Drift current is the concept involved in a doped semiconductor, there are free charges available, once the voltage is applied we can notice the movement of the charge carriers based on the polarity of the charges it gets attracted towards the respective terminals. Hence the electric field is applied due to which the motion of charge carriers observed results in the production of current. The velocity required for the movement of charge carriers is referred to as drift velocity. In order to describe this type of current direction, it is dependent on the collision occurring with the atoms. Generally, the flow of electrons will be in the Brownian motion. But here the electric field is responsible for them to arrange in a single direction.
What is Drift Current?
The current generated because of the application of external voltage that results in the movement of charge carriers is defined as Drift current.
The main reason behind the occurrence of this current is because of the application of external forces it can be either electric field or voltages.
Density in Semiconductors
As the external supply is provided there is the movement in the majority of the concentration of the carriers. Based on the type of semiconductors its majority of the carriers vary.
However, both holes and electrons in the majority or in minority are present in any type of extrinsic semiconductor.
Holes are the positive carriers and electrons being negatively charged, holes always prefer the direction with respect to electric field whereas the direction of the flow of electrons can be evident opposite to the path of an electric field.
This net motion in the concentration of the particles that are either holes or electrons can be referred to as the drift current of that particular semiconductor.
In this way, this current is calculated. It is measured in terms of Amperes. The velocity achieved by the charge carriers due to the application of external voltage is referred to as Drift velocity.
The drift velocity for the carrier concentration of electrons is
Vn = µnE
Where Vn is the drift velocity representing the carrier concentration of electrons.
µn is the mobility of the particular electrons and
E represents the electric field applied to the n-type of the semiconductor.
Drift velocity of the holes concentration is
Here Vp is the drift velocity referred to the concentration of holes
µp refers to the mobility of the concentration of holes and E representing the electric field acting upon it.
General Relation between Current and Drift Velocity
Suppose that there are n number of electrons per cubic centimeter as well as the drift velocity (Vd). In a certain moment of time, the electron tends to move this is referred to as the distance Vd∆t. Its volume is
Then the motion of a number of free electrons in that particular region is
Similarly, the charge crossed across the area is given by
∆q = nqAVd∆t
I= ΔQ/ Δt = nqaVd
Hence the above equation shows the relationship between current and the drift velocity.
Derivation for Density
The current caused by the movement of charge carriers either by free electrons or holes per square unit area is referred to as the density of the drift current or Drift current Density.
The density is variable based on its type of charge carriers. The drift current is preferably measured in terms of Ampere per square centimeter.
The drift current density with respect to the concentration of electrons is
Jn = qnµnE
The drift current density based on the concentration of holes is
Jp = qpµpE
Here n represents the number of electrons present per cubic centimeter
p represents the number of holes per cubic centimeter
q is related to the charge carried by the concentration of carriers
q = 1.6X10-19 coulomb
µn is the mobility of electrons its units are cm2/ Vs
µp is the mobility of holes and its units are cm2/ Vs
E is the electric field applied on it and it is measured in terms of V/ cm
Then the overall density is calculated by the sum of the individual current densities of electrons and holes. Therefore
J = Jn + Jp
J = qnµnE + qpµpE
J = q (nµnE+pµpE)
This the final equation obtained for the overall current density. As per the analysis made can the presence is detected in the unbiased condition of the diode with p-n junction?