ByAya Emen20.05.2013OHMIC CONTACT RESISTANCE RESEARCH

Contact resistance depends on;Epitaxy-Barrier layer (Al concentration)-Barrier layer thickness-GaN cap layerMetal stack-type of metals-thickness-ratiosAnnealing-annealing temperature-annealing time-thermal ramp rate

For AlGaN/GaN HEMT Low contact resistance (Rc) decreases losses reduces self heating and promotes higher fT and FmaxOhmic contact formation in AlGaN/GaN during annealing that, N is extracted from the AlGaN, leaving vacancies behind which act as n-dopants Current flow: thermionic, tunnelingRc

Ti/Al/Ni/Au contacts on Al0,24Ga0,76N/GaN HEMT on SI-SiCNs=1x 1013 cm2, = 1400 cm2/V.S ,Rs=450 ohm/sqAl/Ti= 7.5

Ni thickness affects contact ResiastanceAu thickness affects surface morphology

Best: Ni :1.8X, Au: 1X annealed @ 832 C 30s Rc=0,26 ohm mm Surface Rougness: ~22 nm

Ti/Al/Pt/Au contacts on HEMT on sapphire 200/800/400/1500 A Annealed @900C for 35s.

Rc=0,039 ohm mmHall measurements:Ns=1.85x 1013 cm2, = 460 cm2/V.S ,Rs=450 ohm/sq

Nsx=1,670 x 1016 1/V.S

DC Measurements: Idss=1000mA/mm@ Vgs=0.5 Vgm=200mS/mm @Vg=-4VVbr>70V

Ti/Al/Ni/Au (30/90/30/100 nm) contacts onAl0,25Ga0,75N/GaN HEMT on SI-SiCAnnealed @900C for 30s

RTP ramping rates varying

IDS-Sat decrease by decreasing ramp ratetype I: 420mA/mm type : 260mA/mm

IDS (V=15V) Type : 715 A/mmType II: 86 A/mmType :76 A/mmThermal stress induced N vacancy

IGS leakageType I >by 2 orders of magnitudeIncreased surface/interface charge caused by N vacancy, lowers the barrier height and increases gate leakage

Ti/Al/Ni/Au (200/1200/500/2000 A) contacts onAl0,28Ga0,72N/GaN HEMT on sapphireNs=6x 1012 cm2, = 900 cm2/V.S ,Rs=420,6 ohm/sqAnnealed @900C for 30s.Rc=0,752 ohm mm

Subjected to thermal aging for 3 different temperatures

Themally induced changes in Total rrrrrrresistanceThermally induced changes in total resistance

At T

Ti/Al/Mo/Au (15/65/35/50 nm ) contacts onn-Al0,28Ga0,72N/GaN HEMT on SiCNs=1,2 x 1013 cm2, = 1200 cm2/V.S ,Annealed at diferent temperatures (400,500,650,700,850 ve 950 C) for 30s

Best : Rc=0,176 ohm mm @850 C

At 850 C condition, high density TiN islands were observed tohave nonuniformly formed along threading dislocations andhad penetrated through the AlGaN into the underlying GaNLayer and allowed direct transport of carriers across the AlGaN. The size, density, and distribution of the TiN islands were crucial in promoting and enabling efficient electron transport.

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