Sungjae Lee

High-power broad-band AlGaN/GaN HEMT MMICs on SiC substrates

Broadband, high power cascode AlGaN/GaN HEMT MMIC amplifiers with high gain and power-added efficiency (PAE) have been fabricated on high-thermal conductivity SiC substrates. A cascode gain cell exhibiting 5 W of power at 8 GHz with a small signal gain of 19 dB was realized. A broadband amplifier MMIC using these cascode cells in conjunction with a lossy-match input matching network was designed, fabricated, and evaluated, showing a useful operating range of DC-8 GHz with an output power of 5-7.5 W and a PAE of 20-33% respectively. A nonuniform distributed amplifier (NDA) based on this same process yielded an output power of 3-6 W over a DC-8 GHz bandwidth with an associated PAE of 13-31%.

Intrinsic noise equivalent-circuit parameters for AlGaN/GaN HEMTs

Intrinsic noise sources and their correlation in gallium-nitride high electron-mobility transistors (HEMTs) are extracted and studied. Microwave noise measurements have been performed over the frequency range of 0.8-5.8 GHz. Using measured noise and scattering parameter data, the gate and drain noise sources and their correlation are determined using an equivalent-circuit representation. This model correctly predicts the frequency-dependent noise for two devices having different gate length. Three noise mechanisms are identified in these devices, namely, those due to velocity fluctuation, gate leakage, and traps.

High-power broadband AlGaN/GaN HEMT MMICs on SiC substrates

Broadband, high power cascode AlGaN/GaN HEMT MMIC amplifiers with high gain and power-added efficiency (PAE) have been fabricated on high-thermal conductivity SiC substrates. A cascode gain cell exhibiting 5 W of power at 8 GHz with a small signal gain of 19 dB was realized. A broadband amplifier MMIC using these cascode cells in conjunction with a lossy-match input matching network was designed, fabricated, and evaluated, showing a useful operating range of DC-8 GHz with an output power of 5-7.5 W and a PAE of 20-33% respectively. A nonuniform distributed amplifier (NDA) based on this same process yielded an output power of 3-6 W over a DC-8 GHz bandwidth with an associated PAE of 13-31%.

III–V FET channel designs for high current densities and thin inversion layers

III–V FETs are being developed for potential application in 0.3–3 THz systems and VLSI. To increase bandwidth, we must increase the drive current I<inf>d</inf> = qn<inf>s</inf> v<inf>inj</inf>W<inf>g</inf> per unit gate width W<inf>g</inf>, requiring both high sheet carrier concentrations n<inf>s</inf> and high injection velocities v<inf>inj</inf>. Present III–V NFETs restrict control region transport to the single isotropic Γ band minimum. As the gate dielectric is thinned, I<inf>d</inf> becomes limited by the effective mass m*, and is only increased by using materials with increased m* and hence increased transit times.¹ The deep wavefunction also makes Γ -valley transport in low-m*materials unsuitable for < 22-nm gate length (L<inf>g</inf>) FETs. Yet, the L-valleys in many III–V materials² have very low transverse m<inf>t</inf> and very high longitudinal mass m<inf>1</inf>. L-valley bound state energies depend upon orientation, and the directions of confinement, growth, and transport can be chosen to selectively populate valleys having low mass in the transport direction^3,4. The high perpendicular mass permits placement of multiple quantum wells spaced by a few nm, or population of multiple states of a thicker well spaced by ∼10–100 meV. Using combinations of Γ and L valleys, n<inf>s</inf> can be increased, m* kept low, and vertical confinement improved, key requirements for <20-nm L<inf>g</inf> III–V FETs.

High efficiency monolithic gallium nitride distributed amplifier

The first gallium-nitride monolithic distributed amplifier is demonstrated. A nonuniform design allows the removal of the drain line dummy load with a concomitant increase in efficiency, An optimized nonuniform design shows a 10% increase in efficiency over an optimized uniform design having the dummy termination.

Demonstration of a high efficiency nonuniform monolithic gallium-nitride distributed amplifier

A monolithic gallium-nitride (GaN) dual-gate HEMT distributed amplifier has been designed which offers increased efficiency by removal of the drain line dummy load. This amplifier uses a dual-gate cascode gain cell to provide higher gain and power with a wideband frequency response. A fabricated four-stage nonuniform distributed amplifier has validated this approach.

Scalable large-signal device model for high power-density AlGaN/GaN HEMTs on SiC

A scalable device model for high-power, large periphery AlGaN-GaN HEMTs on SiC has been developed which includes device self-heating. The parameterized model coefficients were evaluated using S-parameters obtained from isothermal bias contours and pulsed I-V measurements. Model scaling with device size was examined by comparing with measurements for peripheries from 0.25 mm to 1.5 mm. The scaled model showed good agreement with measured S-parameters and power sweep data.

Numerical noise model for the AlGaN/GaN HEMT

A numerical approach to simulate the intrinsic noise sources within transistors is described. Using a 2D numerical device solver, spectral densities for the gate and drain noise current sources and their correlation are evaluated using a Greens function approach, an equivalent of Shockleys impedance field method. Case studies with AlGaN/GaN HEMTs compare the numerical simulation results to those from measurements, showing good agreement.

Intrinsic noise characteristics of AlGaN/GaN HEMTs

Intrinsic noise sources and their correlation in AlGaN/GaN HEMTs are extracted and studied. Using three noise parameters obtained from microwave noise measurements and S-parameter data, two intrinsic noise sources and their correlation are specified by applying a noise deembedding technique, and their dependence on frequency and bias point is investigated.

The influence of transistor nonlinearities on noise properties

A nonlinear field-effect transistor equivalent-circuit model is examined to identify the fundamental mechanisms that up-convert baseband 1/f noise to near-carrier sideband noise when the device is operated in the large-signal regime. This model captures all physical noise sources and nonlinearities in the transistor, and thereby allows a general cause-and-effect treatment. The noise sources in the equivalent-circuit model are determined using low-frequency spectrum analyzer and microwave noise-figure meter data. Using the example of an AlGaN/GaN high electron-mobility transistor, the developed model correctly describes both the measured near-carrier sideband amplitude and phase noise simultaneously.