PN Junction Diodes - Forward, Reversed Biased , Unbiased Diodes

What is an Ordinary Silicon Diode:

It is simply a pn junction. One side of a diode is doped with a p-type impurity and the other with an n-type impurity. The symbol looks like an arrow that shows the direction of the current in the circuit. i.e. the current flows from a positive to negative direction.

fig 1 Diode Circuit Symbol

Unbiased Diode:

It is nothing but a piece of a semiconductor material doped with p-type material on one side and n-type material on the other side. The junction is the border where both regions meet. There are many free electrons in the n-type region because of doping and in the same manner, the p-region has many holes because of doping. The holes near the junction diffuse across the junction into the n-type region. Whereas, electrons near the junction diffuse across the junction in the p-type region. The n-type region loses some free electrons because of diffusion across the junction. Due to this, a layer of positive charges is formed near the junction. Similarly, a p-type junction loses some holes because of diffusion across the junction. Due to this, a layer of negative charges formed across the junction.


Fig 3 PN-Junction before diffusion

Figure 4 PN-Junction After Diffusion

Figure 4 shows the pn-junction in equilibrium. At the equilibrium the negative charges in the depletion region (due to potential barrier see figure 4) repels the remaining charges for further diffusion, similarly, positive charges or holes oppose further diffusion. So at equilibrium, the depletion region acts as a potential barrier and hence no further movement of charges.

MODES OF OPERATION Of PN JUNCTION:

When we apply forward bias, the equilibrium disturbs and the potential barriers reduce and start conducting the current if the applied potential is greater than the barrier potential.

Forward Bias:

Applying positive potential at the p-type and negative potential at the n-type.

Fig 5 Forward Bias

The application of positive potential V will Pressure electrons in the n-type material and holes in the p-type material to recombine with the ions near the boundary and reduce the width of the depletion region. The width of the depletion region decreases as the applied bias voltage increase. The small potential barrier will never diminish, so there is a potential drop called CUT IN VOLTAGE.
For the silicon diode, it is 0.7V. For conduction V>0.7V.

Reverse Bias:

Applying negative potential at the p-type and positive potential at the n-type.

For an ideal diode, in reverse bias condition, no current flows through it, and it behaves as an ideal insulator. But this is not true for a practical diode. The current that flows under the reverse bias condition is called REVERSE SATURATION CURRENT.

Fig 6 Reverse bias

DIODE CURRENT UNDER FORWARDING BIAS:

$I = I{_o}(e^{V/\eta*V_T{} }-1)$
I=Current through diode
$I_O$ =Reverse saturation current
V=Applied voltage
$\eta$ =material parameter
$V_T$ =Voltage equivalent to temperature

PROS & CONS:

It is a non-linear device because its IV characteristic curve is non-linear.

It can conduct current only in 1 direction, and this quality makes them helpful in rectification applications

The higher the junction temperature greater will be the saturation current

Why does a diode be a non-linear device?

An electronic device is linear if the current flowing through it is directly proportional to its voltage. As we observed the current and voltage behaviour of a diode, it does not act like a linear device.

If you graph current versus voltage, you will get an exponential curve.

What are the uses of diodes? (Applications of diodes):

As a beginner, you might not aware of the usefulness of this tiny, low-cost semiconductor device. It is a passive two-terminal device. The characteristic of the diode is to flow current only in one direction. There are numerous applications of this tiny device, each of which utilizes the characteristic property of flowing current in one direction. I have written a series of articles on each of the following applications.