Abstract
Free carriers, causing an increase in electrical conductivity, can optically be generated either intrinsically by band-to-band absorption or extrinsically involving defect states in the bandgap. Photoconductivity provides information about carrier excitation and relaxation processes and hence about electronically significant imperfections. Photoconductors can be substantially sensitized by doping with slow recombination centers. An exceedingly long dwell time for carriers captured in traps may induce persistent photoconductivity. A related very small recombination cross-section occurs for deep impurities with a large lattice relaxation. Photoconductivity can be quenched (reduced) by a shift of minority carriers from predominantly slow to fast recombination centers. Such a shift can be induced optically with additional long-wavelength light, as well as thermally or by an electric field.
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- 1.
The principle of detailed balance states that in equilibrium all transitions into a level must equal all transitions out of this level between each group of two states.
- 2.
That is, the Hall effect (Sect. 1.2.2 of chapter “Carriers in Magnetic Fields and Temperature Gradients”) measured for photogenerated majority carriers.
- 3.
The coefficients for downward transitions (capture c) or upward transitions (excitation e) used here are given in units of cm3 s−1, yielding units of cm−3 s−1 for generation or recombination rates. In literature symbols for capture or excitation are also defined differently, meaning transition probabilities (e.g., c × n) measured in s−1.
- 4.
Except when carrier excitation occurs from filled trap levels.
- 5.
These relate to the basic Maxwell’s equation with its condition for the conservation of electrons div j = −dρ/dt; for equilibrium, with dρ/dt ≡ 0, it follows div j ≡ 0. In semiconductors with the introduction of holes, we have two types of currents, j n and j p , and expect with g n = g p and r n = r p that div (j n + j p ) ≡ 0; the sign dilemma in comparing this equation with Eq. 60 can be resolved by replacing the conventional e = |e| with −e for electrons and +e for holes. This is the condition for the conservation of charges. In actuality, however, only a fraction of the electrons and holes are mobile, others are trapped and do not contribute to the currents while participating in the total neutrality account.
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Böer, K.W., Pohl, U.W. (2018). Photoconductivity. In: Semiconductor Physics. Springer, Cham. https://doi.org/10.1007/978-3-319-69150-3_31
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