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Chapter 4:
Intrinsic Direct Bandgap Semiconductors
Direct bandgap semiconductors are mainly compound semiconductors derived from combinations of elements from the II, III, IV, V, and VI groups of the periodic table. The earliest direct bandgap IR detectors involved simple binary compounds, such as InSb (III-€“V) and PbS (IV-€“VI), and were either photoconductive or photovoltaic. In the early 1970s, these binary compounds were superceded in many instances, but not all, by the variable bandgap alloys Hg 1-ˆ’x Cd x Te and Pb 1-ˆ’x Sn x Te , whose compositions could be tuned to cover the important regions of the IR spectrum. HgCdTe, in its photoconductive form, ultimately became the material of choice for first-generation FPAs, and its photovoltaic counterpart is now the de facto standard for today's FPA technology. This alloy can be tuned to cover the complete IR spectrum from 1 to 20 µm for x compositions in the range 0.18 < x < 1.0. PbSnTe is somewhat less versatile, covering the spectral range from 7 to < 20 µm, but this can be extended to include the important MWIR region by the use of another member of the Pb salt family, PbGeTe. The demise of the Pb salt alloys for detection purposes was in part due to the large dielectric constants associated with this materials system, leading to high capacitance values and response time issues in first-generation scanning systems. It is interesting to speculate how this alloy might perform in today's proposed third-generation staring FPAs, although the question of its mechanical strength might still be a cause for concern. The advent of quantum wells has provided bandgap-engineered equivalents to the ternary alloys in the form of the Type II superlattice, such as InAs/GaSb, and the Type III superlattice, such as HgTe/CdTe. These may prove of value in certain applications where HgCdTe faces significant materials physics issues due to tunneling, such as in the detection of very long wavelength IR photons.
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