Abid, I

Bougerol, C.

Comyn, R

Cordier, Y.

Kabouche, R

Medjdoub, F.

Zegaoui, M.

English

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Submitted on 9 Nov 2019
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AlGaN/GaN High Electron Mobility Transistors with
Ultra -Wide Bandgap AlN buffer
I Abid, R Kabouche, M. Zegaoui, C. Bougerol, R Comyn, Y. Cordier, F.
Medjdoub
To cite this version:
I Abid, R Kabouche, M. Zegaoui, C. Bougerol, R Comyn, et al.. AlGaN/GaN High Electron Mobility
Transistors with Ultra -Wide Bandgap AlN buffer. 43rd Workshop on Compound Semiconductor
Devices and Integrated Circuits, WOCSDICE 2019, Jun 2019, Cabourg, France. ￿hal-02356886￿
AlGaN/GaN High Electron Mobility Transistors with Ultra -Wide Bandgap AlN buffer 
I. Abid
1*
, R. Kabouche
1
, M. Zegaoui
1
, C. Bougerol
2
, R.Comyn
3
, Y. Cordier
3
, F. Medjdoub
1* 
1 CNRS-IEMN, UMR8520, Av. Poincaré, 59650 Villeneuve d'Ascq, France 
2 Univ. Grenoble-Alpes, CNRS-Institut Néel, 25 av. des Martyrs, 38000 Grenoble, France 
3 Univ. Côte d’Azur, CNRS, CRHEA, rue Bernard Grégory, 06560 Valbonne, France 
*Email: idriss.abid@ed.univ-lille1.fr; farid.medjdoub@univ-lille.fr
 
Introduction 
AlGaN/GaN high-electron-mobility transistor (HEMT) 
grown on silicon substrate is a promising device for cost-
effective high-power and high-voltage electronic 
applications. In these devices, the breakdown voltage is 
basically limited by the total buffer thickness. In order to 
reach higher performances, a new generation of 
material system is required. In this frame, the ultra-
wide-bandgap (UWBG) material AlN (6.2eV) that has a 
much larger bandgap than GaN (3.4eV) could enable the 
next leap forward in power electronics performance. 
Furthermore, AlN material represents the ideal back 
barrier for high voltage applications due to its large 
electrical breakdown field and high thermal conductivity. 
In turn, the AlN buffer can potentially not only boost the 
breakdown voltage owing to its wider bandgap while 
benefiting from an enhanced thermal dissipation as 
compared to GaN-based devices but also allow 
increasing the electron confinement in the transistor 
channel. 
Experimental 
In this paper, we present the fabrication and the DC / 
high voltage characteristics of two structures including 
thin and thick channels AlGaN/GaN HEMT on AlN buffer 
grown on sapphire. Lateral buffer breakdown voltage 
assessment reveals a remarkable breakdown field of 5 
MV/cm for short contact distances in the case of the 
sub-10 nm channel heterostructure, which is far beyond 
that of GaN-based material system. Furthermore, 
fabricated transistors are fully functional with low 
leakage current and low on-resistance. 
 
The two heterostructures consist in the regrowth of an 
AlN buffer layer by ammonia-MBE on a 6 µm thick AlN 
templates on sapphire substrates (Fig. 1).  
 
 
 
(a) 
 
(b) 
 
(c) 
Fig. 1. Schematic cross-section of AlGaN/GaN HEMT, 
(a) thin channel structure 1, (b) thick channel structure 2 (c) and 
TEM of structure 1. 
 
For the first structure, this is followed by a thin GaN 
channel of 8 nm and a 10 nm AlGaN barrier with a high 
aluminum content (90%) capped with a 7 nm in-situ SiN 
layer. For the second structure, a 240 nm thick GaN 
channel has been grown with a 10 nm AlGaN barrier 
(29% Al) capped with a 2 nm GaN layer. The 2DEG 
properties have been measured on a Van der Pauw 
pattern resulting in a sheet carrier density of 1.9x10
13
 
cm
-2
 and an electron mobility of 200 cm²/V.s for the first 
structure. The rather low mobility can be attributed to 
the thin channel and/or to the high Al content into the 
barrier layer. The second structure shows a charge 
density of 1.1x10
13
 cm
-2
 and an electron mobility slightly 
higher than 1500 cm²/V.s. 
 
 Results and discussion 
Significant lateral buffer breakdown voltage up to 10 kV 
has been measured on isolated ohmic contacts for large 
distances above 90 µm on the thin channel 
heterostructure (Fig. 2).  
 
(a) 
 
(b) 
Fig. 2. Lateral breakdown voltage on 96 µm contact distance 
for (a) thin channel structure 1, (b) thick channel structure 2. 
 
The very high blocking voltage and the low leakage 
current show that the heterostructure does not suffer 
from any parasitic conduction as full depletion down to 
the sapphire substrate occurs. Interestingly, a 
remarkable breakdown field of 5 MV/cm is observed for 
short distances with for instance more than 1000 V for a 
2 µm contact distance (Fig. 3). The thick channel 
structure delivers much lower breakdown voltage for the 
same contact distances. This is even more pronounced 
for a  
2 µm contact distance with only 270V against more than 
1 kV for the thin channel structure. As it could be 
expected, the breakdown field of less than 2 MV/cm is 
limited by the thick GaN channel layer in this case. 
 
.             (a)                          (b) 
Fig. 3. Lateral breakdown voltage on 2 µm contact 
distance, (a) structure 1, (b) structure 2. 
It appears that the breakdown mechanism is not 
limited by the GaN channel when using a sub-10 nm 
thickness enabling to benefit from the AlN bandgap for 
short contacts distances. This paves the way for short 
AlN-based devices delivering low on-resistances above 
1 kV voltage range. 
 Thin channel
 Thick channel
0 20 40 60 80 100
0
2000
4000
6000
8000
10000
L
a
te
ra
l 
b
k
 (
V
)
Distance (µm)
 
Fig. 4. Lateral breakdown voltage for various isolated 
contact distances for the two structures. 
Electrical characterizations were carried out on 
transistors with gate width/length = 50µm/1.5µm and 
gate-to-drain spacing (LGD) varied from 2 to 40µm. For 
the various transistor designs from the thin channel 
structure, the off-state leakage current at VDS = 4V is 
around 200 nA/mm and the static on-resistance RON 
scales as expected with LGD to reach values below 15 
mΩ.cm
2
 for a 5 µm distance. 
 
Fig. 5. DC characteristics of AlN-based transistors with 
different gate drain distances for the structure 1. 
Despite rather poor ohmic contacts for the thick channel 
structure, the off-state leakage current at VDS = 4V is 
below 100 µA/mm and the static on-resistance RON 
scales as expected with LGD to reach values below 12 
mΩ.cm
2
 for a 8 µm distance. 
 
Fig. 6. DC characteristics of AlN-based transistors with 
different gate drain distances for the structure 2. 
 Conclusions  
This work show that ultrathin channel AlN-based 
transistors could provide low on-resistances with 
significantly higher breakdown field far beyond the 
theoretical limits of GaN-based devices. Indeed,  
2-terminal lateral breakdown measurements revealed a 
remarkable breakdown field of 5 MV/cm with more than 
1000 V for a 2 µm contact distance. Furthermore, large 
contact distances (> 90 µm) enabled a blocking voltage 
above 10 kV with less than 0.5 µA/mm leakage current, 
reflecting the absence of parasitic conduction during the 
depletion down to the sapphire substrate. The rather 
limited electron mobility in the 2DEG can still be further 
improved by optimizing the device design and the 
growth interface quality.  
ACKNOWLEDGMENT: This work is supported 
by the French RENATECH network and the French 
National grant ANR project BREAKUP (ANR-17-
CE05-0013). 
 


AlGaN/GaN High Electron Mobility Transistors with Ultra -Wide Bandgap AlN buffer

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AlGaN/GaN High Electron Mobility Transistors with Ultra -Wide Bandgap AlN buffer

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