Drift and diffusion currents :-
→ The flow of charge (ie) current through a semiconductor material are of two types namely drift & diffusion. → (ie) The net current that flows through a (PN junction diode) semiconductor material has two components Drift current (i) (ii) Diffusion current Drift current :-
(a) Exess hole concentration varying along the axis in an N-type semiconductor bar (b) The resulting resulting diffusio diffusion n current
→ When an electric field is applied across the semiconductor material, the charge carriers attain a certain drift velocity V d , which is equal to the product of the mobility of the charge carriers and the applied Electric Field intensity E ; Drift velocity Vd = mobility of the charge carriers X Applied Electric field intensity. → Holes move towards the negative terminal of the battery and electrons move towards the positive terminal of the battery. This combined effect of movement of the charge carriers constitutes a current known as “ the drift current “ . → Thus the drift current is defined as the flow of electric current due to the motion of the charge carriers under the influence of an external electric field. → Drift current due to the charge carriers such as free electrons and holes are the current passing through a square centimeter perpendicular to the direction of flow. (i)
(ii)
Drift current density J n , due to free electrons is given by Jn = q n μn E A / cm2 Drift current density J P, due to holes is given by JP = q p μ p E A / cm2
Where, n - Number of free electrons per cubic centimeter. P - Number of holes per cubic centimeter μ n – Mobility of electrons in cm2 / Vs μ p – Mobility of holes in cm2 / Vs E – Applied Electric filed Intensity in V /cm q – Charge of an electron = 1.6 x 10-19 coulomb. Diffusion current :-
→ It is possible for an electric current to flow in a semiconductor even in the absence of the applied voltage provided a concentration gradient exists in the material. → A concentration gradient exists if the number of either elements or holes is greater in one region of a semiconductor as compared to the rest of the Region. → In a semiconductor material the change carriers have the tendency to move from the region of higher concentration to that of lower concentration of the same type of charge carriers. Thus the movement of charge carriers takes place resulting in a current called diffusion current.
Carrier Actions What Actions? Definitions Drift Current Diffusion Current Mobility & Scattering What causes band bending? Recombination Generation Let's Draw! Related Topics Diffusion Current
Diffusion is the process of particles distributing themselves from regions of high concentration to regions of low concentration. If this process is left unperturbed, there will eventually be a uniform distribution of particles. Diffusion does not need external forces to act upon a group of particles. The particles move about using only thermal motion. If we let the particles be carriers, so as they move around they take charge with them. The moving of charge will result in a current. We call this current due to diffusion. The difference between drift current and diffusion current is that drift current depends on the electric field applied: if there's no electric field, there's no drift current. Diffusion current occurs even though there isn't an electric field applied to the semiconductor. It does not have E as one of its parameters. The constants it does depend on are D p and Dn, and +q and -q, for holes and electrons respectively. The first constants are called the diffusion coefficients, a proportionality factor. We don't worry too much about these because they are constants. We do worry about the gradient of the concentration of p and/or n, though. But, since we are talking about a one dimensional situation when we are solving for current densities, we only worry about the gradient (or derivative) with respect to the x- plane.
The other difference between drift current and diffusion current, is that the direction of the diffusion current depends on the change in the carrier concentrations, not the concentrations themselves. In the equation, the signs are reversed as we are used to seeing them. We usually assign a +q to holes and -q to electrons. In the case of diffusion current, they are reversed to be opposite of the derivative of the concentrations. This occurs because the carriers are diffusing from areas of high concentrations to areas of low concentrations. For example, if the derivative of p with respect to x is positive, then the concentration of holes is growing as you move towards the +x direction. Diffusion current will be the opposite of that, the holes will be diffusing in the -x direction to where there's a lower concentration of holes. If the derivative is negative, the opposite will occur. The concentration of holes is decreasing as you go from the -x to +x direction. Therefore, holes will diffuse to the +x direction where there's a lower concentration of holes. This is why the negative sign is needed in the equation for the hole diffusion current. The same goes for electrons, but in this case, the signs cancel for a positive derivative because the electrons, carrying -q, diffuse to the -x direction where there's less electrons. The sign remains if the derivative is negative, because electrons will be diffusing to the +x direction carrying a -q charge. For these reasons it's not included in the equation for the electron diffusion current. Both drift current and diffusion current make up the total current in a semiconductor. They may not be occurring at the same time, but the equation is still valid. Under equilibrium conditions, the current density should be zero because there shouldn't be any drastic changes occurring, like applying an electric field or changing the carrier concentrations by a large margin. Even so, if the doping is not completely uniform, there will be a change in concentration is some places in the semiconductor, resulting in a gradient. This gradient can in turn give rise to an electric field, which in turn can give rise to non-zero current densities. Drift Current
Drift is, by definition, charged particle motion in response to an applied electric field. When an electric field is applied across a semiconductor, the carriers start moving, producing a current. The positively charged holes move with the electric field, whereas the negatively charged electrons move against the electric field. The motion of each carrier can be described as a constant drift velocity, vd . This constant takes into consideration the collisions and setbacks each carrier has while moving from one place to another. It is considered a constant though, because the carriers will eventually go the direction they are supposed to go regardless of any setbacks, especially if you look at the direction of all the carriers, instead of each one individually. Drift current in a semiconductor is the resultant of carrier drift. Because we are talking about a semiconductor, or specific areas in a semiconductor, we are interested in the current density. When dealing with drift current, we are interested in the current density
due to drift, and drift arises in response to an electric field. Drift current also depends on the ability of the carriers to move around in the semiconductor, or the electron and hole mobility. Another parameter drift current depends on is the carrier concentration, because you have to have carriers in order for there to be current. Each one of these carriers has a charge, but in this case we will only take q as a magnitude. Finally, we have that the current density due to drift depends on four parameters: the electric field, the electron or hole concentration, the mobility constant, and the charge. The reason we use q for both electrons and holes when it's +q for holes and -q for electrons, is that the electric field takes care of the sign, or direction of the current. When a negative electric field is applied, the electrons will go opposite the electric field. The electron charge is -q, so the resulting electron drift current will be positive. On the other hand, when the electric field is negative, the holes will go the direction of the electric field. Their charge is +q, so the resulting hole drift current will be negative. Vice versa when the electric field is positive. Take a look at the demo for the electric field. To run the demo properly, you need Shockwave installed for your browser. Also, for better viewing, use the "full screen" option on your browser. To return to this page, you'll need to use the browser's "back" button. On the demo, if you click on "semiconductor" a potential will be applied, creating an electric field. You can see how the electron does not travel in a straight line, but it makes it to the other end eventually. This is an example of drift current. If you see the demo by clicking on "free space", there isn't anything for the electron to bump into in free space, so it travels in a straight line.
