A High-Density Electron Source that Uses Un-Gated Transistors for Ballasting

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. We are implementing two types of field enhancers: carbon nanofibers (CNFs) and silicon conical tips. Spatial variation of tip radius results in the spatial variation of the emission currents and non-uniform turn-on voltages. 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. 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 allowing 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 un-gated field effect transistors that effectively provide high dynamic resistance with large saturation currents. In the past our research group demonstrated the use of a MOSFET to ballast the emission of electrons from silicon tips. We plan to implement vertical un-gated transistors in series to the field emitters to obtain spatial uniformity in the current emission and I-V characteristics of the array. The ballast structure is an n-doped, single-crystal silicon column, patterned using Deep Reactive Etching, and thinned using wet oxidation. The field emitters are formed on top of the columns. Current efforts focus on device testing.