The outstanding material properties of III-Nitride semiconductors, has prompted intense research efforts in order to engineer resonant tunneling transport within this revolutionary family of wide-bandgap semiconductors. From resonant tunneling diode (RTD) oscillators to quantum cascade lasers (QCLs), III-Nitride heterostructures hold the promise for the realization of high-power ultra-fast sources of terahertz (THz) radiation. However, despite the considerable efforts over last two decades, it is only during the last three years that room temperature resonant tunneling transport has been demonstrated within the III-Nitride family of semiconductors. In this paper we present an overview of our current understanding of resonant tunneling transport in polar heterostructures. In particular we focus on double-barrier III-Nitride RTDs which represents the simplest device in which the dramatic effects of the internal polarization fields can be studied. Tunneling transport within III-heterostructures is strongly influenced by the presence of the intense spontaneous and piezoelectric polarization fields which result from the non-centrosymmetric crystal structure of III-Nitride semiconductors. Advances in heterostructure design, epitaxial growth and device fabrication have led to the first unequivocal demonstration of robust and reliable negative differential conductance. which has been employed for the generation of microwave power from III-Nitride RTD oscillator. These significant advances allowed us to shed light into the physics of resonant tunneling transport in polar semiconductors which had remained hidden until now.
We analyze amplification of terahertz plasmons in a grating-gate semiconductor hetero-structure. The device consists of a resonant-tunneling-diode gated high-electron-mobility transistor (RTD-gated HEMT), i.e. a HEMT structure with a double-barrier gate stack enabling resonant tunneling from gate to channel. In these devices, the key element enabling substantial power gain is the coupling of terahertz waves into and out of plasmons in the RTD-gated HEMT channel, i.e. the gain medium, via the grating-gate itself, part of the active device, rather than by an external antenna structure as in previous works, enabling amplification with associated power gain >> 30 dB at room temperature.
In two-dimensional electron systems with mobility on the order of 1,000 – 10,000 cm<sup>2</sup>/Vs, the electron scattering time is about 1 ps. For the THz window of 0.3 – 3 THz, the THz photon energy is in the neighborhood of 1 meV, substantially smaller than the optical phonon energy of solids where these 2D electron systems resides. These properties make the 2D electron systems interesting as a platform to realize THz devices. In this paper, I will review 3 approaches investigated in the past few years in my group toward THz devices. The first approach is the conventional high electron mobility transistor based on GaN toward THz amplifiers. The second approach is to employ the tunable intraband absorption in 2D electron systems to realize THz modulators, where I will use graphene as a model material system. The third approach is to exploit plasma wave in these 2D electron systems that can be coupled with a negative differential conductance element for THz amplifiers/sources/detectors.