Electrical Degradation of InAlAs/InGaAs Metamorphic High-Electron Mobility Transistors S. D. Mertens and J.A. del Alamo Massachusetts Institute of Technology Sponsor: Agilent Technologies
Outline Introduction Electrical Degradation of mhemts Degradation of TLMs Degradation Mechanisms
Metamorphic HEMT: InP HEMT on GaAs Substrate Ohmics Source Gate Drain Cap Etch-Stop Insulator δ-doping Channel GaAs AlInAs InGaAs AlInAs GaAs Lg= 0.125 µm, ft= 150 GHz, gm= 1.05 S/mm, BV DG,off =4.8 V
Electrical Degradation of InAlAs/InGaAs mhemts Little known about reliability of mhemts Observations in InP HEMTs: Change in R D [Wakita et al] Change in R D and R S [Suemitsu et al] Change in V T [Christianson et al] Linked to: Impact ionization [Rohdin et al] Hot electrons [Menozzi et al] No systematic studies of InP HEMT electrical reliability
Effects of Electrical Stress Stress at V DS =1.5 V and I D =250 ma/mm for 12 hours 0.5 0.4 Before Stress After Stress 1200 Before Stress After Stress I D [ma/mm] 0.3 0.2 0.1 g m [ms/mm] 800 400 0.0 0.0 0.2 0.4 0.6 0.8 1.0 V DS [V] 0-0.4 0.0 0.4 V GS [V] Main effects of bias stress: Increase in R DS Decrease in I Most Worrying D Decrease in gm and ft
Time Evolution of Degradation Stress at V DGo + V T =1.6 V and V GS -V T = 0.3 V 1.6 1.4 1.2 1 R D g mo /R D (0) R S /R S (0) /g mo (0) The degradation of R D is most important is irreversible is initially very fast tends to saturate 0.8 0 200 400 600 time [min]
Electrical Stress Methodology Studied several bias stress schemes: Constant V DS & constant V GS Constant I D & constant I G - Device characteristics change bias point changes Constant V DGo & constant I D - Different devices, different degradation Constant I D & constant V DGo +V T Constant V GS -V T & constant V DGo +V T - Reproducible degradation - Keep impact-ionization constant
R D /R D (0) 1.6 1.4 1.2 R D Degradation Associated with Impact-Ionization? Impact-ionization rate I D exp(- V DGo +V T = 1.65 V 1.0 0 200 400 600 800 1000 Time [min] I D = 450 ma/mm 400 ma/mm 350 ma/mm 300 ma/mm 150 ma/mm R D /R D (0) 1.8 1.6 1.4 1.2 A V DGo +V T ) V DGo +V T = 1.7 V 1.6 V 1.5 V 1.4 V 1.3 V 1.2 V V GS -V T =0.3 V 1.0 0 200 400 600 800 Time [min] Higher impact-ionization Higher degradation
Impact-Ionization behind R D degradation Impact-ionization rate 1.E-03 I D exp(- A V DGo +V T ) drd/dt at 144 min 1.E-04 VGS=V T+0.3 V 1.E-05 0.5 0.6 0.7 0.8 0.9 1/(V DGo +V T ) Degradation rate follows classical impact-ionization behavior
Other Drain-Related Figures of Merit Change BV DGoff /BV DGoff (0) 1.3 1.2 1.1 1.0 V DGo +V T= 1.7 V 1.6 V 1.5 V 1.4 V 1.3 V 1.2 V V GS -V T =0.3 V 1.0 1.2 1.4 1.6 1.8 R D /R D (0) Both BV DGoff and R D depend on n s on drain side Drop in n s probable cause of degradation C dg also degrades
Step-Stress Experiments Improved experimental productivity R D /R D (0) and g mo /g mo (0) 1.15 1.10 1.05 1.00 0.95 Onset of mechanism 1 Onset of mechanism 2 1 0 400 800 1200 1600 time [min] R D /R D (0) g mo /g mo (0) 2 Degradation mechanisms can be identified 4 3 2 V DGo +V T [V]
Simpler Case: TLMs Ohmics Cap Etch-Stop Insulator δ-doping Channel GaAs Substrate I [A/mm] 1.2 1.0 0.8 0.6 0.4 0.2 V=3.8 V L=12 µm t=0 t=12 min t=302 min More Stress 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 V [V] Integrated TLMs : uniform field in channel Only two figures of merit: R, I sat
Time Evolution of TLM Degradation L=12 µm L=12 µm 3.0 5.2 1.5 Mechanism 2 Mechanism 1 5.0 5.0 5.2 R [ohm.mm] 2.5 2.0 4.8 4.6 4.4 V stress [V] I sat [A/mm] 4.8 V stress [V] 4.6 4.4 Mechanism 1 4.2 1.0 4.2 1.5 4.0 0 100 200 300 time [min] Two mechanisms appear: V<4.5 V R and I sat track each other V>4.5 V only R increases 4.0 0 100 200 300 time [min] n s
Increase of Lateral and Contact Resistance 3 5.0 R, R s, R C [ohm.mm] 2 1 R R s R C 4.8 4.6 4.4 4.2 V stress [V] R C R s R=R s +2R C R C 0 4.0 0 50 100 150 200 250 300 time [min] Mechanism 1: Degradation of n s R s Mechanism 2: Degradation of R C only,r C
Field Reversal R [ohm.mm] 2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 0 500 1000 1500 2000 time [min] Flip Polarity Flip Polarity 3.15 1.0 2.35 V stress [V] R [ohm.mm] 0.9 0.8 0.7 3.10 0.6 0.5 0 500 2.30 1000 time [min] Mechanism 1 Mechanism 2 Independent of stress polarity Dependent of stress polarity V stress [V] Uniform degradation Degradation of one ohmic contact
Critical Voltage for Degradation Critical Voltage vs Length 4 Mechanism 1 Mechanism 2 V critical [V] 3 2 1 E th (InGaAs)+2R C I sat 0 0 2 4 6 8 10 12 L TLM [µm] Threshold of degradation Threshold of impact-ionization Degradation exhibits constant-field behavior
Degradation Mechanisms Mechanism 1 Occurs at lower voltage n s drops on drain-side Saturates Occurs at surface (cap plays a role) Correlated with impact-ionization (also temperature) Polarity Independent Mechanism 2 Occurs at higher voltage Drain-ohmic contact degrades Does not saturate Polarity Dependent
Degradation Mechanisms Source Gate Drain Mechanism 2 Mechanism 1 Mechanism 1 Mechanism 2 Hot electron/hole generation by impact-ionization Hot carriers change drain side of device (trapping, recombinationenhanced damage) close to surface Hot electrons Drain-ohmic contact degrades Nothing specific about metamorphic substrate
Conclusions Degradation in mhemts correlated with impact-ionization Two degradation mechanisms identified Extrinsic drain surface Drain ohmic contact No specific degradation mechanism identified specifically associated with metamorphic substrate