Designer germanium quantum dot phototransistor for near infrared optical detection and amplification

We demonstrated a unique CMOS approach for the production of a high-performance germanium (Ge) quantum dot (QD) metal-oxide-semiconductor phototransistor. In the darkness, low off-state leakage (I _off ∼ 0.27 pA μm ^-2 ), a high on-off current ratio (I _on /I _off ∼ 10 ⁶ ), and good switching behaviors (subthreshold swing of 175 mV/dec) were measured on our Ge-QD phototransistor at 300 K, indicating good hetero-interfacial quality of the Ge-on-Si. Illumination makes a significant enhancement in the drain current of Ge QD phototransistors when biased at both the on- and off-states, which is a great benefit from Ge QD-mediated photoconductive and photovoltaic effects. The measured photocurrent-to-dark-current ratio (I _photo /I _dark ) and the photoresponsivities from the Ge QD phototransistor are as high as 4.1 × 10 ⁶ and 1.7 AW ^-1 , respectively, under an incident power of 0.9 mW at 850 nm illumination. A superior external quantum efficiency of 240% and a very fast temporal response time of 1.4 ns suggest that our Ge QD MOS phototransistor offers great promise as optical switches and transducers for Si-based optical interconnects.