built-in potential provides a lower potential energy for an electron on the n-side relative to the
p-side. As a result, the energy bands bend as shown in Fig. 3. In thermal equilibrium there is only
a single Fermi function for the entire structure so that the Fermi levels in the p- and the n-regions
must align.
ƒ No net current flows across the junction. The currents associated with the diffusion and built-in
field (drift current) cancel for both the electrons and holes.
Fig. 3. A p-n junction in the Thermal equilibrium at
T > 0º K. The depletion-layer, energy-band
diagram, and concentrations (on a
logarithmic scale) of the mobile electrons
n(x) and holes p(x) are shown as a functions
of the position x. The built-in potential
difference V corresponds to the energy eV
where e is the electron charge.
0
2
0
The Biased p-n Junction
An externally applied potential will alter the potential difference between the p- and n-regions. This
in turn will modify the flow of majority carriers, so that the junction can be used as a “gate”. If the
junction is forward biased by applying a positive voltage V to the p-region (Fig. 4), its potential is
increased with respect to the n-region, so that an electric field is produced in a direction opposite to
that of the built-in field. The presence of the external bias voltage causes a departure from
equilibrium and a misalignment of the Fermi levels in the p- and n-regions, as well as in the
depletion layer. The presence of the two Fermi levels in the depletion layer, Efc and Efv represents a
state of quasi-equilibrium.
Fig.
4.
Energy band diagram and carrier
concentrations for a forward-biased p-n
junction.
In effect, then, if one were to connect the two terminals of the p-n junction to form a closed circuit,
two currents would be present. First, a small current, called reverse saturation current, is, exists
because of the presence of the contact potential and the associated electric field. In addition, it also
happens that holes and free electrons with sufficient thermal energy can cross the junction. This
current across the junction flows opposite to the reverse saturation current and is called diffusion
current. Of course, if a hole from the p side enters, it is quite likely that it will quickly recombine
with one of the n-type carriers on the n side. (Fig. 4)
The net effect of the forward bias is to reduce the height of the potential-energy hill by an amount
eV. The majority carrier current turns out to increase by an exponential factor exp(eV/kT). So that
the net current becomes i = isexp(eV/kT) – is, where is is nearly a constant. The excess majority
carrier holes and electrons that enter the n and p regions, respectively, become minority carriers and
recombine with the local majority carriers. Their concentration therefore decreases with the
distance from the junction as shown in Fig. 4. This process is known as minority carrier injection.
If the junction is reversed biased by applying a negative voltage V to the p-region, the height of the
potential energy hill is augmented by eV. This impedes the flow of majority carriers. The
corresponding current is multiplied by the exponential factor exp(eV/kT) where V is negative; i.e. it
is reduced. The net result for the current is i = is exp(eV/kT) - is, so that a small current of magnitude
≈ is flows in the reverse direction when IVI » kT/c.
A p-n junction therefore acts as a diode with a current-voltage (i-V) characteristic as illustrated.
⎡ ⎛ eV ⎞ ⎤
i = i s ⎢exp⎜
⎟ − 1⎥
⎣ ⎝ kT ⎠ ⎦
Ideal Diode Characteristic
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The ideal diode characteristic equation is known as the Shockley equation, or simply the diode
equation.
Figure 5 depicts the real diode i-V characteristic for a fairly typical silicon diode for positive diode
voltages. Since the reverse saturation current, is is typically very small (10-9 to 10-15 A), the
expression
⎡ ⎛ eV ⎞⎤
i = is ⎢exp⎜
⎟⎥
⎣ ⎝ kT ⎠⎦
is a good approximation if the diode voltage V is greater than a few tenths of a volt.
Fig. 5. i -V characteristic of a real p-n junction
diode. Note that the conduction takes off
for voltages larger than the contact
potential.
Fig. 6 summarizes the behaviour of the semiconductor diode in terms of its i-V characteristic. Note
that a third region appears in the diode i-V curve that has not been discussed yet. The
reverse-breakdown region to the far left (typically > 50V) of the curve represents the behavior of
the diode when a sufficiently high reverse bias is applied. Under such a large reverse bias, the diode
conducts current again, this time in the reverse direction. To explain the mechanism of reverse
conduction, one needs to visualize the phenomenon of avalanche breakdown. When a very large
negative bias is applied to the p-n junction, sufficient energy is imparted to charge carriers that
reverse current can flow, well beyond the normal reverse, saturation current. In addition, because of
the large electric field, electrons are energized to such levels that if they collide with other charge
carriers at a lower energy level, some of their energy is transferred to the carriers with low energy,
4
and these can now contribute to the reverse conduction process, as well. This process is called
impact ionization. Now, these new carriers may also have enough energy to energize other lowenergy electrons by impact ionization, so that once a sufficiently high reverse bias is provided, this
process of conduction takes place very much like an avalanche: a single electron can ionize several
others.
Fig. 6. The reverse breakdown region
The phenomenon of Zener breakdown is related to avalanche breakdown. It is usually achieved by
means of heavily doped regions in the neighbourhood of the metal-semiconductor junction (the
ohmic contact) .The high density of charge carriers provides the means for a substantial reverse
breakdown current to be sustained at a much lower specific voltage than normal diode, at a nearly
constant reverse bias known as the Zener voltage, Vz. This phenomenon is very useful in
applications where one would like to hold some load voltage constant for example, in voltage
regulators.
The response time of a p-n junction to a dynamic (ac) applied voltage is determined by solving the
set of differential equations governing the processes of electrons and hole diffusion, drift (under the
influence of the built-in and external electric fields), and recombination. These effects are important
for determining the speed at which the diode can be operated. They may be conveniently modeled
by two capacitances, a junction capacitance and diffusion capacitance, in parallel with an ideal
diode. The junction capacitance for the time necessary to change the fixed positive and negative
charges stored in the depletion layer when the applied voltage changes. The thickness l of the
depletion layer turns out to be proportional to √(Vo-V); it therefore increases under the reverse-bias
conditions (negative V) and decreases under the forward-bias conditions (positive V). The junction
capacitance C=ЄA/l (where A is the area of the junction) is therefore inversely proportional to √(VoV). The junction capacitance of a reverse-biased diode is smaller (and the RC response time is
therefore shorter) than that of a forward-biased diode. The dependence of C on V is used to make
voltage-variable capacitors (varactors).
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Experiment l(a) : i-v characteristics of a semiconductor diode
Procedure
Connect the diode according to the circuit diagram as shown in Fig.8.
Fig 8
Vary the voltage V on the power supply between 0-30V. The current i may be deduced using
V
i = R , converting the recorded voltage VR in the Y-input into current scale. The voltage across
R
the diode is recorded as the X-input as V. Use suitable input scales and calibrate the scale on the
recording sheet. Carry out the recording for both forward and reversed bias characteristics.
Calculation
Do a curve fitting using the diode equation. Note down the contact potential.
Experiment l(b): i-v characteristics of a Zener dioide(5.6V).
Procedure
Same as in Experiment l(a) .
Calculation
Just note down the Zener breakdown voltage. Curve fitting is not required.
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EXPERIMENT 2
RECTIFYING AND FILTER CIRCUITS
Objective
To study the operating principles and performance of various rectifying circuits and filters.
Introduction
Many electronic circuits require d.c. power. But our power lines are all a.c. So a way to change one
to the other is necessary. Because of their property of unidirectional conduction, pn-junction diodes
can be used to convert a.c. to d.c. To most common circuits are the full-wave and half-wave
rectifiers. Their circuit are as follows.
1K
1K
7
and the waveform appearing across RL are
FILTERING
The rectified signal may be thought of as a combination of a fixed (d.c.) voltage and a
superimposed alternating voltage. The purpose of the filtering stage is to remove the remaining
alternating portion of the signal. The principle of operation of all filters may be thought of as either
or both of two concepts. The first is that, by offering a low impedance path to ground for the a.c.
portion of the signal while maintaining a higher resistance path for the d.c. portion, the a.c. is
removed. This is. accomplished by a series resistor and capacitor to ground. Alternately, the second
concept is that the blocking action of an inductor stops the a.c. portion while the d.c. portion passes
without much attenuation.
Note:
For filtering, large capacitance (hundreds to tens of hundreds microfarad) is needed. These are generally
electrolytic capacitors, which consist of a repeating sandwich of aluminum sheets and a conducting paste, rolled
into a cylinder for miminmun size. The aluminum sheets are polarized to form thin layers of aluminum oxide, a
dielectric insulating material. The thinner the the dielectric the higher the capacitance will be. The voltage polarity
of the electrolytic capacitance must be observed. The polarity is indicated on the capacitor itself.
If the incorrect polarity is used the oxide will be reduced and conduction (shorting) will occur between the plates.
Since this type of capacitor is sealed to keep the conducting paste from drying out, explosion may result from
wrong polarity.
The most common filter is the RC type, shown in Figure 5(a). The
resistor represents the load on the power supply. The value of the
capacitance is chosen so that the impendance Xc « RL. For example,
if RL = 10 KΩ, then a typical value of Xc is 100.0. Since the
frequency of the full wave rectified signal is 100 Hz (not 50
Hz!), the capacitance needed is
C=
1
1
=
= 16uf
2πf X c (2π)(100)(100)
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Figure 5(b) shows the output of the circuit. In the first cycle, the capacitor is charged to the peak
value, and then begins to decay at a slower rate governed by the RC time constant. In the next cycle
the charge on the capacitor is restored to its peak value, before it has decreased appreciably. If most
of the alternating portion of the signal is to be removed, the RC time constant must be very long
compared to the period of the signal. In this example, RC = 0.16 second, while the period is 1/100
Hz = 0.01 second; correct operation is thus ensured.
With the exception of the first cycle, the output waveform is a good approximation to a triangular
wave. Thus, it is possible to calculate the Fourier series for it. Analysis of the Fourier series for this
waveform is useful in evaluating the magnitudes of d.c. voltage, the fundamental a.c. wave, and its
higher harmonics. This is of particular interest in filtering, where the value of the d.c. portion and
the amount of a.c. signal remaining after filtering are important design considerations.
Fourier analysis of a half wave rectified waveform is
V = 0.318 Vo + 0.5 Vo sin ω t 0.212 Vo cos2 ω t – KK
and for a full wave rectified waveform it is
V
= 0.636 Vo - 0.424 Vo cos2 ω t 0.085 Vo cos4 ω t + KK
When Vo is the peak value of the half waveform, Vo = 1.4Vrms.
Note:
To observe the accurate waveform of the rectified signal on an oscilloscope, use the input marked DC.
The input marked AC has an input coupling capacitor. It will alter the profile of the signal due to the
charging and discharging.
AC
DC
C
Y-Amplifier
R
It serves to block the DC component when desires.
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Note that there is a phase shift between the peak of the ripple voltage and the peak of the sinusoidal
wave. The ripple factor, r, is defined as the ratio of the rms ripple voltage(Vac) to the background dc
voltage(Vdc)
Vac
r = ____
Vdc
1 V
where Vac = RMS of __
ripple (peak to peak)
2
The complete mathematical development of expressions for this quantity is given in many texts.
Our interest is only in the results of this process. For the RC filter introduced earlier, these results
are
r=
and
Vdc =
1
4 3f R L C
Vp
1
1+
4 f R LC
where Vp is the rms voltage of the transformer output
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These equations can be simplified by inserting the frequency of the a.c. signal. For all full wave
rectified situations, the ripple frequency is twice the line frequency, or 100 Hz. Then and
r=
1
680R L C
(2)
and
Vdc =
Vp
1
1+
400 R L C
If the load current is zero (RL→ ∞ ), it will be seen from equation (2) that the ripple factor is zero
arid Vdc →Vp. If the current increases (which is equivalent to a decrease in the load resistance), the
ripple factor will increase and Vdc will decrease. This suggests that the filter lacks regulation in
situations where the load current varies considerably.
Let us rewrite equations 2, using the fact that Idc ≅ Vdc/RL:
Vdc = Vp −
I dc
400C
From this equation, two facts are clear: the output voltage decreases linearly with increasing current
through the load, and the magnitude of the slope of that variation decreases with increasing
capacitance. Thus, better regulation can be achieved by using the largest practical capacitance.
RC π -Section
A significant improvement in filtering is obtained when an additional resistor and capacitor are
connected to the RC filter of Figure 6. This configuration is called a π -section filter, and is shown
schematically in Figure 6. Note that, as the name "section" implies, any number of these units may
be strung together to improve filtering and regulation.
1K
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When RL >> R, as is usually the case, the ripple factor for this filter is approximately
r=
π X C1X C 2
4 3 RR L
(RL >> R)
If, as before, we take the ripple frequency to be 100 Hz, and remember that Xci = l/ ω Ci = 1/2 π fCi,
then
r=
1.16x10-6
C1C2RRL
showing that the voltage regulation is a function of C1 improving with large values. It is usually
said that the major effect of C1 is on the regulation and of C2 is ripple.
Experiment
(i) Construct the half-wave rectifying circuit in Fig.1, using a 6 Vac supply. Connect one channel of
the scope across the secondary of the transformer and the second channel across the load.
Record the waveforms observed.
(ii) In most practical applications full-wave rectification is the norm. Rewire the diode in the full
wave rectifier circuit shown in Fig. 2. Connect one channel of the scope across the secondary
of the transformer and the second channel across the load. Record the waveforms observed
(iii) Construct the full-wave rectifier with the filtering as shown in Fig.5. Record the resulting
waveform for two values of C = (47µf and 470µf), and RL = 1KΩ (> 1watt).
Calculate the ripple factors.
Note: Use the oscilloscope to measure the RMS value of the ripple voltage.
( Do not use the multimeter for ripple voltage )
(iv) Construct the RC π-section as shown in Fig 6. Record the wave form for various value of RC
and calculate the ripple factors, for two values of C where C1 = C2(47f and 470f),
R = 10Ω (> 1watt) and RL = 1k.
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True RMS Multimeters
Certain multimeters will compute the RMS voltage using an IC chip. That is, the meter actually
performs squaring, averaging and taking square root of the signal. This is important if the signal is
not sinusoidal.
Multimeters that do not claimed to be TRUE RMS simply assume a factor of 1.41 for RMS, which
is correct only for sinusoidal signal.
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EXPERIMENT 3 (Optional 4 credit hours)
REGULATION WITH ZENER DIODE
Considerations involved in regulation were presented in the preceding section. Regulation may be
significantly improved by adding a regulator stage to the filtered output. One device often used as a
voltage regulator is the Zener diode.
Our previous discussion of the junction diode omitted an important feature of the diode's operation,
the avalanche or reverse breakdown process. Suppose that the reverse bias on the diode is greatly
increased. When the voltage is sufficient, valence electrons will be freed from their positions
around the nuclei that bind them. Since these electrons possess excess energy, their collisions with
other atoms will knock loose additional electrons; these, in turn, will knock loose more electrons,
and the reverse current becomes an "avalanche." This effect was first noted and utilized by
Clarence Zener, for whom the phenomenon is named. Note that, for reverse voltages greater than
the breakdown voltage, it takes only a very small change in voltage to cause a large change in
current; essentially, the diode goes from OFF to ON as the voltage becomes more negative. The
breakdown or Zener voltage of a diode can be controlled in the manufacturing process, and diodes
are available in various values from 1.2 V to several hundred volts.
It is apparent from Figure 1 that, as long as the Zener diode is reverse biased, any current flow
greater than a few microamperes must be accompanied by a voltage at least as large as the Zener
voltage. This is the key to the use of the Zener diode as a voltage regulator. Figure 2 shows a Zener
diode placed in parallel with a variable load resistance RL; the fixed resistance R is included for
reasons that will become clear below. The unregulated power supply must be designed so that its
filtered output voltage, V, will at an times exceed the desired regulated voltage; the breakdown
voltage, Vz of the diode is chosen to be the same as the desired regulated voltage. A further design
consideration is the maximum allowable current through the diode, Iz, which is given in the
manufacturer's specifications as power rating in Watts. Normally, the diode will be chosen so that
the expected current through the diode is about 20 per cent of Iz. Having these known factors in
hand, we can write, for the circuit of Figure 2, the equation
V = IR + Vz
where
I= I1 + I2 = 0.2Iz + I2
Solving these equations for R, we obtain
R=
V − Vz
0.21z + I 2
(Note: Iz = 5mA)
If we now take I2 to be the maximum current in the load resistance, we
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15
obtain from the value of R needed in the circuit. In operation, the Zener diode will maintain the
voltage Vz between its terminals (and thus across the load) at all times; the excess of V over Vz will
appear across R. The regulation under these conditions is excellent, regardless of the current
demands of the load.
One can place two or more Zener diodes in series to obtain almost any regulated voltage; the values
of the individual Vz's are simply added. Note, however, that the maximum allowable current
through such an arrangement is that of the diode with the lowest Iz. Other arrangements, such as the
one in Figure 3, can be used to obtain regulated voltages of any specified value.
Close examination of the curve in Figure 1 in the Zener region shows that there is a slight variation
in voltage as the current through the diode increases. This variation, however, is so small that one
percent regulation is reasonably easy to achieve. Unless the current is essentially constant, though,
better regulation than this must be accomplished by electronic regulation.
Zener Diode Experiments
(i) Construct the circuit as shown in Fig 2. using a variable DC power supply. Record the output
voltage for a range of input voltages.
(ii) Construct the circuit as shown in Fig 3. using a variable DC power supply. Record the output
voltage for a range of input voltages.
Figure. 3. Circuit using a combination of zener diodes to achieve a voltage not available with
a single zener diode.
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