Ultra Wideband DHBTs using a Graded Carbon-Doped InGaAs Base Mattias Dahlström, Miguel Urteaga,Sundararajan Krishnan, Navin Parthasarathy, Mark Rodwell Department of Electrical and Computer Engineering, University of California, Santa Barbara , fax

Wideband InP/InGaAs/InP Mesa DHBT Mattias Dahlström UCSB Objectives: fast HBTs → mm-wave power, 160 Gb fiber optics desired: 440 GHz f t & f max, 10 mA/  m 2, C cb /I c <0.5 ps/V better manufacturability than transferred-substrate HBTs Approach: narrow base mesa → moderately low C cb very low base contact resistance required → carbon base doping, good base contact process high f t through high current density, thin layers Bandgap engineering: small device transit time with wide bandgap emitter and collector

DHBT Layer Structure and Band Diagram InGaAs 3E19 Si 400 Å InP 3E19 Si 800 Å InP 8E17 Si 100 Å InP 3E17 Si 300 Å InGaAs graded doping 300 Å Setback 2E16 Si 200 Å InP 3E18 Si 30 Å InP 2E16 Si 1700 Å SI-InP substrate V be = 0.8 V V ce = 1.5 V M Dahlstrom Grade 2E16 Si 240 Å InP 1.5E19 Si 500 Å InGaAs 2E19 Si 500 Å InP 3E19 Si 2000 Å UCSB 300 A doping graded base Carbon doped 8*10 19  5* cm Å n-InGaAs setback 240 Å InAlAs-InGaAs SL grade Thin InGaAs in subcollector Emitter Collector Base

P c is immeasurably low: below 10 –7  cm -2 Critical for narrow base mesa HBT Carbon doping 6E19 cm -3 Pd-based p-contacts Careful ashing and oxide etch 300 C, 1 minute InP/InGaAs/InP Mesa DHBT Base contact resistance Mattias Dahlström UCSB The size of the base contacts must be minimized due to C cb  s =722  /sq ???

InP/InGaAs/InP Mesa DHBT Device Results UCSB Mattias Dahlström  =20-25 J=3.5 mA/um2BVCEO=7.5 V No evidence of current blocking or heating

Submicron HBTs have very low C cb Characterization requires accurate measure of very small S12 Standard 12-term VNA calibrations do not correct S12 background error due to probe-to-probe coupling Solution Embed transistors in sufficient length of transmission line to reduce coupling Place calibration reference planes at transistor terminals Line-Reflect-Line Calibration Standards easily realized on-wafer Does not require accurate characterization of reflect standards CPW lines suffer from substrate TE, TM mode coupling: thin wafer, use Fe absorber ! lateral TEM mode on CPW ground plane… present above 150 GHz, must use narrower CPW grounds Accurate Transistor Measurements Are Not Easy Transistor in Embedded in LRL Test Structure 230  m Corrupted GHz measurements due to excessive probe-to-probe coupling Miguel Urteaga Mattias Dahlstrom UCSB

InP/InGaAs/InP Mesa DHBT Device Results Mattias Dahlström UCSB 2.7  m base mesa, 0.54  m emitter junction 0.7  m emitter contact V ce =1.7 V J=3.7E5 A/cm 2 f  = 282 GHz; f max =480 GHz  = 25; BV CEO = 7.5 V MAG/MSG U H 21

InP/InGaAs/InP Mesa DHBT Device Results Mattias Dahlström UCSB A ej =3.4 um 2 J=4.4 mA/  m 2 V cb =0.9 V Emitter contact sizes um, 8 um long. Base extends um on each side of the contact Maximum current density >10 mA/um 2 V ce >1.5 V for best performance Best f t found at current density of 3-5 mA/  m 2 f max measurement above 500 GHz currently not reliable in CPW environment

InP/InGaAs/InP Mesa DHBT Conclusions Mattias Dahlström UCSB Doping-graded base InGaAs/InP Mesa DHBT: High current density Operates up to 10 mA/  m 2 without destruction …Kirk threshold 4.4 mA/  m 2 at 1.5 V f t of 280 GHz with a 220 nm collector f max is 450 GHz or higher R bb is no longer a major factor - excellent base ohmics f max no longer a good measure of C cb or circuit performance C cb reduction a priority 87 GHz static frequency divider circuit already demonstrated

Narrow-mesa DHBT:base design Doping graded base: At degenerate doping levels (>1E19) the variation of the Fermi level in the base is very rapid Exponential doping roll-off not needed, linear roll-off good enough! Many approximate methods for determining E f such as Boltzmann, Joyce-Dixon are insufficient Mattias Dahlström UCSB Energy (eV) Base doping (cm -3 )

Narrow-mesa DHBT:base design Base transit time calculation: -Bandgap narrowing -Fermi-Dirac statistics - doping and bandgap dependent mobility Mattias Dahlström UCSB Transit time (ps) Base width (A) The exit term (electron velocity in top of collector) important for thin bases: use InGaAs, not InP, close to base