Type-II GaAs_0.5Sb_0.5/InP Uni-Traveling Carrier Photodiodes with Sub-Terahertz Bandwidth and High-Power Performance under Zero-Bias Operation

We successfully demonstrate ultrafast uni-traveling carrier photodiodes (PD) with sub-terahertz bandwidth (~170 GHz) and high-power performance under zero bias and at 1.55-μm optical wavelength operation. By using a type-II (GaAs_0.5Sb_0.5/InP) absorption-collector interface and inserting an n-type (1 × 10¹⁸ cm^-3) charge layer in the collector, the current blocking (Kirk) effect can be greatly minimized. A stack of undoped Al_xIn_0.52Ga_0.48-xAs layers with different Aluminum mole fractions (x: 0.2 to 0.08) and bandgaps is adopted as the collector layer. This graded-bandgap design can provide a built-in electric field and further shorten the internal collector transit time. The demonstrated PD structure achieves a 3-dB optical-to-electrical bandwidth of 170 GHz and subterahertz output power-11.3 dBm at 170 GHz, a record among all the reported zero-bias PDs.