Ramakrishna Vetury

The impact of surface states on the DC and RF characteristics of AlGaN/GaN HFETs

GaN based HFETs are of tremendous interest in applications requiring high power at microwave frequencies. Although excellent current-voltage (I-V) characteristics and record high output power densities at microwave frequencies have been achieved, the origin of the 2DEG and the factors limiting the output power and reliability of the devices under high power operation remain uncertain. Drain current collapse has been the major obstacle in the development of reliable high power devices. We show that the cause of current collapse is a charging up of a second virtual gate, physically located in the gate drain access region. Due to the large bias voltages present on the device during a microwave power measurement, surface states in the vicinity of the gate trap electrons, thus acting as a negatively charged virtual gate. The maximum current available from a device during a microwave power measurement is limited by the discharging of this virtual gate. Passivated devices located adjacent to unpassivated devices on the same wafer show almost no current collapse, thus demonstrating that proper surface passivation prevents the formation of the virtual gate. The possible mechanisms by which a surface passivant reduces current collapse and the factors affecting reliability and stability of such a passivant are discussed.

Anisotropic epitaxial lateral growth in GaN selective area epitaxy

Epitaxial lateral mask overgrowth which occurs during GaN selective epitaxy has been studied using linear mask features. The lateral growth varies between its maximum and minimum over a 30° angular span and exhibits hexagonal symmetry. Vertical growth follows an opposite trend, with lateral growth maxima, and vertical growth minima occurring for lines parallel to the GaN 〈10•0〉. Large variations in the lateral growth are also obtained through variations in the growth temperature and NH3 flow. Under proper growth conditions, lateral to vertical growth rate ratios of up to 4.1 can be achieved, resulting in significant lateral mask overgrowth and coalescence of features without excessive growth times.

POLARIZATION-INDUCED CHARGE AND ELECTRON MOBILITY IN ALGAN/GAN HETEROSTRUCTURES GROWN BY PLASMA-ASSISTED MOLECULAR-BEAM EPITAXY

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm2/V s (T=13 K) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012 cm−2.

Deep traps in AlGaN/GaN heterostructures studied by deep level transient spectroscopy: Effect of carbon concentration in GaN buffer layers

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed DLTS measurements on the two SBDs show that (i) different trap species are seen in the two SBDs: electron traps Ax (0.9 eV), A1 (0.99 eV), and A2 (1.2 eV), and a holelike trap H1 (1.24 eV) in the low-[C] SBD; and electron traps A1, A2, and A3 (∼1.3 eV), and a holelike trap H2 (>1.3 eV) in the high-[C] SBD; (ii) for...

High-transconductance self-aligned AlGaN/GaN modulation-doped field-effect transistors with regrown ohmic contacts

Self-aligned AlGaN/GaN modulation-doped field-effect transistors (MODFETs) with high transconductances have been demonstrated on a sapphire substrate. Source and drain were selectively regrown with ∼1700 A of n-GaN adjacent to the gate electrode. Source resistance was reduced to 0.95 Ω mm from 1.4 to 1.8 Ω mm with conventional GaN-based MODFETs. These self-aligned devices show a record high value of extrinsic transconductance ∼400 mS/mm for AlGaN/GaN MODFETs with a gate length of 1.2 μm.

Analysis of the residual stress distribution in AlGaN/GaN high electron mobility transistors

A comparative analysis of the residual stress distributions across the conductive channel of Ga-face AlGaN/GaN high electron mobility transistors (HEMTs) is presented. Stress was measured by means of micro-Raman spectroscopy and micro-photoluminescence (PL). Raman measurements probed the volume average of the stress through the GaN layer whereas the stress near the GaN surface (AlGaN/GaN heterointerface) was acquired via PL. By combining Raman, PL, and x-ray diffraction, a self-consistent method was developed to accurately determine the variation in magnitude of stress throughout the thickness of the GaN layer. Based on this framework, it is observed in AlGaN/GaN HEMTs that a depth variation in the GaN residual stress occurs near the gate and ohmic electrodes. At these regions, the stress near the AlGaN/GaN interface (or GaN surface) exhibits a tensile shift compared to the stress averaged through the entire thickness of GaN. Across the conductive channel (away from the metal pads), the bulk average stres...

Direct measurement of gate depletion in high breakdown (405 V) AlGaN/GaN heterostructure field effect transistors

A new measurement technique that directly reveals the extent of the gate depletion region in FETs by using floating Schottky gates as potential probes is presented. Measurements on GaN heterojunction field heterojunction field effect transistors (HFETs) show that large extension of the depletion width is responsible for the high dc breakdown voltages in these HFETs.

Broadband GaAs MESFET and GaN HEMT resistive feedback power amplifiers

We report 0.2 to 6-GHz MMIC power amplifiers with 12-dB gain, over 23-dBm output power, and more than 25% power-added efficiency (PAE) in a GaAs MESFET technology offering 18 GHz f/sub /spl tau// and 12-V breakdown. These circuits have gain-bandwidth products of /spl sim/1.3/spl middot/f/sub /spl tau// and are more efficient than distributed power amplifiers. A first demonstration of similar circuits in GaN/AlGaN HEMT technology yielded 11-dB gain, 0.2 to 7.5-GHz bandwidth amplifiers with over 31.5-dBm output power and up to 15% PAE. With improved devices and models we expect significantly higher power from the GaN HEMT circuits.

History of GaN: High-Power RF Gallium Nitride (GaN) from Infancy to Manufacturable Process and Beyond

In the early 1990s, gallium nitride (GaN) was deemed an excellent, next generation, semiconductor material for high power/high frequency transistors based on the material parameters of bandgap, electron mobility, and saturated electron velocity (Figure 1). The lack of bulk GaN source material led to the need for GaN growth on mismatched substrates such as Si, SiC and sapphire, but fundamental material development controlled the pace of maturation of GaN technology for both electronic and optoelectronic applications [1]. The development of GaN for RF electronics was significantly aided by the intense development that occurred in the race to first production of blue and, eventually, white light-emitting diodes (LEDs). Ultimately, advancements in the growth of device-grade aluminum gallium nitride (AlGaN)/GaN

Thermometry of AlGaN/GaN HEMTs Using Multispectral Raman Features

In this paper, we utilize micro-Raman spectroscopy to measure temperature and stress in state-of-the-art AlGaN/GaN HEMTs. A rigorous discussion on the physical accuracy, precision, and precautions for diverse Raman thermometry methods is developed. Thermometry techniques utilizing shifts in a single Raman Stokes peak position underpredict the channel temperature due to induction of operational thermoelastic stress in operating devices. Utilizing the change in phonon linewidth by employing a proper reference condition gives true temperature results. Making use of frequency shifts in both the E2(high) and A1(LO) phonon modes offers accurate and time-efficient means to determine the state of temperature and thermal stress in operating AlGaN/GaN HEMTs presuming that linear relations between phonon frequencies and temperature/stress are well determined. Useful applications of this method such as monitoring stress in GaN wafers between fabrication steps and Raman thermography on AlGaN/GaN HEMTs are demonstrated.