1 Introduction 2 The Haynes-Shockley Experiment

where d is the separation of the points A and C. If the electric field is
E = V L
(2)
where L is the length of the bar, then the mobility µ h of the holes will be
µ h = v d
E
(3)
= dL
t d V
(4)
The flash of light creates an excess of both minority and majority carriers, n = p (where n is the
number of electrons created, p the number of holes created). However, the relative increase in the
majority carrier concentration is much less than the relative increase in the minority charge carrier
concentration. Therefore, the arrangement is much more sensitive to minority charge carriers. The
sign of the minority carriers can be determined by reversingthe polarity V of the battery and observing
the conditions under which the minority carriers propagate towards the contact C.
The Haynes-Shockley experiment also provides information on two other very important processes
that occur in semiconductors, namely diffusion and recombination. The pulse on the oscilloscope is
much broader in time than the flash of light that generated the carriers. Furthermore, if the pulse is
monitored as a function of the drift time t d (which can be varied by changing the distance, d), it will
be seen that the pulse becomes broader as the drift time increases (figure
The broadening of the pulse is due to diffusion, which is a movement of charge carriers due to the
presence of a concentration gradient. The diffusion of charge carriers gives rise to a diffusion current, as
distinct from drift currents that arise from teh presence of a voltage gradient. We can write equations
for the electron diffusion current density, J e , and the hole diffusion current density, J h , respectively,
J e = eD e
dn
dx
(5)
and
J h = −eD h
dn
dx . (6)
Here, D e is the electron diffusion coefficient and D h is the hole diffusion coefficient. In contrast, the
equations for the drift current density of electrons and holes respectively are
and
J e = env d (7)
J h = −epv d . (8)
Beware of confusion between these current densities! Make sure you’re explicit about which one you’re
referring to.
The diffusion coefficients are related to their associated carrier mobilities according to the Einstein
relation:
2