280 GHz f T InP DHBT with 1.2  m 2 base-emitter junction area in MBE Regrown-Emitter Technology Yun Wei*, Dennis W. Scott, Yingda Dong, Arthur C. Gossard, Mark Rodwell University of California at Santa Barbara This work was supported by the DARPA TFAST program and by the Office of Naval Research (ONR) * RF Micro Devices, Infrastructure Product Line, GaN Technology Charlotte, North Carolina 28269, Tel: (704) ;

Motivation for Regrown-Emitter HBT: InP vs. Si/SiGe Advantages of InP ~20:1 lower base sheet resistance ~5:1 higher base electron diffusivity ~3:1 higher collector electron velocity ~4:1 higher breakdown at same f  Disadvantages of InP Production devices: large ~ 0.7 µm emitters High emitter resistance: scaling limit Large excess collector capacitance Non-planar device → low IC yield Low integration scales The advantages of InP-based HBTs lie in the material system. The disadvantages lie in the device structure and fabrication technology.

Emitter Resistance is a Key HBT Scaling Limit

Why Emitter Regrowth ? Target Benefits: Eliminate emitter undercut etch Eliminate base-emitter metal liftoff Flared emitter structure → large contact, small junction → low emitter access resistance Thick, ~2*10 20 /cm 3 -doped extrinsic base → low resistance: 250 Ohms/square → tolerant of contact metal migration Thin, ~3*10 19 /cm 3 -doped intrinsic base → low transit time → high current gain (less Auger) Passivated base-emitter junction → reliability Polycrystalline InAs has low resistivity, can play same role in InP as the polysilicon extrinsic emitter in Si/SiGe

Regrown emitter HBT RF fabrication process

0.3 um Intrinsic emitter 0.3 x 4 um 2 regrown-emitter InP DHBT extrinsic base base contact collector contact extrinsic emitter polyimide emitter collector base plug

Initial DC/RF results using CSL (graded) InAlAs emitter * Y. Wei, D. Scott, et al., IEEE EDL, May 2004, pp Peak f τ = 162 GHz, f max = 140 GHz Common-emitter current gain, h 21 = 20 η C = 1.2, η B = 2.2

First DC/RF results with improved surface & InP emitter Peak f τ = 183 GHz, f max = 165 GHz Common-emitter current gain, h21 ~17 Abrupt base-emitter junction and InP emitter * D. Scott, Y. Wei, et al., IEEE EDL, June 2004, pp.360~362.

Breaks in Emitter Growth Increase Emitter Resistance Narrowing or breakage of emitter regrowth - due to facet-dependent growth - due to high surface mobility of indium intrinsic emitter extrinsic emitter SiN

Improving emitter film continuity intrinsic emitter intrinsic emitter Suppress indium migration on the regrowth facets by: orienting abrupt InP emitter 60 o off [110] inserting alloy-graded InGa X As 1-X layers between the InP emitter and InAs cap [110] [100] extrinsic emitter extrinsic emitter Si x N y

HBT layer structure Layer Material Doping (cm -3 ) Thickness (Å) Emitter cap InAs 3e19 Si 800 Cap grade InGa X As 1-X 3e19 Si 500 N+ Emitter InP 3e19 Si 800 N- Emitter InP 8e17 Si 100 N-- Emitter InP 3e17 Si 300 Extrinsic base InGaAs 1~2e20 C 500 Etch stop InP 4e19 Be 20 Intrinsic base InGaAs 4e19 C 400 Set-back InGaAs 2e16 Si 200 Grade InGaAlAs 2e16 Si 240 Delta doping InP 3e18 Si 30 Collector InP 2e16 Si 1030

Regrown-Emitter InP DHBT with 0.3  4 µm 2 junction: 280 GHz f T V CE,sat < 0.9 V at J E =11mA/µm 2 Peak AC current gain=30 Collector breakdown voltage V CEO =5 V Peak f τ = 280 GHz, f max = 148 GHz Emitter access resistance R ex =11 Ohm, R ex A e =13 Ohm-um 2 η B =3.2 η C =1.2

Base Dopant Passivation by Hydrogen Degrades Performance Hydrogen passivation of carbon base doping increases base sheet resistance source of Hydrogen: PECVD-deposited Si x N y Solution: process uses hydrogen-free sputtered Si x N y for surfaces present process still uses PECVD Si x N y for sidewalls need to also used sputtered Si x N y for sidewalls

Hydrogen-Free Sputter-Deposited SiN Sidewalls substrate WW S. SiN Sputter-deposited SiN process development: 4 inch Si wafer uniformity testing refractive index measurement using ellipsometer: RI=2.06 BHF wet etching rate testing: ~8 Å/min - Stoichiometry controllable by heating, gas ratio and pressure

Summary InP HBT with emitter regrowth wide emitter contact, submicron emitter junction→ potential for reduced R ex junction formation by regrowth, not mesa etching thick extrinsic base for reduce base resistance Performance still limited by immature process technology breaks in emitter regrowth→ increased R ex hydrogen passivation from sputtered SiN sidewalls → increased R bb Present results: 0.3 um x 4 um regrown-emitter InP HBT 280 GHz f τ, 148 GHz f max, peak AC current gain=30 V CE,sat < 0.9 V at J E =11mA/µm 2 r ex = R ex A e =13 Ohm-um 2 Remaining improvements needed for 400-GHz-class device: hydrogen-free sputtered SiN sidewall further improvements in regrown emitter film continuity