Electron emission from GaN/LaB₆ cold cathodes

Recent developments in vacuum microelectronics have led to a resurgence of interest into cold cathode emission for applications to a variety of electronic devices. The devices include microwave vacuum transistors and tubes, cathodoluminescent flat panel displays, pressure sensors, high temperature and radiation hardened electron devices. For these new applications, the ideal cold cathode should have the following characteristics: (i) low-voltage operation (5-20 volts), (ii) high current density (5-10 A/cm²), (iii) room temperature operation and (iv) stable and durable operation. Effective Negative-Electron-Affinity (NEA) and Optoelectronic Cold Cathode (OECC) structures have been fabricated using a combination of the wide-bandgap semiconductors, GaN and AlGaN, and the low work function metal, LAB₆. In the NEA structure, electrons are injected from an n-type GaN layer into a thin p-type GaN layer. Appropriate design of the p-type thickness, which was guided by Monte Carlo transport simulations, allows some fraction of the injected electrons to arrive at the p-GaN/LaB₆ interface with enough energy to traverse the thin LaB₆ layer and emit into vacuum. In the OECC, photons are generated at a p-n junction in GaN. The photons are subsequently absorbed by a LaB₆ layer, creating electrons with sufficient energy (3.4 ev) to overcome the LaB₆ work function of approximately 2.5 eV. The GaN and LaB₆ fabrication will be discussed in detail. Results of the photoemission from thin LaB₆ films and electron emission from hybrid and monolithic cold cathodes will be discussed.