Taking into account the discrete nature of the quantum‐well intersubband infrared detectors, we construct a model to calculate the photoconductive gain. It is shown that the photoconductive gain is inversely proportional to the number of quantum wells and that the detector‐current responsivity is independent of the number of wells.

Photoconductive gain mechanism of quantum‐well intersubband infrared detectors

We present a detailed and thorough study of a wide variety of quantum well infrared photodetectors (QWIPs), which were chosen to have large differences in their optical and transport properties. Both n- and p-doped QWIPs, as well as intersubband transitions based on photoexcitation from bound-to-bound, bound-to-quasi-continuum, and bound-to-continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot carrier mean free path; net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity. These results allow a better understanding of the optical and transport physics and thus a better optimization of the QWIP performance.

Photoexcited escape probability, optical gain, and noise in quantum well infrared photodetectors

The first long wavelength quantum well infrared photodetector based on valence band intersubband absorption holes is demonstrated. A normal incidence quantum efficiency of η=28% and detectivity of D*λ=3.1×1010 cm √Hz/W at T=77 K, for a cutoff wavelength λc=7.9 μm, have been achieved.

Normal incidence hole intersubband absorption long wavelength GaAs/AlxGa1−xAs quantum well infrared photodetectors

The photoconductive gain, g, and generation‐recombination (GR) noise of multiple‐quantum‐well infrared detectors are calculated for structures in which the well capture probability, pc, and the fraction of current derived from tunneling are allowed to vary through the structure. For uniform pc and no tunneling current, g=1/(Npc), where N is the number of wells. The GR noise power under the same conditions is 4eĪgB(1−pc/2), where e is electronic charge, I is the mean current, and B is the measurement bandwidth. When pc≪1, the noise contributions from carrier generation and decay (recombination) are equal as in a homogeneous photoconductor. However, when pc→1, the recombination noise decreases to zero, and the noise is equal to the shot noise of N series‐connected, independent junctions.

Photoconductive gain and generation‐recombination noise in multiple‐quantum‐well infrared detectors

We have designed and fabricated an optimized long-wavelength/very-long wavelength two-color quantum well infrared photodetector (QWIP) device structure. The device structure was grown on a 3-in semi-insulating GaAs substrate by molecular beam epitaxy (MBE). The wafer was processed into several 640/spl times/486 format monolithically integrated 8-9 and 14-15 /spl mu/m two-color (or dual wavelength) QWIP focal plane arrays (FPAs). These FPAs were then hybridized to 640/spl times/486 silicon CMOS readout multiplexers. A thinned (i.e., substrate removed) FPA hybrid was integrated into a liquid helium cooled dewar for electrical and optical characterization and to demonstrate simultaneous two-color imagery. The 8-9 /spl mu/m detectors in the FPA have shown background limited performance (BLIP) at 70 K operating temperature for 300 K background with f/2 cold stop. The 14-15 /spl mu/m detectors of the FPA reaches BLIP at 40 K operating temperature under the same background conditions. In this paper we discuss the performance of this long-wavelength dualband QWIP FPA in terms of quantum efficiency, detectivity, noise equivalent temperature difference (NE/spl Delta/T), uniformity, and operability.

640/spl times/486 long-wavelength two-color GaAs/AlGaAs quantum well infrared photodetector (QWIP) focal plane array camera

We demonstrate for the first time that a quantum well infrared photodetector based on bound‐to‐continuum state intersubband transitions can have a photoconductive gain much greater than unity similar to extrinsic photoconductors. An optical gain g=8.1 was determined by comparing the optical absorption, responsivity, and noise characteristics of two multiquantum well detectors which were identical in every respect except for having a different number of periods (2 and 20). The results suggest that since the photocarrier lifetime τL is not transit time limited, detector optimization to increase τL or optical coupling can be expected to lead to an improved detectivity D*.

Large photoconductive gain in quantum well infrared photodetectors

We have measured and theoretically analyzed the optical absorption and hot electron transport physics of a new long wavelength (λ=5–9 μm) infrared miniband photoconductor based on a GaAs/AlxGa1−x superlattice. The optical absorption photoexcites an electron from the lowest miniband below the barriers to the excited state which is designed to be above the top of the barriers. This bound to continuum absorption leads to more than an order of magnitude improvement in electron transport and detector performance, compared with previous bound state miniband photoconductors.

S. D. Gunapala

Papers

Large photoconductive gain in quantum well infrared photodetectors

Bound to continuum superlattice miniband long wavelength GaAs/AlxGa1−xAs photoconductors