The theory of the Shockley-Haynes experiment: Contact effects

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Abstract
Theoretical expressions are derived which describe the excess minority carrier distribution one can expect to observe when a Shockley-Haynes measurement is performed in the presence of a contact. The contact has been modelled by postulating the existence of an interface recombination velocity (S) at the contact/semiconductor boundary. An ohmic contact (S = ∞) and the general case of a finite S contact have both been considered. The variation in minority carrier distribution with S is illustrated using the derived expressions and the material parameters appropriate to a short wavelength (3–5 μm) Cadmium Mercury Telluride photodetector. The applied electric field (E) has been taken equal to 100 V cm−1 and the width of the read-out region, l = 40 μm. It has been shown that in this case an ohmic contact scarcely affects the average carrier distribution in the read-out region, and therefore the ’’classical’’ theory can be applied to derive the mobility ( μ) and diffusion constant (D), from the evolution with time of the conductivity modulation in the read-out region. The same procedure, when applied to a less recombining contact, still leads to a fairly reliable value of μ, but gives an appreciable overestimate of D, due to the broadening of the observed pulse.

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