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Characterization and fabrication of surface junctions with application to photovoltaic energy conversion Camporese, Daniel Stefen


The central topic of this thesis is the semiconductor surface junction of which two types will be discussed here: the metal-insulator-semiconductor (MIS) junction and the insulator charge-induced (ICI) junction. These junctions are investigated by correlating theoretical models for these devices with measurements performed on devices fabricated in the laboratory. The MIS junction is formed when a metal is deposited on a semiconductor which has had a thin insulator formed on its surface. The insulator is so thin that tunneling of electrons between the metal and semiconductor is possible. A difference in work function between the metal and the semiconductor will produce a field in the semiconductor. A metal with the appropriate work function will produce inversion of the semiconductor surface. The thin insulator reduces the normal thermionic emission current which flows between the metal and the semiconductor. If the semiconductor surface is inverted, fewer majority carriers than minority carriers are available to flow into the metal. In this work, the ratio of net minority to majority carrier current flows is measured to establish that MIS junctions can be dominated by minority carrier currents thus making their electrical properties very similar to those of a standard pn junction diode. This hypothesis is demonstrated by using the MIS junction to form the collectors and emitters of bipolar transistors. These transistor structures are also used to measure the minority carrier lifetime in the transistor base. The ICI junction is formed when an insulator is deposited on a semiconductor. The naturally-occuring positive charge in the insulator produces a field in the semiconductor. For insulator charge of a particular sign, the semiconductor doping type can be chosen such that the semiconductor surface is inverted by the insulator charge. Analysis of this type of junction is performed using MOS capacitor techniques. In order to produce a useful device, contact to the inversion layer under the insulator must be made with another type of junction. In this work, the MIS junction is placed beside the ICI junction to contact the inversion layer. The resulting device can be very useful as a high efficiency solar cell (known as an inversion layer solar cell) since the ICI junction can be designed to couple light efficiently into the semiconductor and the junction is at the surface, where most of the short wavelength photons are absorbed. A simple quasi-two dimensional model for the inversion layer cell is presented and the results of this model are used to determine the factors limiting the efficiency of these solar cells. Actual inversion layer solar cells with efficiencies as high as 14% have been fabricated using a unique shadow mask technology developed during this work.

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