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Measurement of
Planck’s constant
Francesco Gonnella
Matteo Mascolo
Our experiment
• The goal of our experiment is to give an
estimation of Planck’s constant;
• With our experimental setup, we expect to achieve
an accuracy of 10-20%
The diode (1/2)
“In electronics, a diode is a two-terminal electronic
component with an asymmetric transfer characteristic, with
low resistance to current flow in one direction, and high
resistance in the other.”
(Wikipedia)
Silicon lattice
N-doped
semiconductor
P-doped
semiconductor
The diode (2/2)
A “hole” behaves exactly like an electron of positive
electric charge:
• Negative charges are available in the n zone
• Positive charges are available in the p zone
Vd
I=0
Depletion zone
V = 0
LED: Light Emitting Diode
I≠0
I=0
V = 0
V > 0
Measuring h with a LED
• If one applies enough voltage to the junction, the LED starts
emitting photons having all the same frequency, f (depending only on
Egap)
• When the LED starts to glow, the energy E lost by every electron/hole
jumping over the junction is converted to the energy of one single photon
• The energy of the charges in these conditions is E = eVth
where e = 1.602x10-19 C and Vth is the LED threshold voltage.
• The energy of the emitted photon will be, according to Planck’s theory,
is E = hf (with h Planck’s constant)
…energy is conserved in the process, so:
eVth = hf
To-do list
You must determine the threshold voltage of the LED, Vth
1) You have to change the voltage applied to the LED, writing
the corresponding current flowing through the junction
1) Draw the plot “I vs V” as shown in figure
1) Extrapolate the value of V
for I= 0 using a linear fit of
the straight part of the data.
That value is a good
estimation of Vth
4) Evaluate h using the relation
eVth=hf, knowing e and f
(the latter is written in the LED datasheet)
Experimental setup
ocular
LED power I/0
Measurement
connectors
Readout
switch
Power
controller
12 V
current
voltage
Pay attention to the instruments
Ammeter
Voltmeter
Let’s get started!