Metal-semiconductor Schottky Contact Research Articles

Negative transconductance in monocrystalline (Al,Ga)As/NiAl/(Al,Ga)As semiconductor/metal/semiconductor tunneling transistors

The authors present the three-terminal transport characteristics of a resonant-tunneling semiconductor-metal-semiconductor (SMS) structure. The buried metal quantum well consists of a 3-nm-thick NiAl layer, epitaxially integrated in (Al,Ga)As, and is contacted by selectively removing the semiconductor overgrowth. The undoped AlAs tunneling barriers are 2 nm thick and are set back by 5 nm of undoped GaAs from the doped GaAs electrodes. The GaAs doping densities were adjusted to allow for the fabrication of emitter-up, collector-up, and symmetric transistors. The metal-semiconductor Schottky contacts between the NiAl and the cladding (Al,Ga)As layers were studied in order to characterize the individual interfaces and also to confirm the independence of the ultrathin buried metal electrode. Transistor action has been observed at room temperature in emitter-up structures with a wide (70 nm) undoped GaAs collector spacer. Room-temperature negative transconductance values as high as 1.4 mS/mm/sup 2/ have been obtained for large-area (80- mu m diameter) devices.

Thickness dependence of the work function in double-layer metallic films

The work function of metallic thin films limited by symmetric surfaces is expected to be thickness dependent at a level of 0.1 eV and a thickness range of about 5 nm. Recent experiments, however, demonstrated that Cu films on glass or Ni substrates show a long ranging (10–20 nm) increase of the work function with increasing film thickness [1]. This effect was attributed to a violation of local charge neutrality in films with unlike surfaces. In this paper we show that the barrier height of thin film diodes like metal-insulator-metal (MIM)-, metal-semiconductor (Schottky contacts)-and metal-vacuum-metal (Kelvin capacitors) structures decreases with increasing thickness of one metal electrode. This metal electrode consists of a double layer whose single layer thicknesses are of the order of few tens of nm. The observed effect can be attributed to a decrease of the work function at the counter limiting interface not exposed to the evaporation beam. A possible explanation can be found again in the violation of the local charge neutrality in films with unlike surfaces.

GaAs semiconductor-insulator-semiconductor field-effect transistor with a planar-doped barrier gate

A new GaAs field-effect transistor structure is proposed and demonstrated. The gate electrode consists of an asymmetric planar-doped barrier (PDB) diode which behaves like a metal-semiconductor Schottky contact. The device allows engineering of the gate energy barrier, and optimization of transconductance and gate capacitance for a given application. The larger gate energy barrier in conjunction with the self-aligned nature of the process holds promise for both large signal analog and digital switching applications.

Observation of Stoichiometry Changes Beneath Metal Contacts on GaAs

The structure of metal-semiconductor Schottky contacts (Au:GaAs, Al:GaAs, and TiSi/sub 2/:GaAs) was investigated by high-resolution and analytical electron microscopy. In all cases investigated, a change in stoichiometry (increase of the As/Ga ratio) in the GaAs beneath the metal contact was observed. This indicates the formation of anion-rich defects at Schottky contacts on GaAs, giving strong support for the defect models of Fermi level pinning.