Forward conduction and minority carrier storage in the Schottky barrier diode

Abstract

The static characteristic of a Schottky barrier diode approaches the ideal diode law, i=I s [exp(qv/kT)-1] about the origin, as does a conventional pn junction diode. Important differences exist, however, which favor the Schottky barrier for high-frequency or fast-transient applications. The technology associated with the Schottky barrier diode permits a great deal more freedom in the selection of barrier height (hence, the scale factor I s ) than is possible in the pn junction. This diode consists of a metal-semiconductor interface, wherein different metals on the same semiconductor can be used to produce different barrier heights. The equivalent pn junction barrier height is limited to the band gaps of the few available semiconductors. Therefore, a wider control of forward conductivity, or alternatively a better impedance match to the circuit, is possible with the Schottky barrier diode. Minority carrier storage can virtually be eliminated in the Schottky barrier diode, because a negligible number of minority carriers is involved in the forward conduction process within certain excitation limits. Reverse recovery times well below 0.1 nanosecond can be readily achieved at low excitation levels. Schottky barrier diodes were made on epitaxial silicon layers having resistivity of about an ohm-cm. These were found, at forward current densities exceeding 103amperes/cm2, to depart considerably from the behavior of an ideal diode in series with a linear resistance. Two successive conduction regimes were observed. At moderate excitation, dynamic resistance is established by scatter limiting velocity of electrons in the epitaxial layer, and at high excitation by the substrate resistance. A negative resistance region may separate these regimes. Associated with this complex forward conduction, a threshold in forward excitation may be observed at about 104amperes/cm2above which minority carrier storage is greatly increased. This threshold is related to the epitaxial layer resistivity and to the barrier height. This paper discusses the observation of this storage threshold, its relation to the mechanism of forward conduction, and the general consequences of storage on the transient behavior of these diodes -- both with respect to turn-on and turn-off.