Data Linearisation - diode I-V characteristic
Introduction
The trend that data follows as a parameter is varied is often quite complex. In such as case it can be difficult to see how well a set of measurements or predictions is following a trend. Linearisation is used to cast a complex relationship into a linear (straight line) form where it is far easier to see if of data is following an expected trend. Graphing and linear regression can be used to gain a mathematical or statistical measure of how well a trend is being followed.
Diodes are commonly used in many circuits, and the commonly used transistor often contains two diode junctions. The relationship between the diode current and the voltage across the diode is called the Diode Law and is not a simple linear relationship. The non-linear relationship is however fairly simple to linearise. The purpose of this laboratory exercise is to measure the current-voltage (I-V) characteristic of a diode. Examining the I-V characteristic data with a simple linearisation allows for simple measurement of two of the main parameters in the Diode Law and illustrates how well the Diode Law models the relationship between current and voltage in a diode. The experiment also complements the material in the EE10168 (Circuit Theory) Unit on diodes.
Objectives
- To measure the I-V characteristic of a 1N4148 silicon diode.
- Assess the leakage current and ideality factor of the diode.
- Assess the influence of measurement accuracy in characterising the diode.
Preparation
- Read this script to find out what is needed to do in the Laboratory session.
- Develop a simple spreadsheet in which to record measurements to be taken in the Procedure section.
- Find out how to produce a plot with logarithmic scales rather than linear scales.
Theory
In its most simple form the diode is an electronic switch that allows current to pass in one direction, but stops it flowing in the opposite direction. When current is allowed to flow the device is said to be forward-biased, while under reverse bias it prevents or blocks current flow. This current versus voltage behaviour is used to characterise a diode.
Theoretically, the current-voltage characteristic for an ideal diode can be described mathematically by the Diode Law as:
where
is the forward bias current through the diode; is the reverse bias leakage current, often called the saturation current; is the ideality factor; is the charge on an electron ( ); is the applied forward voltage; is Boltzmann’s constant ( ) and is the absolute temperature in degrees Kelvin.
This equation is theoretically valid for both forward (
The Diode Law has an implicit exponential form which is non-linear. This is easily linearised by taking the logarithm of the equation:
In the case where
Example I-V characteristics are shown in Figure 1 for two different diodes. In Figure 1a the diode has a very high saturation current in order to be able to see the saturation current on the linear current scale used. In Fig. 1b the saturation current is more realistic. This shows the expected linear trend when
It is also important to recognise that a diode, and most other semiconductor devices, will have a maximum specified current
Test Circuit
The test circuit is shown in Figure 2. Measuring
The use of different sensing resistances
Caution is needed with this circuit in order to prevent damaging the diode. Diodes have a maximum current they can conduct, and in the case of the 1N4148 used in this experiment the maximum current is 150 mA. When using a power supply with a maximum voltage of 6 V to set
Procedure
Initial settings and observation
- Connect the sensing resistor and the 1N4148 diode to the BreadBoard as shown in Figure 3. The 6 V terminals of the PSU should be used. The crocodile clip of the oscilloscope probes attached to the Digital Oscilloscope should be connected to common connection of the diode and the negative PSU supply as shown, while the probes connect to measure
and . On the BreadBoards there are two rows of connected socket holes running horizontally across the top of the board. Below this there and many columns of connected sockets running vertically. The 6 V terminals of the PSU should be used with the negative supply connected to the common Ground. The crocodile clip of the oscilloscope probes attached to the Digital Oscilloscope should be connected to common connection of the diode and the negative PSU supply as shown, while the probes connect to measure and .
-
Make sure that both channels 1 and 2 are active on the Digital Oscilloscope so that two waveforms are visible. Selecting Autoscale on the Digital Oscilloscope should show both traces (although they may overlap).
-
To make recording data rather easier some of the facilities of the Digital Oscilloscope should be used. Add measures under the Measure menu for the DC-RMS values of both channels 1 and 2. Using the Aquire menu set Aquire Mode to Averaging and using the adjuster knob set the averaging factor to 32. Set the oscilloscope probes to
and using the Probe menu under the channel buttons ’1’ and ’2’ buttons set the probe scale to 1.00:1. -
Set the output voltage on the 6 V PSU to about 1 V. Note the DC averages for both channels and determine the diode current. This should be a few milliamps.
-
Measure the diode current and voltage as the source voltage
is set to 0.2 V, 0.4 V, 0.8 V, 1.6 V, 3.2 V and 6 V when using and noting the uncertainties in and and the resulting uncertainty in . Take extra measures at intermediate voltages if needed. -
Using
measure the diode current and voltage as the source voltage is set to 0.2 V, 0.4 V, 0.8 V, 1.6 V noting the uncertainties in and and the resulting uncertainty in . Take extra measures at other voltages if needed but take care to keep the diode current below the 1N4148 diode’s maximum current of 150 mA. -
Plot the data obtained as the logarithm of the diode current against the diode voltage and determine the ideality factor
and the saturation current of the diode measured. Include your estimates of how accurately these values have been determined.
Data and observations submissions for the experiment
To aid the recording of data and observations in this experiment there is a proforma to use. This is available as “ELT3 submission proforma” on the EE10142 Moodle site. There you will also find an Assignment portal called “ELT3 Submission point” where you should submit your completed PDF version of the form. You should record:
- Data table recorded in Initial settings and observations. [6 marks]
- Plot of the current-voltage relationship and associated uncertainties recorded in Initial settings and observations. [14 marks]
- Comments or observations made on the measured current-voltage data. [5 marks]
- The ideality factor and the saturation current determined from the measurements, and observations made concerning accuracy of these values. [10 marks]
The deadline for submission is 16:00 on Friday