Sungjae Lee

Bruce M. Green; Vinayak Tilak; Sungjae Lee; Hyungtak Kim; Joseph A. Smart; Kevin J. Webb; J. R. Shealy; L. F. Eastman

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%.

Sungjae Lee; Kevin J. Webb; Vinayak Tilak; L. F. Eastman

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.

Bruce M. Green; Vinayak Tilak; Sungjae Lee; Hyungtak Kim; Joseph A. Smart; Kevin J. Webb; J. R. Shealy; L. F. Eastman

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%.

Mark J. W. Rodwell; William R. Frensley; Sebastian Steiger; Evgueni Chagarov; Sungjae Lee; H. Ryu; Y. Tan; Ganesh Hegde; Lingquan Wang; Jeremy J. M. Law; T. Boykin; G. Klimek; Peter M. Asbeck; Andrew C. Kummel; J. N. Schulman

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.

Bruce M. Green; Sungjae Lee; K. Chu; Kevin J. Webb; L. F. Eastman

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.

Sungjae Lee; Bruce M. Green; K. Chu; Kevin J. Webb; Lester F. Eastman

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.

Jong-Wook Lee; Sungjae Lee; Kevin J. Webb

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.

Sungjae Lee; Kevin J. Webb

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.

Sungjae Lee; V. Tilak; K.J. Webb; L.F. Eastman

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.

Sungjae Lee; Kevin J. Webb

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.