James G. Champlain

A first principles theoretical examination of graphene-based field effect transistors

This paper presents an in-depth theoretical examination of graphene-based field effect transistors, looking at thermal statistics, electrostatics, and electrodynamics. Using a first principles approach, the unique behavior observed in graphene-based field effect transistors, such as the V-shaped transfer characteristic, limited channel pinch-off, and lack of off-state (under gate modulation), are described. Unlike previous attempts, a description of both drift and diffusion currents in the device is presented. The effect of external resistance on steady-state and high-frequency performance is examined. Comparisons of the theoretical results to experimental results are made and show good agreement. Finally, the theoretical work in this paper is used as a basis to discuss the possible source of some observed behavior in practical graphene-based field effect transistors.

Atomic Layer Deposition of Al2O3 on GaSb Using In Situ Hydrogen Plasma Exposure

In this report, we study the effectiveness of hydrogen plasma surface treatments for improving the electrical properties of GaSb/Al2O3 interfaces. Prior to atomic layer deposition of an Al2O3 dielectric, p-GaSb surfaces were exposed to hydrogen plasmas in situ, with varying plasma powers, exposure times, and substrate temperatures. Good electrical interfaces, as indicated by capacitance-voltage measurements, were obtained using higher plasma powers, longer exposure times, and increasing substrate temperatures up to 250 °C. X-ray photoelectron spectroscopy reveals that the most effective treatments result in decreased SbOx, decreased Sb, and increased GaOx content at the interface. This in situ hydrogen plasma surface preparation improves the semiconductor/insulator electrical interface without the use of wet chemical pretreatments and is a promising approach for enhancing the performance of Sb-based devices.

Sb-Based n- and p-Channel Heterostructure FETs for High-Speed, Low-Power Applications

Heterostructure field-effect transistors (HFETs) composed of antimonide-based compound semiconductor (ABCS) materials have intrinsic performance advantages due to the attractive electron and hole transport properties, narrow bandgaps, low ohmic contact resistances, and unique band-lineup design flexibility within this material system. These advantages can be particularly exploited in applications where high-speed operation and low-power consumption are essential. In this paper, we report on recent advances in the design, material growth, device characteristics, oxidation stability, and MMIC performance of Sb-based HEMTs with an InAlSb upper barrier layer. The high electron mobility transistors (HEMTs) exhibit a transconductance of 1.3S/mm at VDS=0.2V and an fTLg product of 33GHz-μm for a 0.2μm gate length. The design, fabrication and improved performance of InAlSb/InGaSb p-channel HFETs are also presented. The HFETs exhibit a mobility of 1500cm2/V-sec, an fmax of 34GHz for a 0.2μm gate length, a threshold voltage of 90mV, and a subthreshold slope of 106mV/dec at VDS=-1.0V.

Total Ionizing Dose Induced Charge Carrier Scattering in Graphene Devices

We investigate total ionizing dose (TID) effects in graphene field effect transistors comprised of chemical vapor deposition grown graphene transferred onto trimethylsiloxy(TMS)-passivated SiO2 Si substrates. TID exposure with a positive gate bias increases the concentration of positive oxide trapped charges near the SiO2/TMS/graphene interface making Coulomb-potential scatterer limited mobility more apparent. In particular, we observe asymmetric degradation in electron and hole mobility, the former degrading more rapidly. Consistent with the electron-hole puddle description, we observe an increase in intrinsic electron carrier density that varies linearly with the oxide trapped charge density, while the hole carrier density remains largely unaltered. These effects give rise to an increasing minimum conductivity.

Narrow band gap InGaSb, InAlAsSb alloys for electronic devices

Solid source molecular beam epitaxy has been used to grow random alloy quaternary InAlAsSb and ternary InGaSb alloys with a 6.2A lattice constant for use in electronic devices such as p-n junctions and heterojunction bipolar transistors (HBTs). Several p-n hetrojunctions composed of p-type InGaSb and one of several different n-type InAlAsSb alloys have been fabricated and show good rectification with ideality factors near one. In addition, several of these alloys have been used to make an n-p-n HBT that has demonstrated a dc current gain of 25.

High frequency capacitance-voltage technique for the extraction of interface trap density of the heterojunction capacitor: Terman’s method revised

In a multi-layer heterojunction system, the interface responsible for trap charging is spatially displaced from the two-dimensional charge gas, in contrast to the typical SiO2/Si capacitor. This displacement causes the effective trap capacitance to occur in a different configuration than that of the SiO2/Si system that Terman originally considered. The adaptation of Terman’s high frequency capacitance-voltage method for interface trap density extraction is developed for the heterojunction multi-layer capacitor.

On the use of the term “ambipolar”

The term ambipolar has been used extensively in association with carbon nanotube and graphene-based field effect transistors, often in a varied manner, leading to a confused understanding of the term. Through the use of established scientific definitions and theoretical work on device operation, this paper attempts to clarify the understanding of the term and present a discussion of its appropriate use.

Antimonide-based diodes for terahertz mixers

Antimonide-based p+N junctions have been grown by molecular beam epitaxy and processed into diodes. The diodes have good rectification with ideality factors near 1, and high saturation current densities of 2.5×10−2A∕cm2. S-parameter measurements to 50GHz indicate a 1Ω series resistance and a capacitance of 1.2fF∕μm2 for a 5μm diameter mesa diode. A cutoff frequency of 6.5THz is estimated from the RC product. The high saturation current indicates that this diode will reach forward bias currents at substantially lower voltages than GaAs Schottky diodes. These properties suggest using the diode as a terahertz mixer.

Low Resistance, Unannealed, Ohmic Contacts to p-type In0.27Ga0.73Sb

Unannealed Pd∕Pt∕Au contacts to p-type In0.27Ga0.73Sb were fabricated and measured. Relatively high hole mobilities, with respect to similarly doped InP-lattice-matched materials, and associated low sheet resistances were measured for the p-type In0.27Ga0.73Sb material. The unannealed Pd∕Pt∕Au contacts were found to be Ohmic in nature; and for a hole density of 2.9×1019cm−3 and a mobility of 160cm2∕Vs, a specific contact resistance of 7.6×10−8Ωcm2 was measured.

Examination of the temperature dependent electronic behavior of GeTe for switching applications

The DC and RF electronic behaviors of GeTe-based phase change material switches as a function of temperature, from 25 K to 375 K, have been examined. In its polycrystalline (ON) state, GeTe behaved as a degenerate p-type semiconductor, exhibiting metal-like temperature dependence in the DC regime. This was consistent with the polycrystalline (ON) state RF performance of the switch, which exhibited low resistance S-parameter characteristics. In its amorphous (OFF) state, the GeTe presented significantly greater DC resistance that varied considerably with bias and temperature. At low biases (<1 V) and temperatures (<200 K), the amorphous GeTe low-field resistance dramatically increased, resulting in exceptionally high amorphous-polycrystalline (OFF-ON) resistance ratios, exceeding 109 at cryogenic temperatures. At higher biases and temperatures, the amorphous GeTe exhibited nonlinear current-voltage characteristics that were best fit by a space-charge limited conduction model that incorporates the effect of a defect band. The observed conduction behavior suggests the presence of two regions of localized traps within the bandgap of the amorphous GeTe, located at approximately 0.26–0.27 eV and 0.56–0.57 eV from the valence band. Unlike the polycrystalline state, the high resistance DC behavior of amorphous GeTe does not translate to the RF switch performance; instead, a parasitic capacitance associated with the RF switch geometry dominates OFF state RF transmission.