6.3 Ambipolar Transport

The generation and recombination rates of excess carriers are important parameters，but how the excess carriers behave with time and in space in the presence of electric fields and density gradients is of equal importance. As mentioned in the preview section，the excess electrons and holes do not move independently of each other，but they diffuse and drift with the same effective diffusion coefficient and with the same effective mobility. This phenomenon is called ambipolar transport.

If a pulse of excess electrons and a pulse of excess holes are created at a particular point in a semiconductor with an applied electric field，the excess holes and electrons will tend to drift in opposite directions. However，because the electrons and holes are charged particles，any separation will induce an internal electric field between the two sets of particles. This internal electric field will create a force attracting the electrons and holes back toward each other. This effect is shown in Fig.6.7.

This E-field may be written as

E=Eapp+E Eint （6.1）

where Eapp is the applied electric field and Eint is the induced internal electric field. Since the internal E-field creates a force attracting the electrons and hole，this E-field will hold the pulses of excess electrons and excess holes together.

Fig.6.7 The creation of an internal electric field as excess electrons and holes tend to separate.