- Precise measurement of Band-Gap
- Clear physical interpretation
- Self contained unit with
no extra accessory required
Several proposals to measure
the Planck’s Constant for didactical purposes, using
the current – voltage (I-V) characteristics of a light
emitting diode (LED) have been made quite regularly in the
last few years. The reason is that the experiment can be done
easily in any lab and the result are surprisingly good (±10%).
The physical interpretation however is not completely clear
and this has raised many discussions, which has been published
almost with same regularity as the proposals themselves.
The present experiment is based on diode
current for V<Vo , using the diode law.
I = Io exp [- e (Vo–
where, e is electronic charge, k is Boltzmann
constant, T is absolute temperature and h is material constant
which depends on the type of diode, the location of recombination
The correct method to determine the real
height of the potential energy barrier Vo is to directly measure
the dependence of the current on temperature keeping the applied
voltage V slightly below Vo. The idea is that the disturbance
to Vo is as little as possible. The slope of ln I vs. 1/T
curve gives e (Vo - V)hk (Fig. 1). The constant
h is determined from I-V characteristics of the diode (Fig.
2) at room temperature from the relation
h = (e/kt)
Compared with previous methods, this determination
of Vo is more precise and more accurate and at the same time
the physical interpretation is more transparent.
The Planck’s constant is then obtained
by the relation
h = e Vo
The wavelength (l) of the light emitted
by the diode can be measured by a transmission grating spectrometer
normally available in the lab.
The value of Plank’s constant obtained
from this method is within 5% of accepted value (6.62 x 10-34
It is a self contained unit. All the necessary facilities
and measuring devices are built in a single unit, as a result
only minimum of external connections need to be made.
Dependence of current (I) on temperature
(T) at constant applied voltage (V)
The following facilities are built in for
highly stable current meter with 3½ digit display
Range: 0-2mA with resolution
is a small temperature controlled oven with built-in RTD
sensor. The temperature is adjustable from ambient to
65°C and displayed on 3½ digit panel meter.
The stability of temperature is ± 0.2°C.
high stability voltage source with 3½ digit display
Material Constant h
To draw I-V characteristics of LED for
determination of h, a variable
voltage source and current meter are provided with 3½
The wavelength l of light emitted
These are taken from LED datasheet or
as measured by transmission grating are provided with the
set of LED’s