High speed (207 GHz f  ), Low Thermal Resistance, High Current Density Metamorphic InP/InGaAs/InP DHBTs grown on a GaAs Substrate Y.M. Kim, M. Dahlstrǒm, S. Lee, Y. Wei, M.J.W. Rodwell, A.C. Gossard Department of Electrical Engineering, Materials Department, University of California, Santa Barbara

Technical Objective Growth of InGaAs/InAlAs/InP HBTs on GaAs substrates ….with low leakage and high yield ….for low-cost high-volume manufacturing of InP HBT integrated circuits on 6" diameter substrates Gives basic data for growth which is free of lattice constant limit

Why InP-based HBTs ? better device bandwidth than GaAs or Si bipolar transistors  microwave ADCs, DACs, digital frequency synthesis better E max v sat than GaAs  millimeter-wave power Why metamorphic HBTs ?--economic argument low cost, high volume processing: wafer size is critical GaAs substrates, processes: 6" diameter now large InP substrates: high cost, high breakage, only 4" available today breakage much worse with 6" wafers  grow InP-based HBTs on GaAs substrates for cost and manufacturability

Metamorphic HBTs InGaAs/InP or InGaAs/InAlAs HBT on a GaAs substrate Lattice mismatch between substrate and epitaxial device layers Thick intervening buffer layer to capture most defects

Why might M-HBTs be harder than M-HEMTs ? Much thicker depletion regions: base-collector (2kÅ) vs. gate-channel junctions (200 Å) 1, ,000 times more active device area defect density, thermal resistance: more serious concerns HBT HEMT

What are the potential problems ? Defects  collapse in DC gain recombination in e/b junction surface recombination recombination in base generation in collector Thick (ternary) buffer layer poor thermal conductivity

RHEED of metamorphic layer AlGaAsSbInAlAs InP Show the streak lines Indicate good surface morphology

Morphology of metamorphic layer AlGaAsSbInAlAs InP

AFM image of metamorphic layer Metamorphic buffer Surface roughness (nm) AlGaAsSb4.0 InAlAs11.7 InP9.5 AlGaAsSbInAlAs InP

Thermal Conductivity Measurement Pattern a 1x100 μm Pt line – 50 nm thick Measure the resistance with varying input power As the input power increases, the Pt wire gets hot and the resistance increases. Resistance change is determined by the thermal conductivity of underlying layer. Extract thermal conductivity of film from finite element simulation. GaAs subst. Metamorphic layer Pt wire

Results and Junction Temperature Calculation Metamorphic buffer Thermal conductivity (W/mC) AlGaAsSb8.4 InAlAs10.5 InP16.1 GaAs bulk44 InP bulk69 InP buffer has best thermal conductivity though it is smaller than bulk value. GaAs 350 μm Metamorphic layer 1.5 μm HBT 8 μm x 0.5 μm 1000 μm 30 HBTs with 45 μm device separation Solve the 3D Laplace eq. to determine junction temp. as function of thermal conductivity power density : 200 kW/cm 2

Thermal Conductivity vs. HBT Temp. AlGaAsSb (128°C) InAlAs (112°C) InP (89°C) Without metamorphic (65°C) Power density : 200 kW/cm  m x 8  m emitter device 30 HBTs with 45  m device seperation

Power density vs. HBT Temp. High power density is required for future device. Need high thermal conductivity buffer layer

Expected Reliability of HBT InP InAlAs AlGaAsSb Metamorphic buffer Life time relative to AlGaAsSb HBT AlGaAsSb1 InAlAs6.3 InP119 Long life time shows that InP buffer is essential in metamorphic HBT from thermal point of view. Ref) K.Kiziloglu et al. IPRM, 294 (2000)

Mesa structure for RF measurement Advantage of mesa structure Adequate for metamorphic HBT due to the excellent heat flow High speed operation GaAs substrate Metamorphic buffer (InP, InAlAs,AlGaAsSb) In 0.53 Ga 0.47 As subcollector InP collector In 0.53 Ga 0.47 As base InP emitter emitter base collector

Structure of metamorphic M-DHBT Emitter cap In 0.53 Ga 0.47 As : Si (2x10 19 cm -3 ) 300 Ǻ Emitter grade In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As : Si (2x10 19 cm -3 ) 200 Ǻ Emitter InP : Si (1x10 19 cm -3 ) 700 Ǻ InP : Si (8x10 17 cm -3 ) 500 Ǻ Grade In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As : Si (4x10 17 cm -3 ) 280 Ǻ Base In 0.53 Ga 0.47 As : Be (4x10 19 cm -3 ) 400 Ǻ SetBack In 0.53 Ga 0.47 As : Si (2x10 16 cm -3 ) 100 Ǻ Grade In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As : Si (2x10 16 cm -3 ) 240 Ǻ Delta doping InP : Si (5.6x10 18 cm -3 ) 30 Ǻ Collector InP : Si (2x10 16 cm -3 ) 1630 Ǻ Sub collector In 0.53 Ga 0.47 As : Si (1x10 19 cm -3 ) 250 Ǻ Sub collector InP : Si (1x10 19 cm -3 ) 1250 Ǻ BufferInP1.5 μm GaAs (100) semi-insulating substrate 500Ǻ thick and 8e17/cm 3 n-doped emitter1 layer was grown for low C je 400 Ǻ base with 50 meV bandgap grading 100 Ǻ setback layer was introduced 2000 Ǻ collector 1.5 μm InP metamorphic layer was grown at 470 o C on GaAs wafer

InP/InGaAs/InP Metamorphic DHBT on GaAs substrate Growth: 400 Å base, 2000 Å collector GaAs substrate InP metamorphic buffer layer (high thermal conductivity) Processing conventional mesa HBT narrow 2 um base mesa, 0.4 um emitter Results 207 GHz f t, 140 GHz f max, 6 Volt BVCEO,  =76

Gummel curves Large area (60  m x 60  m) Small area (0.4  m x 0.75  m) Small area device shows larger leakage current than large area device.  The leakage current source is not the growth defect.  pad to pad leakage turned out to be the source.  There may be surface leakage through the side wall.  More study is being tried

InP/InGaAs/InP Metamorphic DHBT on GaAs substrate V CE = 1.5V J = 3.2e5 A/cm 2

Summary Several materials were tried for metamorphic buffer layer InP was chosen because of high thermal conductivity Highest speed for MHBT was acquired More study is needed for reducing leakage current