Diffusion and Drift of Carriers: Built-in Fields
Diffusion and Drift of Carriers:
If an electric field is present in addition to the carrier gradient, the current densities will each have a drift component and a diffusion component
and the total current density is the sum of the contributions due to electrons and holes:
- We can best visualize the relation between the particle flow and the current by considering a diagram such as shown in Fig given below.
- In this figure an electric field is assumed to be in the x-direction, along with carrier distributions n(x) andp(x) which decrease with increasing x.
- Thus the derivatives are negative, and diffusion takes place in the x-direction.
- The resulting electron and hole diffusion currents [Jn (diff.) and Jp (diff.)] are in opposite directions. Holes drift in the direction of the electric field Φp,
- whereas electrons drift in the opposite direction because of their negative charge. The resulting drift current is in the x-direction in each case.
- Note that the drift and diffusion components of the current are additive for holes when the field is in the direction of decreasing hole concentration, whereas the two components are subtractive for electrons under similar conditions.
- The total current may be due primarily to the flow of electrons or holes, depending on the relative concentrations and the relative magnitudes and directions of electric field and carrier gradients.
- An important result of the equ of the drift & diffusion component is that minority carriers can contribute significantly to the current through diffusion.
- Since the drift terms are proportional to carrier concentration, minority carriers seldom provide much drift current.
- On the other hand, diffusion current is proportional to the gradient of concentration.
- For example, in n-type material the minority hole concentration p may be many orders of magnitude smaller than the electron concentration n, but the gradient dpldx may be significant.
- As a result, minority carrier currents through diffusion can sometimes be as large as majority carrier currents.