E.M. Chumbes

The effect of surface passivation on the microwave characteristics of undoped AlGaN/GaN HEMTs

Surface passivation of undoped AlGaN/CaN HEMTs reduces or eliminates the surface effects responsible for limiting both the RF current and breakdown voltages of the devices. Power measurements on a 2/spl times/125/spl times/0.5 /spl mu/m AlGaN/GaN sapphire based HEMT demonstrate an increase in 4 GHz saturated output power from 1.0 W/mm [36% peak power-added efficiency (PAE)] to 2.0 W/mm (46% peak PAE) with 15 V applied to the drain in each case. Breakdown measurement data show a 25% average increase in breakdown voltage for 0.5 /spl mu/m gate length HEMTs on the same wafer. Finally, 4 GHz power sweep data for a 2/spl times/75/spl times/0.4 /spl mu/m AlGaN/GaN HEMT on sapphire processed using the Si/sub 3/N/sub 4/ passivation layer produced 4.0 W/mm saturated output power at 41% PAE (25 V drain bias). This result represents the highest reported microwave power density for undoped sapphire substrated AlGaN/GaN HEMTs.

Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm/sup 2//Ns, respectively, For electron sheet density near 1/spl times/10/sup 13//cm/sup 2/, Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f/sub t/ of 106 GHz for 0.15 /spl mu/m gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of /spl sim/4 W/mm, while large periphery devices have /spl sim/2 W/mm, both thermally limited. Performance, without and with Si/sub 3/N/sub 4/ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates.

Microwave performance of AlGaN/GaN metal insulator semiconductor field effect transistors on sapphire substrates

Metal-insulator-semiconductor field effect transistors (MISFETs) from surface-passivated undoped AlGaN/GaN heterostructures on sapphire were fabricated. Measured static output characteristics includes full channel currents (I/sub dss/) of roughly 750 mA/mm with gate-source pinchoff voltages of -10 V and peak extrinsic transconductancies (g/sub m/) of 100-110 mS/mm. Increased surface roughness resulting from a gate recess process to reduce the pinchoff voltage introduces gate leakage currents in the micro-amps regime. With evidence for reduced dc-to-rf dispersion from pulsed gate transfer characteristics, these devices at 4 GHz with 28.0 V bias generated maximum output power densities of 4.2 W/mm with 14.5 dB of gain and 36% power added efficiency.

AlGaN/GaN high electron mobility transistors on Si(111) substrates

AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have for the first time been realized using organometallic vapor phase epitaxy (OMVPE). Using 1 /spl Omega/-cm p-Si(111), these devices exhibited static output characteristics with low output conductance and isolation approaching 80 V. Under microwave rf operation, the substrate charge becomes capacitively coupled and parasitically loads these devices thereby limiting their performance. As a result, typical 0.3 /spl mu/m gate length devices show a 25 GHz cutoff frequency, with near unity f/sub max//f/sub T/ ratio and 0.55 W/mm output power. A small-signal equivalent circuit incorporating elements representing the parasitic substrate loading accurately models the measured S-parameters. Removal of the conductive substrate is one way to effectively eliminate this parasitic loading. Through backside processing, freestanding 0.4-mm HEMT membranes with no thermal management were demonstrated and exhibited a significant improvement in their f/sub max//f/sub T/ ratio up to 2.5 at the cost of lower f/sub T/ and f/sub max/ along with an almost four-fold reduction of I/sub dss/.

Growth and passivation of AlGaN/GaN heterostructures

Abstract The undoped AlGaN/GaN heterostructure (Ga-face) produces a polarization induced two-dimensional electron gas (2DEG) at the heterointerface. The as-grown structure has a fixed positive charge on its surface to oppose the negative polarization charge on the top AlGaN layers surface. Although sensitive to atmospheric conditions this positive surface charge is not sufficient in density to prevent significant depletion of the 2DEG. The ideal surface passivation is an insulator, acting as an encapsulant, which provides a fixed positive charge to neutralize the AlGaN polarization charge. We demonstrate that certain types of silicon nitride thin films can result in a charge balance and virtually eliminate depletion of the underlying 2DEG. A simple polarization model is used to explain the Hall and capacitance–voltage data on AlGaN/GaN heterostructures grown by organometallic vapor phase epitaxy. By changing the passivation layer thickness, the surface depletion can be systematically controlled, allowing the dependence of electron mobility on two-dimensional charge densities to be determined for samples grown on sapphire and SiC substrates. A metal–insulator–semiconductor structure of this type can be used to deplete the electrons induced at the AlGaN/GaN interface allowing insulated gate transistors to be realized.

Single step process for epitaxial lateral overgrowth of GaN on SiC and sapphire substrates

Using flow modulation organometallic vapor phase epitaxy, a process has been developed which produces epitaxial lateral overgrowth of GaN-base materials directly on SiC and sapphire substrates patterned with silicon nitride. The key feature of this single step process is the use of a high temperature AlGaN nucleation layer which wets the exposed substrate surface, without significant nucleation on the mask. This eliminates the need for regrowth while producing smooth growth surfaces in the window opening as well as over the mask. Subsequent GaN deposition results in relatively defect free materials grown laterally over the mask. Using arrays of stripe windows aligned parallel to the 〈11_00〉 crystal direction, the epitaxial films completely planarize after roughly 5 microns of growth. Defect densities estimated from atomic force micrographs indicate a reduction from mid 108 to 105 cm−2 in regions over the window and over the mask, respectively. This process represents a significant simplification over curr...

Influence of oxygen and methane plasma on the electrical properties of undoped AlGaN/GaN heterostructures for high power transistors

Abstract Plasma-induced damage often reduces the electrical and optical performance and the lifetime of compound semiconductor devices. We have investigated the effect of oxygen and methane reactive ion etching plasma on the electrical characteristics of nominally undoped GaN/Al 0.25 Ga 0.75 N/GaN high mobility heterostructures on sapphire substrates grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition (MOCVD). The electrical transport properties of the two-dimensional electron gas in the AlGaN/GaN heterostructures were investigated by a combination of capacitance–voltage profiling and Hall effect measurements. The performance degradation of the heterostructures is attributed to the reduction of the carrier density and mobility due to ion bombardment, which is causing a creation of surface and deep acceptor states. After rapid thermal annealing (RTA) at temperatures between 400°C and 800°C, the electrical properties of the heterostructures exposed at moderated plasma power density and bias were mostly recovered. However, samples exposed at high power density lost the significant part of the electron sheet carrier concentration unrecoverable even after RTA at 800°C.

Microwave performance of AlGaN/GaN high electron mobility transistors on Si(111) substrates

The first AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have been realized using Organometallic Vapor Phase Epitaxy (OMVPE). Static output characteristics with low output conductance and isolation approaching 80 V were observed on devices fabricated on 1 /spl Omega/-cm p-Si(111). At microwave frequencies, a capacitively coupled substrate charge limits the device performance, resulting in a 25 GHz cutoff frequency and 0.5 W/mm output power for 0.3 /spl mu/m gate length devices. A small-signal equivalent circuit incorporating elements representing the parasitic loading from the substrate accurately models the capacitive coupling effect observed on measured S-parameters.

Microwave performance of AlGaN/GaN metal insulator semiconductor field effect transistors

Metal-insulator semiconductor field effect transistors (MISFETs) from surface-passivated undoped AlGaN/GaN heterostructures were fabricated on sapphire using a gate-window process that does not severely impact its performance. Measured static output characteristics include full channel currents (I/sub max/) of roughly 750 mA/mm and peak transconductances (g/sub m/) of 100-110 mS/mm. With evidence for reduced DC-to-RF dispersion from gate-lag measurements, these devices at 4 GHz with 28.0 V bias generated maximum output power densities of 4.2 W/mm and 36% power added efficiency. The process when applied to 2-inch wafers produced with high yield 0.6-/spl mu/m MISFETs exhibiting consistent static performance over the whole wafer with a mean I/sub max/ value of 754 mA/mm and a mean g/sub m/ value of 66 mS/mm at an impressive standard deviation <5%.

Power limits of polarization-induced AlGaN/GaN HEMT's

The present and predicted limits on microwave power performance of undoped AlGaN/GaN HEMTs are presented, based on measured frequency response and drain-source breakdown voltage, both as functions of gate length. The spontaneous and piezoelectric polarization that induce the 2DEG in these HEMTs are covered. Process methods, including Si/sub 3/N/sub 4/ passivation are included. Thermal simulation results are shown for heat dissipation that limits channel temperature to 300/spl deg/C. Microwave cw power density limits of 12.5 W/mm at 10 GHz are predicted for class A operation on thick SiC substrates.