when different top and bottom insulators are used [14]. As for the mechanism of electron

injection, the accepted theory is thermally assisted tunneling. The device current is a

strong function of the phosphor field, which suggests the tunneling mechanism

dominates. Moreover, experimental observations have shown that there is a slight

temperature dependence on the ACTFEL device, which suggests that the tunneling

mechanism is thermally assisted [15]. Finally, the fact that the electrical properties are

only a weak function of temperature rules out a strict thermionic emission process. In

summary, electrons trapped at the phosphor/insulator interface are the carrier source, and

a thermally assisted tunneling mechanism is responsible for their injection.

 The trap depth and the density of energy states at the interface will affect the device

performance. As the applied voltage reaches the threshold voltage, the energy bands are

bent enough that electrons tunnel into the phosphor conduction band from interface

states. The tunneling emission current, J, for Shottky barriers is given by [16]

 S8f 2S m )3/2
 J ~ E2exp 2-3qhE J
 3qhE

where E is the electric field, m is the effective electron mass, q is the charge on an

electron, QB is the barrier height, and h is Planck's height. For interface trap emission,

this equation remains the same except that the barrier height term is replaced with a term

that represents the interface trap depth. The trap depth controls how much energy is

required to tunnel electrons out of the interface states and into the conduction band of the

phosphor. If the trap depth is too shallow, the electrons will be injected out of the

interface at a low electric field and will not be accelerated to the high energy required to

cause luminescence. However, if the trap depth is too deep, it will take too high an