Current Density Limits in InP DHBTs: Collector Current Spreading and Effective Electron Velocity Mattias Dahlström 1 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 (1)Now with IBM Microelectronics, Essex Junction, VT This work was supported by the Office of Naval Research under contracts N and N , and by DARPA under the TFAST program N C-8080.

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

Collector in HBT under current (simulation) and measured effects on f t and C cb At some current density J kirk device performance will degrade due to the Kirk effect Current blocking and base push-out effects f t and C cb – the Kirk effect High current

Observation: The Kirk current density J kirk depends on the emitter width J kirk extracted from f t and C cb vs J e, extracted from S-parameter measurements at 5-40 GHz Collector current spreads laterally in the collector

 =0.14  m for T c =150 nm  =0.19  m for T c =217 nm Sources of error: Coarse I c Ohmic losses reduces J kirk by max 4 % Device heating not important - low V cb Extraction of the current spreading distance  Poisson’s equation for the collector Averaged data points Plot I kirk /L vs. emitter junction width W eb Current spreading important as emitter width W e scales to  J kirk will be much higher ! Poissons equation for the composite collector:

Collector velocity extraction from V cb There is no evidence of velocity modulation ∂J kirk /∂V cb provides effective electron velocity! Method requires  and v eff to be constants with regards to V cb over measured interval Linearity of fit indicates this is correct But how can v eff be constant with regards to V cb ?  -L scattering should lead to velocity modulation! T c =150 nm: v sat = m/s T c =217 nm: v sat = m/s

Why is there no V cb dependence on v eff ? v eff is extracted at the Kirk current condition  near flat-band at bc interface   - L scattering removed from bc interface  minimum V cb influence on v eff  -L scattering occurs when electrons in the  band scatters to the slower L band  v eff reduced Larger V cb   -L scattering closer to the bc interface  v eff reduced e = J kirk V cb changes J e = J kirk (V cb ) e <J kirk V cb changes J e fixed

Mesa DHBT with 0.6 mm emitter width, 0.5 mm base contact width Thicknes s (nm) Material Doping (cm -3 ) Description 40 In 0.53 Ga 0.47 A s 3∙10 19 : SiEmitter Cap 80InP3∙10 19 : SiEmitter 10InP8∙10 17 : SiEmitter 30InP3∙10 17 : SiEmitter 30 In 0.53 Ga 0.47 A s 8-5∙10 19 : CBase 20 In 0.53 Ga 0.47 A s 3∙10 16 : SiSetback 24 InGaAs/ InAlAs SL 3∙10 16 : SiGrade 3InP3∙10 18 : SiDelta doping 100InP3∙10 16 : SiCollector 10InP1∙10 19 : SiSub Collector 12.5 In 0.53 Ga 0.47 A s 2∙10 19 : SiSub Collector 300InP2∙10 19 : SiSub Collector SubstrateSI : InP Typical layer composition DHBT-19 with 150 nm collector Z. Griffith, M Dahlström

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/um 2 these HBTs can be biased....at ECL voltages...while carrying the high current densities needed for high speed T c =150 nm

Conclusions Current spreading 0.14  m for T c =150 nm 0.19  m for T c =217 nm (first experimental determination for InP) v eff =3.2∙10 5 m/s for both 150 and 217 nm T c Large effect on max collector current for sub-  InP HBTs. J kirk increases drastically Must be accounted for in collector isolation by implant or regrowth (provide room for current spreading)