Harvey C. Nathanson

On the separation of bulk and surface components of lifetime using the pulsed MOS capacitor

Abstract A significant component of reverse current can occur due to lateral surface depletion effects in pulsed MOS capacitor experiments. This component of surface current is approximately proportional to the space-charge region width and can therefore appear as an apparent degradation of the bulk lifetime. A simple model, incorporating this lateral effect, permits direct separation of surface-controlled from bulk-controlled generation components.

Fabrication and some applications of large-area silicon field emission arrays

Abstract A method of fabricating large-area arrays of sharply-pointed field emitters at densities up to 1·5 × 10 5 per cm 2 from single crystal silicon wafers is described. The point emitters are formed by etch-undercutting a precision oxide pattern which is delineated on the silicon surface by projection photolithography. Observations indicate that emitters with very small tip dimensions in the 200Arange are formed. In the presence of an external electric field, such as produced by a voltage applied to a closely-spaced, planar anode, multiple-emitter arrays are shown to field-emit electrons uniformly over areas up to 3 cm dia. Two important applications currently being explored, are discussed: (1) High resistivity, p -Si has been utilized to develop experimental field emission photocathodes with which field emission imaging has been demonstrated. These photoemitters exhibit very high photo-sensitivities at visible and near i.r. wavelengths. For example, at 0·86 μm, the measured quantum efficiency is 25 per cent which is about five-times higher than the red-sensitive S-20 photocathode and comparable to the highest reported sensitivities of the III–V photosurfaces; (2) N -type emitter arrays show considerable promise as high current, cold cathodes and total emission currents of 1/4 A from 1 cm 2 areas of 100 Ω-cm n -type emitters have been obtained. Measurements were made under pulse conditions because of anode dissipation considerations.

The semiconductor field-emission photocathode

The recently developed large-area field-emission photocathode is described. It consists of a finely spaced array of point emitters fabricated by etching of p-type silicon or other semiconductor. Uniform emission over areas of 6-7 cm2have been obtained. For Si, the spectral response extends from 0.4 to 1.1 µm. Quantum yields of 25 percent at 0.86 µm have been measured, which is about five times the value reported for the extended S-20 photocathode and comparable to the best III-V photoemitters. Calculations indicate that quantum yields of up to 40 percent at 0.86 µm and 28 percent at 0.9 µm are attainable with the present photocathode structures. For low dark current densities, photocathode cooling to temperatures approaching 77 K must be employed at present. The dark current is shown to be dominated by surface-generated electrons in the space-chargeregion of the emitters. Effects of phosphorus gettering and annealing treatments on dark current are discussed, and the spatial frequency response of the device is determined. The results of a computer study show that the field intensification factor of p-semiconductor field emitters behaves quite differently from that of metallic emitters.

A high field triode

Abstract Single junction insulated-gate silicon triodes exhibiting very high d.c. voltage gain are described. These planar devices consist of a high resistivity n type substrate into which is diffused a p region of suitably high surface concentration. An aluminum gate electrode, positioned over this junction but insulated by an SiO 2 layer, provides a means by which intense surface fields can be applied to the underlying semiconductor. At a surface field of about 10 6 V/cm, the reverse current of the p + n junction markedly softens. At higher values of diode voltage, the reverse current saturates sharply resulting in pentode-like characteristics. A model for the operation of this triode, based on a combination of electrostatically-induced tunneling and substrate pinch-off, is presented.

Voltage controlled minority carrier collection—the exponentially retrograded photodiode

Abstract The large retarding field in an exponentially retrograded photodiode is shown to significantly reduce the base-generated photocurrent at low reverse-bias voltages. Increasing the reverse voltage on the diode reduces the length over which this retarding field is effective, thereby increasing the Photo-Transmission Coefficient of the diode. From avalanche breakdown considerations, the largest ratio of change in photocurrent caused by this effect is shown to be only a function of the resistivity and bulk-lifetime in the base of the diode. This voltage-controlled collection effect is observable only in diodes with high background resistivity (i.e. ϱ silicon > 25 Ω cm.). It is theoretically possible to obtain voltage-controlled collection ratios of better than 50:1 in, typically, a 10 V junction swing. Experimental results compare well with theoretical models.

A simple, inexpensive electrometer used to measure currents of individual silicon vidicon diodes in the 10 -16 to 10 -15 ampere range

A simple, inexpensive, essentially noiseless current-measuring device has been devised with a sensitivity of 10-16to 10-15ampere. Its operation relies on a measurement of the voltage decay of the device under test, the current being proportional to the rate of change of the voltage. Individual diodes of silicon vidicon targets (10-15ampere) and high resistivity layers (1015ohms/square) have been measured. In addition to its simplicity, this technique lends itself to nondestructive in-line testing.