Asymmetric Barrier Engineering of Tunneling Heterostructures for MM-Wave Zero-Bias Sensitivity below the Schottky Limit

Metal-semiconductor (“Schottky”) junctions have defined the “gold standard” in sensitive, room-temperature “RF” (microwave, mm-wave, and THz) rectifiers going back over a century. As such they have already been widely pursued as elements in one- and two-dimensional arrays for “RF” imaging applications, both passive and active. However, they present a limit for the maximum RF current responsivity of ÂI (V = 0) = e/(2a·kBT) (“Schottky limit”) where a is the ideality factor (a ≥ 1.0) and T is the ambient temperature. This limit is imposed by the assumption that the electron transport is dominated by thermionic emission over the metal-semiconductor junction. It has a maximum value of 19.3 A/W at T = 300 K and a = 1.0, which in-turn limits the specific noise-equivalent power (NEP’) to values above ~1.0 pW/Hz1/2 at RF frequencies around 100 GHz. This proposal focuses on a new type of zero-bias "Schottky-like" rectifier based on tunneling rather than thermionic emission, for which in principle the current responsivity can exceed the Schottky limit, and the NEP’ can be << 1.0 pW/Hz1/2 at 100 GHz. It is a double-barrier electron-tunneling heterostructure, similar to resonant-tunneling diodes (RTDs), but designed specifically for higher zero-bias responsivity and better RF impedance matching than typically achieved in Schottky diodes. The Phase-I STTR project will focus on the modeling and optimization of the proposed structure using the well-established transfer-matrix approach (within the effective-mass approximation), followed by fabrication and demonstration of a single pixel having NEP’ < 1.0 pW/Hz1/2 and noise-equivalent temperature difference (NETD) < 1.0 K/Hz1/2, both at 100 GHz.