The total cross section for positronium formation in helium for positrons of energies E from threshold (17.8 eV) to 250 eV has been measured using a gas cell transmission method. The results indicate that the cross section above E=60 eV is satisfactorily described by 0.5e-0.02E pi a02, tending to zero at energies close to 200 eV. The approximate description of the cross section at intermediate energies, falling as E-2.5, is consistent with a number of theoretical predictions but is in conflict with earlier experiments, which show an E-1 behaviour. The implications of these measurements are discussed, with particular reference to the observed merging of the total cross sections for positron- and electron-helium scattering at intermediate energies.

Previous analytical treatments of low-field electron transpolt in the base of bipolar transistors have all made the implicit assumption that the velocity distributions are Gaussian. Using material and scattering parameters appropriate to GaAs and a simple model for the transistor base in which the wllector is replaced by a step potential, we have found that near the collector these distributions are distorted, leading to a diffusion velocity exceeding the thermal velocity. We also investigated the effects of the base-emitler boundary condition and found that standard injection techniques can force non-equilibrium distributions. This becomes critical as the base lengths become submicron and boundary effects dominate. With use of a dead region, where the electron distributions could relax before injection, to overcome this problem we found close agreement with the analytical results of Herbert.

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