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Activity: Diode current vs. voltage curves

Objective:

The purpose of this activity is to investigate the current vs. voltage characteristics of various solid state PN junction diodes such as the conventional Si diode, the Schottky barrier diode, the Zener diode and Light emitting diode (LED).

Background:

The PN junction diode is a device which is commonly used in circuit applications such as rectification where current is allowed to flow in only one direction. When the diode is fabricated in silicon, the forward voltage drop is typically 0.7 V and the VD vs. ID characteristic relating diode voltage and current can be described by an exponential relationship:

where IS and n are scale factors, and kT/q (˜ 25.4 mV at room temperature) is the thermal voltage VT.

Diode schematic symbols:

Each type of diode has a specific schematic symbol which are variations of the conventional diode symbol shown on the left in figure 1. A sort of “Z” shaped cathode denotes a zener diode as in the second symbol in figure 1. An “S” shaped cathode denotes a Schottky diode as in the next symbol. The arrows pointing away from the diode denotes an LED as in the symbol on the right.

Figure 1, Diode schematic symbols

Zener Diode Fundamentals:

A Zener diode is similar in construction and operation to an ordinary diode. Unlike a conventional diode where the intended use is to prevent current in the reverse direction, a zener diode is mostly used in the reverse region above the breakdown voltage. Its I vs, V characteristic curve is similar to ordinary diode. By adjusting the doping of the P and N sides of the junction, it is possible to design a Zener diode that breaks down at anywhere from a few volts to a few hundred volts. See Figure 2. In this breakdown or zener region the diode voltage will remain approximately constant over a wide range of currents. The maximum reverse-bias potential that can be applied before entering the Zener region is called the Peak Inverse Voltage (PIV) or the Peak Reverse Voltage (PRV).

Figure 2, Forward and reverse zener diode I/V characteristics

At voltages above the onset of breakdown, an increase in applied voltage will cause more current to flow in the diode, but the voltage across the diode will stay very nearly at VZ. A Zener diode operated in reverse breakdown can provide a reference voltage for systems like voltage regulators or voltage comparators.

Schottky Diode Fundamentals:

A Schottky barrier diode uses a rectifying metal-semiconductor junction formed by plating, evaporating or sputtering one of a variety of metals onto n-type or p-type semiconductor material. Generally, n-type silicon and n-type GaAs are used in commercially available Schottky diodes. The properties of a forward biased Schottky barrier diode are determined by majority carrier phenomena. A conventional PN junction diode's properties are determined by minority carriers. Schottky diodes are majority carrier devices that can be switched rapidly from forward to reverse bias without minority carrier storage effects.

The normal current vs. voltage (I/V) curve of a Schottky barrier diode resembles that of a PN junction diode with the following exceptions:

1. The reverse breakdown voltage of a Schottky barrier diode is lower and the reverse leakage current higher than those of a PN junction diode made using the same resistivity semiconductor material.

2. The forward voltage at a specific forward current is also lower for a Schottky barrier diode than for a PN junction diode. For example, at 2 mA forward bias current a low barrier silicon Schottky diode will have a forward voltage of ~0.3 volts while a silicon PN junction diode will have a voltage of ~0.7 volts. This lower forward voltage drop can cut the power dissipated in the diode by more than one half. This power savings can be very significant when the diodes need to carry large forward currents.

The current vs. voltage (I/V) relationship for a Schottky barrier diode is given by the following equation known as the Richardson equation. The primary difference from the conventional diode equation is in IS with the addition of the modified Richardson constant A*.

Where:
A = junction area
A* = modified Richardson constant (value varies by material and dopant) = 110
A/(°K2-cm2)
for n-type Si T = absolute temperature in ºK
q = electronic charge = 1.6 * 10-19 C
fB = barrier height in volts
k = Boltzman's constant = 1.37 * 10-23 J/K
n = ideality factor (forward slope factor, determined by metal-semiconductor interface)

LED Fundamentals:

The LED is a junction diode that emits light when forward biased. Actually all PN junction diodes emit photons when forward biased, it is just that the photons are in the infrared band and the physical shape of the diode does not allow the photons to escape the package. To achieve the visible light emitting property, it is necessary to fabricate the LED from materials with larger band-gaps other than silicon. As a result, the forward voltage drop of the LED is greater than 0.7V; usually on the order of 1.5 to 2 volts depending on the wavelength of the emitted light. The LED is also built in a special transparent package as shown in figure 3.

Figure 3, Light emitting diodes

An LED is a semiconductor device that emits electromagnetic radiation at optical and infrared frequencies. The device is a PN junction diode made from p-type and n-type semiconductors, usually GaAs, GaP or SiC. They emit light only when an external applied voltage is used to forward bias the diode above a minimum threshold value. The gain in electrical potential energy delivered by this voltage is sufficient to force electrons to flow out of the n-type material, across the junction barrier, and into the p-type region. This threshold voltage for the onset of current flow across the junction and the production of light is V0. The emission of light occurs after electrons enter into the p-region (and holes into the n-region). These electrons are a small minority surrounded by holes (essentially the anti-particles of the electrons) and they will quickly find a hole to recombine with. Energetically, the electron relaxes from the excited state (conduction band) to the ground state (valence band). The diodes are called light emitting because the energy given up by the electron as it relaxes is emitted as a photon. Above the threshold value, the current and light output increases exponentially with the bias voltage across the diode. The quanta of energy or photon has an energy E = hf. The relation between the photon energy and the turn-on voltage V0, is:

Where:
Eg is the size of the energy gap
V0 is the threshold voltage
f and λ are the frequency and wavelength of the emitted photons
c is the velocity of light
e is the electronic charge
h is Planck's constant

Lab Experiments:

You will be making I vs. V measurements on various types of diodes in this part of the Lab.

Materials:

Analog Discovery Instrument
Solder-less Breadboard
1 - Resistor (100 Ω or any similar value from 100 Ω to 1 KΩ)
1 - Conventional diode (1N4001 or similar)
1 - Schottky diode (1N5817 or similar)
1 - 4.7 volt Zener diode (1N5230 or similar)
Various - LED diode ( 5mm red, yellow, green, blue or white etc.)

Directions:

The current vs. voltage characteristics of the PN junction diode can be measured using the Discovery Board and the following connections shown in figure 4. The green boxes indicate where to connect the Discovery board. Set up the breadboard with waveform generator output, W1, attached to one end of the resistor. The 2+ scope input is also connected here. The other end of the resistor is connected to one end of the diode being measured as shown in the diagram. The 2- scope input as well as the 1+ scope input is also connected to the second end of the resistor. The other end of the diode is connected to ground along with the 1- scope input.

Figure 4, Connection diagram for diode I vs. V curves

Hardware Setup:

The waveform generator should be configured for a 100 Hz triangle wave with 3 volt amplitude and 0 volt offset. The differential input of scope channel 2 (2+,2-) is used to measure the current in the resistor (and diode). The single ended input of scope channel 1 (1+) is used to measure the voltage across the diode (1- input is grounded). The Scope should be setup with channel 1 at 500mV per division and channel 2 set also at 500mV per division. The current flowing through the diode, ID, is the voltage measured by channel 2 divided by the resistor value (100 Ω in this example). Use the XY display mode to plot the voltage across the diode (scope channel 1) on the X axis vs. the current in the diode (scope channel 2) on the Y axis.

Procedure:

Load the captured data for each type of diode into a data analysis software program like MatLab or a spreadsheet (Excel) and calculate the diode current ID. Plot the current vs. the forward bias voltage across the diode. The diode voltage, current relationship should be logarithmic. If plotted on log scale the line should be straight.

LED data analysis:

As was discussed in the background section on LEDs, the I/V relationship of different color (i.e. wavelength) LEDs might be used to measure Planck's constant. The datasheets for your various color LEDs should list the wavelength of the light emitted. Use those numbers in your calculations.

One method to consider begins with plotting the I/V data for each color LED on a semi-log graph. Your data should approximate a straight line, indicative of the exponential nature of the current vs. voltage relationship. An operational definition of the threshold voltage could be the value of the bias voltage when the current reaches 0.01 mA. Extrapolate the I/V curves to where they cross 0.01mA current and use that as the working value of V0.

Construct a table with columns for V0, λ, and f. For each LED, use the measured value of V0 and the value of f to determine a value for Planck's constant and enter it as a column in the table. Find the mean value of Planck's constant and its uncertainty from your experimental values. Compare to the value given earlier.

Questions:

What is the mathematical expression for the diode current, ID, given the voltage across the diode VD?

How do the diode parameters change based on the type of diode, Schottky, LED etc.?

Zener Breakdown voltage measurements:

Directions:

To measure the reverse breakdown voltage of your zener diode modify your measurement set up as shown in figure 5.

Figure 5, Zener diode I vs. V setup

Hardware Setup:

The waveform generator should be configured for a 100 Hz triangle wave with 3 volt amplitude and -2 volt offset such that it swings from -5 volts to +1 volt. This should be enough larger than the breakdown voltage of your 1N5230 4.7 volt zener. Be sure to turn on the -5V power supply before taking your measurements.

Procedure:

Load the captured data for each type of diode into a data analysis software program like MatLab or a spreadsheet (Excel) and calculate the diode current ID. Plot the current vs. the now reverse bias voltage across the diode.

Questions:

What is the slope (effective resistance) of the curve above the breakdown voltage? How much does the voltage change as the current changes from 100uA to 10 mA?

For Further Reading:

http://en.wikipedia.org/wiki/Diode
http://en.wikipedia.org/wiki/Schottky_diode
https://en.wikipedia.org/wiki/Light-emitting_diode

Return to EPS Lab Activity Table of Contents.

university/courses/eps/diode-curves.txt · Last modified: 21 May 2014 14:41 by dmercer

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