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High performance and high reliability AlGaN GaN HEMTs


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High performance and high reliability AlGaN/GaN HEMTs

applications and materials science

Toshihide Kikkawa*, 1, 2, Kozo Makiyama1, 2, Toshihiro Ohki1, 2, Masahito Kanamura1, 2, Kenji Imanishi1, 2, Naoki Hara2, and Kazukiyo Joshin1, 2
1 2

Fujitsu Limited, 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan

Received 1 December 2008, revised 2 March 2009, accepted 4 March 2009 Published online 23 April 2009 PACS 85.30.Tv
*

Corresponding author: e-mail kikkawa.toshi@jp.fujitsu.com, Phone: +81 46 250 8243, Fax: +81 46 250 8168

In this paper, a current status and future technologies of highpower GaN HEMTs was described. First, commercialization roadmap was shown with output power and efficiency status. Power electronics benchmark was also introduced. Reliability improvement technologies were addressed with recent issues such as drift phenomena. Then, future requirements for ex-

panding GaN electronics market were shown with some recent device developments. Novel E-mode recessed GaNHEMT has been developed using the triple cap layer structure. High-k insulated gate HEMTs using Ta2O5 were also developed. Finally, we described the next generation GaN HEMTs for millimeter-wave applications.

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1 Introduction The GaN high electron mobility transistors (HEMTs) have a high breakdown voltage with high cutoff frequency, compared to the other material based devices, leading to high power systems with high efficiency [1–3]. Figure 1 shows the commonly-used commercialization roadmap of GaN electronic devices. Currently GaN HEMTs for transmitter power amplifiers (PAs) of wireless base stations have been commercialized since 2005. Wireless mobile networks are expected to move up to mobile WiMAX, LTE, and 4G technologies from 2009. As transmission speeds will be over 100 Mbps, power consumption of transmission amplifiers (PAs) will be increased drastically. This results in significantly higher power and more physical space in the base station system. Thus, next generation networks will necessitate much higher power efficiency to dramatically reduce the increased power consumption. Switching-mode or envelop tracking PAs are candidates for high-efficiency PAs. In these architectures, transistors will be used at the power saturation region with high-efficiency. Compared with Si-LDMOS, GaN HEMTs showed a higher maximum efficiency, indicating the advantage of GaN HEMT for future PAs. GaN HEMTs for higher frequency application up to X-band have been developed close to commercialization phase. Millimeter-wave amplifiers have also been devel-

oped for new markets. Recently, power electronics using GaN are attracting much attention. In these applications, enhanced-mode operation has been required with high reliability. This commercialization will start after 2010. 2 Experimental procedures in Fujitsu A specific device structure to achieve high efficiency operation for wireless communication application at 2 GHz was developed in Fujitsu. This GaN HEMT consists of metal organic vapour phase epitaxy (MOVPE) grownn-GaN/n-AlGaN/ i-GaN structure, which was called surface-charge-controlled structure as shown in Fig. 2 [4]. The n-type doped GaN cap layer can suppress dispersion in I–V characteristics up to Vds of 100 V. Figure 3 shows device simulation results of un-doped AlGaN-cap case and n-GaN cap case. The n-AlGaN electron supplying layer was used for both cases. In the AlGaN-cap layer device, its built-in electric field almost reaches the breakdown electric field strength, causing low reliability. When using n-GaN cap layer, built-in electric field can be suppressed by piezoelectric charge between the n-GaN cap layer and n-AlGaN layer. Recessed ohmic technique was used to reduce the ohmic contact resistance [5]. SiN passivation layer was optimized to obtain lower trap device structures. SiC substrates were used to obtain good thermal managements.
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Phys. Status Solidi A 206, No. 6, 1135– 1144 (2009) / DOI 10.1002/pssa.200880983

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100G 40G 10G

Frequency (Hz)

POUT (W)

Millimeter wave communication Fixed wireless access / Base station communication Satellite Satellite communication (VSAT) / broadcasting / Rader Space electronics

1000

L-S Band
800 600 400

Wireless communication

1G 100M

W-CDMA WiMAX (IMT2000)

LTE

4th Generation

Power electronics PFC/ Diode PC/ Home Car/ Railway

200 0

100K 2005 Current status 2010 Year 2015 2020

(a)
2000 2002 2004 2006

Year

RF power density (W/mm)

Figure 1 (online colour at: www.pss-a.com) Commercialization roadmap of GaN HEMT for several applications divided by the frequency.

100
Cu rre

nt t he r ma ll

imi

t

10

Gate Source
0.5 - 0.8 (mm) 4 - 6 (mm) 1 (mm)

Drain
20 - 40 (nm) 2 - 10 (nm)

SiN n-GaN n-AlGaN

1 1

(b)
10 100 1000 RF total output power (W)

10 - 30 (nm)

Figure 4 (online colour at: www.pss-a.com) (a) Trends of output power of GaN HEMT power amplifiers (PAs) at 2 – 3 GHz. (b) Power density vs. total output power at 2 – 3 GHz.

u-GaN buffer layer

1 - 2 (mm)

S.I.-SiC substrate
Figure 2 (online colour at: www.pss-a.com) A cross-section of surface-charge-controlled structure.

Gate

SiN AlGaN

3 Results and discussions 3.1 Current status Figure 4(a) shows the trends of output power of GaN HEMTs from the literatures. An over 1 kW output power could be obtained on SiC substrates [6]. Figure 4(b) shows power densities with total output power. At the lower output power level, power densities up to 40 W/mm could be available. At the higher output power level over 10 W, power density of 10 W/mm has been the highest power density due to the thermal management issues.
Table 1 Recent developed technologies for high performance to eliminate gate leakage current and current collapse. device structure example of reported affiliations UCSB, NEC, Cree, Toshiba, HRL, Nitronex, etc. Oki, HRL, Sharp, UCSB, Fraunhofer, etc. SCSU, IMEC, UCSB, NICT, Panasonic, etc. Mitsubishi UCSD, Triquint, Nitronex, etc. Fujitsu, Eudyna

(a) un-doped AlGaN-cap

Gate

SiN GaN
(b) n-GaN-cap

field plate recessed gate insulated gate annealed gate i-GaN cap epilayer n-GaN cap epilayer

Figure 3 (online colour at: www.pss-a.com) Device simulation results of electric field distribution for (a) un-doped AlGaN-cap case and (b) n-GaN cap case.
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Phys. Status Solidi A 206, No. 6 (2009) 1137

Table 2 Drain efficiency of several different PAs using GaN HEMTs for base station applications around 2 GHz. PA type class AB class D class E/J class F class S Doherty envelope tracking maximum efficiency (%) @CW mode 65 – 75 [1] 78 [7] 82 [8] 90 [9] – 65 – 75 [11] – efficiency (%) @modulation signal 40 [1] – 45 [8] – 90 (without driver) [10] 80 (with driver) [10] 45 – 50 [11] 75 (without ET amp.) [12] 55 (with ET amp.) [12]
Ids

Idle

Power on

Power off

Initial Idsq drift 0 0 Time

Idsq recovery rate (%)

Two problems were mainly studied in the initial stage. One problem was the large gate leakage current and the other was large current collapse. Table 1 shows the typical technology list for obtaining low gate leakage current and small current collapse. Field plate technologies were mainly used by the many affiliations. Two different field plates such as the source-connected field plate and the gate-connected field plate were investigated. Field plate could reduce the electric field at the gate edge, enhancing reliability. Recessed gate structures were also investigated to obtain low on-resistance. Insulated gate and annealed gate could effectively reduce the gate leakage current. GaN cap structures were also introduced to modify the electric field. We mainly used n-type doped GaN cap structures to suppress gate leakage current and current collapse. Table 2 shows the drain efficiency comparison using GaN HEMTs. PA types were changed in this table. Class AB has been commonly used in PAs. Currently, new PA types in Table 1 have been developed using GaN-HEMTs. High maximum efficiency at continuous wave (CW) mode could be obtained using the new PA types. High drain efficiency was attributed to high breakdown voltage and high output impedance of GaN-HEMT. In the power electronics, the benchmark of the onresistance as a function of the breakdown voltage is comSpecific on-resistance (mW cm–2)
100

Figure 6 Method to investigate Idsq drift. Saturation power was applied at 50 V. After power was turned off, Idsq was decreased from the initial value. Then the Idsq was recovered slowly to the initial value.

monly used for comparing materials as shown in Fig. 5. Low on-resistance of less than 10 mΩ cm2 with breakdown voltages of 400–1000 V was obtained using GaN. In this study, Lgd was expanded to over 10 μm. Thus, onresistance became large. This will be the main issued to be solved in the future. One candidate was vertical devices. However, current breakdown voltage of vertical device was lower than 100 V. 3.2 Efficiency improvement To enhance the efficiency of GaN HEMT power amplifiers, we further optimized the device buffer structure. In the conventional GaN HEMTs, the Idsq-drift was observed at an end of RF power saturation operation. This drift phenomenon was different from the current collapse. Photoluminescence was used for evaluating deep traps in the GaN buffer layer and we found that yellow luminescence from the GaN buffer layer had strong correlation with this drift phenomenon. A carefully
Initial Idsq = 2% Imax 100 80 60 40 20 0 0 20 40 60 80 8 100 00 Time (sec)
Figure 7 (online colour at: www.pss-a.com) Conventional Idsq transient phenomena. Initial Idsq was settled at 2% maximum current (Imax) at 50 V. Device topology was shown in Fig. 2. Total gate width was 1 mm. Saturation power was 5 W. Deep class-AB was used in this study for base station applications. Three different lines correspond to different wafers. Recovery rate was varied by run to run.
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t iu mj jin !lm j i S T
GaN results

10

Si

C

t ji lim it

1

N Ga

lim

0.1 100

1000 Breakdown voltage (V)

10000

Figure 5 (online colour at: www.pss-a.com) Benchmark of materials for power electronics.
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Recovery rate (%)

80 60 40 20 0 Initial Ids = 1.4% Imax Vds : 50 V to 30 V

Recovery rate (%)

100 80 60 40 20 0 Newly developed results Conventional Initial Idsq = 1.4%Imax 0 20 40 Time (sec) 60 80

0

50 Time (sec)

100

Figure 8 Drain lag effect at Ids of 2% Imax. Device topology was shown in Fig. 2. Total gate width was 1 mm. Saturation power was 5 W at 50 V.

Figure 10 (online colour at: www.pss-a.com) Newly developed Idsq drift results in this study. Faster recovery time could be realized by focusing on yellow luminescence improvement.

AM-PM (deg.)

controlled GaN buffer layer with low yellow luminescence could extremely reduce this Idsq-drift, which improved efficiency. Figure 6 shows the method to investigate Idsq drift. Figure 7 shows typical Idsq recovery phenomena, i.e., drift phenomena. Three different lines show the run to run wafer variation in the Idsq drift study. Idsq recovery rates as a function of time after power measurements were shown. Idsq was decreased from initial Idsq of idling stage just after power measurements. Then Idsq recovered slowly. It took over 1 min to recover to original value. Even if these phenomena were observed, GaN HEMTs could provide over 100 W with over 60% drain efficiency at 50 V. These have never been found at GaAs based field effect transistors (FETs). In addition, Idsq drift phenomena were different from saturation current (Idss) drift which has been reported by Si-LDMOS [13]. Current collapse is a μs-order phenomenon and usually observed at pulsed current– voltage (I – V) measurements from the pinched-off bias point [14]. This Idsq drift is quite different from the current

collapse. Idsq drift caused larger memory effect and lower efficiency with instability. When initial Idsq was increased, Idsq recovery time became shorter drastically from over 1 min to less than 30 s. Thus, when GaN-HEMT was used at deep class AB, these Idsq drift phenomena became obvious. Idsq drift was also affected by ambient temperature. Higher ambient temperature resulted in fast recovery time from over 1 min to less than 30 s, suggesting that deep traps caused Idsq drift. To investigate the origin of these traps, drain lag effect was investigated. Drain lag effect was measured as follows.
40

0

1
Intensity (a.u.)

Deep level PL bands

-40

30

(a) Conventional

35 Pin (dBm) (a) Conventional

40

GaN
(b) Newly developed results

AM-PM (deg.)

40

0

0 340 440 540 640 740 840 Wavelength (nm)
Figure 9 (online colour at: www.pss-a.com) Photoluminescence of GaN-HEMT epi-layers. Excitation power of He – Cd laser was 0.1 mW. Yellow luminescence around 500 – 600 nm range was improved by optimizing growth conditions. Laser intensity was small as (a) conventional layer condition and (b) improved layer condition.
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-40

30

35 40 Pin (dBm) (b) Newly-developed

Figure 11 (online colour at: www.pss-a.com) AM – PM measurements of GaN-HEMT by 20 MHz 2.5 GHz WiMAX signals. Device topology was shown in Fig. 2. Total gate width was 28.8 mm. Saturation power was 100 W at 50 V. Memory effect became smaller by improving Idsq drift.
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Phys. Status Solidi A 206, No. 6 (2009) 1139

D rain E fficiency (% )

50 40 30 20 10 0 5 6 7 8 9 10 11 12 13 14 1 15 5 Back off from saturation power (dB) (a) Conventional (b) Newly developed

Thermal resistance (?C/W)

10 on Si (www.nitronex.com) 1 on SiC

0.1 10

100 1000 Saturation power (W)

Figure 12 (online colour at: www.pss-a.com) Drain efficiency as a function of output power. Improved Idsq drift resulted in higher efficiency at WiMAX signal. Device topology was shown in Fig. 2. Total gate width was 28.8 mm. Saturation power was 100 W at 50 V.

Figure 14 (online colour at: www.pss-a.com) Difference of thermal resistance between substrates.

1) Ids was settled at same value of Idsq for power measurements such as 2% maximum current (Imax). Thermal issues can be ignored because Ids was too small. 2) Then only Vds was changed from 50 V to 30 V. Vgs was not changed during this measurements. 3) Ids change was monitored after Vds change. Ids was dropped when Vds was decreased rapidly from 50 V. Then Ids recovered for several minutes as shown in Fig. 8. This phenomenon was similar to Idsq drift after power measurements. Thus, Idsq drift was attributed to drain lag effect when Ids was small compared with Imax. We also measured GaN channel quality using PL. Deep traps of GaN channel layer can be detected by PL. The He–Cd laser was used to evaluate PL characteristics of GaN. Excitation power of the He–Cd laser was decreased to as low as 0.1 mW to detect deep level PL bands such as yellow luminescence with high sensitivity. Yellow luminescence around 550 nm from GaN channel layer was mainly focused on investigating drain lag effect. As shown

in Fig. 9(a), strong yellow luminescence was observed, suggesting that the origin was located in the GaN buffer. Origin of yellow luminescence is considered as Ga vacancy and carbon impurity, which might cause deep electron traps [15]. We improved yellow luminescence of GaN channel layer by changing MOVPE growth conditions, as shown in Fig. 9(b). Figure 10 shows Idsq drift of GaNHEMT with the improved yellow luminescence. Idsq recovery time became improved, indicating that low yellow luminescence origin affected Idsq drift. Influence of Idsq drift on AM-PM (amplitude modulation/phase modulation) memory effect was firstly investigated, which is most important characteristics for digital pre-distortion PAs. Mobile WiMAX signal (64QAM) at 2.5 GHz was used. Signal band width was 20 MHz. Figure 11(a) shows AM–PM characteristics of conventional GaN-HEMT with large Idsq drift. Scattered AM–PM characteristics were observed, suggesting large memory effect. Figure 11(b) shows the AM–PM characteristics of improved GaN-HEMT. Improved drift devices proved smaller memory effect. This indicates that Idsq drift influenced memory effect of power amplifier. Figure 12 shows drain efficiency as a function of backed-off output power under mobile WiMAX signal
Source Gate NiSi Drain

Drain efficiency (%)

70 60 50 40 ACLR 30 41 43 45 Power (dBm) 50%

-20
ACLR (dB)

Efficiency -30 -40 -50 -60 47

SiN n-GaN Gate leakage path n-AlGaN -----------------------i-GaN

S.I.-SiC substrate

Figure 13 (online colour at: www.pss-a.com) Performance of newly-developed GaN-HEMT power amplifier for mobile WiMAX. ACLR of –50 dB for 20 MHz signal was realized with 50% record-efficiency. Device topology was shown in Fig. 2. Two 100 W packaged devices were used at 50 V.
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Figure 15 (online colour at: www.pss-a.com) Cross section of GaN-HEMT with the NiSi formation at the side edge of the Ni gate electrode.
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Imax current collapse ratio

Source

Gate

Drain

1 0.8 0.6 0.4 0.2 0 0.01

High reliability

SiN n-GaN n-AlGaN i-GaN

NiO barrier

------------------------

Low reliability

S.I.-SiC substrate
Figure 16 (online colour at: www.pss-a.com) Cross section of GaN-HEMT with the NiO barrier to suppress gate leakage current.

0.1

1

10

100

Gate leakage current @ 40 V (mA/mm)

Figure 18 (online colour at: www.pss-a.com) Imax current collapse ratio as a function of gate leakage current.

(16 QAM) at 2.5 GHz. Improved drift devices showed higher backed-off efficiency. Drain lag effect might cause difficulties of high efficiency matching. Figure 13 shows developed GaN-HEMT power amplifier with DPD for mobile WiMAX signal (20 MHz–16 QAM). Record average drain efficiency of over 50% and liner gain of 17.2 dB were obtained at 45 dBm, satisfying full specification of mobile WiMAX. This efficiency was over 10 point higher than that of a conventional PA. 3.3 Thermal issues Substrates are key technologies to decrease the cost. GaN on Si is a good candidate for realizing low cost. However, thermal conductance of Si is lower than SiC. Thus, the thermal resistance of commercially available packaged GaN HEMTs on Si and those on SiC were compared as shown in Fig. 14. The packaged GaN HEMTs on SiC showed a half thermal resistance compared with those on Si. Currently, SiC substrate is most appropriate substrate to suppress the increase of channel temperature. 3.4 Reliability Reliability is the most significant issue to be discussed for manufacturing. First, initial degradation

or infant failure should be improved. We have found NiSi formation between Ni gate electrode metal and SiN passivation layer. NiSi contributed to initial degradation as shown in Fig. 15 [16]. To suppress the NiSi formation, NiO barrier insertion between Ni electrode and SiN layer was developed as shown in Fig. 16. DC stress test was applied at 200 °C. With NiO barrier case, no sudden degradation was observed as shown in Fig. 17. Current collapse was also affected by gate leakage current. Imax current collapse ratio was defined as the following equation,
I max current collapse ratio = I max B . I max A

In the above equation, Imax A was defined as the drain current at Vg of 2 V and Vd of 5 V under pulsed measurement with the bias conditions at Vg of 0 V and Vd of 0 V.
Table 3 Key features of future GaN technologies for wireless communication and power electronics. parameters current proven technology @2 GHz D-mode >300 future technology wireless >1 GHz D/E-mode power electronics <100 MHz E-mode

10-0 Pinched-off DC stress test

Ig (A/mm)

10-1 10-2 10-3 10-4

NiSi

Vds= 100 V Vgs= -5 V 250 ∞C

NiO(method#1)

NiO(method#2)

0

1000 2000 Time (sec)

3000

pinched-off voltage breakdown voltage without collapse (V ) operation voltage (V ) Lg (?m) gate device type cost

>100@ Lg = 0.1 ?m >1000

50 0.5 ? 1.0 Schottky lateral –

>50@2 GHz >30@ >10 GHz 0.1 ? 0.5 MIS lateral to lower 4?6

100 ? 600 >1.0 MIS vertical extremely lower 6?8

Figure 17 (online colour at: www.pss-a.com) DC stress test for investigating initial degradation at 100 V.
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substrate size 3 (inch)

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Gate 0.8 mm Source Ta2O5 SiN Drain

Transconductance, gm (mS/mm)

250 200 150 100 50

Schottky gate HEMT Ta2O5 MIS-HEMT (10 nm) SiN MIS-HEMT (20 nm)

n-GaN n-AlGaN i-GaN S.I.-SiC

2DEG

Vds=10 V

0 -6

-4

-2

0

2

Gate voltage, Vgs (V)

Figure 19 (online colour at: www.pss-a.com) Cross section of the Ta2O5 MIS-HEMTs.

Figure 20 (online colour at: www.pss-a.com) Transfer characteristics of MIS-HEMTs and Schottky gate HEMT.

ImaxB was defined as the drain current at Vg of 2 V and Vd of 5 V under pulsed measurement with bias conditions at Vg of –3 V and Vd of 50 V. This Imax current collapse definition was more sensitive compared with Ron (on-resistance) definition. Figure 18 shows the current collapse ratio as a function of gate leakage current. When gate leakage current was large, current collapse could be suppressed easily. Because when gate leakage was large, electric field at the gate edge was decreased. In addition, captured electrons at the surface traps under pinched-off case could be easily emitted to 2DEG when operation became on. However, in those cases, poor reliability was obtained due to gate interface degradation by gate leakage current [14]. To obtain a high reliability, low gate leakage current of less than 1 × 106 A/mm with small current collapse was essential. By carrying out the gate leakage current screening method, long MTTF (mean time to failure) of 106 h at 200 °C was obtained [17]. 3.5 Future prospects Table 3 shows the key features of future GaN technologies for wireless communication and power electronics. MIS HEMTs, E-mode HEMTs and millimeter wave HEMTs will be the important target for future technologies. 3.6 MIS-HEMT MIS HEMTs have been required to realize low forward leakage current. Our developed structures are shown in Fig. 19 [18]. SiN was used as the pasTable 4 Material parameters of dielectric materials. Eg (eV) SiO2 SiN Al2O3 HfO2 ZrO2 Ta2O5
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sivation layer between electrodes. The Ta2O5 was used as the gate insulating layer, since the thermal expansion coefficient of Ta2O5 was close to SiN passivation, as shown in Table 4. Ta2O5 MIS-HEMTs showed the small current collapse, the large gm of 200 mS/mm as shown in Fig. 20. Lgd was 5 ?m. Low gate-leakage current of less than 1 × 10 A–8/mm with a high breakdown voltage (BVgd) of 400 V was observed. Both a high output power over 100 W and high gain of 16 dB were successfully achieved at 2.5 GHz [18]. 3.7 E-mode HEMT Novel recessed gate GaN-HEMT using 2DEG-induced structure with GaN/AlN/GaN triple cap layer was developed as shown in Fig. 21 [19]. Piezoelectric charge at the interface between AlN and GaN laySource SiN
++++++++ ++++++++

Gate

Drain

1st layer: n-GaN (2 nm) 2nd layer: i-AlN (2 nm) 3rd layer: n-GaN (2 nm)

n-AlGaN ----------------------i-GaN buffer S.I.-SiC substrate

Gate Recess

Figure 21 (online colour at: www.pss-a.com) Cross section of the recessed gate GaN-HEMT with GaN/AlN/GaN triple cap layer.

ε 3.9 7 9 30 25 25

coefficient of thermal expansion (ppm/K) 8 2 5 9 9 5

melting point (°C) 1600 1900 2020 2800 2700 1800

MIS-HEMT

9 5 8 6 7.8 5.3

Hokkaido Univ. Nagoya Univ. UCSB this work

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n-GaN n-AlGaN i-GaN

n-GaN i-AlN n-GaN

600 500
I d (mA/mm)
i-GaN

Vds= 10 V

300 250 200 150 100
gm (mS/mm)

n-AlGaN 2DEG

400 300 200 100 0 -1 0 1 V gs (V) 2 3

2DEG

80% larger 2DEG

Ef

(a)

(b)

Vth= +0.25 V

50 0

Figure 22 (online colour at: www.pss-a.com) Simulated band diagram of GaN-HEMT. (a) GaN-cap structure and (b) GaN/ AlN/GaN triple cap structure.

100

Pout (dBm) and Gain (dB)

ers induced additional two dimensional electron gas as shown in Fig. 22. GaN surface cap layer prevented AlN surface from cracking during the cooling down procedure from growth temperature to room temperature as shown in Fig. 23. We investigated if the cracks were screened by GaN cap growth. Using TEM, no cracks were observed when using GaN cap. However, when AlN-terminated cap was grown over 2 nm, in-situ cracks at growth temperature happened, increasing sheet resistance significantly. Even if we used GaN-cap, no effect of surface smoothing effect was observed in this in-situ cracking case. When Lgd was 5 ?m, high Imax of over 500 mA/mm with threshold voltage (Vth) of +0.25 V was obtained with a high breakdown voltage of 336 V as shown in Fig. 24. High power of 126 W was obtained using 36 mm gate periphery GaN-HEMT power amplifier at 2.5 GHz as shown in Fig. 25. This is the first demonstration of over 100 W output power using E-mode GaN-HEMT. Figure 26 shows the benchmark of E-mode GaN-HEMT. High Imax with high breakdown voltage was demonstrated. 3.8 Millimeter-wave HEMT Figure 27 shows our developed millimeter-wave HEMTs. 0.08 ?m to 0.1 ?m Y-gate structure and GaN cap layer were used to obtain high breakdown voltage [20]. Figure 28 shows the I–V characteristics. High gm of 350 mS/mm was obtained with

10-1 10-2 10-3 10-4 10-5 BVgd= 336 V 10-6 -7 10 10-8 10-9 -400 -300 -200 -100 V gd (V)

I g (A/mm)

0

Figure 24 (a) Transfer characteristics and (b) reverse gate leakage current of the E-mode-HEMTs. Lgd was 5 μm.
60 50 40 30 20 10 0 10 30 P in (dBm) 50

Pout

126 W

Wg = 36 mm Wgu = 300 mm Lg = 0.5 mm Psat= 126 W GL = 14.7 dB at Vds= 50 V, Idsq= 280 mA Freq.= 2.5 GHz

Gain

Figure 25 (online colour at: www.pss-a.com) Power characteristics of E-mode GaN-HEMT.

1200 1000
I dmax (mA/mm)

Target area

800 600 400 200 0 10

This work
Previous reports

Figure 23 (online colour at: www.pss-a.com) AFM top view image of GaN-HEMT. (a) AlN terminated cap structure and (b) GaN/AlN/GaN triple cap structure.
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100 1000 BV gd (V)

10000

Figure 26 (online colour at: www.pss-a.com) Benchmark of E-mode GaN-HEMT.
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Phys. Status Solidi A 206, No. 6 (2009) 1143

Y-shaped Gate
Gain (dB)

60 50 40 30 20 10 0 1 10 Frequency (GHz) 100 fT=85 GHz

Vds = 10 V

n-AlGaN i-GaN S.I. SiC Sub.

n-GaN n-AlGaN i-GaN S.I. SiC Sub.

(a)

(b)

fmax=200-230 GHz
Figure 30 (online colour at: www.pss-a.com) Small signal RF characteristics of 0.08 μm gate HEMTs measured at Vds of 10 V.

Figure 27 (online colour at: www.pss-a.com) Cross section of the millimeter-wave GaN-HEMTs. (a) Conventional structure and (b) newly-developed Y-shape structure.

50
1.0 0.8 Lg = 0.1 mm Vg=2 V Vg=1 V Vg=0 V Vg=-1 V Vg=-2 V 0 10 20 30 40 Vds (V) 50 60 VGS= -3 ~ +2 V VGS STEP= 1 V

mm Lg Lg=0.08

40

Vds = 5 V
U Gamax |H |2 21

Gain (dB)

Ids (A/mm)

0.6 0.4 0.2 0.0

30 20 10 0 1 1 10 Frequency (GHz)

Figure 28 (online colour at: www.pss-a.com) I – V characteristics of 0.1 μm gate HEMT.

100 00 fT=100 GHz fmax=200 GHz

Figure 31 (online colour at: www.pss-a.com) Small signal RF characteristics of 0.08 μm gate HEMTs measured at Vds of 5 V.

100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 0 20

250 200

Ids (A/mm)

fmax (GHz)

150 100 50
Previous work

Fujits Fujitsu

Recent improvement

Vg=-5 V Vg=-10 V 40 60 Vds (V) 80 100

0 0

50 100 150 200 Breakdown voltage BVgd (V)

250

Figure 29 (online colour at: www.pss-a.com) Three-terminal off-breakdown voltage of 0.1 μm gate HEMT.
www.pss-a.com

Figure 32 (online colour at: www.pss-a.com) Benchmark of the millimeter-wave HEMTs.
? 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

pss
status solidi

1144

physica

a

T. Kikkawa et al.: High performance and high reliability AlGaN/GaN HEMTs

Pout (dBm), Gain (dB)

Freq. = 76.5 GHz 20 Wg = 240 mm Vds = 30 V

25

Pout = 130 mW Pout

Acknowledgement This work was partially supported by the Ministry of Internal Affairs and Communications, Japan.

15 10 5 0 0 5

References
[1] T. Kikkawa et al., CS-MANTECH Digest (2006), p. 171. [2] T. Inoue et al., IEEE Trans. Microw. Theory Tech. (2005), p. 74. [3] Y.-F. Wu et al., IEEE IEDM Technical Digest (2005), p. 583. [4] T. Kikkawa et al., IEEE IEDM Technical Digest (2001), p. 585. [5] M. Kanamura et al., Extended Abstracts of Int. Conf. Solid State Devices and Materials (2003), p. 916. [6] E. Mitani et al., European Microwave IC Conf. Technical Digest (2007), p. 176. [7] U. Gustavsson, Master’s Thesis, ?rebro University (2006). [8] N. Ui et al., IEEE Radio Wireless Symp. Technical Digest (2006), p. 718. [9] D. Schmelzer et al., IEEE Compound Semiconductor Integrated Circuit Symposium Technical Digest (2006), p. 96. [10] C. Meliani et al., IEEE MTT-S Int. Microwave Symp. Technical Digest (2008), p. 751. [11] T. Yamamoto et al., IEEE MTT-S Int. Microwave Symp. Technical Digest (2007), p. 1263. [12] D. F. Kimball et al., IEEE Compound Semiconductor Integrated Circuit Symposium Technical Digest (2005), p. 89. [13] J. Olsson et al., IEEE Electron Device Lett. 2, 206 (2002). [14] S. C. Binari et al., IEEE Trans. Electron Devices 48, 465 (2001). [15] K. Saarinen et al., Phys. Rev. Lett. 79, 3030 (1997). [16] T. Ohki et al., CS-MANTECH Digest (2008), p. 249. [17] Y. Inoue et al., IEEE MTT-S Int. Microwave Symp. Technical Digest (2007), p. 639. [18] M. Kanamura et al., Phys. Status Solidi C 5, 2037 (2008). [19] T. Ohki et al., Extended Abstract Int. Symp. on Compound Semiconductors (2008), Tu.1.7. [20] K. Makiyama et al., Phys. Status Solidi A 204, 2054 (2007). [21] K. Makiyama et al., Abstracts of Int. Workshop Nitride Semiconductors (2008), p. 288.

Gain 10 15 20 Pin (dBm) 25

Figure 33 (online colour at: www.pss-a.com) Power characteristics of millimeter-wave GaN-HEMT without optimizing source impedance.

good pinched-off characteristics. Buffer layer improvement suppressed the short channel effect, resulting in high three-terminal off-breakdown voltage of over 100 V, when Lg was 0.1 ?m as shown in Fig. 29 [21]. High fmax of 200 GHz was obtained with high breakdown voltage. When Lg was decreased to 0.08 ?m, a high fmax of 230 GHz was confirmed at Vds of 10 V without decreasing breakdown voltage as shown in Fig. 30. At Vds of 5 V, fT of 100 GHz was obtained as shown in Fig. 31. Figure 32 shows the benchmark of GaN HEMT. Buffer improvement caused higher BVgd of 230 V. Figure 33 shows the power characteristics by the load-pull power measurement of the millimeter-wave GaN-HEMT in Fig. 31. Source impedance was not matched to minimize the input power loss from driver amplifier. A 130 mW power was successfully obtained, which is a high enough power level to fabricate GaN MMIC at W-band. 4 Conclusions Performance of GaN HEMTs has been improved with high reliability. Drift issues and sudden degradation were important to increase the yield of GaN HEMT chip die. Next generation technologies such as E-mode, MIS, and millimeter-wave HEMTs have been currently researched to extend the application area.

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