Electrical Degradation of InAlAs/InGaAs Metamorphic High-Electron Mobility Transistors

Transcription

1 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

2 Outline Introduction Electrical Degradation of mhemts Degradation of TLMs Degradation Mechanisms

3 Metamorphic HEMT: InP HEMT on GaAs Substrate Ohmics Source Gate Drain Cap Etch-Stop Insulator δ-doping Channel GaAs AlInAs InGaAs AlInAs GaAs Lg= µm, ft= 150 GHz, gm= 1.05 S/mm, BV DG,off =4.8 V

4 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

5 Effects of Electrical Stress Stress at V DS =1.5 V and I D =250 ma/mm for 12 hours Before Stress After Stress 1200 Before Stress After Stress I D [ma/mm] g m [ms/mm] V DS [V] V GS [V] Main effects of bias stress: Increase in R DS Decrease in I Most Worrying D Decrease in gm and ft

6 Time Evolution of Degradation Stress at V DGo + V T =1.6 V and V GS -V T = 0.3 V 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 time [min]

7 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

8 R D /R D (0) R D Degradation Associated with Impact-Ionization? Impact-ionization rate I D exp(- V DGo +V T = 1.65 V Time [min] I D = 450 ma/mm 400 ma/mm 350 ma/mm 300 ma/mm 150 ma/mm R D /R D (0) 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 Time [min] Higher impact-ionization Higher degradation

9 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 /(V DGo +V T ) Degradation rate follows classical impact-ionization behavior

10 Other Drain-Related Figures of Merit Change BV DGoff /BV DGoff (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 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

11 Step-Stress Experiments Improved experimental productivity R D /R D (0) and g mo /g mo (0) Onset of mechanism 1 Onset of mechanism time [min] R D /R D (0) g mo /g mo (0) 2 Degradation mechanisms can be identified V DGo +V T [V]

12 Simpler Case: TLMs Ohmics Cap Etch-Stop Insulator δ-doping Channel GaAs Substrate I [A/mm] V=3.8 V L=12 µm t=0 t=12 min t=302 min More Stress V [V] Integrated TLMs : uniform field in channel Only two figures of merit: R, I sat

13 Time Evolution of TLM Degradation L=12 µm L=12 µm Mechanism 2 Mechanism R [ohm.mm] V stress [V] I sat [A/mm] 4.8 V stress [V] Mechanism time [min] Two mechanisms appear: V<4.5 V R and I sat track each other V>4.5 V only R increases time [min] n s

14 Increase of Lateral and Contact Resistance R, R s, R C [ohm.mm] 2 1 R R s R C V stress [V] R C R s R=R s +2R C R C time [min] Mechanism 1: Degradation of n s R s Mechanism 2: Degradation of R C only,r C

15 Field Reversal R [ohm.mm] time [min] Flip Polarity Flip Polarity V stress [V] R [ohm.mm] time [min] Mechanism 1 Mechanism 2 Independent of stress polarity Dependent of stress polarity V stress [V] Uniform degradation Degradation of one ohmic contact

16 Critical Voltage for Degradation Critical Voltage vs Length 4 Mechanism 1 Mechanism 2 V critical [V] E th (InGaAs)+2R C I sat L TLM [µm] Threshold of degradation Threshold of impact-ionization Degradation exhibits constant-field behavior

17 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

18 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

19 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