Ultra Wideband DHBTs using a Graded CarbonDoped In

Ultra Wideband DHBTs using a Graded Carbon-Doped In. Ga. As Base Mattias Dahlström, Miguel Urteaga, Sundararajan Krishnan, Navin Parthasarathy, Mark Rodwell Department of Electrical and Computer Engineering, University of California, Santa Barbara mattias@ece. ucsb. edu 805 -893 -8044, 805 -893 -3262 fax

UCSB Wideband In. P/In. Ga. As/In. P Mesa DHBT Mattias Dahlström Objectives: fast HBTs → mm-wave power, 160 Gb fiber optics desired: 440 GHz ft & fmax, 10 m. A/ m 2, Ccb/Ic<0. 5 ps/V better manufacturability than transferred-substrate HBTs Approach: narrow base mesa → moderately low Ccb very low base contact resistance required → carbon base doping, good base contact process high ft through high current density, thin layers Bandgap engineering: small device transit time with wide bandgap emitter and collector

DHBT Layer Structure and Band Diagram UCSB M Dahlstrom In. Ga. As 3 E 19 Si 400 Å Emitter Collector In. P 3 E 19 Si 800 Å In. P 8 E 17 Si 100 Å In. P 3 E 17 Si 300 Å In. Ga. As graded doping 300 Å Setback 2 E 16 Si 200 Å Grade 2 E 16 Si 240 Å In. P 3 E 18 Si 30 Å In. P 2 E 16 Si 1700 Å In. P 1. 5 E 19 Si 500 Å In. Ga. As 2 E 19 Si 500 Å In. P 3 E 19 Si 2000 Å SI-In. P substrate Vbe = 0. 8 VBase Vce = 1. 5 V • 300 A doping graded base • Carbon doped 8*1019 5* 1019 cm-2 • 200 Å n-In. Ga. As setback • 240 Å In. Al. As-In. Ga. As SL grade • Thin In. Ga. As in subcollector

In. P/In. Ga. As/In. P Mesa DHBT Base contact resistance UCSB Mattias Dahlström • Carbon doping 6 E 19 cm-3 • Pd-based p-contacts • Careful ashing and oxide etch • RTP @ 300 C, 1 minute The size of the base contacts must be minimized due to Ccb Pc is immeasurably low: below 10 – 7 cm-2 s=722 /sq ? ? ? Critical for narrow base mesa HBT

In. P/In. Ga. As/In. P Mesa DHBT Device Results UCSB Mattias Dahlström b=20 -25 No evidence of current blocking or heating J=3. 5 m. A/um 2 BVCEO=7. 5 V

Accurate Transistor Measurements Are Not Easy UCSB Miguel Urteaga Mattias Dahlstrom • Submicron HBTs have very low Ccb Characterization requires accurate measure of very small S 12 • Standard 12 -term VNA calibrations do not correct S 12 background error due to probe-to-probe coupling Solution Embed transistors in sufficient length of transmission line to reduce coupling 230 m Transistor in Embedded in LRL Test Structure 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 Corrupted 75 -110 GHz measurements due to excessive probe-to-probe coupling

In. P/In. Ga. As/In. P Mesa DHBT Device Results UCSB Mattias Dahlström • 2. 7 m base mesa, • 0. 54 m emitter junction • 0. 7 m emitter contact • Vce=1. 7 V • J=3. 7 E 5 A/cm 2 U MAG/MSG H 21 ft = 282 GHz; fmax=480 GHz b = 25; BVCEO = 7. 5 V

In. P/In. Ga. As/In. P Mesa DHBT Device Results Vcb=0. 9 V • Emitter contact sizes 0. 5 -2. 0 um, 8 um long. • Base extends 0. 25 -1. 0 um on each side of the contact • Maximum current density >10 m. A/um 2 UCSB Mattias Dahlström Aej=3. 4 um 2 J=4. 4 m. A/ m 2 fmax measurement above 500 GHz currently not reliable in CPW environment • Vce >1. 5 V for best performance • Best ft found at current density of 3 -5 m. A/ m 2

In. P/In. Ga. As/In. P Mesa DHBT Conclusions UCSB Mattias Dahlström Doping-graded base In. Ga. As/In. P Mesa DHBT: • High current density Operates up to 10 m. A/ m 2 without destruction …Kirk threshold 4. 4 m. A/ m 2 at 1. 5 V • ft of 280 GHz with a 220 nm collector • fmax is 450 GHz or higher • Rbb is no longer a major factor - excellent base ohmics • fmax no longer a good measure of Ccb or circuit performance • Ccb reduction a priority • 87 GHz static frequency divider circuit already demonstrated

Narrow-mesa DHBT: base design UCSB Mattias Dahlström Energy (e. V) Many approximate methods for determining Ef such as Boltzmann, Joyce-Dixon are insufficient Base doping (cm-3) Doping graded base: At degenerate doping levels (>1 E 19) the variation of the Fermi level in the base is very rapid Exponential doping roll-off not needed, linear roll-off good enough!

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