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Open issues in GaN-based HEMTs: performances, parasitics and reliability
Full text linkAbstract
Gallium nitride based high electron mobility transistors (HEMTs) are excellent candidates for high frequency and power applications. Due to high breakdown field, mobility, saturation velocity and thermal conductivity of GaN-based materials, HEMTs may operate at voltage and temperature ranges far beyond conventional semiconductor as Si, GaAs or InP; they also have a Baliga figure of merit many times higher and a lower resistance and hence reduced switching times and losses leading to improved effciency. Still, they are affected by (i) parasitics phenomena and (ii) reliability issues: defects and dislocations may induce high leakage currents, kink effect and soft breakdown, while, in the reliability field, hot electrons, high electric fields and power are still under investigation.
In the first part of this work parasitics have been investigated. In particular, great interest has been devoted to trapping phenomena, which mainly influence the on-resistance (Ron); transient and pulsed measurements help to extrapolate useful information as location in the epilayer structure, activation energy and cross section of the traps responsible of the Ron collapse; we also studied leakage phenomena, proving that both the phenomena can be significantly reduced with the introduction of a AlGaN back-barrier layer which, thanks to the additional band offset that prevents electrons from traveling and being trapped deep into the buffer. Finally, kink effect has been characterized; main results show it becomes almost negligible when a capping layer is grown over the AlGaN barrier and a semi-insulating substrate is used.
An extensive analysis of the electrical and optical properties of HEMTs biased in a non-destructive breakdown regime is the main topic of the second part. HEMTs can reach a sustainable breakdown condition with a Vg lower than the pinch-off voltage. Phenomena are mainly activated by two mechanisms, depending on the gate voltage applied: when Vg is close to the pinch-off, space charge injection of electrons occurs and a parasitic path between source and drain is formed; if a more negative voltage is applied, breakdown occurs due to electrons injection through the gate. Tests reveal that HEMTs can emit a weak electroluminescence (EL) signal: this is localized at the edge of the gate when a low current is
owing; it shifts to drain edge and the intensity reaches its maximum at higher Id when breakdown conditions are met. Moreover, the breakdown has a non-monotonic dependence from the temperature; this result confirms that two different mechanisms jointly interact at high voltage levels, one dominating on the other depending on the thermal condition. Single-heterostructure (SH) has a soft breakdown due to a poor ability to confine electrons into the channel and the consequent punch-through, independently from the gate to drain distance (Lgd). Many approaches have been successfully tested in order to improve the breakdown voltage (BV): GaN buffer doped with C or Fe, application of double-heterostructure (DH) epitaxy and devices with an AlGaN back-barrier grown on a doped buffer. These solution efficiently increase the BV, which also becomes dependent from Lgd distance with a slope that ranges from 30 V/um to 50 V/um.
The third part deals with reliability issues. Results of accelerated life tests show that in SH devices a quick degradation of the electrical properties is visible in off-state even at low drain voltage biasing condition: the punch-through leakage path increases defects formation, causing a strong device degradation even in short life tests. DH devices present improved reliability due likely to (i) lower leakage currents (ii) less sub-surface DIBL (iii) higher breakdown values.
The last section is devoted to the NPI Project. The purpose of the third placement has been the analysis of the performances and of the reliability behavior of GH25 technology. DC measurements show that technology process is quite mature: low off-state and leakage currents, good output current and very few devices with non-standard behavior. Still, the devices suffer from kink effect as confirmed by pulsed measurements; moreover, pulsed characterization enlightens a consistent trapping phenomena, the current collapse being 30%, mainly related to traps under the gate into the buffer. Maximum gain available MAG analysis from RF tests reveals that the source terminated field plate (STFP) positively increases the gain, thanks to an extended depletion region that reduces current lag due surface effects. The application of the field plate brings an additional capacitance, affecting the cross-over frequency which shifts from 25GHz to 20GHz.
Current controlled breakdown measurements enlightened how, when a high Vd is applied, a parasitic paths between source and drain is formed due to sub-surface DIBL (punch-through). The critical voltage which the phenomena take place at depends from many factors: (i) the longer the gate drain distance is, the less the punch-through is likely to occur (ii) it shifts toward lower voltages when increasing Vg due to the reduction of the depletion region under the gate (iii) STFP seems to have no meaningful effects.
These results seems to be related with those obtained from off-state step stress, where a fast degradation of the gate takes place for Vd higher than 70 V until sub-surface DIBL occurs; when a parasitic source-drain channel is formed, the degradation rate reduces significantly because most of the current is sustained by the source. The only relevant visible change is the increase in off-state and leakage currents and parameters. A comparison with breakdown test results suggests that the cause may be the same described for current controlled breakdown.
Life tests have been carried out selecting three different biasing conditions (i) with high current and low field (Id = 660mA/mm, Vd = 10V), (ii) high field and low current (Id = 5mA/mm, Vd = 60V), and (iii) class A bias point (Id = 400mA/mm, Vd = 30V) at 423K to assess the reliability along the load line. Class A results show a fast degradation of the output current and a steep increase of on-resistance within 100 hours; similar results are visible when the sample is biased at high current and low voltage, even if at a much lower degree. When the device is biased at high voltage and low currents, only a small decrease of output characteristic is reported; on the other hand, both off-state and leakage currents significantly increase. The Class A condition is the worst working condition. The degradation can be caused (i) by high power and visible only when both high voltage and high current are applied (ii) by high temperature to which both the power dissipation and the high temperature jointly contribute. Additional tests at room temperature could help to understand the failure mechanisms
Evidence for breakdown luminescence in AlGaN/GaN HEMTs
No full textWith this paper we present a detailed analysis of the electrical and optical
properties of AlGaN/GaN HEMTs in the breakdown regime. By means of a wide set of
measurements carried out at different voltage and current conditions, we show that: (i) when
HEMTs are biased in pinch-off conditions, and in current-controlled mode, they can reach a
non-destructive breakdown condition; (ii) during non-destructive breakdown devices emit a weak
luminescence signal, in proximity of the drain edge; (iii) breakdown voltage (BDV) has a
non-monothonic dependence on the gate voltage level and on temperature
Degradation of AlInN/AlN/GaN High Electron Mobility Transistors under Proton Irradiation
No full textWe measured the effects of 3-MeV proton irradiation on
AlInN/AlIn/GaN HEMTs. After fluences larger or equal to
1013 p/cm2, the devices experience threshold voltage shifts and
transconductance degradation, visible both in DC and pulsed
measurements, within the range of the observed degradation in
conventional GaN HEMTs. Some differences in the post-rad
response are visible between the two types of tested devices,
likely due to a different initial concentration of defects.
Source field plates are shown to have a very limited impact on
the radiation response
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Electrical and Electroluminescence Characteristics of AlGaN/GaN High Electron Mobility Transistors Operated in Sustainable Breakdown Conditions
No full textThis paper reports on an extensive analysis of the electrical and optical properties of GaN-based high electron mobility transistors (HEMTs) biased in a non-destructive breakdown regime. By means of a number of measurements carried out with varying voltage and temperature conditions, we show that: (i) HEMTs can reach a sustainable breakdown condition, when they are biased in current-controlled mode, with a gate voltage smaller than the pinch-off voltage; (ii) when biased in sustainable breakdown, HEMTs can emit a weak luminescence signal, localized in proximity of the drain edge; (iii) the breakdown voltage (BDV) is strongly dependent on temperature. Through a careful investigation of the drain, source and gate current components, we demonstrate that breakdown originates from two different mechanisms, depending on the gate voltage: for gate voltages close to the pinch-off, breakdown current originates from the space charge injection of electrons from the source to the drain. On the other hand, for more negative gate voltages, breakdown current originates from the injection of electrons from the gate. Finally, the analysis of the temperature dependence of the breakdown current confirms that two different mechanisms significantly contribute to current conduction at high drain voltage level
Characterization Of Gan-Based Single- And Double-Heterostructure Devices
No full textThis work focuses on a comparison of two GaN high-electron mobility transistors (HEMTs) epilayer structures, a single- (SH) and a double-heterostructure (DH). DC and pulsed tests reveal lower ID and Ileak in DH devices. Kink effect is also reported; none shows relevant current collapse. DH HEMTs show a much higher breakdown voltage (VBR) and slope. Off-state step-stresses confirm good reliability for these devices thanks to improved back-barrier confinement; SH suffers from punch-through resulting in premature breakdown
Comparison of the performances of an InAlN/GaN HEMT with a Mo/Au gate or a Ni/Pt/Au gate
No full textIn this paper GaN based HEMT structures characterized by a 6.3 nm InAlN barrier layer and a different gate contact (Mo/Au and Ni/Pt/Au) are described. The effects of a different gate contact structure on DC and pulsed main characteristics are discussed. Despite no meaningful variation is noticed during DC analysis, pulsed evaluation demonstrates that the use of a Mo/Au gate contact leads to an improvement of trapping characteristics. Reliability performances of the devices are finally investigated by means of a OFF state three terminals step stress, proving that the use of a Mo/Au stack does not affect significantly the device stability during the stress
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