Martin Neuburger

High-sheet-charge–carrier-density AlInN∕GaN field-effect transistors on Si(111)

AlInN∕GaN heterostructures have been proposed to possess advantageous properties for field-effect transistors (FETs) over AlGaN∕GaN [Kuzmik, IEEE Electron Device Lett. 22, 501 (2001); Yamaguchi et al., Phys. Status Solidi A 188, 895 (2001)]. A major advantage of such structures is that AlInN can be grown lattice-matched to GaN while still inducing high charge carrier densities at the heterointerface of around 2.7×1013cm−3 by the differences in spontaneous polarization. Additionally, it offers a higher band offset to GaN than AlGaN. We grew AlInN FET structures on Si(111) substrates by metalorganic chemical vapor phase epitaxy with In concentrations ranging from 9.5% to 24%. Nearly lattice-matched structures show sheet carrier densities of 3.2×1013cm−2 and mobilities of ∼406cm2∕Vs. Such Al0.84In0.16N FETs have maximum dc currents of 1.33A∕mm for devices with 1μm gate length.

Piezoelectric GaN sensor structures

Free-standing GaN and AlGaN/GaN cantilevers have been fabricated on (111) silicon substrate using dry etching. On these cantilevers, a piezoresistor and a high-electron-mobility transistor (HEMT) structure have been realized, and the piezoresponse has been characterized. Cantilever bending experiments resulted in a Youngs modulus of approximately 250 GPa, a sensitivity of K/spl sim/90, and a modulation of the HEMT current of up to 50%. It is seen that the piezoresponse could be related to both the bulk properties and the properties of the heterostructure interface.

Transient thermal characterization of AlGaN/GaN HEMTs grown on silicon

We studied a temperature increase and a heat transfer into a substrate in a pulsed operation of 0.5 length and 150 /spl mu/m gate width AlGaN/GaN HEMTs grown on silicon. A new transient electrical characterization method is described. In combination with an optical transient interferometric mapping technique and two-dimensional thermal modeling, these methods determine the device thermal resistance to be /spl sim/70 K/W after 400 ns from the start of a pulse. We also localized the high-electron mobility transistor heat source experimentally and we extracted a thermal boundary resistance at the silicon-nitride interface of about /spl sim/7/spl times/10/sup -8/ m/sup 2/K/W. Thermal coupling at this interface may substantially influence the device thermal resistance.

Transient characteristics of GaN-based heterostructure field-effect transistors

DC current-switching and power-switching transients of various GaN-based FET structures are investigated. Two different characteristics are compared, namely, thermal and electronic transients. While the thermal transients are mainly reflected in changes in channel carrier mobility, the electronic transients are dominated by charge instabilities caused by the polar nature of the material. The discussion of the electronic transients focuses, therefore, on instabilities caused by polarization-induced image charges. Three structures are discussed, which are: 1) a conventional AlGaN/GaN heterostructure FET; 2) an InGaN-channel FET; and 3) an AlGaN/GaN double-barrier structure. In structures 2) and 3), field-induced image charges are substituted by doping impurities, eliminating this source of related instability. This is indeed observed.

Unstrained InAlN/GaN HEMT structure

InAlN has been investigated as barrier layer material for GaN-HEMT structures, potentially offering higher sheet charge densities (Kuzmik, 2002) and higher breakdown fields (Kuzmik, 2001). Lattice matched growth of the barrier layer can be achieved with 17% in content, avoiding piezo polarization. In this configuration the sheet charge density is only induced by spontaneous polarization. First experimental results of unpassivated undoped samples realized on 111-Si substrate exceed a DC output current density of 1.8 A/mm for a gate length of 0.5 /spl mu/m. Small signal measurements yield a f/sub t/ = 26 GHz and f/sub max/ = 14 GHz, still limited by the residual conductivity of the Si-substrate. A saturated output power at 2 GHz in class A bias point yielded a density of 4.1W/mm at V/sub DS/ = 24 V.

Coplanar AlGaN/GaN HEMT power amplifier MMIC at X-band

A power amplifier MMIC based on AlGaN/GaN HEMTs was fabricated and measured. The coplanar balanced amplifier consists of two 8/spl times/100/spl mu/m transistors. Wilkinson splitters were used to divide and combine the power. BY biasing the amplifier at V/sub DS/=30V a maximum CW output power of 39dBm corresponding to 5W/mm with a maximum power added efficiency (PAE) of 33.8% was achieved at 10GHz. Biasing the amplifier at V/sub DS/=20V resulted in 36.7% PAE with 37.2dBm CW output power at 10GHz. To the authors knowledge these results represent the highest output power density so far achieved for GaN-based MMICs at X-band in CW mode.

Analysis of surface charging effects in passivated AlGaN-GaN FETs using a MOS test electrode

Passivated AlGaN-GaN high electron mobility transistor (HEMT) structures are modified by adding a MOS test gate placed between gate and drain to identify surface charge phenomena by stress experiments. A new method is described to identify the vertical position of the charge centroid of charge injected from the gate. In the case investigated, this is within the passivation layer.

Diamond-MESFETs - synthesis and integration

We report the utilization of synthetic diamond grown by chemical-vapour-deposition (CVD) for use as metal-semiconductor field-effect-transistors (MESFETs). The lack of a shallow n-type donor means that diamond-based electronic devices are unipolar (p-type). The devices presented in this paper are based on delta-doping. Delta-doping stands for the use of very thin (<5 mm) highly doped (NA 1020 cm-3 ) buried layers. This approach poses a huge challenge in terms of synthesis as well as processing. First successful attempts of fully integrating working delta-doped diamond MESFETs are presented

GaN based piezo sensors

This work presents a technology which has been developed to fabricate free-standing GaN membrane and cantilever structures. First experiments have enabled us to verify the piezo response of these GaN based cantilever structures. Especially, the bulk polarization doping generated in the base layer is a new important contribution. GaN heterostructures grown on 111-oriented Si wafers have been used. Free standing cantilevers and membranes have been fabricated using RIE and ICP dry etching. Cantilevers have been etched from the rear side or from the surface. It is expected that this technology will enable new device concepts based on stress induced pn-junction effects.

A novel wireless converter topology for dynamic EV charging

This paper presents a novel topology, based on inductive power transfer (IPT) that is suitable for dynamic charging of electric vehicles (EVs). The proposed topology is novel in that a super capacitor (SC) is used in conjunction with the EV IPT secondary converter to combine power transfer and energy storage in a single converter. In comparison to existing dynamic EV IPT systems, this topology requires no additional switching components, while offering the benefit of a SC energy buffer. Operational modes of the system are described in terms of the switching waveforms required to transfer energy between the grid powered primary IPT pad and the EV. A mathematical model that describes the power transfer within the system is presented along with a simple control strategy to regulate power flow. Simulated and experimental performance of a small scale prototype system in a typical dynamic charging scenario is presented to confirm the validity of the mathematical model and the control strategy.