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Schematic cross-sections of the (a) ungraded and (b) graded structures investigated.
Depletion width versus reverse bias determined by capacitance-voltage measurements at 120 K for the ungraded and graded structures, with three samples reported in Ref. 3 shown for comparison. The ungraded and graded structures exhibited rapid depletion of the i-region at a rate corresponding to background doping concentrations below 2 × 1014 cm−3—significantly lower than previous reports.
Representative room-temperature gain-normalized dark current densities for the ungraded and graded structures compared with recent results from Ref. 7. The graded structure exhibited a significant, ∼3×, improvement due to high p-doping in the top contact and a thick p-region which reduced thermally injected electron diffusion into the multiplication region.
Measured room temperature multiplication gain versus reverse bias for the ungraded and graded structures compared to the best previously reported results found in Ref. 3. The reduced background doping in the ungraded and graded structures resulted in increased gain at a given bias, and the reduced dark current allowed stable low-noise amplification at higher biases, culminating in a peak gain of ∼80 at 12 V reverse bias.
Representative graded structure excess noise factor versus multiplication gain measurements taken at room temperature (filled squares), compared to McIntyre's local field model16 for different values of k, the hole to electron ionization coefficient ratio (dashed lines). The extremely low noise characteristics observed are consistent with McIntyre's history-dependent model for electron-dominated impact ionization,17 as previously reported for HgCdTe and InAs based APDs.15,18
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