22.2 Photoconductors
593
S/N =
i
2
ph
i
2
G + i
2
GR
=
η m
2
(P opt / hν)
8 B
1 + β
−1 t r
τ
(1 + ω
2
τ
2
)
G
I 0
−1
.
(22.9)
An important quantity is the noise equivalent power (NEP). This is the light power (m P opt /
√
2) for
which the S/N ratio is equal to 1 (for B = 1). The responsivity of a detector to light has been termed
’detectivity’ and is the inverse of the noise equivalent power. It typically depends on the square root of
the detector area A and the bandwidth B [1825]. Thus the detectivity D
∗ (D star) has been introduced
defined by [1826]
D
∗
=
√
A B
NEP
,
(22.10)
in order to make various detectors comparable. The unit of D
∗ is cm Hz
1/2 W
−1 , also known as Jones.
The detectivity should be stated together with the modulation frequency. It can be given for monochromatic radiation of a particular wavelength λ or a blackbody spectrum of given temperature T . As
refined measure for detectivity, D
∗∗ (D double star) has been defined to take into account the solid
angle from which radiation can reach the detector [1827],
D
∗∗
=
/π D
∗
;
(22.11)
for a Lambertian characteristic D
∗∗
= D
∗ .
22.2.3 Electrophotography
The principle of the Xerox copy machine is based on a photoconductive layer (Fig. 22.5). This layer is
normally insulating such that both sides of the layer can be oppositely charged. If light hits the layer
it becomes photoconductive and neutralizes locally. This requires a small lateral diffusion of charge
carriers. Initially amorphous selenium (E g = 1.8 eV) was used. The conductivity in the dark of a-Se
is 10
16
/cm. Se was subsequently replaced by organic material. The highest performance is currently
achieved with amorphous silicon.
On the charged areas of the photo-sensitive layer toner can be attached. The toner pattern is subsequently transferred to the copy sheet and fixated. A copy takes typically more than one rotation of
the drum. The principle was invented in 1938 by Chester F. Carlson (1906–1968) with sulfur as the
photoconductor.
5
22.2.4 QWIPs
Quantum-well intersubband photodetectors (QWIPs) are based on the absorption of photons between
two quantum well subbands (Fig. 22.6). A review can be found in [1829]. Quantized electron or hole
states can be used. Besides an oscillator strength for this transition, the lower level must be populated
and the upper level must be empty in order to allow this process. The Fermi level is typically chosen
by appropriate doping such that the lower subband is populated.
5 In 1947 the Haloid company bought the rights to this process, renamed itself XeroX and brought the first copy machine
to the market in 1958 based on amorphous selenium. The word ‘xerography’ stems from the Greek word ξ ´
ρoς (dry).
The last ‘X’ in XeroX was added to mimic the name of the KodaK corporation.
593
S/N =
i
2
ph
i
2
G + i
2
GR
=
η m
2
(P opt / hν)
8 B
1 + β
−1 t r
τ
(1 + ω
2
τ
2
)
G
I 0
−1
.
(22.9)
An important quantity is the noise equivalent power (NEP). This is the light power (m P opt /
√
2) for
which the S/N ratio is equal to 1 (for B = 1). The responsivity of a detector to light has been termed
’detectivity’ and is the inverse of the noise equivalent power. It typically depends on the square root of
the detector area A and the bandwidth B [1825]. Thus the detectivity D
∗ (D star) has been introduced
defined by [1826]
D
∗
=
√
A B
NEP
,
(22.10)
in order to make various detectors comparable. The unit of D
∗ is cm Hz
1/2 W
−1 , also known as Jones.
The detectivity should be stated together with the modulation frequency. It can be given for monochromatic radiation of a particular wavelength λ or a blackbody spectrum of given temperature T . As
refined measure for detectivity, D
∗∗ (D double star) has been defined to take into account the solid
angle from which radiation can reach the detector [1827],
D
∗∗
=
/π D
∗
;
(22.11)
for a Lambertian characteristic D
∗∗
= D
∗ .
22.2.3 Electrophotography
The principle of the Xerox copy machine is based on a photoconductive layer (Fig. 22.5). This layer is
normally insulating such that both sides of the layer can be oppositely charged. If light hits the layer
it becomes photoconductive and neutralizes locally. This requires a small lateral diffusion of charge
carriers. Initially amorphous selenium (E g = 1.8 eV) was used. The conductivity in the dark of a-Se
is 10
16
/cm. Se was subsequently replaced by organic material. The highest performance is currently
achieved with amorphous silicon.
On the charged areas of the photo-sensitive layer toner can be attached. The toner pattern is subsequently transferred to the copy sheet and fixated. A copy takes typically more than one rotation of
the drum. The principle was invented in 1938 by Chester F. Carlson (1906–1968) with sulfur as the
photoconductor.
5
22.2.4 QWIPs
Quantum-well intersubband photodetectors (QWIPs) are based on the absorption of photons between
two quantum well subbands (Fig. 22.6). A review can be found in [1829]. Quantized electron or hole
states can be used. Besides an oscillator strength for this transition, the lower level must be populated
and the upper level must be empty in order to allow this process. The Fermi level is typically chosen
by appropriate doping such that the lower subband is populated.
5 In 1947 the Haloid company bought the rights to this process, renamed itself XeroX and brought the first copy machine
to the market in 1958 based on amorphous selenium. The word ‘xerography’ stems from the Greek word ξ ´
ρoς (dry).
The last ‘X’ in XeroX was added to mimic the name of the KodaK corporation.