David A. Deen

High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates

AlN/GaN heterostructures with 1700-cm²/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L_G) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L_G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f_T of 165 GHz, a maximum frequency of oscillation f_max of 171 GHz, and intrinsic average electron velocity v_e of 1.5 ×10⁷ cm/s. The 40-GHz load-pull measurements of L_G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v_e, and high RF performance in AlN/GaN transistors.

Atomic layer deposited Ta2O5 gate insulation for enhancing breakdown voltage of AlN/GaN high electron mobility transistors

AlN/GaN heterostructures with a 3.5 nm AlN cap have been grown by molecular beam epitaxy followed by a 6 nm thick atomic layer deposited Ta2O5 film. Transistors fabricated with 150 nm length gates showed drain current density of 1.37 A/mm, transconductance of 315 mS/mm, and sustained drain-source biases up to 96 V while in the off-state before destructive breakdown as a result of the Ta2O5 gate insulator. Terman’s method has been modified for the multijunction capacitor and allowed the measurement of interface state density (∼1013 cm−2 eV−1). Small-signal frequency performance of 75 and 115 GHz was obtained for ft and fmax, respectively.

Impact of barrier thickness on transistor performance in AlN/GaN high electron mobility transistors grown on free-standing GaN substrates

A series of six ultrathin AlN/GaN heterostructures with varied AlN thicknesses from 1.5–6 nm have been grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. High electron mobility transistors (HEMTs) were fabricated from the set in order to assess the impact of barrier thickness and homo-epitaxial growth on transistor performance. Room temperature Hall characteristics revealed mobility of 1700 cm2/V s and sheet resistance of 130 Ω/□ for a 3 nm thick barrier, ranking amongst the lowest room-temperature sheet resistance values reported for a polarization-doped single heterostructure in the III-Nitride family. DC and small signal HEMT electrical characteristics from submicron gate length HEMTs further elucidated the effect of the AlN barrier thickness on device performance.

Suppression of surface-originated gate lag by a dual-channel AlN/GaN high electron mobility transistor architecture

A dual-channel AlN/GaN high electron mobility transistor (HEMT) architecture is demonstrated that leverages ultra-thin epitaxial layers to suppress surface-related gate lag. Two high-density two-dimensional electron gas (2DEG) channels are utilized in an AlN/GaN/AlN/GaN heterostructure wherein the top 2DEG serves as a quasi-equipotential that screens potential fluctuations resulting from distributed surface and interface states. The bottom channel serves as the transistors modulated channel. Dual-channel AlN/GaN heterostructures were grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. HEMTs fabricated with 300 nm long recessed gates demonstrated a gate lag ratio (GLR) of 0.88 with no degradation in drain current after bias stressed in subthreshold. These structures additionally achieved small signal metrics ft/fmax of 27/46 GHz. These performance results are contrasted with the non-recessed gate dual-channel HEMT with a GLR of 0.74 and 82 mA/mm current collapse wi...

Effect of GaN buffer thickness on the electrical properties of RF-MBE grown AlGaN/GaN HEMTs on free-standing GaN substrates

We have previously demonstrated that excellent dc and RF properties can be obtained from AlGaN/GaN high electron mobility transistor (HEMT) structures grown by RF-plasma assisted molecular beam epitaxy (RF-MBE) on free-standing, bulk GaN substrates. For example, we have measured Hall mobilities as high as 1920 cm2/Vs on these structures, [1] and HEMTs fabricated on them exhibit excellent small-signal performance and output power densities of nearly 5 W/mm at 10 GHz [2]. The device heterostructures consist of 1-μm beryllium-doped GaN isolation layers grown directly on semi-insulating, freestanding GaN:Fe substrates, followed by 500-nm unintentionally doped (UID) GaN buffers and capped by 250-Å Al0.3Ga0.7N barriers. While the dependence of interdevice leakage on compensation-doped profiles in RF-MBE grown AlGaN/GaN HEMTs on SiC has been investigated [3], to date there has been no systematic study of the effect on device properties caused by the proximity of the two dimensional electron gas to the compensation-doped isolation layer in AlGaN/GaN heterostructures grown on bulk GaN substrates.

Modeling the small signal characteristics of an ALD Al 2 O 3 insulated-gate AlN/GaN high electron mobility transistor

The ultra-thin barrier AlN/GaN high electron mobility transistor (HEMT) has recently demonstrated over 2 A/mm dc drain current and an appreciable 480 mS/mm transconductance [1] owing to the high mobility and enhanced polarization effects of the binary AlN/GaN heterointerface [2]. Due to a large lattice mismatch the strained AlN barrier is limited in thickness to less than 7 nm. Therefore, in order to mitigate high gate currents in an AlN/GaN HEMT a gate insulator is essential. Atomic Layer Deposited (ALD) oxides offer a means to accurately control oxide thickness down to the angstrom scale while simultaneously achieving a dense, high dielectric constant oxide. In this work we have grown, fabricated, and analyzed dc and RF characteristics of an ALD Al2O3 gate-insulated, 3.5 nm thick barrier AlN/GaN HEMT. In particular, small signal modeling was performed using standard ColdFET and on-state techniques which allow the extraction of bonding pad and parasitic element influences on RF operation. The extraction of these elements becomes even more necessary for either of two conditions: 1) the electron-beam defined sub-micron gate length reduces gate capacitance and causes the bonding pad capacitance to dominate RF operation or 2) the presence of high-k Al2O3 in the access and inter-pad regions of the device further increases parasitic and pad-to-pad capacitance. Both conditions occur for our devices.

Self-aligned ALD AlO x T-gate footprint insulator for gate leakage current suppression in SiN x passivated AlGaN/GaN HEMTs

GaN based transistors have recently demonstrated record high RF output power density at 4 and 8 GHz through refinements in materials growth and improved device design involving field plates [1]. However, to push GaN HEMT performance into the mm-wavelength frequency range (30–300 GHz) will require the use of other innovative solutions that maintain high device breakdown voltages while avoiding the incurrence of additional parasitic capacitance. High-κ dielectric gate insulators can reduce gate leakage current and improve breakdown voltage [2], but often require a blanket deposition that may degrade frequency performance due to fringing capacitance or trapping/charging effects in the gate-source and gate-drain access regions [3]. Removing the high-K dielectric in the access regions with plasma etching can present challenges in reproducibility or induce crystalline damage/surface modification that can degrade 2DEG channel properties. The devices in this study demonstrate a novel process methodology that allows for a self-aligned ALD AlOx gate insulator to be placed directly under the T-gate footprint and not in the access regions. After gate contact formation, standard PECVD SiNx passivation can be applied just as in a conventional HEMT process. Preliminary devices show promising results that this technique can be used to achieve high power mm-wave performance.