| 1. |
- Axelsson, Olle, 1986, et al.
(author)
-
Impact of Trapping Effects on the Recovery Time of GaN Based Low Noise
- 2016
-
In: IEEE Microwave and Wireless Components Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-1764 .- 1531-1309. ; 26:1, s. 31-33
-
Journal article (peer-reviewed)abstract
- This study investigates recovery time of the gain of AlGaN/GaN HEMT based low noise amplifiers (LNA) after an input overdrive pulse. Three LNAs, fabricated in two commercial MMIC processes and a Chalmers in-house process, are evaluated. The Chalmers process has an unintentionally doped buffer instead of the intentional Fe doping of the buffer which is standard in commercial GaN HEMT technologies. It is shown that the LNAs from the two commercial processes experience a severe drop in gain after input overdrive pulses higher than 28 dBm, recovering over a duration of around 20 ms. In contrast the LNA fabricated in-house at Chalmers experienced no visible effects up to an input power of 33 dBm. These results have impact for radar and electronic warfare receivers, which need to be operational immediately after an overdrive pulse. The long time constants suggest that these effects are due to trapping in the transistors with the Fe doped buffer playing an important role.
|
|
| 2. |
|
|
| 3. |
- Westlund, Andreas, 1985, et al.
(author)
-
Graphene self-switching diodes as zero-bias microwave detectors
- 2015
-
In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:9, s. 093116-
-
Journal article (peer-reviewed)abstract
- Self-switching diodes (SSDs) were fabricated on as-grown and hydrogen-intercalated epitaxial graphene on SiC. The SSDs were characterized as zero-bias detectors with on-wafer measurements from 1 to 67 GHz. The lowest noise-equivalent power (NEP) was observed in SSDs on the hydrogen-intercalated sample, where a flat NEP of 2.2 nW/Hz½ and responsivity of 3.9 V/W were measured across the band. The measured NEP demonstrates the potential of graphene SSDs as zero-bias microwave detectors.
|
|
| 4. |
- Winters, Michael, 1986, et al.
(author)
-
Hysteresis modeling in graphene field effect transistors
- 2015
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 117:7, s. Art. no, 074501-
-
Journal article (peer-reviewed)abstract
- Graphene field effect transistors with an Al2O3 gate dielectric are fabricated on H-intercalated bilayer graphene grown on semi-insulating 4H-SiC by chemical vapour deposition. DC measurements of the gate voltage nu(g) versus the drain current i(d) reveal a severe hysteresis of clockwise orientation. A capacitive model is used to derive the relationship between the applied gate voltage and the Fermi energy. The electron transport equations are then used to calculate the drain current for a given applied gate voltage. The hysteresis in measured data is then modeled via a modified Preisach kernel.
|
|
| 5. |
- Axelsson, Olle, 1986, et al.
(author)
-
Application Relevant Evaluation of Trapping Effects in AlGaN/GaN HEMTs With Fe-Doped Buffer
- 2016
-
In: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 63:1, s. 326-332
-
Journal article (peer-reviewed)abstract
- This paper investigates the impact of different iron (Fe) buffer doping profiles on trapping effects in microwave AlGaN/gallium nitride (GaN) high electron mobility transistors (HEMTs). We characterize not only the current collapse due to trapping in the buffer, but also the recovery process, which is important in the analysis of suitable linearization schemes for amplitude modulated signals. It is shown that the simple pulsed dc measurements of current transients can be used to investigate transient effects in the RF power. Specifically, it is revealed that the design of the Fe-doping profile in the buffer greatly influences the recovery time, with the samples with lower Fe concentration showing slower recovery. In contrast, traditional indicators, such as S-parameters and dc as well as pulsed $I$-$V$ characteristics, show very small differences. An analysis of the recovery shows that this effect is due to the presence of two different detrapping processes with the same activation energy (0.6 eV) but different time constants. For highly doped buffers, the faster process dominates, whereas the slower process is enhanced for less doped buffers.
|
|
| 6. |
- Axelsson, Olle, 1986, et al.
(author)
-
The Effect of Forward Gate Bias Stress on the Noise Performance of Mesa Isolated GaN HEMTs
- 2015
-
In: IEEE Transactions on Device and Materials Reliability. - : Institute of Electrical and Electronics Engineers (IEEE). - 1530-4388 .- 1558-2574. ; 15:1, s. 40-46
-
Journal article (peer-reviewed)abstract
- This study investigates degradation of gallium nitride (GaN) high-electron mobility transistor (HEMT) noise performance after both dc and RF stress with forward gate current. The results are used to facilitate optimization of the robustness of GaN low-noise amplifiers (LNAs). It is shown that forward biasing the gate of a GaN HEMT results in permanent degradation of noise performance and gate current leakage, without affecting S-parameters and drain current characteristics. The limit of safe operation of the 2 x 50 mu m devices in this study is found to be between 10 and 20 mW dissipated in the gate diode for both dc and RF stress. We propose that degradation could be caused by excessive leakage through the mesa sidewalls at the edges of each gate finger. Circuit simulations may be used together with device robustness rating to optimize LNAs for maximum input power tolerance. Using a resistance in the gate biasing network of 10 k Omega, it is estimated that an LNA utilizing a 2 x 50 mu m device could withstand input power levels up to 33 dBm without degradation in noise performance.
|
|
| 7. |
- Bergsten, Johan, 1988, et al.
(author)
-
AlGaN/GaN high electron mobility transistors with intentionally doped GaN buffer using propane as carbon precursor
- 2016
-
In: Japanese Journal of Applied Physics. - : IOP Publishing. - 1347-4065 .- 0021-4922. ; 55:5
-
Journal article (peer-reviewed)abstract
- AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on a heterostructure grown by metalorganic chemical vapor deposition using an alternative method of carbon (C) doping the buffer are characterized. C-doping is achieved by using propane as precursor, as compared to tuning the growth process parameters to control C-incorporation from the gallium precursor. This approach allows for optimization of the GaN growth conditions without compromising material quality to achieve semi-insulating properties. The HEMTs are evaluated in terms of isolation and dispersion. Good isolation with OFF-state currents of 2 x 10(-6)A/mu m, breakdown fields of 70V/mu m, and low drain induced barrier lowering of 0.13mV/V are found. Dispersive effects are examined using pulsed current-voltage measurements. Current collapse and knee walkout effects limit the maximum output power to 1.3W/mm. With further optimization of the C-doping profile and GaN material quality this method should offer a versatile approach to decrease dispersive effects in GaN HEMTs. (C) 2016 The Japan Society of Applied Physics
|
|
| 8. |
- Bergsten, Johan, 1988, et al.
(author)
-
Electron Trapping in Extended Defects in Microwave AlGaN/GaN HEMTs with Carbon-Doped Buffers
- 2018
-
In: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1557-9646 .- 0018-9383. ; 65:6, s. 2446-2453
-
Journal article (peer-reviewed)abstract
- This paper investigates AlGaN/GaN high-electron mobility transistors (HEMTs) fabricated on epistructures with carbon (C)-doped buffers. Metalorganic chemical vapor deposition is used to grow two C-doped structures with different doping profiles, using growth parameters to change the C incorporation. The C concentration is low enough to result in n-type GaN. Reference devices are also fabricated on a structure using iron (Fe) as dopant, to exclude any process related variations and provide a relevant benchmark. All devices exhibit similar dc performance. However, pulsed I - V$ measurements show extensive dispersion in the C-doped devices, with values of dynamic R-mathrm-scriptscriptstyle ON 3 -4 times larger than in the dc case. Due to the extensive trapping, the devices with C-doped buffers can only supply about half the output power of the Fe-doped sample, 2.5 W/mm compared to 4.8 W/mm at 10 GHz. In drain current transient measurements, the trap filling time is varied, finding large prevalence of trapping at dislocations for the C-doped samples. Clusters of C around the dislocations are suggested to be the main cause for the increased dispersion.
|
|
| 9. |
- Bergsten, Johan, 1988, et al.
(author)
-
Low resistive Au-free, Ta-based, recessed ohmic contacts to InAlN/AlN/GaN heterostructures
- 2015
-
In: Semiconductor Science and Technology. - : IOP Publishing. - 1361-6641 .- 0268-1242. ; 30:10, s. 105034-
-
Journal article (peer-reviewed)abstract
- The formation of recess etched Au-free ohmic contacts to an InAlN/AlN/GaN heterostructure is investigated. A Ta/Al/Ta metal stack is used to produce contacts with contact resistance (R-c) as low as 0.14 Omega mm. It is found that R-c decreases with increasing recess depth until the InAlN barrier is completely removed. For even deeper recesses R-c remains low but requires annealing at higher temperatures for contact formation. The lowest R-c is found for contacts where the recess etch has stopped just above the 2D electron gas channel. At this depth the contacts are also found to be less sensitive to other process parameters, such as anneal duration and temperature. An optimum bottom Ta layer thickness of 5-10 nm is found. Two reliability experiments preliminary confirm the stability of the recessed contacts.
|
|
| 10. |
- Bergsten, Johan, 1988, et al.
(author)
-
Performance Enhancement of Microwave GaN HEMTs Without an AlN-Exclusion Layer Using an Optimized AlGaN/GaN Interface Growth Process
- 2016
-
In: IEEE Transactions on Electron Devices. - : Institute of Electrical and Electronics Engineers (IEEE). - 1557-9646 .- 0018-9383. ; 63:1, s. 333-338
-
Journal article (peer-reviewed)abstract
- The impact of the sharpness of the AlGaN/GaN interface in high-electron mobility transistors (HEMTs) is investigated. Two structures, one with an optimized AlGaN/GaN interface and another with an unoptimized, were grown using hot-wall metal-organic chemical vapor deposition. The structure with optimized sharpness of the interface shows electron mobility of 1760 cm(2)/V . s as compared with 1660 cm(2)/V . s for the nonoptimized interface. Gated Hall measurements indicate that the sharper interface maintains higher mobility when the electrons are close to the interface compared with the nonoptimized structure, indicating less scattering due to alloy disorder and interface roughness. HEMTs were processed and evaluated. The higher mobility manifests as lower parasitic resistance yielding a better dc and high-frequency performance. A small-signal equivalent model is extracted. The results indicate a lower electron penetration into the buffer in the optimized sample. Pulsed-IV measurements imply that the sharper interface provides less dispersive effects at large drain biases. We speculate that the mobility enhancement seen AlGaN/AlN/GaN structures compared with the AlGaN/GaN case is not only related to the larger conduction band offset but also due to a more welldefined interface minimizing scattering due to alloy disorder and interface roughness.
|
|
