The Effect of Germanium Fraction on High-Field Band-to-Band Tunneling in ${\rm p}^{+}$-SiGe/${\rm n}^{+}$-SiGe Junctions in Forward and Reverse Biases

The dependence of band-to-band tunneling in
${\rm p}^{+}\hbox{-}{\rm Si}_{1-x}{\rm Ge}_{x}/{\rm n}^{+}\hbox{-}{\rm Si}_{1-x}{\rm Ge}_{x}$
homojunctions on Ge fraction and electric field is investigated in the range
$2\hbox{--}3\times 10^{8}~{\rm V}/{\rm m}$
. Negative differential resistance (NDR) in forward bias is observed for each device with the highest peak tunneling-current density of 8.2
${\rm kA}/{\rm cm}^{2}$
without any postannealing step. Reverse-biased band-to-band tunneling, as relevant for tunneling field-effect transistors, is also measured. Tunneling via junction defects can mask band-to-band tunneling and the observation of NDR at forward bias confirms negligible tunneling via those defects. Both forward-biased and reverse-biased data are compared with models versus electric field and Ge fraction.