Current Density Limits in InP DHBTs: Collector Current Spreading and Effective Electron Velocity

1. Current Density Limits in InP DHBTs: Collector Current Spreading and Effective Electron Velocity Mattias Dahlström1 and Mark J.W. Rodwell Department of ECE University of California, Santa Barbara USA • Special thanks to: • Zach and Paidi for processing and development work • Now with IBM Microelectronics, Essex Junction, VT • This work was supported by the Office of Naval Research under contracts N00014-01-1-0024 and N0001-40-4-10071, and by DARPA under the TFAST program N66001-02-C-8080. mattias@us.ibm.com 802-769-4228

2. Introduction What limits the current density in a HBT? • Heating • High thermal conductivity InP ☺ • Low thermal conductivity InGaAs • Low Vce☺ • Kirk effect • Injected electron charge in collector deforms the conduction band  current blocking • thin the collector, increase collector doping

3. Collector in HBT under current (simulation)and measured effects on ft and Ccb High current Current blocking and base push-out effects ft and Ccb – the Kirk effect At some current density Jkirk device performance will degrade due to the Kirk effect

4. Observation: The Kirk current density Jkirk depends on the emitter width Jkirk extracted from ft and Ccb vs Je, extracted from S-parameter measurements at 5-40 GHz Collector current spreads laterally in the collector

5. Extraction of the current spreading distance D Poisson’s equation for the collector Poissons equation for the composite collector: Plot Ikirk/L vs. emitter junction width Web D=0.14 mm for Tc=150 nm D=0.19 mm for Tc=217 nm Current spreading important as emitter width We scales to D ! Jkirk will be much higher ! Sources of error: Coarse Ic Ohmic losses reduces Jkirk by max 4 % Device heating not important - low Vcb Averaged data points

6. Collector velocity extraction from Vcb ∂Jkirk/∂Vcb provides effective electron velocity! There is no evidence of velocity modulation Tc=150 nm: vsat= 3.2 105 - 3.9 105 m/s Tc=217 nm: vsat=2.3 105 - 3.2 105m/s Method requires D and veff to be constants with regards to Vcb over measured interval Linearity of fit indicates this is correct But how can veff be constant with regards to Vcb? G-L scattering should lead to velocity modulation!

7. Why is there no Vcb dependence on veff? G-L scattering occurs when electrons in the G band scatters to the slower L band  veff reduced Larger Vcb  G-L scattering closer to the bc interface  veff reduced Simulated @Je<Jkirk Vcb changes Je fixed veff is extracted at the Kirk current condition near flat-band at bc interface  G-L scattering removed from bc interface  minimum Vcb influence on veff Simulated @Je= Jkirk Vcb changes Je= Jkirk(Vcb)

8. Mesa DHBT with 0.6 mm emitter width, 0.5 mm base contact width Typical layer composition Z. Griffith, M Dahlström DHBT-19 with 150 nm collector

9. Device results at high current density higher than original Kirk current threshold Low-current breakdown is > 6 Volts this has little bearing on circuit design Safe operating area is > 10 mW/um2 these HBTs can be biased ....at ECL voltages ...while carrying the high current densities needed for high speed Tc=150 nm

10. Conclusions • Current spreading 0.14 mm for Tc=150 nm 0.19 mm for Tc=217 nm (first experimental determination for InP) • veff=3.2∙105 m/s for both 150 and 217 nm Tc • Large effect on max collector current for sub-m InP HBTs. Jkirk increases drastically • Must be accounted for in collector isolation by implant or regrowth (provide room for current spreading)