Ionization Rates of Holes and Electrons in Silicon

The ionization rates of charge carriers in silicon have been measured and fit to the recent theoretical calculations of Baraff; in contrast, none of the existing published data could be fit to these theoretical curves. The study has been made using microplasma-free junctions of demonstrably high, uniform local multiplication. A new and considerably simplified approach to the problem of extracting the ionization rates from the multiplication data has been used. By employing much more precise control of the electron and hole currents used to initiate the multiplication process, the hole ionization rate at electric fields less than 300 kV/cm is found to be more than an order of magnitude smaller than any previously published measurements. Hole and electron ionization rates have been measured in the same junction and consequently in the identical scattering environment. The threshold energy is determined to be $E_g\le E_i\le 1.5E_g$, and the mean free path for scattering of high-energy electrons is $50 \AA \le {\lambda }_e\le 70 \AA$ and for energetic holes $30 \AA \le {\lambda }_h\le 45 \AA$. Measurement of ionization rates at various temperatures substantiates the assumption that the energy-loss mechanism is the emission of optical phonons. In addition, significant differences of the electrical breakdown characteristics of microplasma-free junctions are discussed as well as their preparation.