Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2003-03-12
2004-11-16
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S184000, C257S440000
Reexamination Certificate
active
06818917
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an infrared photodetector, more particularly, the present invention relates to a structure with voltage-tunable and voltage-switchable photoresponses constructed of superlatuices and blocking barriers.
2. Description of Related Art
2-1. Semiconductor Infrared Photodetectors
Infrared photodetectors can be cataloged into two main groups. In one group, called thermal detectors, a change in some electrical property of the detector is induced by the temperature increment due to the absorption of the incident infrared radiation. In the other group, called photon or quantum detectors, the carriers in the material can be excited by photons with appropriate energy and become detectable by the external circuit. Although the photon detectors for the FIR detection need to be operated at cryogenic temperatures, they exhibit better detectivity and temporal response than the thermal detectors. For applications in which the sensitivity and response time are highly emphasized, photon detectors are the better choices.
The photon detectors can be further divided into several types—intrinsic, extrinsic, internal photoemission and intersubband photodetectors. The extrinsic photodetectors need to be cooled to relatively low temperatures (<40 K). The main disadvantage of the internal photon emission photodetectors is their low quantum efficiency because the carriers are activated by free carrier absorption. The other type of photon detectors that use bandgap absorption is classified as intrinsic photodetector. The material choices for intrinsic photodetectors are limited to those semiconductors with bandgap absorption in the FIR region. Among them, HgCdTe is the most important one. However, the growth of large area films of low bandgap materials with high uniformity still remains challenging. Therefore, wide bandgap based intersubband photodetectors with more mature growth and processing technologies are more suitable for the fabrication of large photodetector arrays.
Please refer to
FIG. 1
, which shows intersubband photodetectors that are made of multiple quantum wells or superlattices. For multiple quantum wells
10
, adjacent wells
10
are separated by thick barriers
11
. Because the coupling of electron wavefunctions between adjacent wells is negligible, electron energies in the quantum wells
10
are quantized into discrete levels. On the contrary, adjacent wells
10
in a superlattice
12
are separated by thin barriers. As a result of the coupling of electron wavefunctions between adjacent wells
10
, minibands form in the superlattice region
12
.
The superlattice
12
has broader spectral response than the quantum well
10
because of the transition between minibands instead of discrete states. In addition, the is coupling of electrons in adjacent wells
10
makes the superlattice
12
a low impedance structure. In this invention, the low impedance characteristic of superlattices
12
is utilized to design multicolor infrared photodetector.
2-2. Some State of the Art Intersubband Multicolor Infrared Photodetectors
Because imaging systems capable of multicolor detection are valuable in various applications including astronomical observation, military and medical science, the development of intersubband multicolor infrared photodetectors has drawn much attention in recent years. In the following, some state of the art multicolor infrared photodetector structures are recited.
Please refer to
FIG. 2
, which shows a Multi-stack infrared photodetector with separating conducting layers. As shown in
FIG. 2
, multistack infrared photodetector
2
is fabricated by stacking multiple quantum wells
21
designed for different wavelengths on the same substrate
21
during the growth of the structure. This kind of multicolor photodetector
2
can be further divided into two types (A. Kock, E. Gomick, G. Abstreiter, G. Bohm, M. Walther and G. Weimann, Appl. Phys. Lett 60, 2011, 1992; Sarath D. Gunapala, Sumith V. Bandara, A. Singh, John K. Liu, Sir B. Rafol, E. M. Luong, et al, IEEE Transactions on Electron Devices, 47, 970, 2000). (i) The different detector stacks are isolated by thick conducting layers. The operation of each stack is actually the same as if the stack is grown as a single photodetector
2
. This structure simply integrates several different photodetectors
2
onto the same substrate
22
.
FIG. 2
shows a multicolor focal plane array realized with the multi-stack infrared photodetectors
2
. Obviously, some extra process steps are needed to make only one detector stack
21
of a pixel to connect to the corresponding unit cell of the readout circuit. For short wavelength detection, the upper stacks designed for long wavelength are disabled by metal gratings
211
; while for short wavelength detection, the bottom stacks are disabled by the lateral metal
212
. The pixels shown in
FIG. 2
for long and short wavelength interlaces spatially. Because pixels for different wavelengths occupy different physical locations, the chip area is utilized inefficiently. The maximum achievable resolution is therefore limited. In addition, extra process steps also increase cost and decrease yield rate.
(ii) Please refer to FIG.
3
(
b
), which shows the structure of Multi-stack infrared photodetector with separating conducting layer. As shown in FIG.
3
(
b
) (L. C. Lenchyshyn, H. C. Liu, M. Bunchanan and Z. R. Wasilewski, J. Appl. Phys. 79, 8091, 1996), two stacks
31
,
33
for different wavelength regions are stacked on the same substrate
34
and are separated by a thick conducting layer
32
. This kind of photodetector is operated as a two terminal device. The difference from the multi-stack photodetector mentioned in (i) is that the middle contact only serves as an internal connection for the two stacks
31
,
33
. The photodetector operates as if there is two discrete photodetectors connect in series.
Please refer to FIG.
3
(
a
), which shows the small signal equivalent circuit. The measurable photocurrent generated in one of the stacks
31
,
33
is determined by the dynamic resistance of stack
31
or
33
. Because the current-voltage relation is not a linear, the dynamic resistance of the photodetector stack
31
or
33
changes with the operating point. As a result, by changing the total external bias, the dynamic resistance of stack
31
or
33
and therefore the portion of the generated photocurrent can be varied by the external bias. Although this structure can achieve multicolor detection, there are several disadvantages. First, the lack of accurate model for the current-voltage relation hinders the prediction in the design phase of the photoresponses versus bias voltage. Second, because the dynamic resistance of the short wavelength stack
31
is generally higher than the long wavelength stack
33
under low bias, short wavelength dominates the spectral response under low bias. In order to observe long wavelength photoresponses, the photodetector needs to be operated in a high bias voltage to saturate the dynamic resistance of the short wavelength stack
31
. However, both dark current and noise increase under such high bias condition. Third, the dynamic resistance changes with the operating temperature and background radiation. Therefore the photoresponses are sensitive to the variation of the environment. Fourth, the long wavelength spectral response is unavoidable accompanied by strong photoresponses in the short wavelength region.
Please refer to
FIG. 4
, which shows a Multi-stack infrared photodetector without separating conducting layers. As shown in
FIG. 4
(K. L. Tsai, K. H. Chang, C. P. Lee, K. F. Huang, J. S. Tsang and H. R. Chen, Appl. Phys. Lett. 62, 3504, 1993), this kind of photodetector also stacks photodetectors for different wavelengths. However, there are no conducting layers separating the different stacks
43
. Multicolor detection in this structure is achieved by the electric field domains formed in the different stacks under different bias voltages. At low bias voltage, more of
Chen Chun-Chi
Chen Hsin-Cheng
Kuan Chieh-Hsiung
Lin Sheng-Di
Lu Jen-Hsiang
Jackson Jerome
National Taiwan University
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