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What Is a Semiconductor?

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By Alexis Alcazar

 Semiconductors are commonly known as devices used in electrical and electronics engineering to act as either a conductor or an insulator. One example of these devices are diodes. A diode is a two-terminal device that lets current pass in one direction and blocks it from the other direction.

To better understand it, think of resistors. A resistor resists the flow of current to a certain level depending on the value of its "resistance", and it works the same way no matter the polarity of the applied voltage.


In the above example, the resistor's value is 100Ω. The voltage applied to it is 100VAC peak-to-peak. I used alternating current (AC) to show the waveform of the current passing through the resistor when applied with positive and negative voltage.

As we can see from the waveform, the current passes through the resistor regardless of the polarity of the applied voltage. When the applied voltage is positive*, the current passes through the resistor from its right terminal to the left. When the applied voltage switches to negative, the current passes through the resistor from the other direction—left to right.
*The blue arrow under the resistor indicates the direction of positive current.

Now, let us observe how the current flows when a diode is inserted in the circuit.

As we can see, the waveform is now different. The current no longer passes through the resistor in both directions. When the applied voltage is positive, the diode serves as a conductor and allows current to pass through the resistor. The diode in the forward bias (positive applied voltage) is almost just like a piece of wire that lets current pass without resistance. The waveform shows that the current during the positive half-cycle of the applied AC voltage is directly proportional to the value of the voltage at any given time.

On the other hand, when the applied voltage switches to negative, the diode no longer acts as a conductor, but an insulator, almost like an open switch. With the diode acting as an insulator in the reverse bias (negative applied voltage), the circuit is practically broken, and the current does not flow. The waveform shows that the current during the negative half-cycle is 0 mA.

This is a good display of how a semiconductor can act as either an electrical conductor or an insulator in a circuit, depending on the configuration. But diodes are not the only semiconductor devices out there. There are thyristors, transistors, and many others.

In the next post, I will explain the science behind the semiconductor's behavior and how it can allow and block the flow of current.

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