V. N. Sokolov

Terahertz generation in submicron GaN diodes within the limited space-charge accumulation regime

The conditions for microwave power generation with hot-electron transport are investigated in a submicron GaN diode when it operates in the limited space-charge accumulation (LSA) mode. Applying a transport model based on the local quasistatic approximation, the analysis shows that the nitride diodes can support the LSA mode of oscillation in the terahertz-frequency range. For a 100nm n-GaN diode with a cross section of 500μm2 and the electron density of 1×1017cm−3, the generated microwave power is estimated to be as high as ≈0.6W with the corresponding dc-to-rf conversion efficiency of ≈9% and the negative differential resistance of ≈−1.3Ω; which thus provides an efficient mechanism to achieve very high-frequency microwave generation in the nitrides.

Large-Signal Analysis of Terahertz Generation in Submicrometer GaN Diodes

The conditions for microwave power generation in a submicrometer GaN diode, with a relatively lightly doped active channel, coupled to an external resonant circuit are investigated. Applying a high-field electron transport model based on the local quasi-static approximation, we show that oscillations in group III-nitride diodes can be supported in the terahertz-frequency range near the limited space-charge accumulation regime. The shape of the diode voltage and electronic current waveforms are examined in terms of the circuit parameters and operating frequencies over the bandwidth of active generation. Based on a Fourier series analysis of the diode voltage and current, the generated power and dc-to-RF conversion efficiency at the fundamental and the second or higher order harmonic frequencies are estimated. The calculation results clearly indicate that submicrometer GaN diodes (channel doping of 1 × 1017 cm-3) can achieve large output powers (> 1 W) in the absence of Gunn domain formation, over a wide range of frequencies, near 0.5 THz.

Terahertz generation in GaN diodes in the limited space-charge accumulation mode

The conditions for terahertz power generation are investigated theoretically in a nanoscale GaN-based diode coupled to an external resonant circuit for operation in the limited space-charge accumulation (LSA) mode under the high-field transport regime. The generation criteria are revisited in terms of a phase plane analysis of the diode high-field transport and circuit equations. Based on a Fourier series analysis, the waveforms of the diode voltage and current are examined and the generated power and conversion efficiencies are estimated at the fundamental and lowest harmonic frequencies. The advantages of group-III nitride LSA diodes are elucidated including their ability to simultaneously achieve large output powers (>10 mW) and high dc-to-rf conversion efficiencies (>1%) over a wide range of frequencies near 1 THz.

High-frequency small-signal conductivity of hot electrons in nitride semiconductors

We investigate the small-signal conductivity of the electrons in group-III nitrides under a high steady-state electric field. The resulting calculation indicates a frequency dependence of the conductivity that is drastically different from that given by the Drude formula. A large and very fast response of the hot electrons in the nitrides is revealed. The complex conductivity is found to be finite up to the frequency of about 10 THz. For the fields above the threshold corresponding to the peak drift velocity, the calculation also predicts a frequency interval with a negative conductivity. A detailed analysis is provided on the field and frequency dependence of the dynamic conductivity at the high electric fields.

Terahertz generation in GaN diodes operating in pulsed regime limited by self-heating

The conditions for pulsed regime operation of terahertz power generation in vertical nanoscale GaN-based diodes are investigated via self-consistent simulation of the high-field electron transport in the active channel and thermal transport in the entire device structure. The combined electrothermal model allows for a detailed analysis of the dynamical local distributions of the electric field, drift-velocity, and lattice temperature. We show that stable generation is achievable with a self-heating limited output power of 2.25 W at an operation frequency of 0.71 THz for a pulse width of 3 ns with a few tens of nanosecond duty cycle.

Terahertz emission mediated by surface plasmon polaritons in doped semiconductors with surface grating

Doped semiconductors with one-dimensional surface gratings are studied theoretically for application to terahertz emission. The presence of free carriers supports thermally excited surface plasmon polaritons at the semiconductor-vacuum interface whose resonance frequency can be controlled by doping. The calculation based on the fluctuational electrodynamics shows that the near-field spectral energy density of this surface excitation can be many orders of magnitude larger than those in the blackbody radiation. At the same time, a rigorous coupled-wave analysis with a properly designed surface microstructure clearly indicates narrow angular lobes in the directional emissivity at the targeted frequencies, illustrating strong coupling between surface plasmon polaritons and propagating waves. Thus, doped semiconductors with engineered radiation may provide an efficient terahertz source with spatial and spectral control.

Negative small-signal impedance of nanoscale GaN diodes in the terahertz frequency regime

A physical mechanism of electrical instability in the terahertz frequency range is explored in a nanoscale two-terminal configuration of nitride-based structures under a high-field transport regime. The investigated instability utilizes the negative differential resistance effect that is known to occur in bulk group-III nitride semiconductors. A distinctive feature of the proposed mechanism for terahertz generation is that the diode steady-state I-V characteristics maintain a positive differential resistance for a stable operation, while the small-signal response provides negative impedance within the desired frequency range. An additional advantage is that the frequency window of negative small-signal impedance can be tuned electrically.

Phase-plane analysis and classification of transient regimes for high-field electron transport in nitride semiconductors

We present a detailed theoretical analysis of steady-state, transient time-dependent, and spatially dependent electron transport in the group-III nitrides at high and ultrahigh electric fields. To develop an analytical model, we derive time-dependent differential equations describing the hot-electron rates of momentum and energy relaxation in electron–polar-optical-phonon scattering and analyze them by employing phase-plane analysis. From the structure of the phase-plane partitioning based on the phase trajectories, the transient regimes are investigated and classified depending on various initial conditions. We have studied different subpicosecond regimes and found a considerable velocity overshoot effect. One of our findings is that when the velocity reaches the maximum, the electron temperature is of a moderate magnitude but increases considerably in the subsequent stage. Dynamic regimes with high electron temperature overshoot have been revealed. For the dominant electron–polar-optical-phonon scatteri...

Quasimonochromatic emission spectra in the near field by polar semiconductor thermal sources

The near-field spectra of the electromagnetic field emitted from a planar surface are theoretically investigated for a number of polar semiconductor and dielectric materials that support surface phonon polaritons. All of the studied materials, InP, GaAs, GaN, SiC, and α-Al2O3 (sapphire), exhibit quasimonochromatic thermal emission symbolized by strong peaks of evanescent modes at well-defined frequencies in the near field that correspond to the appropriate peaks in the density of states for surface phonon polaritons. It is also found that the materials with lower polariton frequencies (e.g., InP and GaAs) generally demonstrate a higher peak spectral energy density compared to those with higher frequencies (e.g., SiC). This trend is maintained over the entire range of temperature (300–600K) and the distance from the surface (⩽10μm) considered in the calculation. Thus, the results clearly indicate that among the studied materials InP and GaAs are the best candidates to provide the quasicoherent thermal emis...

Coulombic effects of electron-hole plasma in nitride-based nanostructures

We study the Coulombic effects of electron-hole plasma on the ground and first excited energy levels and carrier wave functions in a GaN∕AlGaN quantum well (QW) structure. The coupled Schrodinger equations for electron and hole states are solved self-consistently in the Hartree-Fock approximation along with the Poisson equation. As expected, the decreasing QW width and increasing plasma density diminish the charge separation in the QW induced by the internal field, leading to the relatively reduced contribution of the Hartree interaction to the energy-level shift. In contrast, the calculation also demonstrates that the contribution of many-body effects becomes more pronounced. The resulting competition between the many-body and Hartree contributions causes a nonmonotonous dependence of the electron and hole energies on the plasma density at a given field. These findings are applied to explore the potential bistable behavior in the QW electroabsorption under near-band-edge photoexcitation.