N-polar GaN-based MIS-HEMTs for mixed signal applications
Dept. of Electr. & Comput. Eng., Univ. of California, Santa Barbara, CA, USADOI: 10.1109/MWSYM.2010.5518145 Conference: Microwave Symposium Digest (MTT), 2010 IEEE MTT-S International
Source: IEEE Xplore
GaN-based transistors are attractive for the next-generation RF power and mixed signal electronics due to their high breakdown field and high carrier saturation velocity. III-N high electron mobility transistors (HEMTs) fabricated on the N-face of GaN are well-suited to address the problems of poor electron confinement and high ohmic contact resistance in the highly scaled transistors. At 4 GHz, N-polar metal-insulator-semiconductor (MIS)-HEMTs with a gate length of 0.7 μm exhibited a highest output power density (Pout) of 8.1 W/mm and a highest power-added efficiency (PAE) of 71%, while a Pout of 4.2 W/mm and a PAE of 49% were achieved at 10 GHz. A high speed N-polar MIS-HEMT fabricated with a gate-first self-aligned InGaN-based ohmic contact regrowth technology was characterized, demonstrating an ultra-low contact resistance of 23 Ω-μm and a state-of-the-art fT·LG product of 16.8 GHz-μm with a gate length of 130 nm.
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ABSTRACT: An anomalous output conductance that resembled short-channel effects was observed in long-channel N-polar GaN-channel/AlGaN-back-barrier/GaN-buffer high electron mobility transistors. The phenomenon could not be reasonably explained by drain-induced barrier lowering, leakage currents, or impact ionization events. We propose that the output conductance was caused by the ionization of a donorlike hole trap state at the negatively polarized AlGaN-back-barrier/GaN-buffer interface that shifted the threshold voltage at the drain side of the gate, where a high-field depletion region developed beyond current saturation. No evidence of increased output conductance or related device performance degradation was apparent under small-signal high-frequency conditions. The output conductance was suppressed by introducing photogenerated holes that compensated the traps. The effect of several typical back-barrier designs on the dc output conductance was examined.
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ABSTRACT: In this paper, we report the recent progress in the high-frequency performance of enhancement-mode devices in the novel N-polar GaN technology and provide a pathway for further scaling. The intrinsic advantages of electron confinement, polarization doping of the back-barrier and the absence of a source barrier in N-polar GaN technology were leveraged with polarization engineering with a top barrier for enhancement mode operation and advanced self-aligned source/drain technology for low parasitic access resistances. The scalability of the device structures are explored in terms of short-channel effects and high-frequency performance. Low-field electron mobility in vertically scaled channel was also investigated providing insights on the scattering mechanism.
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