Laboratory work № 8-5 studying photoconductivity of homogeneous semiconductors

1 Goal of the work

1.1. Learning basic of internal photoeffect theory.

1.2. Acquaintance with the method of investigation of photoelectric properties of semiconductor.

1.3. Measurement of the lux-ampere characteristic in order to elucidate recombination mechanism of nonequilibrium charge carriers.

2 Main concepts

Photoconductivity is a phenomenon of change of electrical conductivity of crystals under the action of light due to the internal photoeffect. The internal photoelectric effect is a process of generation of charge carriers under the action of light, leading to increase of electrical conductivity.

The primary process for the internal photoeffect is absorption of a photon with energy, sufficient for the transition of electron from the valence band and donor level to conductivity band, as well as from the valence band to acceptor level. On a Fig. 1 represented possible types of transitions, leading to changes in electrical conductivity. Transition 1 leads to formation of electron - hole pair, transitions 2 and 3 produce charge carriers of one sign (only electrons for transition 2, and only holes for 3).

The intrinsic photoconductivity is conductivity caused by optical transition of electrons from the valence band to the conductivity band. In this case, obviously,that the photon energy hν should satisfy the condition hν Е_g. Consequently, there is some boundary frequency ν_RB, at which the photoconductivity still observed. It is determined by the ratio hν_RB = Е_g and is called the photoconductivity red border .For example, the boundary frequency for a semiconductor with a bandgap Е_g = 2 eV is equal ν_RB = Е_g/h = 5ּ10¹⁴ s^-1, which corresponds to a wavelength λ_RB = 0,6ּ10^-6 m. Transitions 2 and 3 cause the appearance of the extrinsic photoconductivity.

Detailed studying of the internal photoeffect shows that the effective concentration of emerging photocarriers at given intensity of light flux depends on its spectral composition. That is why each semiconductor has its own spectral characteristics (range of photosensitivity).

The generation of free charge carriers under the action of light leads to increase in electrical conductivity of the semiconductor, which, in the presence of nonequilibrium electrons Δn and holes Δр, can be written as:

(1)

where n₀, р₀ – concentration of equilibrium electrons and holes.

The excess (nonequilibrium) conductivity, equal to the difference of conductivities of semiconductor at the presence of illumination (γ) and in its absence (γ ₀), is photoconductivity γ _PHOTO:

(2)

Concentration of nonequilibrium carriers Δn and Δр depend from the intensity of illumination.

With the continuous illumination of semiconductor with light of constant intensity, it is establishes steady state, characterized by concentration of nonequilibrium carriers Δn₀ and Δр₀, determined by the expressions:

,

(3)

where τ_n и τ_р – lifetime of nonequilibrium electrons and holes respectively (time over which the carrier concentration decreases in e times at the absence of illumination); β –quantum efficiency, determining the number of pairs of nonequilibrium carriers, formed by one absorbed quantum of light; Φ – quantum intensity of light (photon flux density), α – absorption coefficient. Dependence γ_PHOTO from light intensity, under steady illumination, have a look:

(4)

If one of the terms in parentheses of equation (4) is significantly larger than the other (due to the difference in values of mobility or the lifetime of electron and hole), then, the photoconductivity is determined by the carriers of one sign, and called monopolar. In this case

(5)

Value τ determined by conditions of recombination of charge carriers in the semiconductor and depends on many factors. In the simplest case it is a constant.

Investigating the dependance of γ_PHOTO from light intensity (measuring the lux-ampere characteristics of the semiconductor) we can get some information about the conditions of recombination of charge carriers in a semiconductor. When semiconductor is illuminated with monochromatic light, it can be considered, that α = const and β = const. Linear dependence γ_PHOTO from Φ means, that the lifetime τ = const , i.e. it does not depend on the concentration of nonequilibrium carriers of charge. This is co-called linear recombination.

If the dependence γ_PHOTO fromΦ sublinear, i.e. γ_PHOTO ~ Ι^а, where а < 1, it means, that τ decreases with increasing light intensity, or, that is the and same, with increasing concentration of nonequilibrium carriers of charge. It is observed at high intensities of illumination.

Under certain conditions, dependence γ_PHOTO from Φ can be superlinear. This means, that τ increases with increasing of light intensity, due to the complex processes of recharging of trapping centers in the semiconductor, leading to significant changes in the conditions of recombination of charge carriers.

On a Fig. 2 represented plot of photoconductivity versus light flux. The phenomenon of photoconductivity in semiconductors is widely used for measuring parameters of semiconductor materials, as well as in engineering to create a light-sensitive devices - photoresistors.