1 Introduction 2 The Haynes-Shockley Experiment

A = Ht b , (22)
where H is the pulse height. It is possible to use this quantity to determine the lifetime of the minory
charge carriers, τ.
Assume that when excess carriers are present, the rate of recombination is proportional to the excess
carrier concentration. That is,
− ∂n
∂t
∝ n. (23)
The minus sign indiates that, as t increases, the excess concentration decreases. We can integrate the
above equation to give
n(t) = n 0 e ( −t
τ
) . (24)
Here, τ is the time required for the number of excess carriers to drop to 1/e times their original value.
We call this characteristic time the lifetime of the minority charge carriers.
The pulse area is proportional to the excess charge concentration, so it follows the same form as
equation (24) above:
Hence,
A(t) = A 0 e ( −t
τ
) . (25)
ln A(t) = ln A 0 − t τ
ln e (26)
= ln A 0 − 1 τ
.t (27)
h. Plot the graph of the logarithm of the pulse area A versus the drift time t d .
i. From the graph, find the lifetime of the minority charge carriers, τ.
3 Optical Characterisation and Band Gap of Materials
References - the optical properties handout and kittel...
So far we have used the electrical properties of semiconductors to help characterise them, but the
electronic structure of materials can also be investigated by considering their optical properties. Light
incident on a sample may be reflected, absorbed and/or transmitted. Each of these processes is
governed by the electronic structure of the material. For example, the dominant absorption mechanism
in semiconductors and insulators is usually the excitation of an electron from the valence to the
conduction band across the forbidden band gap.
(a) Before proceeding, make sure that you understand the following terms:
(i) band gap
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