In the color-by-white approach, each pixel has a phosphor layer that produces white

light that is subsequently filtered by a red, green, or blue filter (Fig 2.3(b)). Since only

one phosphor layer is used, the processing is simple. However, as the phosphor layer is

filtered, there will be a loss in efficiency.

 There are also other approaches to the development of full color displays.

Interested readers are referred to Ono [12].

 2.3 Device Physics

 The processes leading to EL (Fig 2.4) from a device can be divided into four steps,

namely:

1. As the applied voltage is raised above the threshold voltage, electrons are tunnel
 injected into the conduction band from electronic states at the insulator-phosphor
 interface.

2. Due to the large electric field, these electrons are accelerated to ballistic energies in
 the phosphor layer. They are called "hot electrons."

3. The hot electrons with sufficient energy impact excite the electrons in the
 luminescent centers in the phosphor layer.

4. The excited electrons of the luminescent centers relaxed back to the ground state by
 radiative recombination and thus generate photons.

2.3.1 Tunnel Emission

 To understand tunnel emission, one must consider two questions: One is the

source of hot electrons; the other is the mechanism of electron injection. In ACTFEL

device, the origin of the hot electrons is generally defect states at the insulator/phosphor

interface. Experimental observations have shown that the threshold voltage is a function

of the polarity of the applied voltage, so devices have demonstrated asymmetric currents