Unity Power Gain Frequency Research Articles

DC and RF Performance of an N-channel Monolayer Black Phosphorus Nanoribbon Transistor

Two-dimensional black phosphorus is a relatively new discovery. There are numerous studies on black phosphorus two-dimensional transistors that focus on analog and RF performance. However, the RF performance of black phosphorus nanoribbon transistors is yet to be explored. We use a four-band tight binding Hamiltonian in conjunction with a non-equilibrium Green's function quantum transport simulator to investigate both the DC and RF performance of a monolayer black phosphorus nanoribbon transistor. We found that electron intra-band tunneling is responsible for current flow in the off-state, while in the on-state, the electrons flow over the top of the channel barrier potential. With a VDD of 0.4 volt and a gate length of 5 nm, our black phosphorus nanoribbon transistor has DC performance metrics of 510 µA/µm on-state current, 105 on/off current ratio, and 65 mV/dec inverse subthreshold slope. The device's RF performance characteristics are as follows: cut-off (unity current gain) frequency of 772.84 GHz, maximum oscillation (unity power gain) frequency of 1.15 THz, and open circuit voltage gain of 26.7 dB with transistor operating in the on-state. The RF performance of the device is found to be significantly impacted by the source and drain contact resistances. With source and drain resistances set to zero, the cut-off frequency increases to 995.23 GHz and the unity power gain frequency increases to 4.16 THz. The device shows unconditional stability above 893 GHz and it is conditionally stable below this frequency.

Assessment of High-Frequency Performance Limit of Black Phosphorus Field-Effect Transistors

Recently, gigahertz frequencies have been reported with black phosphorus (BP) field-effect transistors (FETs), yet the high-frequency performance limit has remained unexplored. Here we project the frequency limit of BP FETs based on rigorous atomistic quantum transport simulations and the small-signal circuit model. Our self-consistent nonequilibrium Green’s function (NEGF) simulation results show that semiconducting BP FETs exhibit clear saturation behaviors with the drain voltage, unlike zero-bandgap graphene devices, leading to >10 THz frequencies for both intrinsic cutoff frequency ( ${f}_{T}$ ) and unity power gain frequency ( ${f} _{{\mathsf {max}}}$ ). To develop keen insight into practical devices, we discuss the optimization of ${f} _{T}$ and ${f} _{{\mathsf {max}}}$ by varying various device parameters such as channel length ( ${L} _{{\mathsf {ch}}}$ ), oxide thickness, device width, gate resistance, contact resistance, and parasitic capacitance. Although extrinsic ${f}_{T}$ and ${f} _{{\mathsf {max}}}$ can be significantly affected by the contact resistance and parasitic capacitance, they can remain near THz frequency range ( ${f} _{T} = \mathsf {900}$ GHz; ${f} _{{\mathsf {max}}} = \mathsf {1.2}$ THz) through proper engineering, particularly with an aggressive channel length scaling ( ${L} _{{\mathsf {ch}}} \approx \mathsf {10}$ nm). Our benchmark against the experimental data indicates that there still exists large room for optimization in fabrication, suggesting further advancement of high-frequency performance of state-of-the-art BP FETs for the future analog and radio-frequency applications.

Flexible Graphene Field-Effect Transistors Encapsulated in Hexagonal Boron Nitride.

Flexible graphene field-effect transistors (GFETs) are fabricated with graphene channels fully encapsulated in hexagonal boron nitride (hBN) implementing a self-aligned fabrication scheme. Flexible GFETs fabricated with channel lengths of 2 μm demonstrate exceptional room-temperature carrier mobility (μFE = 10 000 cm(2) V(-1) s(-1)), strong current saturation characteristics (peak output resistance, r0 = 2000 Ω), and high mechanical flexibility (strain limits of 1%). These values of μFE and r0 are unprecedented in flexible GFETs. Flexible radio frequency FETs (RF-FETs) with channel lengths of 375 nm demonstrate μFE = 2200 cm(2) V(-1) s(-1) and r0 = 132.5 Ω. Unity-current gain frequencies, fT, and unity-power gain frequencies, fmax, reach 12.0 and 10.6 GHz, respectively. The corresponding ratio of cutoff frequencies approaches unity (fmax/fT = 0.9), a record value for flexible GFETs. Intrinsic fT and fmax are 29.7 and 15.7 GHz, respectively. The outstanding electronic characteristics are attributed to the improved dielectric environment provided by full hBN encapsulation of the graphene channel in conjunction with an optimized, self-aligned device structure. These results establish hBN as a mechanically robust dielectric that can yield enhanced electronic characteristics to a diverse array of graphene-based flexible electronics.

A computational study of high-frequency behavior of graphene field-effect transistors

High Frequency potential of graphene field-effect transistors (FETs) is explored by quasi-static self-consistent ballistic and dissipative quantum transport simulations. The unity power gain frequency fMAX and the cut-off frequency fT are modeled at the ballistic limit and in the presence of inelastic phonon scattering for a gate length down to 5 nm. Our major results are (1) with a thin high-κ gate insulator, the intrinsic ballistic fT is above 5 THz at a gate length of 10 nm. (2) Inelastic phonon scattering in graphene FETs lowers both fT and fMAX, mostly due to decrease of the transconductance. (3) fMAX and fT are severely degraded in presence of source and drain contact resistance. (4) To achieve optimum extrinsic fMAX performance, careful choice of DC bias point and gate width is needed.

High-Frequency Performance of Submicrometer Transistors That Use Aligned Arrays of Single-Walled Carbon Nanotubes

The unique electronic properties of single-walled carbon nanotubes (SWNTs) make them promising candidates for next generation electronics, particularly in systems that demand high frequency (e.g., radio frequency, RF) operation. Transistors that incorporate perfectly aligned, parallel arrays of SWNTs avoid the practical limitations of devices that use individual tubes, and they also enable comprehensive experimental and theoretical evaluation of the intrinsic properties. Thus, devices consisting of arrays represent a practical route to use of SWNTs for RF devices and circuits. The results presented here reveal many aspects of device operation in such array layouts, including full compatibility with conventional small signal models of RF response. Submicrometer channel length devices show unity current gain (f(t)) and unity power gain frequencies (f(max)) as high as approximately 5 and approximately 9 GHz, respectively, with measured scattering parameters (S-parameters) that agree quantitatively with calculation. The small signal models of the devices provide the essential intrinsic parameters: saturation velocities of 1.2 x 10(7) cm/s and intrinsic values of f(t) of approximately 30 GHz for a gate length of 700 nm, increasing with decreasing length. The results provide clear insights into the challenges and opportunities of SWNT arrays for applications in RF electronics.

Fringing effects of V-shape gate metal on GaAs/InGa/PInGaAs doped-channel field-effect transistors

V-gate heterojunction doped-channel field-effect transistors (HDCFETs) with a 0.12 µm gate-metal footprint were fabricated through a 1 µm patterned photoresist using conventional optical lithography. The measured transconductance and output conductance are 200 and 3.5 mS mm−1, respectively, resulting in a high open-drain voltage gain of 57. In spite of higher unity-current gain and unity-power gain frequencies (21 and 32 GHz) as compared with those (9.5 and 17 GHz) of a 1 µm planar-gate HDCFET, the frequency enhancement for a V-gate HDCFET with such a short 0.12 µm gate-metal footprint is not very significant. Simulated results reveal that both the triangular-metal fringe and sacrificial layer play an important role in HDCFET dc and ac performances. Experimental and simulated results reveal that (1) the triangular-metal fringe is advantageous to improve short-channel effects and to enhance the dc performance, (2) the gate fringing capacitance due to the triangular-metal fringe and the 0.455 µm sacrificial layer contributes 60% of capacitance to the total gate-to-source capacitance, and (3) even if there is no sacrificial layer upon the Schottky layer, the triangular-metal fringe still contributes 15% of capacitance to the total gate-to-source capacitance.

Technological requirements for a lateral SiGe HBT technology including theoretical performance predictions relative to vertical SiGe HBTs

A new class of bipolar transistors, lateral SiGe HBTs, is considered from a theoretical point of view with regard to their feasibility and potential radio frequency (RF)/microwave performance relative to existing vertical SiGe HBT technologies. Using numerical analysis, it is found that the low terminal parasitics of a lateral SiGe HBT structure can provide at least twice the unity power gain frequency f/sub max/ compared to vertical structures for a given minimum lithography. Methods to fabricate a lateral SiGe HBT with superior RF/microwave characteristics compared to existing vertical technologies are explored. Extension of the numerical analysis results using simple scaling rules reveals that it should be possible to fabricate lateral SiGe HBTs with unity power gain frequencies in the neighborhood of 500 GHz operating in the micro-ampere range.

High-frequency AlGaN/GaN polarization-induced high electron mobility transistors grown by plasma-assisted molecular-beam epitaxy

High-quality AlGaN/GaN heterostructures have been grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. Polarization effects are exploited to achieve a two-dimensional electron-gas sheet density of 8.8×1012 cm−2 and greater on intentionally undoped material with a measured room-temperature mobility as high as 1478 cm2/V s. Transistors were then fabricated from this material, yielding a unity current gain frequency of 50 GHz and a unity power gain frequency of 97 GHz. By increasing the buffer layer thickness, output powers of 1.88 W/mm at 4 GHz with an efficiency of 34% were achieved. These results prove that the polarization effects in the nitrides are as enormous as theory predicts. The key to the improved mobility and operation of the devices of the all-molecular-beam-epitaxy-grown material, the AlN nucleation layer, will be discussed.

New self-aligned T-gate InGaP/GaAs field-effect transistors grown by LP-MOCVD

This paper reports on self-aligned T-gate InGaP/GaAs FETs using n/sup +//N/sup +///spl delta/(P/sup +/)/n structures. N/sup +/-InGaP//spl delta/(P/sup +/)-InGaP/n-GaAs forms a planar-doped barrier. The inherent ohmic gate of camel-gate FETs together with a highly selective etch between an InGaP and a GaAs layers offers a self-aligned T-shape gate with a reduced effective length. A fabricated device with a reduced gate dimension of 1.5/spl times/100 (0.6/spl times/100) /spl mu/m/sup 2/ obtained from 2/spl times/100 (1/spl times/100) /spl mu/m/sup 2/ gate metal exhibits an extrinsic transconductance, unity-current gain frequency, and unity-power gain frequency of 78 (80) mS/mm, 9 (19.5), and 28 (30) GHz, respectively.

High-Frequency Limits of Heterostructure Field-Effect Transistors (HFETs)

This paper present a simplified theoretical basis for an understanding of the high frequency limits of the HFETs, and compares the performance of InP based short gate-length devices reported by various groups. For this comparison, the average velocity (Vav) in each device is extracted using the effective gate-length that includes the laterally extended gate-to-drain depletion length. The results indicate that Vav increases approximately from 2.46 x 107 to 3.0 x 107 cm/sec when the gate-length is reduced from 0.15 μm to 0.05 μm. This comparison clearly suggests that although reduction of the gate-length can increase the unity current-gain frequency (fτ), it does not necessarily lead to a high unity power-gain frequency (fmax). The latter requires a low gate series resistance (rgg'), a low source series resistance (rss'), and a very low drain-to-gate feedback capacitance cdg. Since all these parameters are adversely affected by the reduction of the gate-length, the challenge of realizing practical HFETs with useful power gain of 4dB at and beyond 300 GHz, appears to be a formidable one.

Parametric investigation of InGaAs/InAlAs HEMTs grown by CBE

The InAlAs/InGaAs high electron mobility transistor offers excellent high frequency, low noise operation for amplifiers. While this material system has been grown primarily by conventional MBE, other growth techniques have been examined for improved throughput. The flexibility of chemical beam epitaxy offers semi-infinite sources, good source stability, efficient phosphorus utilization, and extended uptime (more than 560 growth runs over 1.5 years). However, CBE has only recently been shown to produce excellent quality InAlAs suitable for the growth of InAlAs/InGaAs HEMTs [1]. This is the first parametric investigation of the properties of InAlAs/InGaAs HEMTs grown by CBE. A series of lattice matched, pulse doped HEMTs have been grown in which the dopant dose, spacer layer, and channel thickness were systematically varied. Low field 300 K Hall mobilities as high as 8700 cm 2/V·s for a sheet carrier concentration of 3x10 12 cm -2have been measured. This mobility is somewhat lower than uniformly doped HEMTs, which have shown mobilities over 10,000 cm 2/V·s at room temperature. A figure of merit, the low field conductivity, has been correlated among the device structure, gateless saturation currents, and DC and microwave device performance. Its applicability as a rough predictor of device performance will be discussed. For a given spacer thickness, the mobility improves as the pulse dose is decreased up to a mobility somewhat below that for uniformly doped structures. As the dopant to channel thickness is increased, this saturated mobility also increases. Secondary ion mass spectroscopy has shown no increase in carbon or oxygen levels at the dopant pulse. This has led to speculation that interface scattering at the top InAlAs/InGaAs interface may be important; however, initial SIMS results do not conclusively show intermixing of the Group III elements at this interface. It is possible that a reduction in the substrate temperature during growth may improve any interface roughness. Results of this modification in growth conditions shall be reported. Self-aligned 0.15 μm HEMTs fabricated from these layers have shown external DC transconductances over 1000mS/mm, unity current gain cutoff frequencies as high as 190 GHz and unity power gain frequencies above 300 GHz. These results and those of more conventional 0.1 μm gate length HEMTs demonstrate the potential of InAlAs/InGaAs HEMTs grown by CBE.