GaN microelectronics technology
Keeping the gate-to-channel distance small compared to the gate length increases device transconductances, and leads to a faster charging and discharging of intrinsic and parasitic capacitances, and ultimately, to faster transistors.
Lattice-matched barriers? The conventional form of GaN-based HEMT, which involves the pairing of AlGaN and GaN, has undergone more than a decade-and-a-half of frantic development. However, question marks are now hanging over the reliability of this device (see, for example [1, 2]). These are thought to stem from inherent strain in the heterostructure, due to a combination of lattice-mismatch and piezoelectric contributions.
Another concern with this AlGaN/GaN incumbent is surface depletion effects in the 2DEG that arise when the top barrier thickness is thinned much below 15 nm [3]. This can be addressed with AlN thin top barriers that increase channel sheet densities [4], but it is unclear how this impacts the strain-related device degradation. Other research groups have countered surface depletion effects with aluminum-rich AlGaN barriers and various dielectrics, an approach that has yielded impressive cutoff
frequencies, with fT values reaching 190 GHz with 60 nm gate MISFET structures [3, 5].
One promising alternative to the AlGaN/GaN HEMT is a variant based on a heterostructure between AlInN and GaN. This potential successor, which was proposed by Jan Kuzmík from the Technical University of Vienna, can be fabricated with a lattice-matched barrier and thus addresses some of the strain-related reliability concerns associated with conventional lattice-mismatched, AlGaN/GaN heterostructures [6]. Kuzmík has also argued that the pairing of AlInN and GaN should reduce surface depletion effects and potentially unlock the door to the fabrication of GaN HEMTs with excellent channel aspect ratios down to very short gate lengths.
Early experimental efforts suggested that the AlInN/GaN pairing is capable of fulfilling many expectations. The feasibility of ultrathin barrier AlInN/GaN HEMTs has been verified down to 3 nm thick AlInN barriers [7], and such devices have also demonstrated phenomenal stability, even surviving operation at temperatures of 1000 °C [8]. Such high-temperature survival is clearly outside the reach of traditional AlGaN/GaN devices.
To put it simply, it seems that AlInN/GaN heterostructure channels are capable of driving evolutionary improvements reminiscent of those achieved by the transition from GaAs-based pHEMTs to their InP counterparts. And it seems that these benefits can be realized without paying the penalty of more fragile materials, as was the case when moving from GaAs-based HEMTs to high-indium- content GaInAs alloys on InP.
One key benefit that results from the weaker surface depletion effects in AlInN/GaN HEMTs is a vertical scaling advantage − this is very similar to that seen for AlInAs/GaInAs HEMTs over AlGaAs/GaInAs equivalents [9]. In addition, HEMTs based on AlInN were reported to exhibit higher carrier velocities in the channel, thanks to faster thermalization and decay of hot longitudinal-optic phonons. The upshot of this is higher electron velocities in strong electric fields [10].
Although the analogy to InP pHEMT development is very good, there is one difference between GaN and InP devices. With InP, higher channel velocities stemmed from improved transport properties in GaInAs alloys with a high indium content, which made the devices more fragile, due to the lower temperature stability of high indium content materials.
In comparison, while the AlInN/GaN system does contain some indium, it is the higher aluminum content that is responsible for the ruggedness advantage that AlInN/GaN HEMTs have over their AlGaN/GaN equivalents. The aluminum content in the AlInN alloy that forms the lattice-matched barrier on GaN is 83 percent. This is far higher than the mere 30 percent in strained AlGaN barriers, a fundamental difference that appears to confer a far greater stability of the newer form of HEMT.
By putting some faith into the analogy between AlInN/GaN HEMTs and InP pHEMTs, it is just a small further step to reach the idea that an AlInN/GaN HEMT should yield superior cutoff frequencies compared to its conventional predecessor - just as the case was for InP pHEMTs compared to GaAs pHEMTs.
However, until recently, AlInN/GaN HEMTs have failed to fulfill this promise, conceding a significant bandwidth advantage to their forerunners. But the good news is that thanks to material and process improvements, the ultimate cutoff frequency of AlInN/GaN HEMTs has more than doubled in the last year [11, 12].
August / September 2010
www.compoundsemiconductor.net 17
Figure 3. Atomic force microscopy (AFM)
measurements indicate that the AlInN barrier is very smooth. The root-mean- square
roughness is just 0.5 nm
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