C. Kadow, J.-U. Bae, M. Dahlstrom, M. Rodwell, A. C. Gossard *University of California, Santa Barbara G. Nagy, J. Bergman, B. Brar, G. Sullivan Rockwell Scientific, Thousand Oaks, Ca AlSb/InAs/InAsP/AlSb Composite-Channel HFETs Heng-kuang Lin* Disadvantages: Small E g (InAs) : low V br High kink current, high I g Degraded noise performance AlSb/InAs/AlSb HFETs Advantages: High , v sat f t, f max ~162 GHz * ( L g = 0.25 m ) * J. Bergman, et al., IPRM, 2003. Our Approach for Breakdown : Composite Channel Concentration Profile Be Te bias Source Gate Drain InAs channel subchannel High field regionLow field region Motivation: ABCS HFETs need higher V br. Approach: Composite channel Electrons move in high- mobility material at low fields InAs. Electrons move in high-V br, high-v sat material at high fields What material ? Why InAsP as subchannel ? InAlAs: L separation decreases. Results in poor v sat. InGaAs: Strain limits available bandgap. InAsP: Large L separation, high v sat. Manageable levels of strain. Characterization of Composite Channel - I First step : Characterize InAs channels InAs / InAsP 2 DEGEnergy band diagram / Carrier profile Characterization of Composite Channel - II 430 C : Optimal InAsP growth temperature > 20,000 cm 2 / V-s for P ratios up to 0.4 R-T Hall N s vs P ratio R-T Hall Mobility vs P ratio Characterization of Composite Channel - III Second step : Characterize InAsP subchannels 430 C is again optimal InAsP growth temperature > 7,000 cm 2 / V-s in InAs 0.8 P 0.2 subchannels X Energy band diagram / Carrier profile R-T Hall Mobility & N s vs P ratio Carrier Transfer vs Impact Ionization Source Gate Drain InAs InAsP Wave functions InAsInAsPSurfacesubstrate E y ~2.1X10 5 V/cmE y ~2. 3X10 5 V/cm V gs e y x 1. Transfers to InAsP 2. Impact-ionizes E c (InAsP) E v (InAsP) E v (InAs) E c (InAs) e h E~E g (InAs) x direction y direction E y, max, transfer ~ 2.2X10 5 V/cm E c (InAsP) E c (InAs) e h ? ? Our Approach : Introduction of a Doping Dipole Dipole : P-doping (Be) plus N-doping (Te) Advantages : 1.Reduced external bias to reach E y, max, transfer Easier electron transfer Improve breakdown 2. Hole barrier in buffer Prevents hole accumulation Reduces kink current Suggested Composite-Channel Device Layer Structure InSb-like interfaces 7% strain, dislocations, buffer 0.9x10 12 cm x10 12 cm -2 Digital alloy: 20% P Si: 1x10 19 cm % strain As-grown material: =14,500 cm 2 / V-s (R-T Hall) N s =1.7x10 12 cm -2 InAs Channel 10 nm InAsP Subchannel 10 nm Standard Processing Flow 1) Ohmic contacts 1)2) 3) TLM : R sh = 220 /square R c = -mm 4) 2) Mesa isolation by dry etch 3) Optical recessed gate 4) Metal 1 Comparison between Single and Composite Channel : DC Characteristics - I HFETs : 0.7x40 m 2 Blue : composite channel Red : single channel g ds ( Red) : g ds ( Blue) ~ 14 :1 ( V gs = V, V ds = 0.6 V ) No impact ionization peaks Comparison between Single and Composite Channel : DC Characteristics - II HFETs : 0.7x40 m 2 Blue : composite channel Red : single channel RF Performance for Composite Channel f T L g = 32 GHz-m : composite channel f T L g = 44 GHz-m : single channel Slight degradation on RF performance Max f T for single channel Max f T for composite channel Conclusion Growth : - Develop materials for InAs/InAsP composite-channel HFETs ( InAs) > 20,000 cm 2 / V-s for 40%P in InAsP subchannels Device performance : DC :- Increased V br (~0.5V to ~0.8V) - Reduced kink currents ( g ds ~ 14:1) - Suppressed impact ionization in I g RF : - f T,max = 46 GHz, f max,max = 61 GHz (0.7-m gate) - f T L g degrades from 44 to 32 GHz-m InAs/InAsP composite-channel HFETs are a promising enhancement to InAs single-channel HFETs.