We report high-temperature ͑240-300 K͒ operation of a tunneling quantum-dot infrared photodetector. The device displays two-color characteristics with photoresponse peaks at ϳ6 m and 17 m. The extremely low dark current density of 1.55 A / cm 2 at 300 K for 1 V bias is made possible by the tunnel filter. For the 17 m absorption, the measured peak responsivity is 0.16 A / W ͑300 K͒ for a bias of 2 V and the specific detectivity D * is 1.5ϫ 10 7 cm Hz 1/2 /W ͑280 K͒ for a bias of 1 V. Excellent performance characteristics are also measured for the 6 m photoresponse.

The temperature-dependent characteristic of band offsets at the heterojunction interface was studied by an internal photoemission (IPE) method. In contrast to the traditional Fowler method independent of the temperature (T), this method takes into account carrier thermalization and carrier/dopant-induced band-renormalization and band-tailing effects, and thus measures the band-offset parameter at different temperatures. Despite intensive studies in the past few decades, the T dependence of this key band parameter is still not well understood. Reexamining a p-type doped GaAs emitter/undoped Al x Ga 1−x As barrier heterojunction system disclosed its previously ignored T dependency in the valence-band offset, with a variation up to ∼−10 −4 eV/K in order to accommodate the difference in the T-dependent band gaps between GaAs and AlGaAs. Through determining the Fermi energy level (E f), IPE is able to distinguish the impurity (IB) and valence bands (VB) of extrinsic semiconductors. One important example is to determine E f of dilute magnetic semiconductors such as GaMnAs, and to understand whether it is in the IB or VB.

The spectral response of common optoelectronic photodetectors is restricted by a cutoff wavelength limit (λ c) that is related to the activation energy (or band-gap) of the semiconductor structure (or material) (∆) through the relationship: λ c = hc/∆. This spectral rule dominates device design and intrinsically limits the long wavelength response of a semiconductor photodetector. Here, we report a new, long wavelength photodetection principle based on a hot-cold hole energy transfer mechanism that overcomes this spectral limit. Hot carriers injected into a semiconductor structure interact with cold carriers and excite them to higher energy states. This enables a very long-wavelength infrared response. In our experiments, we observe a response up to 55 µm, which is tunable by varying the degree of hot-hole injection, for a GaAs/AlGaAs sample with ∆ = 0.32 eV (equivalent to 3.9 µm in wavelength).

A heterojunction interfacial work function internal photoemission ͑HEIWIP͒ detector with a threshold frequency ͑f 0 ͒ of 2.3 THz ͑ 0 = 128 m͒ is demonstrated. The threshold limit of ϳ3.3 THz ͑92 µm͒ due to the Al fraction being limited to ϳ0.005, in order to avoid control and transition from alloy to isoelectronic doping behavior, was surpassed using AlGaAs emitters and GaAs barriers. The peak values of responsivity, quantum efficiency, and the specific detectivity at 9.6 THz and 4.8 K for a bias field of 2.0 kV/ cm are 7.3 A / W, 29%, 5.3ϫ 10 11 Jones, respectively. The background-limited infrared photodetector temperature of 20 K with a 60°field of view was observed for a bias field of 0.15 kV/ cm. The f 0 could be further reduced toward ϳ1 THz regime ͑ϳ300 m͒ by adjusting the Al fraction to offset the effect of residual doping, and/or lowering the residual doping in the barrier, effectively lowering the band bending.

Design, modeling, and optimization principles for GaAs/ AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) infrared detectors for a broad spectral region are presented. Both n-type and p-type detectors with a single emitter or multiemitters, grown on doped and undoped substrates are considered. It is shown that the absorption, and therefore responsivity, can be increased by optimizing the device design. Both the position and the strength of the responsivity peaks can be tailored by varying device parameters such as doping and the thickness. By utilizing a resonant cavity architecture, the effect of a buffer layer on the response is discussed. Model results, which are in good agreement with the experimental results, predict an optimized design for a detector with a peak response of 9 A / W at 26 m with a zero response threshold wavelength 0 = 100 m. For a 0 =15 m HEIWIP detector, background limited performance temperature (BLIP temperature), for 180°field of view (FOV) is expected around 80 K. For a 0 =70 m optimized design, a highly doped n-type substrate could increase the peak detectivity from 1.7ϫ 10 10 to 3.4ϫ 10 10 Jones at a FOV= 180°operated at temperatures below T Ͻ T BLIP =13 K. Intrinsic response times on the order of picoseconds are expected for these detectors.