Field effect transistors
The field-effect transistors have four terminals which are known as the gate, drain, source and body. The current flows along a semiconductor path called the channel. Although the physical diameter of this channel is fixed, its conductivity can be varied. This is achieved through the application of a voltage to a control electrode called the gate. The change in gate voltage causes a large variation in the current between the two ends of the channel, namely from the source to the drain. There are two types of channels, depending on the semiconductor material, which are the n-channels and the p-channels. In n-type material the charge carriers are electrons whereas in p-type material electric charges are carried mainly in the form of electron deficiencies called holes. The demand for high current density has been the main difficulty to face in realizing an organic injection laser. Efforts focus on maximizing the current-carrying capability of the device and at the same time, limiting the required current density. In the first case, charge transport crystalline organic material in a thin-film transistor (TFT) configuration is used. To achieve the second goal, a metal complex, which comprises the energy acceptor and emitting site, acts as a highly efficient energy transfer light emitting system. Combining these two techniques has led to FETs with ambipolar current characteristics utilising both an n-channel and a p-channel. This is the result of the use of an organic ambipolar transport layer, composed of a co-evaporated thin film of two organic materials, a hole-transport and an electron-transport. The light intensity is controlled by both the drain - source voltage the gate voltage. The device structure shows the way for adjusting electron and hole mobilities by co-evaporation of two different organic semiconductors.
