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Materials Science
Light Emitting Diodes
Solids can be classified as metals, semiconductors, or insulators based on the amount of
energy needed to excite electrons from localized bonds to a higher energy state where they
are free to move about the solid. This energy is called the band gap energy, E. In metals,
the band gap energy is very small, resulting in many delocalized electrons and high
conductivity. In insulators, the band gap energy is very large, so insulators are poor
conductors. Semiconductors are the intermediate case corresponding to a moderate band
gap.
empty orbitals
(conduction
band)
filled orbitals
(valence band)
Metal
empty orbitals
(conduction
band)
empty orbitals
(conduction
band)
E
filled orbitals
(valence band)
Semiconductor
E
filled orbitals
(valence band)
Insulator
Fig. 1 Energy diagrams for metals, semiconductors, and insulators
Light emitting diodes, or LEDs, are semiconductors that produce light (Fig. 2). When a
high enough voltage is applied to an LED, electrons in the valence band are excited into the
conduction band. When an electron returns to the valence band, it releases this energy in
the form of a photon with energy E, the band gap energy. Therefore, the color of light
that is produced by an LED depends on the size of the band gap.
Fig. 2 Structure of an LED
The distance between the nuclei in a solid is one factor that affects the band gap energy. In
the solid elements C, Si, Ge, and Sn, the band gap energy decreases as the size of the unit
cell increase (Table 1).
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Table 1. Periodic Properties of the Group 14 Solids
Element
Length of Unit
Cell (pm)
Band Gap
Energy (kJ/mol)
Wavelength of
Emitted Light (nm)
C
357
531
230
Si
543
106
1100
Ge
566
63.7
1900
sSn
649
< 10
12000
Pure semiconductors like Si and Ge have band gap energies that are outside of the visible
spectrum (they both emit infrared light), which is not useful for making LEDs. In LEDs,
the semiconducting element is replaced with two other elements that result in the same
number of electrons (Fig. 3). For example, if two Ge atoms are replaced with one Ga atom
and one As atom, the total number of valence electrons in the solid remains the same.
GaAs is therefore isoelectronic with Ge and has the same crystal structure. Other solids
that are isoelectronic with Ge are ZnSe and CuBr.
13 14 15 16 17
B C N O F
11 12
Al Si P S Cl
Cu Zn Ga Ge As Se Br
Ag Cd In Sn Sb Te I
Au Hg Tl Pb Bi Po At
Fig. 3 A portion of the periodic table emphasizing the formation of solids that are
isoelectronic with Group 14 solids. Isoelectronic pairs are indicated with similar
shading.
Because band gap energy decreases with the size of the unit cell, replacing larger atoms
with smaller atoms will increase E. For example, if As in GaAs is replaced with P, the
resulting GaP will have a larger band gap energy. If only half of the As is replaced with P,
the resulting GaAs0.5P0.5 will have a band gap energy somewhere in between GaAs and
GaP. As the ratio of As to P is varied, and as other isoelectronic elements are used, the
band gap energy can be tuned to almost any value. In this way, LEDs of any color in the
visible spectrum can be made.
Reference: Adapted from http://mrsec.wisc.edu/Edetc/nanolab/LED/index.html and
“Teaching General Chemistry: A Materials Science Companion”, American Chemical
Society, 1993.
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In this experiment you will relate the color and excitation voltage of LEDs to their
composition and periodic trends. In addition, you will observe how electrical resistance of
a metal changes with temperature and how electrical resistance of a semiconductor changes
under illumination. A key objective of this experiment is to relate the solid-state
structure to physical properties of these materials.
Pre-Lab Questions
1. Does P or As have a larger atomic radius? Considering only atomic radius, would GaP
or GaAs have a larger band gap energy?
2. Considering only atomic radii, rank the following semiconductors in order of increasing
band gap energy:
GaP0.65As0.35
GaP
GaP0.85As0.15
GaP0.40As0.60
3. What usually happens to the bond distance of a material when it is cooled? Considering
only bond distance, would a material’s bad gap energy be larger when warm or cold?
Materials
LEDs of various composition
9-volt battery
battery snap with 1000  resistor
alligator clips
copper coil
styrofoam cup
liquid nitrogen
CdS photocell
voltmeter
Procedure
Part 1: LEDs
1. Obtain five different LEDs. They look identical, so be careful not to mix them up.
Connect the 9-V battery to the battery snap (which is already attached to a 1000  resistor).
Complete the circuit by attaching an LED to the two wire leads with alligator clips. Does it
matter which way the LED is attached? Record the composition and color of the LED in
the data table. Repeat this for all five LEDs.
Composition of
LED
Predicted E
(Rank from lowest
to highest)
Color emitted
Measured
Voltage
Color in Liquid
Nitrogen
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2. Voltage is a measurement of electrical energy. The voltage across an LED in a circuit
can be used to compare band gap energies. To measure voltage, set the multimeter to the
read DC volts to the nearest 0.01 V. Measure the voltage across each LED when it is lit.
3. For this step, you will need to work with another group. Both groups should attach the
same type of LED to their circuit. Use one as a reference. Dip the other LED into a foam
cup of liquid nitrogen for a few seconds. Does the color change? Record what you see.
Try this will all five LEDs. CAUTION: Liquid nitrogen is extremely cold. Do not
allow it to come in contact with your skin or clothing, or severe frostbite may result.
Part 2: Electrical Resistance in Metals vs. Semiconductors
4. Resistance measures the difficulty with which an electron moves through a material.
Use a multimeter on the resistance setting (ohms or ) to record the resistance of a very
long (about 150 m) length of thin copper wire wound in a coil. Next, dip the copper coil in
liquid nitrogen and measure its resistance again. Does it change?
5. Now measure the resistance of a short length (about 6 cm) of a CdS semiconductor.
This semiconductor will crack if placed in liquid nitrogen, so instead, add energy to it by
exposing it to a light source. Does its resistance change?
Postlab Questions
1. How are the color and voltage of an LED related to its chemical composition? Write a
paragraph explaining your observations.
2. Why did the LEDs change color in liquid nitrogen? Did the wavelength of light they
emitted become longer or shorter? Why?
3. What happens to the resistance of a metal when the temperature increases? Why?
4. What happens to the resistance of a semiconductor when the excitation energy
increases? Why? (Hint: Look at Fig. 1 – where do the electrons go when energy is
added?)
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