22.2 Photoconductors
591
22.2 Photoconductors
22.2.1 Introduction
Charge carriers can be generated in the semiconductor through the absorption of light with a photon
energy above or below the band gap (Fig. 22.3). Absorption involving impurities occurs typically in the
mid- and far-infrared spectral regimes (cf. Sect. 9.8). The additional charge carriers cause an increase
in the conductivity (8.11).
22.2.2 Photoconductivity Detectors
In stationary equilibrium for constant illumination of power P opt and photon energy E = hν the
generation rate G is given by
G =
n
τ
= η
P opt / hν
V
,
(22.1)
where V is the volume (V = wd L, see Fig. 22.4) and τ denotes the charge-carrier lifetime. η is the
quantum efficiency, i.e. the average number of electron-hole pairs generated per incoming photon. The
photocurrent between the electrodes is
I ph = σ E w d ≈ e μ n n E w d ,
(22.2)
assuming that μ n μ p and with E = V /L denoting the electric field in the photoconductor, V being
the voltage across the photoconductor. We can then also write
I ph = e
η
P opt
hν
μ n τ E
L
= g I p .
(22.3)
With the primary photocurrent I p = e
η
P opt
hν
we deduce a gain
g =
I ph
I p
=
μ n τ E
L
=
τ
t r
,
(22.4)
Fig. 22.3 Absorption and
charge-carrier generation
in a photoconductor: a
band-band transition, b
valence-band to acceptor
and c donor to
conduction-band transition
(a)
(b)
(c)
591
22.2 Photoconductors
22.2.1 Introduction
Charge carriers can be generated in the semiconductor through the absorption of light with a photon
energy above or below the band gap (Fig. 22.3). Absorption involving impurities occurs typically in the
mid- and far-infrared spectral regimes (cf. Sect. 9.8). The additional charge carriers cause an increase
in the conductivity (8.11).
22.2.2 Photoconductivity Detectors
In stationary equilibrium for constant illumination of power P opt and photon energy E = hν the
generation rate G is given by
G =
n
τ
= η
P opt / hν
V
,
(22.1)
where V is the volume (V = wd L, see Fig. 22.4) and τ denotes the charge-carrier lifetime. η is the
quantum efficiency, i.e. the average number of electron-hole pairs generated per incoming photon. The
photocurrent between the electrodes is
I ph = σ E w d ≈ e μ n n E w d ,
(22.2)
assuming that μ n μ p and with E = V /L denoting the electric field in the photoconductor, V being
the voltage across the photoconductor. We can then also write
I ph = e
η
P opt
hν
μ n τ E
L
= g I p .
(22.3)
With the primary photocurrent I p = e
η
P opt
hν
we deduce a gain
g =
I ph
I p
=
μ n τ E
L
=
τ
t r
,
(22.4)
Fig. 22.3 Absorption and
charge-carrier generation
in a photoconductor: a
band-band transition, b
valence-band to acceptor
and c donor to
conduction-band transition
(a)
(b)
(c)