High Al-content AlGaN/GaN MODFETs for ultrahigh performance

The use of an AlGaN layer with high Al mole-fraction is proposed to increase the equivalent figures of merit of the AlGaN/GaN MODFET structure. It is shown that the room temperature mobility has little degradation with increasing Al mole-fraction up to 50%. 0.7-μm gate-length Al_0.5Ga_0.5N/GaN MODFETs by optical lithography exhibit a current density of 1 A/mm and three-terminal breakdown voltages up to 200 V. These devices on sapphire substrates without thermal management also show CW power densities of 2.84 and 2.57 W/mm at 8 and 10 GHz, respectively, representing a marked performance improvement for GaN-based FETs     

Effects of Sc2O3 and MgO passivation layerson the output power of AlGaN/GaN HEMTs

The low temperature (100°C) deposition of Sc₂O₃ or MgO layers is found to significantly increase the output power of AlGaN/GaN HEMTs. At 4 GHz, there was a better than 3 dB increase in output power of 0.5×100 μm² HEMTs for both types of oxide passivation layers. Both Sc₂ O₃ and MgO produced larger output power increases at 4 GHz than conventional plasma-enhanced chemical vapor deposited (PECVD) SiN_x passivation which typically showed ⩽2 dB increase on the same types of devices. The HEMT gain also in general remained linear over a wider input power range with the Sc₂O₃ or MgO passivation. These films appear promising for reducing the effects of surface states on the DC and RF performance of AlGaN/GaN HEMTs     

Trapping effects and microwave power performance in AlGaN/GaN HEMTs

The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 μm gate length, an f_T of 30 GHz and an f_max of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz     

Short channel AlGaN/GaN MODFET's with 50-GHz fT and1.7-W/mm output-power at 10 GHz

A thin barrier-donor layer of 200 Å was used to increase the active input capacitance and improve the extrinsic current-gain cutoff frequency (f_t) of short-gate-length AlGaN/GaN MODFETs. 0.2-μm gate-length devices fabricated on such an epi-structure with sheet carrier density of ~8×10¹² cm^-2 and mobility of 1200 cm²/Vs showed a record f_t of 50 GHz for GaN based FETs. High channel saturation current and transconductance of 800 mA/mm and 240 mS/mm respectively were also achieved along with breakdown voltages of 80 V per μm gate-drain spacing. These excellent characteristics translated into a CW output power density of 1.7 W/mm at 10 GHz, exceeding previous record for a solid-state HEMT     

Performance comparison of wide bandgap semiconductor rf power devices

Impressive radio frequency power performance has been demonstrated by three radically different wide bandgap semiconductor power devices, SiC metal semiconductor field effect transistors (MESFETs), SiC static induction transistors (SITs), and AlGaN heterojunction field effect transistors (HFETs). AlGaN HFETs have achieved the highest f_max of 97 GHz. 4H-SiC MESFETs have achieved the highest power densities, 3.3 W/mm at 850 MHz (CW) and at 10 GHz (pulsed). 4H-SiC SITs have achieved the highest output power, 450 W (pulsed) at 600 MHz and 38 W (pulsed) at 3 GHz. Moreover, a one kilowatt, 600 MHz SiC power module containing four multi-cell SITs with a total source periphery of 94.5 cm has been demonstrated.     

Metamorphic AlInAs/GaInAs HEMTs on GaAs Substrates by MOCVD

Metamorphic AlInAs/GaInAs high-electron mobility transistors with very good device performance have been grown by metal-organic chemical vapor deposition (MOCVD), with the introduction of an effective multistage buffering scheme. Measured room-temperature Hall mobilities of the 2-DEG were over 8000 cm²/V ldr s with sheet carrier densities larger than 4 times 10¹² cm^-2. Transistors with 1-mum gate length exhibited transconductance up to 626 mS/mm. The unity current gain cutoff frequency f_T and the maximum oscillation frequency f_max were 39.1 and 71 GHz, respectively. These results are very encouraging toward the manufacturing of metamorphic devices on GaAs substrates by MOCVD.     

Silicon carbide high-power devices

In recent years, silicon carbide has received increased attention because of its potential for high-power devices. The unique material properties of SiC, high electric breakdown field, high saturated electron drift velocity, and high thermal conductivity are what give this material its tremendous potential in the power device arena. 4H-SiC Schottky barrier diodes (1400 V) with forward current densities over 700 A/cm² at 2 V have been demonstrated. Packaged SITs have produced 57 W of output power at 500 MHz, SiC UMOSFETs (1200 V) are projected to have 15 times the current density of Si IGBTs (1200 V). Submicron gate length 4H-SiC MESFETs have achieved f_max=32 GHz, f_T=14.0 GHz, and power density=2.8 W/mm @ 1.8 GHz. The performances of a wide variety of SiC devices are compared to that of similar Si and GaAs devices and to theoretically expected results     

Delta-doped AlGaN/AlN/GaN microwave HFETs grown by metalorganicchemical vapour deposition

The performance of an innovative delta-doped AlGaN/AlN/GaN heterojunction field-effect transistor (HFET) structure is reported. The epitaxial heterostructures were grown on semi-insulating SiC substrates by low-pressure metalorganic chemical vapour deposition. These structures exhibit a maximum carrier mobility of 1058 cm²/V s and a sheet carrier density of 2.35×10¹³ cm^-2 at room temperature, corresponding to a large n_s μ_n product of 2.49×10¹⁶ V s. HFET devices with 0.25 μm gate length were fabricated and exhibited a maximum current density as high as 1.5 A/mm (at V_G=+1 V) and a peak transconductance of g_m=240 mS/mm. High-frequency device measurements yielded a cutoff frequency of f_t≃50 GHz and maximum oscillation frequency f_max≃130 GHz     

Carrier transport related analysis of high-power AlGaN/GaN HEMTstructures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm²/V/s and a sheet concentration of 9×10¹² cm at 300 K (7900 cm²/V/s and 8×10¹² cm^-2 at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n_SdH=7.8×10¹² cm^-2 and a high quantum scattering lifetime of τ_q=1.5×10^-13 s at 4.2 K compared to n_SdH=8.24×10¹² cm^-2 and τ_q=1.72×10^-13 s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance     

Large periphery high-power AlGaN/GaN metal-oxide-semiconductorheterostructure field effect transistors on SiC with oxide-bridging

We report on AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor (HFET) over SiC substrates with peripheries from 0.15 to 6 mm. These multigate devices with source interconnections were fabricated using a novel oxide-bridging approach. The saturation current was as high as 5.1 A for a 6 mm wide device with a gate leakage of 1 μA/cm² for 1.5 μm gate length in a 5 μm source-drain opening. The cutoff frequency of around 8 GHz was practically independent of the device periphery. Large-signal output rf-power as high as 2.88 W/mm was measured at 2 GHz. Both the saturation current and the rf-power scaled nearly linearly with the gate width     

Power performance of InP-based single and double heterojunctionbipolar transistors

The microwave and power performance of fabricated InP-based single and double heterojunction bipolar transistors (HBTs) is presented. The single heterojunction bipolar transistors (SHBTs), which had a 5000 Å InGaAs collector, had BV_CEO of 7.2 V and J_Cmax of 2×10⁵ A/cm². The resulting HBTs with 2×10 μm² emitters produced up to 1.1 mW/μm2 at 8 GHz with efficiencies over 30%. Double heterojunction bipolar transistors (DHBTs) with a 3000-Å InP collector had a BV_CEO of 9 V and J_{c max} of 1.1×10⁵ A/cm², resulting in power densities up to 1.9 mW/μm² at 8 GHz and a peak efficiency of 46%. Similar DHBTs with a 6000 Å InP collector had a higher BV_CEO of 18 V, but the J _{c max} decreased to 0.4×10⁵ A/cm² due to current blocking at the base-collector junction. Although the 6000 Å InP collector provided higher f_max and gain than the 3000 Å collector, the lower J_{c max} reduced its maximum power density below that of the SHBT wafer. The impact on power performance of various device characteristics, such as knee voltage, breakdown voltage, and maximum current density, are analyzed and discussed     

An Al0.3Ga0.7N/GaN undoped channelheterostructure field effect transistor with Fmax of 107 GHz

An Al_0.3Ga_0.7N/GaN heterostructure field effect transistor (HFET) grown on semi-insulating SiC with an 0.2-μm gate length is reported. A source-drain ohmic contact resistance of 0.15-Ω-mm was achieved through the use of high Al content and high n-type doping (1E19 cm^-3) in the AlGaN donor layer and optimized metallization procedures. We obtained a maximum transconductance of 260 mS/mm, a saturated current density of 1.2 A/mm, and a maximum oscillation frequency in excess of 107 GHz in the devices. The results are one of the best achieved up to now, and they will open up the potential for the applications of AlGaN/GaN HFET's in high-power microwave radar, remote sensing, and communications     

Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm ²/Ns, respectively, For electron sheet density near 1×10¹³/cm², Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f_t of 106 GHz for 0.15 μm gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of ~4 W/mm, while large periphery devices have ~2 W/mm, both thermally limited. Performance, without and with Si₃N₄ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates     

2.1 A/mm current density AlGaN/GaN HEMT

The electrical performance of high current density AlGaN/GaN HEMTs is reported. 2 /spl times/ 75 /spl mu/m /spl times/ 0.7 /spl mu/m devices grown on sapphire substrates showed current densities up to 2.1 A/mm under 200 ns pulse condition. RF power measurements at 8 GHz and V/sub DS/=15 V exhibited a saturated output power of 3.66 W/mm with a 47.8% peak PAE.     

The effect of surface passivation on the microwave characteristicsof undoped AlGaN/GaN HEMTs

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 2×125×0.5 μm 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 μm gate length HEMT's on the same wafer. Finally, 4 GHz power sweep data for a 2×75×0.4 μm AlGaN/GaN HEMT on sapphire processed using the Si₃N₄ 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     

Ion-implanted high microwave power indium phosphide transistors

Encapsulated rapid thermal annealing (RTA) has been used in the fabrication of indium phosphide (InP) power metal-insulator-semiconductor field-effect transistors (MISFETs) with ion-implanted source, drain, and active channel regions. The MISFETs had a gate length of 1.4 μm. Six to ten gate fingers per device, with individual gate finger widths of 100 or 125 μm, were used to make MISFETs with total gate widths of 0.75, 0.8, or 1 mm. The source and drain contact regions and the channel region of the MISFETs were fabricated using silicon implants in semi-insulating InP at energies from 60 to 360 keV with doses from 1×10¹² to 5.6×10¹⁴ cm^-2. The implants were activated using RTA at 700°C for 30 s in N₂ or H₂ ambients using a silicon nitride encapsulant. The high-power, high-efficiency MISFETs were characterized at 9.7 GHz, and the output microwave power density for the RTA conditions used was as high as 2.4 W/mm. For a 1-W input at 9.7 GHz gains up to 3.7 dB were observed, with an associated power-added efficiency of 29%. The output power density was 70% greater than that reported for GaAs MESFETs     

Microwave performance of AlGaN/GaN inverted MODFET's

A continuous wave output power of 1.5 W/mm with a power added efficiency of 17.5% has been achieved at 4 GHz in inverted AlGaN/GaN MODFET's (IMODFET's) with 2 μm gate lengths and 78 μm gate widths. The current gain and available power gain cutoff frequencies were 6 and 11 GHz, respectively. We suggest that the input characteristics of GaN-based FET's play an important role in the output power that can be obtained. In the present devices, high transconductance, 100 mS/mm, retained over a 5 V input swing is thought to alleviate the limitations imposed by the input characteristics. Moreover, the buried AlGaN buffer layer is suggested as having assisted in the reduction of the output conductance which aids the power gain     

AlGaN/AlN/GaN high-power microwave HEMT

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 (~1 nm) maintains high mobility at high sheet charge densities by increasing the effective ΔE_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_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     

A Compact 6.5-W PHEMT MMIC Power Amplifier for Ku-Band Applications

A compact 6.5-W AlGaAs/InGaAs/GaAs PHEMT monolithic microwave integrated circuit (MMIC) power amplifier (PA) for Ku-band applications is proposed. This two-stage amplifier with chip size of 8.554mm² (3.64mmtimes2.35mm) is designed to fully match 50-Omega input and output impedance. Under 8V and 2000mA dc bias condition, the PA deliver 38.1dBm (6.5W) saturated output power, 10.5-dB small signal gain and peak power added efficiency of 24.6% from 13.6 to 14.2GHz. This MMIC also achieved the best power densities (760mW/mm²) at Ku band reported to date     

AlGaN/GaN MOS-HEMT With $hbox{HfO}_{2}$ Dielectric and $hbox{Al}_{2}hbox{O}_{3}$ Interfacial Passivation Layer Grown by Atomic Layer Deposition

We have developed a novel AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor using a stack gate HfO₂/Al₂O₃ structure grown by atomic layer deposition. The stack gate consists of a thin HfO₂ (30-A) gate dielectric and a thin Al₂O₃ (20- A) interfacial passivation layer (IPL). For the 50-A stack gate, no measurable C-V hysteresis and a smaller threshold voltage shift were observed, indicating that a high-quality interface can be achieved using a Al₂O₃ IPL on an AlGaN substrate. Good surface passivation effects of the Al₂O₃ IPL have also been confirmed by pulsed gate measurements. Devices with 1- mum gate lengths exhibit a cutoff frequency (fT) of 12 GHz and a maximum frequency of oscillation (f _MAX) of 34 GHz, as well as a maximum drain current of 800 mA/mm and a peak transconductance of 150 mS/mm, whereas the gate leakage current is at least six orders of magnitude lower than that of the reference high-electron mobility transistors at a positive gate bias.