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A system for measuring, with reduced photocurrent, the excess noise associated with the gain in avalanche photodiodes (APDs), using a transimpedance amplifier front-end and based on phase-sensitive detection is described. The system can reliably measure the excess noise power of devices, even when the un-multiplied photocurrent is low (~10 nA). This is more than one order of magnitude better than previously reported systems and represents a significantly better noise signal to noise ratio. This improvement in performance has been achieved by increasing the value of the feedback resistor and reducing the op-amp bandwidth. The ability to characterise APD performance with such low photocurrents enables the use of low power light sources such as light emitting diode rather than lasers to investigate the APD noise performance.

The impact ionization characteristics of (Al x Ga 1−x ) 0.52 In 0.48 P have been studied comprehensively across the full composition range. Electron and hole impact ionization coefficients (α and β, respectively) have been extracted from avalanche multiplication and excess noise data for seven different compositions and compared to those of Al x Ga 1−x As. While both α and β initially decrease gradually with increasing bandgap, a sharp decrease in β occurs in (Al x Ga 1−x ) 0.52 In 0.48 P when x > 0.61, while α decreases only slightly. α and β decrease minimally with further increases in x and the breakdown voltage saturates. This behavior is broadly similar to that seen in Al x Ga 1−x As, suggesting that it may be related to the details of the conduction band structure as it becomes increasingly indirect in both alloy systems.

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