Co-investigator Luis Velasquez-Garcia (Heller)
Electrons are field-emitted from the surface of metals and semiconductors when the potential barrier (work function) that holds electrons within the metal or semiconductor is deformed by the application of a high electrostatic field. Field emitters use high-aspect-ratio structures with tips that have nanometer dimensions to produce a high electrostatic field with a low applied voltage. Small changes in the tip radius result in huge changes in the current density because of the exponential dependence of the emitted current on the bias voltage, as described by the Fowler-Nordheim theory. Also, tip radii variation in an array results in non-uniform turn-on voltages. If the emitters are ballasted, the spatial non-uniformity can then be substantially decreased. Furthermore, ballasting individual emitters prevents destructive emission from the sharper tips, resulting in higher overall current emission because of the inclusion of duller tips. Ballasting also results in more reliable operation. The use of large resistors in series with the field emitters is an unattractive ballasting approach because of the resulting low emission currents and power dissipation in the resistors.
A better approach for ballasting field emitters is the use of ungated field-effect transistors (FETs) that effectively provide high dynamic resistance with large saturation currents. In the past the research group demonstrated the use of a MOSFET to ballast the emission of electrons from silicon tips. We have implemented a large and dense array of plasma-enhanced chemical-vapor-deposited (PECVD) carbon nanotubes (CNTs) (1-million elements in 1 cm2), in which each emitter is individually ballasted by a high-aspect-ratio column that acts as an ungated FET. For an n-Si substrate with a high-enough doping level, the Fowler- Nordheim (FN) plot of the data shows no saturation. Current research focuses in systematically studying the dependence of the ballasting on the doping level, showing current limiting data using CNT FEAs, and implementing a gated version of the field-emitter array.