Resonant plasmonic detection of terahertz radiation in field-effect transistors with the graphene channel and the black-As[Formula: see text]P[Formula: see text] gate layer

We propose the terahertz (THz) detectors based on field-effect transistors (FETs) with the graphene channel (GC) and the black-Arsenic (b-As) black-Phosphorus (b-P), or black-Arsenic-Phosphorus (b-As[Formula: see text]P[Formula: see text]) gate barrier layer. The operation of the GC-FET detectors is associated with the carrier heating in the GC by the THz electric field resonantly excited by incoming radiation leading to an increase in the rectified current between the channel and the gate over the b-As[Formula: see text]P[Formula: see text] energy barrier layer (BLs). The specific feature of the GC-FETs under consideration is relatively low energy BLs and the possibility to optimize the device characteristics by choosing the barriers containing a necessary number of the b-As[Formula: see text]P[Formula: see text] atomic layers and a proper gate voltage. The excitation of the plasma oscillations in the GC-FETs leads to the resonant reinforcement of the carrier heating and the enhancement of the detector responsivity. The room temperature responsivity can exceed the values of [Formula: see text] A/W. The speed of the GC-FET detector's response to the modulated THz radiation is determined by the processes of carrier heating. As shown, the modulation frequency can be in the range of several GHz at room temperatures.

Manifestation of plasmonic response in the detection of sub-terahertz radiation by graphene-based devices

We report on the sub-terahertz (THz) (129-450 GHz) photoresponse of devices based on single layer graphene and graphene nanoribbons with asymmetric source and drain (vanadium and gold) contacts. Vanadium forms a barrier at the graphene interface, while gold forms an Ohmic contact. We find that at low temperatures (77 K) the detector responsivity rises with the increasing frequency of the incident sub-THz radiation. We interpret this result as a manifestation of a plasmonic effect in the devices with the relatively long plasmonic wavelengths. Graphene nanoribbon devices display a similar pattern, albeit with a lower responsivity.

Negative Differential Conductivity in AlGaN/GaN HEMTs: Real Space Charge Transfer from 2D to 3D GaN States?

We report on non-thermal negative differential conductivity (NDC) in AlGaN/GaN HEMTs grown on sapphire substrates by low-pressure MOCVD. The sheet electron density was on the order of few times 1012cm−2 and the Hall mobility was 1,000 cm2/V.s. The HEMTs had threshold voltage close to zero and could operate at high positive gate bias up to 3 to 3.5 Volts, with a very low gate leakage current. NDC was observed at the gate bias larger than 1.5V and at the drain biases between approximately 0.5Vg and Vg. We excluded the possibility of self-heating as the cause, since the NDC occurs at relatively small power levels where self-heating effects are negligible.

An explanation we provided for the NDC effect is the new mechanism of real space charge transfer from 2D to 3D GaN states, which leads to a decrease in the channel mobility at large 2D electron gas densities. The observed low leakage can be explained by an enhanced molar fraction of aluminum at the heterointerface that results in a larger conduction band discontinuity. Our model that accounts for the piezoelectric and pyroelectric effects is consistent with the observed NDC effect. The Hall mobility dependence on the gate bias and sheet carrier concentration [1] is consistent with the real space transfer mechanism.

This NDC effect in GaN/AlGaN HEMTs may find applications in high-performance digital circuits at elevated temperatures.

Pyroelectric Effect in Wurtzite Gallium Nitride

We report on the measurements of the pyroeffect in wurtzite n-type GaN films deposited over basal plane sapphire substrates. The voltage drop between the contacts was measured while the sample was subjected to uniform heating. Our results show that the pyroelectric effect in GaN can be partially attributed to the secondary pyroelectricity, caused by the development of strain in the material due to thermal expansion.

A Semi-Analytical Interpretation of Transient Electron Transport in Gallium Nitride, Indium Nitride, and Aluminum Nitride

The energy dependent momentum and energy relaxation times, and the effective single valley energy dependent effective mass, are extracted from Monte Carlo simulations of gallium nitride, indium nitride, and aluminum nitride. A simple semi-analytical energy model, which uses these dependencies, is in good agreement with the results of transient Monte Carlo simulations. Both the Monte Carlo and the semi-analytical simulations show that the overshoot effects are most pronounced when the electric field abruptly changes from a value below a critical field to one above. This is attributed to the relatively large difference between the effective energy and momentum relaxation times for such a variation of electric field. Our calculations indicate that gallium nitride and indium nitride should have the most pronounced transient effects. A calculation of the transit times as a function of the gate length shows that an upper bound for the maximum expected cut-off frequencies are 260 GHz and 440 GHz for 0.2 μm gallium nitride and indium nitride field effect transistors, respectively.

Polar Optical Phonon Instability and Intervalley Transfer in Gallium Nitride

We develop a simple, one-dimensional, analytical model, which describes electron transport in gallium nitride. We focus on the polar optical phonon scattering mechanism, as this is the dominant energy loss mechanism at room temperature. Equating the power gained from the field with that lost through scattering, we demonstrate that beyond a critical electric field, 114 kV/cm at T = 300 K, the power gained from the field exceeds that lost due to polar optical phonon scattering. This polar optical phonon instability leads to a dramatic increase in the electron energy, this being responsible for the onset of intervalley transitions. The predictions of our analytical model are compared with those of Monte Carlo simulations, and are found to be in satisfactory agreement.

Temperature Dependence of Breakdown Field in p-π-n GaN Diodes

We present the results of the study of the electric breakdown in p-π-n GaN diodes. The breakdown is observed at reverse biases above 40 V and is accompanied by the formation of microplasmas. The study shows that the observed breakdown field in GaN (on the order of 1 to 2 MV/cm) increases with the temperature. This feature makes GaN very promising for high power devices and avalanche photodetectors, operating at elevated temperatures.

The Velocity-Field Characteristic Of Indium Nitride

We determine the velocity-field characteristic of wurtzite indium nitride using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high room temperature peak drift velocity, 4.2 × 107 cm/s, at a doping concentration of 1 × 1017 cm−3. This exceeds that of gallium nitride, 2.9 × 107 cm/s, by approximately 40 %. For our nominal parameter selections, the saturation drift velocity of indium nitride is found to be 1.8 × 107 cm/s. The device performance of this material, as characterized by the cut-off frequency, is found to superior to that of gallium nitride, gallium arsenide, and silicon.

GaN And Related Materials For High Power Applications

Unique properties of GaN and related semiconductors make them superior for high-power applications. The maximum density of the two-dimensional electron gas at the GaN/AlGaN heterointerface or in GaN/AlGaN quantum well structures can reach 5×1013 cm−2, which is more than an order of magnitude higher than for traditional GaAs/AlGaAs heterostructures. The mobility-sheet carrier concentration product for these two dimensional systems might also exceed that for GaAs/AIGaAs heterostructures and can be further enhanced by doping the conducting channels and by using “piezoelectric” doping, which takes advantage of high piezoelectric constants of GaN and related materials. We estimate that current densities over 20 A/mm can be reached in GaN-based High Electron Mobility Transistors (HEMTs). These high current values can be combined with very high breakdown voltages in high-power HEMTs. These breakdown voltages are expected to reach several thousand volts. Recent Monte Carlo simulations point to strong ballistic and overshoot effects in GaN and related materials, which should be even more pronounced than in GaAs-based compounds but at much higher electric fields. This should allow us to achieve faster switching, minimizing the power dissipation during switching events. Selfheating, which is unavoidable in power devices, raises operating temperatures of power devices well above the ambient temperature. For GaN-based devices, the use of SiC substrates having high thermal conductivity is essential for ensuring an effective heat dissipation. Such an approach combines the best features of both GaN and SiC technologies; and GaN/SiC-based semiconductors and heterostructures should find numerous applications in power electronics.

Mechanism of self-excitation of terahertz plasma oscillations in periodically double-gated electron channels

We develop a device model for a heterostructure device with an electron channel and with a periodic system of interdigitated gates. Using this model, we find the conditions of the self-excitation of plasma oscillations in portions of the channel. It is shown that the self-excitation of plasma oscillations in these devices and the terahertz emission observed in the experiments (Otsuji et al 2006 Appl. Phys. Lett. 89 263502; Meziani et al 2007 Appl. Phys. Lett. 90 061105; Otsuji et al 2007 Solid-State Electron. 51 1319) might be attributed to the electron-transit-time effect in the barrier regions.

Plasma wave instability and amplification of terahertz radiation in field-effect-transistor arrays

We show that the strong amplification of terahertz radiation takes place in an array of field-effect transistors at small DC drain currents due to hydrodynamic plasmon instability of the collective plasmon mode. Planar designs compatible with standard integrated circuit fabrication processes and strong coupling of terahertz radiation to plasmon modes in FET arrays make such arrays very attractive for potential applications in solid-state terahertz amplifiers and emitters.

An ultra-stable non-coherent light source for optical measurements in neuroscience and cell physiology

We demonstrate that high power light-emitting diodes (LED's) exhibit low-frequency noise characteristics that are clearly superior to those of quartz tungsten halogen lamps, the non-coherent light source most commonly employed when freedom from intensity variation is critical. Their extreme stability over tens of seconds (combined with readily selectable wavelength) makes high power LED's ideal light sources for DC recording of optical changes, from living cells and tissues, that last more than a few hundred milliseconds. These optical signals (DeltaI/I(0)) may be intrinsic (light scattering, absorbance or fluorescence) or extrinsic (absorbance or fluorescence from probe molecules) and we show that changes as small as approximately 8 x 10(-5) can be recorded without signal averaging when LED's are used as monochromatic light sources. Here, rapid and slow changes in the intrinsic optical properties of mammalian peptidergic nerve terminals are used to illustrate the advantages of high power LED's compared to filament bulbs.