Francesco Gonnella
Matteo Mascolo

• 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%

“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

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

V d
Depletion zone
I = 0

V = 0

I = 0

V = 0
I ≠ 0

V > 0

• If one applies enough voltage to the junction, the LED starts emitting photons having all the same frequency, f (depending only on
E gap
)
• 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 = eV th where e = 1.602x10
-19 C and V th 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: eV th
= hf

You must determine the threshold voltage of the LED, V th
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 V th
4) Evaluate h using the relation eV th
=hf , knowing e and f
(the latter is written in the LED datasheet)

ocular
Readout switch
Power controller
LED power I/0
Measurement connectors
12 V current voltage

Pay attention to the instruments
Ammeter
Voltmeter