ionization. After this collision, the electrons are again accelerated and the process
continues. Once an electron travels across the phosphor from either the interface or from
impact ionization, it will be captured by interface states on the other side of the phosphor.
It is possible that electrons can be trapped in bulk states creating a space charge on the
other side of or throughout the phosphor. Once the next voltage pulse arrives, the
polarities of the electrodes are switched and the process begins again in the opposite
direction.
The interface between the insulator and the phosphor can be modeled after a
Schottky barrier. The tunnel emission for a Schottky barrier is given by [32]
J & E2 exp- 8s- 2~m (qB)2
3qhE
where E is the electric field, m* is the electron effective mass, q is the charge of an
electron, (B is the barrier height, and h is Planck's constant. For interface state
emission, the equation must be modified by replacing the barrier height with the interface
trap depth. While the tunneling is temperature independent, the device current is
temperature dependent. Thermionic emission has been suggested as an additional
mechanism for charge injection. The Richard-Dushman equation for thermionic
emission is [33]
=r 2mq e -w/
where Je is the electric charge flux, c is the temperature multiplied by Boltzmann's
constant, m is the mass of an electron, q is the charge of an electron, < is Planck's