Transport Properties of GaAs

Abstract

The transport properties of the electrons in GaAs have been investigated; i.e., the absolute values of the electron drift velocity, the diffusion coefficient, and the trapping time have been measured for the first time as a function of the electric field. To measure the velocity at the electron, the response of a reversed-bias Schottky barrier-$I-n^{+}$ GaAs device to a short pulse (0.1 nsec) of high-energy electrons was measured. The incident electrons create a sheet of charges in the semiconductor very close to the cathode. The electrons move across the diode under the influence of the applied electric field and induce a current in the contacts until they reach the anode. The width of the induced current pulse is a measure of the transit time of the electrons. With a knowledge of the width of the field region ($I$ layer), the drift velocity corresponding to the particular bias field can be accurately determined. The specimen used in this experiment consists of a slab of semi-insulating boat-grown GaAs cut in the [100] and [111] directions. Thin contacts were evaporated on each face; one, the cathode contact, less than 1000 Å thick, forms the noninjecting Schottky barrier. The other, the anode, is ohmic. The experimental results are in excellent agreement with the Butcher-Fawcett theory, with a low-field mobility of 7500 ${\mathrm{cm}}^2$/V sec, a threshold field of 3300 V/cm, and an initial negative mobility of 2600 ${\mathrm{cm}}^2$/V sec. Signs, but no strong evidence, of the minimum velocity being reached up to the highest field used, 14 kV/cm, were observed. We have also measured by the same method the velocity-field relation over a range of ambient temperatures from 160 to 340°K. From the measurement of the difference between the rise and the fall time of the induced current pulse, which is a measure of the spread of the electron layer created at the cathode by electron bombardment, it was possible to obtain the diffusion coefficient as a function of the electric field. The diffusion coefficient is sharply peaked (900 ${\mathrm{cm}}^2$/sec) at the threshold field and decrease to a value slightly less than 200 ${\mathrm{cm}}^2$/sec at high field. The experimental results are in considerable disagreement with the theoretical prediction. This disagreement may possibly be due to the velocity fluctuation arising from the rapid electron intervalley transfers which have not been taken into account in the theory by Butcher and Fawcett. Finally, measurement of the number of electrons trapped during the electron transit time across the specimen yields the variation of the trapping time as a function of the electric field.