22.2 Photoconductors
597
(a)
10
-4
10
10
10
10
10
10
10
10
-5
-6
-7
-8
-9
-10
-11
-12
0
1
2
3
4
5
77
68
46
58
39
4
T(K)
(b)
7
6
5
4
3
2
1
0
6
7
9
8
1 0
1 1
1 2
Fig. 22.11 a Dark current of a QWIP at 10.7 µ m, experimental (solid lines) and theoretical (dashed lines) response.
b QWIP responsivity as a function of the applied voltage. The solid line (dashed line) is the theoretical dependence with
(without) the effect of avalanche multiplication. Adapted from [1829]
22.2.5 Blocked Impurity-Band Detectors
Impurity absorption allows photoconductivity detectors in the mid- and far-infrared regions to be made.
In particular, for THz spectroscopy in medicine and astronomy the extension to longer wavelengths
is interesting. For conventional photoconductors the impurity concentration is well below the critical
dopant concentration (cf. Sect. 7.5.7). Long-wavelength response can be achieved by going to impurity/host systems with smaller ionization energy, such as Si:B (45 meV) → Ge:As (12.7 meV) →
GaAs:Te (5.7 meV). By applying stress to Ge the energy separation between impurity and conduction
bands can be lowered and subsequently the detector response is shifted towards longer wavelengths.
For high doping the impurity level broadens to an impurity band and thus allows smaller ionization
energy and thus stronger long-wavelength detector response. However, conduction in the impurity band
leads to dark current and makes such detectors unfeasible. In a blocked impurity band (BIB) detector
[1833–1835] an additional intrinsic blocking layer is sandwiched between the absorption layer and
the contact (Fig. 22.12a). Such a structure is similar to a MIS diode, the insulator being the intrinsic
semiconductor. We assume in the following an n-type semiconductor, such as Si:As or GaAs:Te, but
also p-type BIBs can be made, e.g., from Ge:Ga.
The semiconductor is highly doped (N D ) and partly compensated (N A ). Typically, the acceptor
concentration must be small, about 10
12 cm
−3 , and controls the formation of the electric field as shown
below. The doping is so high that the impurities form an impurity band. Some of the electrons recombine
with the acceptors N
−
A = N A and leave some donors charged N
+
D = N A . For GaAs, e.g., the donor
concentration in the doped semiconductor is >10
16 cm
−3 and ∼10
13 cm
−3 in the i-layer.
Under an external forward bias V , i.e. the positive pole is at the insulator, part of the applied voltage
drops over the blocking layer of thickness b. If ideally no charges are present here, the electric field is
constant. In the n-doped material electrons move in the impurity band towards the insulator, forming
neutral donors in an electron accumulation layer of thickness w in the presence of the charged acceptors
N
−
A . This layer is the absorption layer. The mechanism can also be considered as if positive charge (the
charged donors, N
+
D ) moves (via hopping conduction) towards the back contact. In the literature the
layer close to the insulator is thus also termed a ‘depletion layer’. The band diagram and the electric
field are shown in Fig.22.12b, c. Due to the blocking layer the carriers on the donors in the n-type
material cannot spill via the impurity band into the contact but must be lifted (by photoabsorption) into
the conduction band.
597
(a)
10
-4
10
10
10
10
10
10
10
10
-5
-6
-7
-8
-9
-10
-11
-12
0
1
2
3
4
5
77
68
46
58
39
4
T(K)
(b)
7
6
5
4
3
2
1
0
6
7
9
8
1 0
1 1
1 2
Fig. 22.11 a Dark current of a QWIP at 10.7 µ m, experimental (solid lines) and theoretical (dashed lines) response.
b QWIP responsivity as a function of the applied voltage. The solid line (dashed line) is the theoretical dependence with
(without) the effect of avalanche multiplication. Adapted from [1829]
22.2.5 Blocked Impurity-Band Detectors
Impurity absorption allows photoconductivity detectors in the mid- and far-infrared regions to be made.
In particular, for THz spectroscopy in medicine and astronomy the extension to longer wavelengths
is interesting. For conventional photoconductors the impurity concentration is well below the critical
dopant concentration (cf. Sect. 7.5.7). Long-wavelength response can be achieved by going to impurity/host systems with smaller ionization energy, such as Si:B (45 meV) → Ge:As (12.7 meV) →
GaAs:Te (5.7 meV). By applying stress to Ge the energy separation between impurity and conduction
bands can be lowered and subsequently the detector response is shifted towards longer wavelengths.
For high doping the impurity level broadens to an impurity band and thus allows smaller ionization
energy and thus stronger long-wavelength detector response. However, conduction in the impurity band
leads to dark current and makes such detectors unfeasible. In a blocked impurity band (BIB) detector
[1833–1835] an additional intrinsic blocking layer is sandwiched between the absorption layer and
the contact (Fig. 22.12a). Such a structure is similar to a MIS diode, the insulator being the intrinsic
semiconductor. We assume in the following an n-type semiconductor, such as Si:As or GaAs:Te, but
also p-type BIBs can be made, e.g., from Ge:Ga.
The semiconductor is highly doped (N D ) and partly compensated (N A ). Typically, the acceptor
concentration must be small, about 10
12 cm
−3 , and controls the formation of the electric field as shown
below. The doping is so high that the impurities form an impurity band. Some of the electrons recombine
with the acceptors N
−
A = N A and leave some donors charged N
+
D = N A . For GaAs, e.g., the donor
concentration in the doped semiconductor is >10
16 cm
−3 and ∼10
13 cm
−3 in the i-layer.
Under an external forward bias V , i.e. the positive pole is at the insulator, part of the applied voltage
drops over the blocking layer of thickness b. If ideally no charges are present here, the electric field is
constant. In the n-doped material electrons move in the impurity band towards the insulator, forming
neutral donors in an electron accumulation layer of thickness w in the presence of the charged acceptors
N
−
A . This layer is the absorption layer. The mechanism can also be considered as if positive charge (the
charged donors, N
+
D ) moves (via hopping conduction) towards the back contact. In the literature the
layer close to the insulator is thus also termed a ‘depletion layer’. The band diagram and the electric
field are shown in Fig.22.12b, c. Due to the blocking layer the carriers on the donors in the n-type
material cannot spill via the impurity band into the contact but must be lifted (by photoabsorption) into
the conduction band.