The high potential at microwave frequencies of AlGaN/GaN HEMTs on sapphire substrate for power application has been demonstrated in this paper. An output power density close to 5W/mm has been measured on a 2x25x0.5µm² HEMT on sapphire substrate. This result is very interesting because the devices have not been passivated. At present time, it is the best power result in Europe on this substrate

Boudart, B.

de Jaeger, J.C.

Ducatteau, D.

Gaquiere, C.

Guhel, Y.

Vellas, N.

Werquin, M.

English

AMS Acta

  1 High Power Performances of AlGaN/GaN HEMTs On Sapphire Substrate At F=4GHz  N. Vellas, C. Gaquiere, Y. Guhel, M. Werquin, D. Ducatteau, B. Boudart1, J. C. de Jaeger  Institut d’Electronique et de Microélectronique du Nord – U.M.R. – C.N.R.S. 8520 Département Hyperfréquences et Semiconducteurs Cité Scientifique – Avenue Poincaré – B.P. 69 – 59652 Villeneuve d’Ascq Cedex – France 1Now in : EIC-LUSAC, Site universitaire BP 78 50130 Octeville Z. Bougrioua2, M.Germain**, M. Leys**, I. Moerman, S. Borghs**  INTEC, IMEC- Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium **IMEC, Kapeldreef, 75, B-3001 Leuven, Belgium 2Now in : CRHEA, Rue Bernard Gregory, Sophia Antipolis F-06560 Valbonne – France  Abstract: The high potential at microwave frequencies of AlGaN/GaN HEMTs on sapphire substrate for power application has been demonstrated in this paper. An output power density close to 5W/mm has been measured on a 2x25x0.5µm² HEMT on sapphire substrate. This result is very interesting because the devices have not been passivated. At present time, it is the best power result in Europe on this substrate.  INTRODUCTION  AlGaN/GaN high electron mobility transistors (HEMTs) have recently received considerable attention for power applications at microwave frequencies. In fact, the GaN material presents a wide band gap, a high saturation velocity and a high thermal stability, so it constitutes an ideal candidate for such applications. At present time, silicon carbide (SiC) and sapphire materials are the main substrates used for the GaN HEMT device production. Recently, a power density of about 10.7 W/mm has been obtained with AlGaN/GaN HEMTs on silicon carbide at 10 GHz and 6.6 W/mm at 20 GHz [1], [2]. Nowadays, these results constitute the state of the art on silicon carbide substrate. On the other hand, the results obtained on sapphire substrate are lower. The state of the art is about 6.5 W/mm at 8 GHz and 3.3 W/mm at 18 GHz [3], [4].  In this paper, we report small and large-signal results of AlGaN/GaN HEMTs on sapphire substrate. First, a brief description of GaN HEMT fabrication is presented. The main DC and small signal microwave results are summarised. Secondly, the static results are compared to those obtained by pulsed measurements and thermal effects are shown. Then, the large signal characterisation is described, including measurement conditions and power results obtained.    II. DEVICE DESCRIPTION AND SMALL SIGNAL RESULTS     The AlGaN/GaN structures (figure 1) have been grown by LP-MOCVD [5] on sapphire substrate using ammonia, TMGa and TMAl as precursors. After deposition of a thin nucleation layer at low temperature, the wafer is brought to high temperature for recrystallisation. A 3.12 µm thick GaN layer is then grown at 1200 °C at a pressure of 100 Torr. The AlxGa1-xN layer is deposited at lower pressure. The aluminum content in the AlGaN layer is 36 % and its thickness is 180 Å.  SAPPHIREGaN 3.12µmAl0.36Ga0.74N   20nmSource Gate DrainMESA MESANIDNID Figure 1: Device structure     2 The devices have gate-drain and gate-source spacings of 1 µm, a gate width of 50 µm and a gate length of 0.5 µm. The mesa isolation is made by reactive ion etching (RIE) with SiCl4. An etch rate of 180 Å/min has been obtained with the following conditions: a RF power of 200 W, a pressure of 20 mTorr and a flow rate of 4 sccm. The ohmic contact metallization is Ti/Al/Ni/Au with respective thickness of 150/2200/400/500 Å. A small etching (RIE) is realized before deposing metals and after it, this contact is annealed under nitrogen atmosphere at 900°C during 40 s. The Schottky contact metallization is Pt/Au (100/2000 Å) then bonding pads deposition are achieved with Ti/Au (1000/4000 Å) [6]. The devices are not passivated. Table 1 summarizes the electron mobility, the sheet carrier concentration at T=300 K, the main DC and small signal microwave performances of a device with a geometry of 2x25x0.5 µm².  µ (cm2/Vs) at T=300 K 1170 ns (cm-2) at T=300 K 1.5 1013 ID (A/mm) at VGS=1V and VDS= 27V 1.15 gmext (mS/mm) at VGS=0.5V and VDS= 15V 160 VP  (V) -6 fT (GHz) at VGS=-2V and VDS= 10V 18 fMAG (GHz) at VGS=-2V and VDS= 10V 50 fMAX  (GHz) at VGS=-2V and VDS= 10V 75 PDC (W/mm) expected at VDS=27V and forIDsat=1.2A/mm 6.25  Table 1: Device parameters  The cut-off frequencies fT, fmax and fMAG determined from the scattering parameters on a Vector Network Analyzer (VNA) HP8510 are respectively 18 GHz, 75 GHz, 50 GHz. The breakdown voltage in transistor configuration (at open and close channel) appears at a drain-source bias voltage higher than 30 V. The drain current density is 1.15 A/mm at VGS=1 V and the DC extrinsic transconductance is 160 mS/mm. The static and pulsed drain current characteristic versus drain-source bias voltage for several gate-source voltages are shown figure 2.  VDS(V)0 5 10 15 20I D(mA/mm)0200400600800100012001400Pulsed characteristic with a quescient bias point VDS0=0V and VGS0=0V.Static characteristicVGS=-6 to 1V step 1V  Figure 2: Pulsed and static characteristics  The pulsed I(V) measurement has been carried out at a quiescent bias point of VGS0=0 V and VDS0=0 V. This quiescent bias point (cold polarization) permits to eliminate the thermal effects. The pulsed set up is described elsewhere [7]. On the ID(VDS) static characteristic, the channel conductance is negative. This phenomenon is attributed to thermal effects because the channel conductance is nearly zero in pulsed configuration. Moreover, the maximum drain current (at VGS=1 V) on the pulsed characteristic is higher than the one coming from the static characteristic. The output power density expected from the static ID(VDS) characteristic is 6.25 W/mm (DI*DV/8). This one has been calculated at VDS=27 V with IDSAT=1.2 A/mm and Vknee=7 V at VGS=2 V.  III. LARGE SIGNAL CHARACTERIZATION  III.1 Set up description and measurement conditions   This set up permits the device measurement at microwave frequencies on wafer or in fixture and gives the possibility to observe the power performances, the signal forms of the drain-source voltage versus biases and load impedances conditions simultaneously. It is an automatic passive load-pull system. The calibration procedures are described elsewhere [8]. A VNA allows to determine the load impedance presented at the output plane of the DUT (Devices Under Test). The load impedances are carried out with a double slugs tuner.   III.2 Power results at 4GHz    3 The power measurements have been performed near the breakdown voltage in order to obtain a high drain-source voltage swing in power condition.   The DUT has been biased at VGS=-2.9 V, VDS=27 V and the optimal power load impedance presented at the output plane of this device is Zload (W ) =1200 + j200 in large signal condition.  Figure 3 presents the power gain, the power added efficiency (PAE), and the output power for a HEMT with a geometry of 2x25x0.5µm² versus the absorbed input power.   Pabs (dBm)-15 -10 -5 0 5 10 15P out (dBm),  GP (dB)0510152025hPAE (%)01020304050PoutGPhPAEPOUT = 5W/mm Figure 3: Power results of a 2x25x0.5µm² device at VDS=27 V and VGS=-2.9 V (Zload (W )=1200+j200) at F=4 GHz  The maximum output power reaches 5 W/mm, the respective power gain is about 10dB (on the other hand the linear power gain is 16dB) and the associated PAE is 39%. Now, this power result represents the state of the art to our knowledge in Europe on sapphire substrate. Moreover, the output power density expected of 6.25W/mm (see table 1) is not reached. The difference between the DC output power density expected and the RF output power measured can be explained by thermal effects or/and by the presence of traps [8], [9]. A passivation step with SixNy can be used on this sample in order to limit trap effects and so to increase the microwave output power [10].It is well know that sapphire has not a good thermal conductivity (0.46W/cm.K). For this reason, to increase the thermal dissipation, the sapphire substrate can be thinned and power performances can be improved.   IV. CONCLUSION  An output power density of 5 W/mm and a linear power gain close to 16 dB have been measured with AlGaN/GaN HEMTs on sapphire substrate at 4 GHz. These results constitute the state of the art in Europe. The high potential of AlGaN/GaN/Al2O3 HEMT devices for power applications at microwave frequencies has been demonstrated. There is still a weak difference between the output power density expected and the measured one. This difference can be explained by thermal effects and trap effects.  ACKNOWLEDGEMENT: This work has been carried out with the financial help of DGA (French Army), contract n°0134050, and the ESA contract 14205/00/NL/PA.  REFERENCES   [1] Vinayak Tilak, Bruce Green, Val Kaper, Hyungtak Kim, Tom Prunty, Joe Smart, James Shealy, and Lester Eastman, “Influence of Barrier Thickness on the High-Power Performances of AlGaN/GaN HEMTs”, IEEE Electron Device Letters, Vol. 22, No. 11, pp 504-506,  November 2001.  [2] J. S. Moon, M. Micovic, P. Janke, P. Hashimoto, W.-S. Wong, R.D. Widman, L. McCray, A. Kurdoghlian and C. Nguyen , “GaN/AlGaN HEMTs operating at 20GHz with continuous -wave power density > 6W/mm,” Electron. Lett., Vol. 37, No. 8, pp. 528-530, April 2001.  [3] Y.-F. Wu, D. Kapolnek, J. P. Ibbetson, P. Parikh, B. P. Keller, and U. K. Mishra, “Very-High Power Density AlGaN/GaN HEMTs,” IEEE Trans. Electron. Dev., Vol. 48, No. 3, pp. 586-590, March 2001.  [4] Y.-F. Wu, B. P. Keller, P. Fini, J. Pusl, M. Le, N. X. Nguyen, C. Nguyen, D. Widma n, S. Keller, S. P. Denbaars and U. K. Mishra, “Short-channel Al0.5Ga0.5N/GaN MODFETs with power density > 3W/mm at 18GHz,” Electron. Lett.,  Vol. 33, pp. 1742-1743, July 1997.  [5] Z. Bougrioua, I. Moerman, N. Sharma, R.H. Wallis, J. Cheyns, K. Jacobs, E.J. Thrush, L.Considine, R. Beanland, J.-L. Farvacque, C. Humphreys, « Material Optimization for AlGaN/GaN HFET applications », J. Crystal Growth, Vol. 230, pp 573-578, 2001.  [6] V. Hoël, N. Vellas, C. Gaquière, J.C. De Jaeger, Y.Cordier, F.Semond, F.Natali and J.Massies, “High power AlGaN/GaN HEMTs on a Resistive Silicon Substrate,” will be published in Electronics Letters.  [7] C.Gaquière, J.P. Lafont, Y. Crosnier, “Pulsed bias/pulsed RF characterization measurements system of FET at constant intrinsic voltages,” IEE Microwave and Optical Technology Letters, Vol 20, n°5, pp 349-352, March 1999.  [8] N. Vellas, C. Gaquiere, F. Bue, Y. Guhel, B. Boudart, J.C. De Jaeger, and M. A. Poisson, “Load Impedance Influence On The ID(VDS) Characteristics Of AlGaN/GaN HEMTs In Large Signal Regime At 4GHz,”   4 IEEE Electron. Dev. Lett., Vol. 23, No 5, pp 246-248, May 2002  [9] S. Trassaert, B. Boudart, C. Gaquière, D. Théron, Y. Crosnier, F. Huet and M.A. Poisson, ”Trap effects studies in GaN MESFETs by pulsed measurements,” Electronics Letters, Vol. 35, No. 16, pp 1386-1388, August 2002.  [10] H. Leier, A. Wieszt, R. Behtash, H. Tobler, A. Vescan, R. Dietrich, A. Schurr, H. Sledzik, JCH. Birbeck, RS. Balmer and T. Martin, “ RF Power Performance Of Passivated AlGaN/GaN HFETs Grown On SiC And Sapphire,” GAAS 2001 Conference, pp 49-52, London 2001  

High Power Performances of AlGaN/GaN HEMTs On Sapphire Substrate At F=4GHz

AMS Acta - Alm@DL - Università di Bologna

  1 
High Power Performances of AlGaN/GaN HEMTs On Sapphire 
Substrate At F=4GHz 
 
N. Vellas, C. Gaquiere, Y. Guhel, M. Werquin, D. Ducatteau, B. Boudart1, J. C. de Jaeger 
 
Institut d’Electronique et de Microélectronique du Nord – U.M.R. – C.N.R.S. 8520 
Département Hyperfréquences et Semiconducteurs 
Cité Scientifique – Avenue Poincaré – B.P. 69 – 59652 Villeneuve d’Ascq Cedex – France 
1Now in : EIC-LUSAC, Site universitaire BP 78 50130 Octeville 
Z. Bougrioua2, M.Germain**, M. Leys**, I. Moerman, S. Borghs** 
 
INTEC, IMEC- Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium 
**IMEC, Kapeldreef, 75, B-3001 Leuven, Belgium 
2Now in : CRHEA, Rue Bernard Gregory, Sophia Antipolis F-06560 Valbonne – France 
 
Abstract: 
The high potential at microwave frequencies of AlGaN/GaN HEMTs on sapphire substrate for power 
application has been demonstrated in this paper. An output power density close to 5W/mm has been measured 
on a 2x25x0.5µm² HEMT on sapphire substrate. This result is very interesting because the devices have not 
been passivated. At present time, it is the best power result in Europe on this substrate. 
 
INTRODUCTION  
AlGaN/GaN high electron mobility transistors 
(HEMTs) have recently received considerable attention 
for power applications at microwave frequencies. In fact, 
the GaN material presents a wide band gap, a high 
saturation velocity and a high thermal stability, so it 
constitutes an ideal candidate for such applications. At 
present time, silicon carbide (SiC) and sapphire materials 
are the main substrates used for the GaN HEMT device 
production. Recently, a power density of about 10.7 
W/mm has been obtained with AlGaN/GaN HEMTs on 
silicon carbide at 10 GHz and 6.6 W/mm at 20 GHz [1], 
[2]. Nowadays, these results constitute the state of the art 
on silicon carbide substrate. On the other hand, the 
results obtained on sapphire substrate are lower. The state 
of the art is about 6.5 W/mm at 8 GHz and 3.3 W/mm at 
18 GHz [3], [4].  
In this paper, we report small and large-signal results 
of AlGaN/GaN HEMTs on sapphire substrate. 
First, a brief description of GaN HEMT fabrication is 
presented. The main DC and small signal microwave 
results are summarised. Secondly, the static results are 
compared to those obtained by pulsed measurements and 
thermal effects are shown. Then, the large signal 
characterisation is described, including measurement 
conditions and power results obtained. 
 
  
II. DEVICE DESCRIPTION AND SMALL SIGNAL RESULTS  
  
 The AlGaN/GaN structures (figure 1) have been 
grown by LP-MOCVD [5] on sapphire substrate using 
ammonia, TMGa and TMAl as precursors. After 
deposition of a thin nucleation layer at low temperature, 
the wafer is brought to high temperature for 
recrystallisation. A 3.12 µm thick GaN layer is then 
grown at 1200 °C at a pressure of 100 Torr. The AlxGa1-
xN layer is deposited at lower pressure. The aluminum 
content in the AlGaN layer is 36 % and its thickness is 
180 Å. 
 
SAPPHIRE
GaN 3.12µm
Al0.36Ga0.74N   20nm
Source Gate Drain
MESA MESA
NID
NID
 
Figure 1: Device structure  
 
  2 
The devices have gate-drain and gate-source spacings 
of 1 µm, a gate width of 50 µm and a gate length of 0.5 
µm. The mesa isolation is made by reactive ion etching 
(RIE) with SiCl4. An etch rate of 180 Å/min has been 
obtained with the following conditions: a RF power of 
200 W, a pressure of 20 mTorr and a flow rate of 4 sccm. 
The ohmic contact metallization is Ti/Al/Ni/Au with 
respective thickness of 150/2200/400/500 Å. A small 
etching (RIE) is realized before deposing metals and after 
it, this contact is annealed under nitrogen atmosphere at 
900°C during 40 s. The Schottky contact metallization is 
Pt/Au (100/2000 Å) then bonding pads deposition are 
achieved with Ti/Au (1000/4000 Å) [6]. The devices are 
not passivated. 
Table 1 summarizes the electron mobility, the 
sheet carrier concentration at T=300 K, the main DC and 
small signal microwave performances of a device with a 
geometry of 2x25x0.5 µm². 
 
µ (cm2/Vs) at T=300 K 1170 
ns (cm
-2) at T=300 K 1.5 1013 
ID (A/mm) at VGS=1V and VDS= 27V 1.15 
gmext (mS/mm) at VGS=0.5V and VDS= 15V 160 
VP  (V) -6 
fT (GHz) at VGS=-2V and VDS= 10V 18 
fMAG (GHz) at VGS=-2V and VDS= 10V 50 
fMAX  (GHz) at VGS=-2V and VDS= 10V 75 
PDC (W/mm) expected 
at VDS=27V and forIDsat=1.2A/mm 
6.25 
 
Table 1: Device parameters 
 
The cut-off frequencies fT, fmax and fMAG 
determined from the scattering parameters on a Vector 
Network Analyzer (VNA) HP8510 are respectively 18 
GHz, 75 GHz, 50 GHz. The breakdown voltage in 
transistor configuration (at open and close channel) 
appears at a drain-source bias voltage higher than 30 V. 
The drain current density is 1.15 A/mm at VGS=1 V and 
the DC extrinsic transconductance is 160 mS/mm. The 
static and pulsed drain current characteristic versus drain-
source bias voltage for several gate-source voltages are 
shown figure 2. 
 
VDS(V)
0 5 10 15 20
I D
(m
A
/m
m
)
0
200
400
600
800
1000
1200
1400
Pulsed characteristic with a quescient bias point
 VDS0=0V and VGS0=0V.
Static characteristic
VGS=-6 to 1V step 1V 
 
Figure 2: Pulsed and static characteristics 
 
The pulsed I(V) measurement has been carried 
out at a quiescent bias point of VGS0=0 V and VDS0=0 V. 
This quiescent bias point (cold polarization) permits to 
eliminate the thermal effects. The pulsed set up is 
described elsewhere [7]. 
On the ID(VDS) static characteristic, the channel 
conductance is negative. This phenomenon is attributed 
to thermal effects because the channel conductance is 
nearly zero in pulsed configuration. Moreover, the 
maximum drain current (at VGS=1 V) on the pulsed 
characteristic is higher than the one coming from the 
static characteristic. The output power density expected 
from the static ID(VDS) characteristic is 6.25 W/mm 
(DI*DV/8). This one has been calculated at VDS=27 V 
with IDSAT=1.2 A/mm and Vknee=7 V at VGS=2 V. 
 
III. LARGE SIGNAL CHARACTERIZATION 
 
III.1 Set up description and measurement conditions 
 
 This set up permits the device measurement at 
microwave frequencies on wafer or in fixture and gives 
the possibility to observe the power performances, the 
signal forms of the drain-source voltage versus biases 
and load impedances conditions simultaneously. It is an 
automatic passive load-pull system. The calibration 
procedures are described elsewhere [8]. 
A VNA allows to determine the load impedance 
presented at the output plane of the DUT (Devices Under 
Test). The load impedances are carried out with a double 
slugs tuner.  
 
III.2 Power results at 4GHz 
 
  3 
The power measurements have been performed near 
the breakdown voltage in order to obtain a high drain-
source voltage swing in power condition.  
 The DUT has been biased at VGS=-2.9 V, VDS=27 V 
and the optimal power load impedance presented at the 
output plane of this device is Zload (W ) =1200 + j200 in 
large signal condition. 
 Figure 3 presents the power gain, the power added 
efficiency (PAE), and the output power for a HEMT with 
a geometry of 2x25x0.5µm² versus the absorbed input 
power.  
 
Pabs (dBm)
-15 -10 -5 0 5 10 15
P o
ut
 (d
B
m
), 
 G
P 
(d
B
)
0
5
10
15
20
25
h
PA
E 
(%
)
0
10
20
30
40
50
Pout
GP
hPAE
POUT = 5W/mm
 
Figure 3: Power results of a 2x25x0.5µm² device at 
VDS=27 V and VGS=-2.9 V (Zload (W )=1200+j200) at 
F=4 GHz 
 
The maximum output power reaches 5 W/mm, the 
respective power gain is about 10dB (on the other hand 
the linear power gain is 16dB) and the associated PAE is 
39%. Now, this power result represents the state of the 
art to our knowledge in Europe on sapphire substrate. 
Moreover, the output power density expected of 
6.25W/mm (see table 1) is not reached. The difference 
between the DC output power density expected and the 
RF output power measured can be explained by thermal 
effects or/and by the presence of traps [8], [9]. A 
passivation step with SixNy can be used on this sample in 
order to limit trap effects and so to increase the 
microwave output power [10].It is well know that 
sapphire has not a good thermal conductivity 
(0.46W/cm.K). For this reason, to increase the thermal 
dissipation, the sapphire substrate can be thinned and 
power performances can be improved.  
 
IV. CONCLUSION 
 
An output power density of 5 W/mm and a 
linear power gain close to 16 dB have been measured 
with AlGaN/GaN HEMTs on sapphire substrate at 4 
GHz. These results constitute the state of the art in 
Europe. The high potential of AlGaN/GaN/Al2O3 HEMT 
devices for power applications at microwave frequencies 
has been demonstrated. There is still a weak difference 
between the output power density expected and the 
measured one. This difference can be explained by 
thermal effects and trap effects. 
 
ACKNOWLEDGEMENT: This work has been carried out 
with the financial help of DGA (French Army), contract 
n°0134050, and the ESA contract 14205/00/NL/PA. 
 
REFERENCES  
 
[1] Vinayak Tilak, Bruce Green, Val Kaper, Hyungtak 
Kim, Tom Prunty, Joe Smart, James Shealy, and Lester 
Eastman, “Influence of Barrier Thickness on the High-
Power Performances of AlGaN/GaN HEMTs”, IEEE 
Electron Device Letters, Vol. 22, No. 11, pp 504-506,  
November 2001. 
 
[2] J. S. Moon, M. Micovic, P. Janke, P. Hashimoto, 
W.-S. Wong, R.D. Widman, L. McCray, A. Kurdoghlian 
and C. Nguyen , “GaN/AlGaN HEMTs operating at 
20GHz with continuous -wave power density > 6W/mm,” 
Electron. Lett., Vol. 37, No. 8, pp. 528-530, April 2001. 
 
[3] Y.-F. Wu, D. Kapolnek, J. P. Ibbetson, P. Parikh, B. 
P. Keller, and U. K. Mishra, “Very-High Power Density 
AlGaN/GaN HEMTs,” IEEE Trans. Electron. Dev., Vol. 
48, No. 3, pp. 586-590, March 2001. 
 
[4] Y.-F. Wu, B. P. Keller, P. Fini, J. Pusl, M. Le, N. X. 
Nguyen, C. Nguyen, D. Widma n, S. Keller, S. P. 
Denbaars and U. K. Mishra, “Short-channel 
Al0.5Ga0.5N/GaN MODFETs with power density > 
3W/mm at 18GHz,” Electron. Lett.,  Vol. 33, pp. 1742-
1743, July 1997. 
 
[5] Z. Bougrioua, I. Moerman, N. Sharma, R.H. Wallis, 
J. Cheyns, K. Jacobs, E.J. Thrush, L.Considine, R. 
Beanland, J.-L. Farvacque, C. Humphreys, « Material 
Optimization for AlGaN/GaN HFET applications », J. 
Crystal Growth, Vol. 230, pp 573-578, 2001. 
 
[6] V. Hoël, N. Vellas, C. Gaquière, J.C. De Jaeger, 
Y.Cordier, F.Semond, F.Natali and J.Massies, “High 
power AlGaN/GaN HEMTs on a Resistive Silicon 
Substrate,” will be published in Electronics Letters. 
 
[7] C.Gaquière, J.P. Lafont, Y. Crosnier, “Pulsed 
bias/pulsed RF characterization measurements system of 
FET at constant intrinsic voltages,” IEE Microwave and 
Optical Technology Letters, Vol 20, n°5, pp 349-352, 
March 1999. 
 
[8] N. Vellas, C. Gaquiere, F. Bue, Y. Guhel, B. 
Boudart, J.C. De Jaeger, and M. A. Poisson, “Load 
Impedance Influence On The ID(VDS) Characteristics Of 
AlGaN/GaN HEMTs In Large Signal Regime At 4GHz,” 
  4 
IEEE Electron. Dev. Lett., Vol. 23, No 5, pp 246-248, 
May 2002 
 
[9] S. Trassaert, B. Boudart, C. Gaquière, D. Théron, Y. 
Crosnier, F. Huet and M.A. Poisson, ”Trap effects 
studies in GaN MESFETs by pulsed measurements,” 
Electronics Letters, Vol. 35, No. 16, pp 1386-1388, 
August 2002. 
 
[10] H. Leier, A. Wieszt, R. Behtash, H. Tobler, A. 
Vescan, R. Dietrich, A. Schurr, H. Sledzik, JCH. 
Birbeck, RS. Balmer and T. Martin, “ RF Power 
Performance Of Passivated AlGaN/GaN HFETs Grown 
On SiC And Sapphire,” GAAS 2001 Conference, pp 49-
52, London 2001 
 


  1 
High Power Performances of AlGaN/GaN HEMTs On Sapphire 
Substrate At F=4GHz 
 
N. Vellas, C. Gaquiere, Y. Guhel, M. Werquin, D. Ducatteau, B. Boudart1, J. C. de Jaeger 
 
Institut d’Electronique et de Microélectronique du Nord – U.M.R. – C.N.R.S. 8520 
Département Hyperfréquences et Semiconducteurs 
Cité Scientifique – Avenue Poincaré – B.P. 69 – 59652 Villeneuve d’Ascq Cedex – France 
1Now in : EIC-LUSAC, Site universitaire BP 78 50130 Octeville 
Z. Bougrioua2, M.Germain**, M. Leys**, I. Moerman, S. Borghs** 
 
INTEC, IMEC- Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium 
**IMEC, Kapeldreef, 75, B-3001 Leuven, Belgium 
2Now in : CRHEA, Rue Bernard Gregory, Sophia Antipolis F-06560 Valbonne – France 
 
Abstract: 
The high potential at microwave frequencies of AlGaN/GaN HEMTs on sapphire substrate for power 
application has been demonstrated in this paper. An output power density close to 5W/mm has been measured 
on a 2x25x0.5µm² HEMT on sapphire substrate. This result is very interesting because the devices have not 
been passivated. At present time, it is the best power result in Europe on this substrate. 
 
INTRODUCTION  
AlGaN/GaN high electron mobility transistors 
(HEMTs) have recently received considerable attention 
for power applications at microwave frequencies. In fact, 
the GaN material presents a wide band gap, a high 
saturation velocity and a high thermal stability, so it 
constitutes an ideal candidate for such applications. At 
present time, silicon carbide (SiC) and sapphire materials 
are the main substrates used for the GaN HEMT device 
production. Recently, a power density of about 10.7 
W/mm has been obtained with AlGaN/GaN HEMTs on 
silicon carbide at 10 GHz and 6.6 W/mm at 20 GHz [1], 
[2]. Nowadays, these results constitute the state of the art 
on silicon carbide substrate. On the other hand, the 
results obtained on sapphire substrate are lower. The state 
of the art is about 6.5 W/mm at 8 GHz and 3.3 W/mm at 
18 GHz [3], [4].  
In this paper, we report small and large-signal results 
of AlGaN/GaN HEMTs on sapphire substrate. 
First, a brief description of GaN HEMT fabrication is 
presented. The main DC and small signal microwave 
results are summarised. Secondly, the static results are 
compared to those obtained by pulsed measurements and 
thermal effects are shown. Then, the large signal 
characterisation is described, including measurement 
conditions and power results obtained. 
 
  
II. DEVICE DESCRIPTION AND SMALL SIGNAL RESULTS  
  
 The AlGaN/GaN structures (figure 1) have been 
grown by LP-MOCVD [5] on sapphire substrate using 
ammonia, TMGa and TMAl as precursors. After 
deposition of a thin nucleation layer at low temperature, 
the wafer is brought to high temperature for 
recrystallisation. A 3.12 µm thick GaN layer is then 
grown at 1200 °C at a pressure of 100 Torr. The AlxGa1-
xN layer is deposited at lower pressure. The aluminum 
content in the AlGaN layer is 36 % and its thickness is 
180 Å. 
 
SAPPHIRE
GaN 3.12µm
Al0.36Ga0.74N   20nm
Source Gate Drain
MESA MESA
NID
NID
 
Figure 1: Device structure  
 
  2 
The devices have gate-drain and gate-source spacings 
of 1 µm, a gate width of 50 µm and a gate length of 0.5 
µm. The mesa isolation is made by reactive ion etching 
(RIE) with SiCl4. An etch rate of 180 Å/min has been 
obtained with the following conditions: a RF power of 
200 W, a pressure of 20 mTorr and a flow rate of 4 sccm. 
The ohmic contact metallization is Ti/Al/Ni/Au with 
respective thickness of 150/2200/400/500 Å. A small 
etching (RIE) is realized before deposing metals and after 
it, this contact is annealed under nitrogen atmosphere at 
900°C during 40 s. The Schottky contact metallization is 
Pt/Au (100/2000 Å) then bonding pads deposition are 
achieved with Ti/Au (1000/4000 Å) [6]. The devices are 
not passivated. 
Table 1 summarizes the electron mobility, the 
sheet carrier concentration at T=300 K, the main DC and 
small signal microwave performances of a device with a 
geometry of 2x25x0.5 µm². 
 
µ (cm2/Vs) at T=300 K 1170 
ns (cm
-2) at T=300 K 1.5 1013 
ID (A/mm) at VGS=1V and VDS= 27V 1.15 
gmext (mS/mm) at VGS=0.5V and VDS= 15V 160 
VP  (V) -6 
fT (GHz) at VGS=-2V and VDS= 10V 18 
fMAG (GHz) at VGS=-2V and VDS= 10V 50 
fMAX  (GHz) at VGS=-2V and VDS= 10V 75 
PDC (W/mm) expected 
at VDS=27V and forIDsat=1.2A/mm 
6.25 
 
Table 1: Device parameters 
 
The cut-off frequencies fT, fmax and fMAG 
determined from the scattering parameters on a Vector 
Network Analyzer (VNA) HP8510 are respectively 18 
GHz, 75 GHz, 50 GHz. The breakdown voltage in 
transistor configuration (at open and close channel) 
appears at a drain-source bias voltage higher than 30 V. 
The drain current density is 1.15 A/mm at VGS=1 V and 
the DC extrinsic transconductance is 160 mS/mm. The 
static and pulsed drain current characteristic versus drain-
source bias voltage for several gate-source voltages are 
shown figure 2. 
 
VDS(V)
0 5 10 15 20
I D
(m
A
/m
m
)
0
200
400
600
800
1000
1200
1400
Pulsed characteristic with a quescient bias point
 VDS0=0V and VGS0=0V.
Static characteristic
VGS=-6 to 1V step 1V 
 
Figure 2: Pulsed and static characteristics 
 
The pulsed I(V) measurement has been carried 
out at a quiescent bias point of VGS0=0 V and VDS0=0 V. 
This quiescent bias point (cold polarization) permits to 
eliminate the thermal effects. The pulsed set up is 
described elsewhere [7]. 
On the ID(VDS) static characteristic, the channel 
conductance is negative. This phenomenon is attributed 
to thermal effects because the channel conductance is 
nearly zero in pulsed configuration. Moreover, the 
maximum drain current (at VGS=1 V) on the pulsed 
characteristic is higher than the one coming from the 
static characteristic. The output power density expected 
from the static ID(VDS) characteristic is 6.25 W/mm 
(∆I*∆V/8). This one has been calculated at VDS=27 V 
with IDSAT=1.2 A/mm and Vknee=7 V at VGS=2 V. 
 
III. LARGE SIGNAL CHARACTERIZATION 
 
III.1 Set up description and measurement conditions 
 
 This set up permits the device measurement at 
microwave frequencies on wafer or in fixture and gives 
the possibility to observe the power performances, the 
signal forms of the drain-source voltage versus biases 
and load impedances conditions simultaneously. It is an 
automatic passive load-pull system. The calibration 
procedures are described elsewhere [8]. 
A VNA allows to determine the load impedance 
presented at the output plane of the DUT (Devices Under 
Test). The load impedances are carried out with a double 
slugs tuner.  
 
III.2 Power results at 4GHz 
 
  3 
The power measurements have been performed near 
the breakdown voltage in order to obtain a high drain-
source voltage swing in power condition.  
 The DUT has been biased at VGS=-2.9 V, VDS=27 V 
and the optimal power load impedance presented at the 
output plane of this device is Zload (Ω ) =1200 + j200 in 
large signal condition. 
 Figure 3 presents the power gain, the power added 
efficiency (PAE), and the output power for a HEMT with 
a geometry of 2x25x0.5µm² versus the absorbed input 
power.  
 
Pabs (dBm)
-15 -10 -5 0 5 10 15
P o
ut
 (d
B
m
), 
 G
P 
(d
B
)
0
5
10
15
20
25
η
PA
E 
(%
)
0
10
20
30
40
50
Pout
GP
ηPAE
POUT = 5W/mm
 
Figure 3: Power results of a 2x25x0.5µm² device at 
VDS=27 V and VGS=-2.9 V (Zload (Ω )=1200+j200) at 
F=4 GHz 
 
The maximum output power reaches 5 W/mm, the 
respective power gain is about 10dB (on the other hand 
the linear power gain is 16dB) and the associated PAE is 
39%. Now, this power result represents the state of the 
art to our knowledge in Europe on sapphire substrate. 
Moreover, the output power density expected of 
6.25W/mm (see table 1) is not reached. The difference 
between the DC output power density expected and the 
RF output power measured can be explained by thermal 
effects or/and by the presence of traps [8], [9]. A 
passivation step with SixNy can be used on this sample in 
order to limit trap effects and so to increase the 
microwave output power [10].It is well know that 
sapphire has not a good thermal conductivity 
(0.46W/cm.K). For this reason, to increase the thermal 
dissipation, the sapphire substrate can be thinned and 
power performances can be improved.  
 
IV. CONCLUSION 
 
An output power density of 5 W/mm and a 
linear power gain close to 16 dB have been measured 
with AlGaN/GaN HEMTs on sapphire substrate at 4 
GHz. These results constitute the state of the art in 
Europe. The high potential of AlGaN/GaN/Al2O3 HEMT 
devices for power applications at microwave frequencies 
has been demonstrated. There is still a weak difference 
between the output power density expected and the 
measured one. This difference can be explained by 
thermal effects and trap effects. 
 
ACKNOWLEDGEMENT: This work has been carried out 
with the financial help of DGA (French Army), contract 
n°0134050, and the ESA contract 14205/00/NL/PA. 
 
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High Power Performances of AlGaN/GaN HEMTs On Sapphire Substrate At F=4GHz

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