Potential, field and carrier distribution in the channel of junction field-effect transistors

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AbstractThe operation mechanism of field-effect transistors (FET) was investigated by the measurements of the potential distribution in the channel using a tungsten needle as a probe in alloyed junction type FET in operation. The field distribution and the free carrier distribution were obtained as the first and the second derivative of the measured potential distribution. It is clear that, before the current saturation, from the source to the drain the electrically neutral channel exists. In this region the ionized impurity density and the carrier density are equal and the field toward the gate can be neglected. Contrary to this, in the channel at the drain side after the current saturation, the space charge layer extending from the gates reaches near the channel centre and the field to the gate direction becomes extremely high. In this region, an effective channel is formed in which the free carriers decrease toward the gate. At the centre of the effective channel, the electrical neutrality almost holds. Almost all of the voltage after the current saturation are spent in this region which is independent of the source side. In this short high field region, the field seems to be even in the velocity saturation region (E>1•5×104V/cm) of the carriers. The highest field region is rather outside of the gates. The potential and the carrier distribution in the channel at the drain side show a fairly good agreement with the theoretical calculation analysed in this paper.