Low-noise high-gain tunneling staircase photodetector

Avalanche photodiodes (APD) are important components in short-wave and mid-wave infrared detection systems (imaging, laser radar, communications, etc.) because their internal gain can improve receiver sensitivity and enable the detection of weak photon fluxes. However, gain originates from impact ionization, a stochastic process that results in excess noise and limits the gain-bandwidth product. The staircase APD was proposed [1, 2] as the solid-state analog of the photomultiplier tube where impact ionization events occur proximate to the sharp bandgap discontinuity of each step, as shown in Fig. 1. As a result, the gain process is more deterministic, with concomitant reduction in gain fluctuations and, thus, lower excess noise. An additional advantage of the staircase structure is that the kinetic energy change required to initiate impact ionization events is supplied by band engineering and a modest applied field, rather than large bias, which is typically 10's of Volts for conventional APDs. Unfortunately, initial studies of staircase APDs used the Al x Gai_ x As material system, which has inadequate band offsets and the projected noise characteristics were never achieved [3], We recently demonstrated the first staircase APDs, where a single step exhibits a constant gain of ∼2x over a range of bias, temperature, and excitation wavelength [4, 5], enabled by the digital alloy growth of high-quality AlInAsSb, lattice-matched to GaSb across the full range of direct bandgap compositions [6, 7].