Semiconductor Diode and 4 of its bizarre Switching Operation

Hello everyone, I hope that you all are doing good in your lives. The topic for today is based is semiconductor diode and their switching mode operations. As we all know that the semiconductor devices and diodes are nearly applicable in all electrical and digital devices. Thus it is very necessary to understand the working and the character of a semiconductor diode. So that it becomes easier to apply it in real-world projects.

Introduction to Semiconductor diode

If we make a digital system requiring hundreds of gates using switches, perhaps it may not operate as per our desire. Because using many gates and switches resulting in increasing the complexity of the device. This will also reduce the speed. However, to overcome the speed we can use relays. But the issue with this is that they are bulky and consume large power. Thus to overcome all of these disadvantages semiconductor devices like p-n junction diode, bipolar junction transistor (BJT), and unipolar transistor (for example MOSFET) is used as switches.

Thus these devices are much faster than relays and switches and act as the best alternative. As a result of these devices, there’s a considerable saving in terms of cost, size, weight, and power.

Semiconductor diode

The flow of current in a material is due to the flow of electrons and the conductivity of the material (k) is directly proportional to the number of free charge carriers (n).

K ∝ n

For example, a good conductor (copper, silver, etc.) has a very large value of n (~10²⁸) and an insulator has a very small n (~10⁷). There’s another class of materials, semiconductors, for which the conductivity lies between the conductor and insulators. Silicon and germanium are the two most important semiconductors

These elements have four valence electrons, which are not free to move about as they are in a conductor. Instead, they combine with the neighboring atom and form electron pairs, known as a covalent bond. Thus the outermost shell has all eight electrons in total which means it is completely filled.

At 0 K, all bonds are intact and the conductivity is zero. If the energy in the form of heat or light is supplied to this sample, some of the bonds break away and form free electrons and holes. Because of the generation of free electrons and holes, the conductivity of the material increases.

The energy require to break the covalent bond is about 1.1 eV for silicon and 0.72 eVfor germanium at room temperature.

P-N Junction Diode

A p-n junction diode is form by introducing n-type of impurity into one side and p-type impurity on the other side of a single semiconductor crystal. The concentration of the charges is same on the both sides.

Because of the concentration gradient across the junction, the hole from the p-side diffuses to the n-side and the electrons from the n-side to the p-side. This process results in the combination of electrons and holes near the junction becomes devoid of charge carriers. This area near the junction free of charge carrier is referred to as depletion, space charge, or transition region. However, the charge density near the junction because, of the negativity of the p-side and the positive polarity on the n-side. This results in developing an electric field intensity and consequently, the potential difference is developed. This leads to the formation of a potential barrier between the two regions for further diffusion.

The symbol of the P-N Junction diode is below;

Forward Bias

Forward bias is the connection if the positive terminal of the battery is in connection with the p-side and the negative terminal to the n-side of the junction. This causes both the holes in the p-side and the electrons in the n-side to move closer to the junction. Consequently, the width of the depletion region decreases, the height of the potential energy barrier at the junction decreases; and thus the equilibrium present initially changes. Hence the holes cross the junction from the p-side into the n-side, where they are referred to as minority charge carriers. Similarly, the electrons are also known as minority charge carriers in the p-side. This results in current I_f in the direction from p-side to n-side.

Reverse Bias

In reverse bias the polarity of the battery is opposite. This causes both, holes in the p-side and electrons in the n-side to move away from the junction. Consequently, the depletion layer width increases which prevent the holes from the p-side to cross over to the n-side and electrons from n-side to p-side.

However, the minority charge carriers, the electrons from the p-side, and the holes from the n-side can easily cross the junction; and constitute a reverse current I_R. In the direction from the n-side to the p-side. Nut this current is very small.

The minority carriers near the junction cross over to the other side, recombine, and the concentration goes to zero at the junction. Far away from the junction, the minority carrier is equal to the thermal equilibrium value.

Characteristics

The characteristic graph of the p-n junction diode is;

Zener Diode

From the V-I character above we can see that a large amount of current flows in the reverse direction as the reverse voltage across the diode increases beyond a voltage known as Zener breakdown voltage.

When the diode is operating in the breakdown mode, the voltage across the diode is most constant, V_z and the current in the diode is dependent on the external resistor. These diodes are use for voltage reference or constant-voltage sources.

Its symbol is;

Schotty Diode

The speed of operation of a semiconductor diode is reduced due to the storage of the minority carriers as discuss above. For reducing the storage time significantly we can use a junction made up of semiconductors and a metal. For example, the junction between aluminum and n-type semiconductor constitutes a diode known as Schottky diode.

The V-I character of the diode is similar to that of the semiconductor diode, except for the cut-in voltage, which is in the range of 0.2 to 0.5 V, depending on the metal present. The aluminum n-type Schottky diode has a cut-in voltage of about 0.35 V.

When the diode is in forward bias, the positive terminal of the battery is in connection with the metal and the negative terminal is to the semiconductor. The current flows across the junction due to the flow of electrons from the semiconductor to the metal. Electrons thus entering the metal are indistinguishable from the plentiful electrons already present in the metal. And hence these do not constitute minority carriers. Therefore, when the junction voltage is reverse by the problem of removal of the excess minority charge does not exist. Hence, the Schottky diode exhibits a negligible storage time.

Bipolar Junction Transistor

A bipolar junction transistor consists of a semiconductor crystal in which either a thin layer of p-type silicon is sandwich between two layers of n-type or a layer of n-type sandwich between two p-type silicons. The former is known as n-p-n transistor and the latter as p-n-p transistor. The two types of transistor along with their symbol are;

This is a three-terminal device design as;

1. Emitter (E)
2. Base (B)
3. and lastly Collector (C)

The arrow on the emitter lead indicates the direction of the current flow. The emitter, base, and collector current IE, IB, and IC respectively are assumed to be positive if the current flows into the device.

The operation of BJT depends upon the biasing between the two junctions. The range of its operation mainly consists of three regions:

1. Cutoff.
2. Active.
3. and lastly Saturation.

The cutoff region is when both the junctions are in reverse bias, and as a result very small reverse saturation current flows through the device. This region is below the curve for I_E = 0

The active region is when the emitter-base junction is in forward biasing and the collector-base junction is in reverse biasing. The output current here is almost linearly independent of the input current.

The saturation region has the largest application in the transistor switches. For the transistor to be in the saturation region, both the junctions must in forward biasing.

Conclusion

However here we are at the end of the blog. I hope that you are satisfied with the content and liked the blog. If you did like then please mention below the part that fascinates you the most. Besides if you do have any doubt then please feel free to mention it below in the comments or reach out to us through our contact us page. And also if have any suggestion regarding the topics that you would like to read next on the please do mention.

Regards,

Have a nice day 🙂

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