Tunnelling planar silicon triode with pentode-like characteristics

When a strong field is applied to the surface of a planar p⁺/n junction, the free carrier concentration at the surface of the lightly doped side may be drastically changed. In fact, degeneracy can be readily induced on the n side by using fields on the order of 10⁶volts/cm. If the planar junction is properly designed, the junction current in the conventional reverse direction can be almost entirely due to tunnelling. A device incorporating this mechanism has been developed in which the electric field is applied by means of a gate electrode insulated from the p-n junction by a conventional oxide. Operational input resistances in excess of 10¹²ohms have been observed. For potentials of a few volts applied across the reversed-biased p-n junction, a soft V-I characteristic, similar to that of a reverse-biased tunnel diode is present. At higher voltages, the diode current saturates sharply, yielding a pentode-like characteristic with an output resist. ance on the order of two megohms. Small devices with 2 pf. input capacitance have yielded transconductance values of 20 µmhos and open-circuit voltage gains greater than 40. Larger devices have been made with gmof 5000 µmhos and µ greater than 300. Present understanding of the device mechanism indicates that the length of the active region in the triode is extremely small. Thus, low device input capacities should be possible, subject only to limits dictated by current fabrication technology. A model for operation of the device will be discussed, with particular emphasis on gate-controlled tunnelling, on the mechanisms for current saturation, and on speed and stability.