Experimental Methods
PY2108
Practical Session 4:
Semiconductor diodes
M.P. Vaughan
Contents
1 Learning objectives
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2 Pre-lab preparation
2.1 Theory . . . . . . . . . . . . . . . . . . .
2.1.1 The ideal diode . . . . . . . . . .
2.1.2 Full rectifier circuit . . . . . . . .
2.2 Plotting the IV curve of an ideal diode
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3 Methods
3.1 Measurement of the IV characteristic of a diode . . . . . . . . .
3.2 Construction of a rectifier circuit . . . . . . . . . . . . . . . . . .
3.3 Measuring the input and output voltages of the rectifier circuit .
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4 Post-lab
4.1 Fitting the ideality factor . . . . . . . . . . . . . . . . . . . . . .
4.2 Error analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Report requirements . . . . . . . . . . . . . . . . . . . . . . . . .
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Experimental Methods PY2108
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Learning objectives
The objectives of this practical section are to
1. Understand the physics of a p-n junction and how a junction diode is
created
2. Understand the behaviour of a diode under forward and reverse bias and
be able to plot the IV characteristic of an ideal diode
3. Measure the IV characteristic of a real diode and compare to theory
4. Understand the use of diodes in a rectifying circuit
5. Construct a full rectifier circuit and measure the input and output voltage
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2.1
2.1.1
Pre-lab preparation
Theory
The ideal diode
Figure 1: IV characteristic for an ideal diode at room temperature with an
ideality factor of unity and I0 = 0.01 A (this is taken to be large for the sake of
visualisation). The insert shows the symbol for a diode, with the p and n sides
indicated. The arrow indicates the direction that the component allows current
to pass in.
In this section, we discuss the ideal semiconductor diode based on the p-n junction (another type of diode is based on a metal-semiconductor junction known
as a Schottky junction). When blocks of p and n-doped semiconductors are
Experimental Methods PY2108
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brought together, electrons diffuse across the junction from the n-type side and
holes diffuse from the p-type side. This flow of charge is called the diffusion
current. As the diffusion proceeds, the n-type side becomes positively charged
and the p-type side becomes negatively charged, producing electric field across
the junction. This electric field sets up drift currents whereby electrons on the
p-type side are swept back to the n-type side and holes are swept from the
n-type to the p-type side.
Eventually, the drift and diffusion currents balance each other and no further
net charge flow takes place. Between the two sides of the diode is an area swept
clear of charge carriers known as the depletion region and across this there
remains a junction voltage VJ due to the internal field.
When the diode is connected in reverse bias (i.e. with the negative terminal
of a power source connected to the p-side and the positive terminal connected
to the n-side) the applied voltage VD increases the junction voltage and widens
the depletion region. This inhibits charge transport due to the lack of charge
carriers and the energy barrier they must surmount. Hence, no (or very little)
current flows.
In forward bias (i.e. with the positive terminal of a power source connected
to the p-side and the negative terminal connected to the n-side) the applied
voltage VD acts in opposition to the junction voltage. Once the VD exceeds VJ
(i.e. when VD equals the turn-on voltage) the depletion region closes up and
charge carriers are free to take part in conduction. Hence a drift current flows
consisting of electrons being swept towards the positive terminal and holes being
swept towards the negative terminal.
The IV characteristic for an ideal diode are given by
eVD
−1 ,
(1)
I (V ) = I0 exp
βkB T
where
I0 is the reverse saturation current,
e is the magnitude of the elementary charge (charge on an electron),
VD is the applied voltage,
β is the ideality factor (between 1 and 2),
kB is Boltzmann’s constant,
T is the absolute temperature (about 300 K for room temperature).
Note that e and kB are universal constants with values
e = 1.603 × 10−19 C,
kB = 1.381 × 10−23 JK−1 .
An example IV curve is shown in Fig. 1. The symbol for a diode showing
the direction that current may flow in is shown in the insert.
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Figure 2: A full rectifier circuit showing the current flow when the voltage source
is positive (black arrows) and negative (grey arrows). Note that the direction
of the current into the RC part of the circuit is always the same.
2.1.2
Full rectifier circuit
Figure 2 shows the use of diodes in a rectifier circuit. This is a circuit that takes
an AC input and outputs a DC voltage (although with some rippling). The
current flowing through the circuit is indicated by the black and grey arrows,
for the cases where the AC input are positive and negative respectively.
During the positive voltage phase of the AC input, current flows from the
positive terminal through diode (1), being blocked by diode (4). It then passes
into the RC portion of the circuit, shown in detail in Fig. 3. The current then
flows back from this arm of the circuit to the node between diodes (3) and (4).
Since the voltage is positive on the other side of diode (4), the current flows
through diode (3) to the negative terminal of the power source.
During the negative voltage phase of the input, the bottom terminal is positive, so current flows from this through diode (2), being blocked by diode (3).
From the node between diodes (1) and (2), the current flows in the same direction into the RC portion of the circuit as before. On the way out, the current
then passes through diode (4) on its way to the negative terminal.
The capacitor C in parallel with the load resistance R acts to smooth the
rectified current. The current through R is always in the same direction but
without C would undergo large changes in magnitude. With the smoothing
capacitor in place, when the magnitude of the V is rising, the capacitor is
charging and C shunts a current IC through it. As the magnitude of V falls, C
discharges and this extra current is added to I. Thus, the large changes in IR
are smoothed out, leaving only a residual ripple.
Experimental Methods PY2108
(a)
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(b)
Figure 3: The RC portion of the rectifier circuit of Fig. 2 showing the current when (a) the capacitor is charging and IR = I − IC ; (b) the capacitor is
discharging and IR = I + IC . The capacitor therefore acts to smooth out the
changes in current.
2.2
Plotting the IV curve of an ideal diode
Using Eq. (1), plot the IV curve of an ideal diode in a spreadsheet. You should
take care to restrict the range of applied voltages to keep the current I in a
realistic range (I grows exponentially with VD . Currents should not be allowed
to become too large, since the resultant power dissipation may burn components
out, so ensure that your maximum current is on the order of 0.1 A.
The ideality factor should be entered as a separate column (i.e. as a function
of VD ) and be adjustable for each row. This is so that it may be fitted to your
measured data.
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3.1
Methods
Measurement of the IV characteristic of a diode
For the measurement of the diode IV curve, you will need to solder a diode
and resistor (of about 47 Ω in series on a copper strip board. The resistor is to
limit the current drawn from the power supply and hence keep the power down.
Ensure that you know in which direction the current will flow through the diode
(See Fig. 4 for an example of the package markings on a typical diode).
Note that a Si diode has a turn-on voltage of about 0.7 V, whereas a Ge
diode turns on at about 0.3 V. Hence, the applied forward bias must be greater
than this to see the diode IV characteristic. A range of around -4 V to 4 V is
suggested for the bias (check with your demonstrator).
Record your results and estimated errors in your lab book or directly in your
spreadsheet.
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Figure 4: Example of the package markings on a real diode, indicating the
direction that current is allowed to flow in.
3.2
Construction of a rectifier circuit
Construct the circuit shown in Fig. 2, soldering the components on to a copper
strip board.
Important point Your demonstrator may supply you with an electrolytic capacitor, which must be connected with the correct bias (one side to positive, the
other to negative). Check this with your demonstrator. An incorrect connection
may lead to damage of the capacitor (the component may start smoking).
3.3
Measuring the input and output voltages of the rectifier circuit
The input voltage will be supplied by a function generator which should be monitored along with the output over the load resistor on a dual trace oscilloscope
(as you did for the frequency response measurements of RLC circuits).
Take measurements over a range of frequencies, bearing in mind the RC time
constant of the circuit. You should record input frequencies and amplitudes as
well as the DC component of the output and the degree of ripple it exhibits, i.e.
the amplitude of the ripple. Also sketch (or if you have a camera, photograph)
the shape of the output voltage at various frequencies. Note that you should
use DC coupling on the output trace.
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4.1
Post-lab
Fitting the ideality factor
Try to determine the reverse saturation current and turn on voltage of the
measured IV data for the diode. Also try to fit your calculated diode IV
characteristic to your measured data by adjusting the ideality factor. Hence,
produce a graph of ideality factor against input voltage.
Experimental Methods PY2108
4.2
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Error analysis
Using the analytical tools in the supplementary manual ‘Error Analysis’ (available on blackboard), perform an error analysis on your data and calculated
values. You should show the working of this in your report. Hence, add error
bars to your data points (or argue that the errors would be too small to be
noticed if this is the case).
4.3
Report requirements
You should type up your report. Preferably, equations should be typeset using
suitable software (e.g. Equation Editor in Word, using Scientific Word or Latex).
The report should have the following structure:
Title This should have the main heading ‘PY2108 Experimental Methods: Session (4)’ and then an appropriate sub-heading of your choice.
Author Your name and student number.
Abstract A very brief description of the report.
Introduction A short introduction. Possible applications of the methods employed could be included here.
5%
Theory This should include any theory that you actually use to either make
testable predictions (e.g. plot theoretical graphs) or analyse the data. Try
to keep this as brief as possible when describing experimental work.
10 %
Method A description of the experimental setup and how you took the data.
You should also include your error analysis in this section.
35 %
Results / Discussion Presentation of the results.
For this experiment, you should plot your measured IV data against (i)
the theoretical IV characteristic for an ideal diode and (ii) your attempt
to fit the theoretical curve by adjusting the ideality factor. Hence plot the
ideality factor against V and comment on any trends that you observe.
Ensure that you give your estimates of the reverse saturation current and
turn-on voltage. Is the turn-on voltage consistent with the type of diode
used?
You should also report your findings with the rectifier circuit. Present your
data quantifying the input amplitude and frequency and the DC output
and ripple. State any qualitative observations that you have made. If
you have any scanned sketches or photographs of the output, include a
sample here. What is the RC time constant of this circuit (ignoring the
capacitance and resistance of the diodes). Approximately, what frequency
does this correspond to? Does this have any bearing on your observations?
Experimental Methods PY2108
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35 %
Discussion / Conclusions Discuss your experimental findings and attempt
to offer explanations for your observations. Give particular attention to
the reverse saturation current, turn-on voltage and ideality factor when
discussing the diode IV characteristic.
For the rectifier circuit, discuss the relationship between the input and
output signals. Explain your observations. How might you go about
improving the rectifier circuit to give a more stable output?
5%
A further 10 % will be given for presentation, including the correct structure
and grammar.