What is Drift Current?
In a general sense, the term drift signifies motion or movement towards something. So, the use of the word “drift” with current corresponds to the generation of current due to some specific motion or movement.
The drift current is defined as an electric current produced as a result of the motion of charge carriers.
Now you must be thinking that what is the reason for the movement of charge carriers and where it is possible. So, don’t worry, this article will help you to clear all your doubts related to drift current.
Where drift current is noticed?
Generally, the term drift current is used in reference to electrons and holes in semiconductor materials. However, also applies to metals, electrolytes, etc.
We know that semiconductor is a type of crystalline solids that possesses conductivity less than conductors but more than insulators. Semiconductors are the materials that allow the movement of charge carriers under special circumstances as these are neither good conductors nor good insulators.
In the case of semiconductors it is generally said that with the rise in temperature, its resistance falls down.
A semiconductor material has 2 types of charge carriers, one is holes that possess positive polarity. While the other is electrons that exhibit negative polarity.
So, drift current is the outcome of the movement of these charge carriers within the semiconductors.
The term drift velocity is used in reference to drift current. Till now you all must be familiar with the fact that movement of charges within the semiconductor material results in drift current.
However, we must note a point over here that whenever we talk about any type of motion then some specific velocity is also associated with that motion. In a similar way when we talk about drift current then the average velocity with which the charge carriers move is called the drift velocity.
It is generally denoted as Vd.
How drift current is generated?
We have recently discussed that the semiconductors are the materials that allow conduction at specific conditions.
So, to understand how a drift current is produced, consider that we have an n-type of semiconductor material as shown below across which a battery is connected.
You must note that in the case of n-type semiconductor material, electrons and holes are the majority and minority carriers respectively. On the contrary, in the case of p-type semiconductor material, holes are the majority charge carriers while the electrons act as minority charge carriers.
So, when a battery is connected across an n-type semiconductor material then it generates an electric field. This electric field is responsible for the movement of the charge carriers present within the material. Now you must be thinking how?
So, as it is clear from the figure shown above that a battery is connected across the material. This battery generates an electric field.
It is known to us that charges of the same polarity repel while the charges of opposite polarity attract each other. So, the electrons of the n-type material i.e., the majority charge carriers get attracted towards the positive terminal of the battery. At the same time, the holes i.e., the minority charge carriers drift towards the negative terminal of the battery.
Hence, the net motion of the free charge carriers due to the generated electric field results in the flow of current through the material. This flowing current is called drift current.
This simply means that the externally applied potential causes the generation of the electric field which results in drift current.
You must note here that with the increase in electric field intensity the charge carriers drift with more velocity towards the opposite polarity thereby causing more current.
Unit of Drift Current
It is measured in Amperes.
Derivation for Drift Current Density
Consider the drift velocity of electrons is given as:
: Vn and μn are the drift velocity and mobility of free electrons respectively.
Also, the drift velocity of holes is given as:
: Vh and μh are the drift velocity and mobility of holes respectively.
Furthermore, the current density due to free electrons is given as:
And due to holes,
: n and p denote the number of electrons and holes respectively,
Jn and Jp is the current density due to electrons and holes respectively,
e is the charge on electron i.e., 1.6 * 10-19 C
Hence, the overall current density due to both electrons and holes will be given as:
Now, putting the values in the above equation, we will get,
So, the above equation provides the overall current density due to the flow of charge carriers in the semiconductor material.