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Quantum well infrared photodetectors (QWIPs) have been widely investigated for the 3–5 μm mid-wavelength infrared (MWIR) and 8–12 μm long-wavelength infrared (LWIR) atmospheric spectral windows as well as very long wavelength infrared (VLWIR: λ_c > 14 μm) imaging array applications in the past decade. The mature III-V compound semiconductor growth technology and the design flexibility of device structures have led to the rapid development of various QWIP structures for infrared focal plane arrays (FPAs) applications. In addition to the single-color QWIP with narrow bandwidth, multi-color or broadband QWIPs required for advanced IR sensing and imaging applications have also emerged in recent years. Using band gap engineering approach, the multi-color (2, 3, and 4-color) QWIPs with multi-stack quantum wells and voltage-tunable asymmetrical coupled quantum well structures for detection in the MWIR, LWIR, and VLWIR bands have been demonstrated recently. The triple-coupled (TC-) QWIP employs the quantum confined Stark effect to tune the peak detection wavelength by the applied bias voltage, A typical single-color QWIP exhibits a rather narrow spectral bandwidth of 1 to 2 μm. For certain applications, such as spectroscopy, sensing of a broader range of infrared radiation is highly desirable. Using the stacked quantum wells with different well width and depth, the digital-graded superlattice barrier (DGSLB) or the linear-graded barrier (LGB) structures, broadband (BB-) QWIPs covering the 8–14 μm atmospheric spectral window have been reported recently.

In this chapter, the basic operation principles of a QWIP, and the design, fabrication, and characterization of multi-color and broadband QWIPs based on the GaAs/AlGaAs and InGaAs/AlGaAs material systems for the MW/LW/VLWIR applications are depicted.

We have investigated commercially available photodiodes and also recent developed Sb-based phototransistors in order to compare their performances for applications to laser remote sensing. A custom-designed phototransistor in the 0.9- to 2.2-μm wavelength range has been developed at AstroPower and characterized at NASA Langley's Detector Characterization Laboratory. The phototransistor's performance greatly exceeds the previously reported results at this wavelength range in the literature. The detector testing included spectral response, dark current and noise measurements. Spectral response measurements were carried out to determine the responsivity at 2-μm wavelength at different bias voltages with fixed temperature; and different temperatures with fixed bias voltage. Current versus voltage characteristics were also recorded at different temperatures. Results show high responsivity of 2650 AIW corresponding to an internal gain of three orders of magnitude, and high detectivity (D*) of 3.9×10¹¹cm.Hz^1/2/W that is equivalent to a noise-equivalent-power of 4.6×10^-14W/Hz^1/2 (-4.0 V @ -20°C) with a light collecting area diameter of 200-μm. It appears that this recently developed 2-μm phototransistor's performances such as responsivity, detectivity, and gain are improved significantly as compared to the previously published APD and SAM APD using similar materials. These detectors are considered as phototransistors based-on their structures and performance characteristics and may have great potential for high sensitivity differential absorption lidar (DIAL) measurements of carbon dioxide and water vapor at 2.05-μm and 1.9-μm, respectively.

A CMOS cascode amplifier, biased near the threshold voltage of a MOSFET, for terahertz direct detection is proposed. A CMOS terahertz imaging circuit (size: 250 × 180 ìm) is designed and fabricated on the basis of low-cost 180-nm CMOS process technology. The imaging circuit consists of a microstrip patch antenna, an impedance-matching circuit, and a direct detector. It achieves a responsivity of 51.9 kV/W at 0.915 THz and a noise equivalent power (NEP) of 358 pW/Hz^1/2 at a modulation frequency of 31 Hz. NEP is estimated to be reduced to 42 pW/Hz^1/2 at 100 kHz. These results suggest that cost-efficient terahertz imaging is possible in the near future.

In this paper, results of investigations on the static and dynamic characteristics, responsivity, internal noises, detectivity and noise equivalent power of the forward biased p-i-n photodiode made on wide band gap semiconductors operating in double injection regime are presented. Numerical simulations were made for 4H-SiC and GaN. It is shown that forward biased p-i-n photodiodes have high values of responsivity (~ 0.03 A/W for 4H-SiC and ~ 0.15 A/W for GaN), detectivity (~10¹¹ cm Hz^1/2 W^-1 for 4H-SiC and ~ 8×10¹³ cm Hz^1/2 W^-1 for GaN) and low level of internal noises at room temperature. It is shown also that dynamic negative resistance on the current–voltage characteristic can come into existence and that the reactive part of the impedance is the sign-changed function versus frequency.

Semiconductor p-i-n structure with long i-base region doped with shallow donors/acceptors can be successfully used and have significant advantages in millimeter wave range as cooling THz receivers. It is shown that such detectors working in double current injection mode can have high current photosensitivity and detectivity, linear dependencies of sensitivity versus wavelength and applied voltage. Simulation results can be successfully used for detectors fabricated on the basis of several semiconductors: Si:Li, Si:O, Ge:B, Ge:Al, Ge:Ti, Ge:Be, Ge:P, Ge:As, Ge:Sb, GaAs:Li, GaAs:Zn, GaAs:Cd, GaAs:X, GaP:N, which covered wavelengths 40–200 μm. The simulation results made in this paper have demonstrated the ability of double injection structures to perform as practical and sensitive power detectors for THz radiation.

Thin films ${\mathrm{\text{Mn}}}_{1.5}{\mathrm{\text{Co}}}_1{\mathrm{\text{Ni}}}_{0.5}{\mathrm{\text{O}}}_4$ (MCNO), ${\mathrm{\text{Mn}}}_{1.5}{\mathrm{\text{Co}}}_1{\mathrm{\text{Ni}}}_{0.35}{\mathrm{\text{Cu}}}_{0.15}{\mathrm{\text{O}}}_4$ (MCNCuO) and ${\mathrm{\text{Mn}}}_{1.5}{\mathrm{\text{Co}}}_1{\mathrm{\text{Ni}}}_{0.2}{\mathrm{\text{Cu}}}_{0.15}{\mathrm{\text{Sc}}}_{0.15}{\mathrm{\text{O}}}_4$ (MCNCuScO) are prepared by Chemical Solution Deposition method. The results show that the addition of Cu and Cu/Sc elements can reduce the grain boundary energy and the grain boundary angle to improve the single crystal degree of MCNO thin film. Through the analysis of MCNCuScO thin film, it is found that the stability of spinel structure mainly depends on the octahedron rather than tetrahedron. The bandgap of the samples from small to large is separately MCNCuScO, MCNCuO and MCNO films. The absorptivity within the waveband of $\lambda \le 1.5\mu \mathrm{\text{m}}$ plays a decisive role in the performance of the detector. At the same frequency, the MCNCuO thin film detector has the highest voltage responsivity, followed by the MCNCuScO thin film detector, while the MCNO film detector has the lowest responsivity.

Hybrid Ultraviolet photodetector (UVPD) was fabricated by depositing reduced graphene oxide on zinc oxide nanorods (ZnO NRs) array-coated FTO glass. Structural and optical properties of ZnO NRs were studied using TEM, SEM, XRD and UV–Vis spectroscopy. Graphene oxide (GO) sheets were synthesized from graphite powder using the modified Hummer’s method. rGO films were obtained after illuminating the GO in UV radiation. The formation of rGO and GO films was studied using AFM surface topography and UV–Vis spectroscopy. External quantum efficiency (EQE), responsivity and transient response of rGO/ZnO NR-based UV photodetector were studied in detail. The photodetector device shows very high EQE ($\sim$45 257%) and responsivity ($R$$\sim$130 A/W).

We have investigated commercially available photodiodes and also recent developed Sb-based phototransistors in order to compare their performances for applications to laser remote sensing. A custom-designed phototransistor in the 0.9- to 2.2-μm wavelength range has been developed at AstroPower and characterized at NASA Langley's Detector Characterization Laboratory. The phototransistor's performance greatly exceeds the previously reported results at this wavelength range in the literature. The detector testing included spectral response, dark current and noise measurements. Spectral response measurements were carried out to determine the responsivity at 2-μm wavelength at different bias voltages with fixed temperature; and different temperatures with fixed bias voltage. Current versus voltage characteristics were also recorded at different temperatures. Results show high responsivity of 2650 A/W corresponding to an internal gain of three orders of magnitude, and high detectivity (D*) of 3.9×10¹¹cm⋅Hz^½/W that is equivalent to a noise-equivalent-power of 4.6×10^-14W/Hz^½ (-4.0 V @ -20 °C) with a light collecting area diameter of 200-μm. It appears that this recently developed 2-μm phototransistor's performances such as responsivity, detectivity, and gain are improved significantly as compared to the previously published APD and SAM APD using similar materials. These detectors are considered as phototransistors based-on their structures and performance characteristics and may have great potential for high sensitivity differential absorption lidar (DIAL) measurements of carbon dioxide and water vapor at 2.05-μm and 1.9-μm, respectively.

Quantum well infrared photodetectors (QWIPs) have been widely investigated for the 3–5 μm mid-wavelength infrared (MWIR) and 8–12 μm long-wavelength infrared (LWIR) atmospheric spectral windows as well as very long wavelength infrared (VLWIR: λ_c > 14μm) imaging array applications in the past decade. The mature III-V compound semiconductor growth technology and the design flexibility of device structures have led to the rapid development of various QWIP structures for infrared focal plane arrays (FPAs) applications. In addition to the single-color QWIP with narrow bandwidth, multi-color or broadband QWIPs required for advanced IR sensing and imaging applications have also emerged in recent years. Using band gap engineering approach, the multi-color (2, 3, and 4-color) QWIPs with multi-stack quantum wells and voltage-tunable asymmetrical coupled quantum well structures for detection in the MWIR, LWIR, and VLWIR bands have been demonstrated recently. The triple-coupled (TC-) QWIP employs the quantum confined Stark effect to tune the peak detection wavelength by the applied bias voltage. A typical single-color QWIP exhibits a rather narrow spectral bandwidth of 1 to 2 μm. For certain applications, such as spectroscopy, sensing of a broader range of infrared radiation is highly desirable. Using the stacked quantum wells with different well width and depth, the digital-graded superlattice barrier (DGSLB) or the linear-graded barrier (LGB) structures, broadband (BB-) QWIPs covering the 8–14 μm atmospheric spectral window have been reported recently.

In this chapter, the basic operation principles of a QWIP, and the design, fabrication, and characterization of multi-color and broadband QWIPs based on the GaAs/AlGaAs and InGaAs/AlGaAs material systems for the MW/LW/VLWIR applications are depicted.

Having long been the realm of molecular chemistry, astronomy, and plasma diagnostics, the upper millimeter-wave band (∼100 to 300 GHz) and the THz region above it have recently become the subject of heightened activity in the engineering community because of exciting new technology (e.g., sub-picosecond optoelectronics) and promising new “terrestrial” applications (e.g., counter-terrorism and medical imaging). The most challenging of these applications are arguably those that demand remote sensing at a stand-off of roughly 10 m or more between the target and the sensor system. As in any other spectral region, remote sensing in the THz region brings up the complex issues of sensor modality and architecture, free-space electromagnetic effects and components, transmit and receive electronics, signal processing, and atmospheric propagation. Unlike other spectral regions, there is not much literature that addresses these issues from a conceptual or system-engineering viewpoint. So a key theme of this chapter is to review or derive the essential engineering concepts in a comprehensive fashion, starting with fundamental principles of electromagnetics, quantum mechanics, and signal processing, and building up to trade-off formulations using system-level metrics such as noiseequivalent power and receiver operating characteristics. A secondary theme is to elucidate aspects of the THz region and its incumbent technology that are unique, whether advantageous or disadvantageous, relative to other spectral regions. The end goal is to provide a useful tutorial for graduate students or practicing engineers considering the upper mm-wave or THz regions for system research or development.

A CMOS cascode amplifier, biased near the threshold voltage of a MOSFET, for terahertz direct detection is proposed. A CMOS terahertz imaging circuit (size: 250 × 180 μm) is designed and fabricated on the basis of low-cost 180-nm CMOS process technology. The imaging circuit consists of a microstrip patch antenna, an impedance-matching circuit, and a direct detector. It achieves a responsivity of 51.9 kV/W at 0.915 THz and a noise equivalent power (NEP) of 358 pW/Hz^½ at a modulation frequency of 31 Hz. NEP is estimated to be reduced to 42 pW/Hz^½ at 100 kHz. These results suggest that cost-efficient terahertz imaging is possible in the near future.