Avalanche ionization rates measured in silicon and germanium at low electric fields

Ionization rates in semiconductors can be measured at low values of electric field using a new method involving a field-effect transistor (FET) structure which offers greater sensitivity than reverse biased p-n junction diode methods. Carriers of only one polarity cause ionization in the FET and no correction is required for ionization caused by carriers of the opposite polarity. Since secondary carriers resulting from ionization are attracted to a different terminal (gate or substrate) than that used to collect the primary carriers (drain), very small ionization currents can be detected. Values of electron ionization rate α
n
and hole ionization rate α
p
as low as 10
^-3
cm
^-1
have been obtained for silicon. The approximate relationship
\alpha = \alpha_{\infin}e^{-b/E}
is observed and the values of α at high fields correspond to those obtained conventionally. Values of α
p
from 0.04
^-1
to 0.4 cm
^-1
have been obtained for germanium. Analytical determination of electric field was provided by a solution of Poisson's equation for the field-effect structure. Difficulty in accurately determining the FET channel doping introduces a ± 30 percent uncertainty in electric field values. The method is applicable to any semiconductor material where junction, MOS, or Schottky barrier techniques can be used to construct field-effect transistors.