The impact of surface states on the DC and RF characteristics of AlGaN/GaN HFETs
TL;DR: In this article, the authors show that the cause of current collapse is a charging up of a second virtual gate, physically located in the gate drain access region, thus acting as a negatively charged virtual gate.
Abstract: 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.
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07 Nov 2002
TL;DR: This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems of the AlGaN/GaN high-electron mobility transistor.
Abstract: Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.
1,849 citations
Cites methods from "The impact of surface states on the..."
...The charge distribution is then composed of the polarization dipole and an opposing dipole comprising of a surface hole gas and a two-dimensional electron gas (2DEG) at the heterointerface . In practice, experiments [ 4 ] have shown that the 2DEG is induced before a surface hole gas formation by the ionization of a deep surface donor resulting in a charge distribution, shown in Fig. 8(b)....
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07 Nov 2002
TL;DR: This paper reviews the various trapping phenomena observed in SiC- and GaN-based FETs that contribute to compromised power performance and the measurement techniques utilized to identify these traps.
Abstract: It is well known that trapping effects can limit the output power performance of microwave field-effect transistors (FETs). This is particularly true for the wide bandgap devices. In this paper we review the various trapping phenomena observed in SiC- and GaN-based FETs that contribute to compromised power performance. For both of these material systems, trapping effects associated with both the surface and with the layers underlying the active channel have been identified. The measurement techniques utilized to identify these traps and some of the steps taken to minimize their effects, such as modified buffer layer designs and surface passivation, are described. Since similar defect-related phenomena were addressed during the development of the GaAs technology, relevant GaAs work is briefly summarized.
466 citations
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...This has been done as a function of frequency [59]–[61] and transition frequencies varying over a very large range (10 Hz to 10 GHz) were observed....
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TL;DR: In this paper, a novel heterojunction AlGaN/AlN/GaN high-electron mobility transistor (HEMT) is discussed, where the insertion of the very thin AlN interfacial layer (/spl sim/1 nm) maintains high mobility at high sheet charge densities by increasing the effective /spl Delta/E/sub C/ and decreasing alloy scattering.
Abstract: In this letter, a novel heterojunction AlGaN/AlN/GaN high-electron mobility transistor (HEMT) is discussed. Contrary to normal HEMTs, the insertion of the very thin AlN interfacial layer (/spl sim/1 nm) maintains high mobility at high sheet charge densities by increasing the effective /spl Delta/E/sub C/ and decreasing alloy scattering. Devices based on this structure exhibited good DC and RF performance. A high peak current 1 A/mm at V/sub GS/=2 V was obtained and an output power density of 8.4 W/mm with a power added efficiency of 28% at 8 GHz was achieved.
418 citations
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...to eliminate DC to RF dispersion [9], [10]....
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TL;DR: In this paper, the fabrication and electrical characteristics of highmobility field effect transistors (FETs) using ZnO nanorods were reported, and the role of the polymer coating in the enhancement of the devices was discussed.
Abstract: We report on fabrication and electrical characteristics of high-mobility field-effect transistors (FETs) using ZnO nanorods. For FET fabrications, single-crystal ZnO nanorods were prepared using catalyst-free metalorganic vapor phase epitaxy. Although typical ZnO nanorod FETs exhibited good electrical characteristics, with a transconductance of ∼140nS and a mobility of 75cm2∕Vs, the device characteristics were significantly improved by coating a polyimide thin layer on the nanorod surface, exhibiting a large turn-ON/OFF ratio of 104–105, a high transconductance of 1.9μS, and high electron mobility above 1000cm2∕Vs. The role of the polymer coating in the enhancement of the devices is also discussed.
370 citations
TL;DR: In this paper, the authors present a methodology to study trapping characteristics in GaN HEMTs that is based on current-transient measurements and identify several traps inside the AlGaN barrier layer or at the surface close to the gate edge and in the GaN buffer.
Abstract: Trapping is one of the most deleterious effects that limit performance and reliability in GaN HEMTs. In this paper, we present a methodology to study trapping characteristics in GaN HEMTs that is based on current-transient measurements. Its uniqueness is that it is amenable to integration with electrical stress experiments in long-term reliability studies. We present the details of the measurement and analysis procedures. With this method, we have investigated the trapping and detrapping dynamics of GaN HEMTs. In particular, we examined layer location, energy level, and trapping/detrapping time constants of dominant traps. We have identified several traps inside the AlGaN barrier layer or at the surface close to the gate edge and in the GaN buffer.
370 citations
Cites background from "The impact of surface states on the..."
...Although fast components have also been reported [9], [10], the recovery time from current collapse is generally long (> 100 s or even a few days) [6], [7], [11], [12]....
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...On the other hand, in the OFF-state under high VDG, surface states between the gate and the drain can trap electrons that tunnel from the gate metal [6]–[8]....
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...These trapped electrons change the electrostatics such that they deplete the channel carrier concentration in the extrinsic drain, resulting in a reduction of the drain current [6]....
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References
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TL;DR: In this article, the authors investigated the role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces.
Abstract: Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15
2,581 citations
"The impact of surface states on the..." refers background in this paper
...Surface states are thought to be unavoidable in this material system due to the strong polarization fields [ 5 ]‐[8]....
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...It is well known that spontaneous and piezoelectric polarization effects lead to charge sheets of opposite polarity at the top and bottom surfaces of the AlGaN layer in an AlGaN/GaN heterostructure [ 5 ]....
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TL;DR: In this paper, the origin of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructure field effect transistors is examined theoretically and experimentally.
Abstract: The origin of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructure field effect transistors is examined theoretically and experimentally. Based on an analysis of the electrostatics, surface states are identified as an important source of electrons. The role of the polarization-induced dipole is also clarified. Experimental Hall data for nominally undoped Al0.34Ga0.66N/GaN structures indicate that ∼1.65 eV surface donors are the actual source of the electrons in the 2DEG, which forms only when the barrier thickness exceeds 35 A.
1,015 citations
TL;DR: In this paper, the authors reported the highest reported microwave power density for undoped sapphire substrated AlGaN/GaN HEMT's on the same wafer.
Abstract: Surface passivation of undoped AlGaN/CaN HEMT's reduces or eliminates the surface effects responsible for limiting both the RF current and breakdown voltages of the devices. Power measurements on a 2/spl times/125/spl times/0.5 /spl mu/m AlGaN/GaN sapphire based HEMT demonstrate an increase in 4 GHz saturated output power from 1.0 W/mm [36% peak power-added efficiency (PAE)] to 2.0 W/mm (46% peak PAE) with 15 V applied to the drain in each case. Breakdown measurement data show a 25% average increase in breakdown voltage for 0.5 /spl mu/m gate length HEMT's on the same wafer. Finally, 4 GHz power sweep data for a 2/spl times/75/spl times/0.4 /spl mu/m AlGaN/GaN HEMT on sapphire processed using the Si/sub 3/N/sub 4/ passivation layer produced 4.0 W/mm saturated output power at 41% PAE (25 V drain bias). This result represents the highest reported microwave power density for undoped sapphire substrated AlGaN/GaN HEMT's.
752 citations
"The impact of surface states on the..." refers result in this paper
...Similar improvements were recently reported [ 14 ]....
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TL;DR: In this article, the formation of the 2DEG in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated.
Abstract: 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.
405 citations
TL;DR: In this paper, a 0.25 μm gate length AlGaN/GaN heterostructure field effect transistor (HFET) with a maximum extrinsic transconductance of 27 mS/mm (at room temperature) limited by the source series resistance was fabricated.
Abstract: We fabricated a 0.25 μm gate length AlGaN/GaN heterostructure field effect transistor (HFET) with a maximum extrinsic transconductance of 27 mS/mm (at room temperature) limited by the source series resistance. The device exhibited an excellent pinch‐off and a low parasitic output conductance in the saturation regime. We measured the cutoff frequency fT and the maximum oscillation frequency fmax as 11 and 35 GHz, respectively. These values are superior to the highest reported values for field effect transistors based on other wide band‐gap semiconductors such as SiC. These results demonstrate an excellent potential of AlGaN/GaN HFETs for microwave and millimeter wave applications.
342 citations