Minority carrier injection and charge storage in epitaxial Schottky barrier diodes

Minority carrier injection by so-called noninjecting metal-semiconductor contacts is considered under conditions of moderate to heavy forward bias. The injection ratio, γ (ratio of minority carrier current to total current), rises linearly with forward current for sufficiently large applied bias. The reason for this rise in γ is that the minority carrier current is enhanced by a drift-field component much larger than the diffusion current which dominates at low bias. In this range the injection ratio is given by γ = {n ²_ij }/{bN _D²j _ns} where n_i and N_D are the intrinsic and doping concentrations, b the mobility ratio, j_ns the Schottky diode saturation current density, and j the diode forward current density. As an example a 5 Ω-cm n-type silicon-gold diode will obtain an injection ratio of 5% at a current density of 350 A/cm ². The minority carrier stored charge per unit area ( Q), for Schottky diodes made on thin epitaxial layers, depends upon the characteristics of the epitaxi-substrate interface and can become very significant when this interface is highly reflecting (i.e., has a low value of surface recombination velocity). For large applied bias and negligible bulk recombination the stored charge is given by Q = {qn _i²D _pj }/{N _Dj _nsσ } where q is the electronic charge, D_p the diffusion constant, and σ the surface recombination velocity. In measurements on experimental epitaxial diodes the interface is not found to be highly reflecting but is characterised by a recombination velocity of about 2000 cm/sec. This value applied to the 5 Ω-cm silicon-gold diode yields a storage time ( {Q}/{j}) of about {1}/{3} nsec. Normalised curves of γ and{Q}/{j} vs. forward current are presented with parameters chosen in the range expected for silicon epitaxial Schottky barrier diodes. The theory has been verified by experiments on diodes made by evaporating gold contacts on silicon.