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The diode’s we’ve been talking about all this time do have some really cool applications in signal processing, power supply & a lot of other places.

And yes, the AOD stands for “Applications of Diodes” & we’re gonna use that for the rest of the apps as well.

Diodes are used in rectifiers which are devices that convert an AC to a DC signal, not a pure dc signal though… And as a matter of fact, the only source of pure DC is a battery.

And this process of converting AC to DC is called rectification. Consider a simply sine wave as the input from an AC source. Then the following summarizes the overall process of rectification…

So, there is no output for the negative half as the diode is in the reverse bias condition(considered OFF). Also, the circuit we use here is just what the half wave rectifier really is.

Now, considering the two halves collectively, we get an output signal like so…

Notice here that the peaks of the positive cycle appear at equal distances, and this distance is same for the input & the output signal. Hence, the frequency of the signal does not change.

(f_in=f_out)

Getting into the mathematical view of things, we can now calculate the RMS & Average values of the voltage for our new output signal.

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In the previous post on the P-N junction, we looked at the formation of a P-N junction. The resulting device is called a diode.

The semiconductor diode is closest to the ideal diode. To study electronic devices made up of the P-N junctions, we look into their VI characteristics.

The VI characteristics of a device is simply a plot of the V vs I curve for the device. For instance, the resistor follows ohm’s law & hence, we obtain a liner VI characteristic for resistance.

The curve below shows the VI characteristics of an ideal & a real diode…

The curve in the first quadrant represents the diode in its forward bias. The diode starts conducting at the voltage V_c called the Cut In Voltage. This voltage is about 0.6 V for Silicon & 0.2 V for Germanium.

In case of the reverse bias, a small current flows. This current, independent of the applied voltage & present only due to the minority charge carries, is called the reverse saturation current.

In reverse bias, the external applied voltage breaks the covalent bonds in the junction region. This leads to the breakdown of the junction at a specific voltage called the breakdown voltage(V _b).

Notice that we only consider V_b in the real case. Hence, ideally, there should be almost no current in the reverse bias state, & hence, no breakdown of the junction.

The breakdown simply means that the diode now allows all the current to flow through it. It is just a malfunctioning of the diode & not the damage of the diode. The diode is otherwise supposed to allow current through just one direction & stop all the current in the other.

Further, two kinds of resistances exist in the diode corresponding to the direct & alternating currents respectively.

The diode can also behave as a capacitor, and even as a variable capacitor. There are two different ways of looking at a diode as a capacitor.

The current that flows through the diode depends upon the applied voltage & the temperature(& hence, the voltage due to the temperature). It is given by…

where I₀ is the reverse saturation current, e is the electronic charge, k is the Boltzmann constant, V the applied voltage & T is the temperature. The term kT/e is also called the voltage equivalent temperature. Also, η is a constant = 1 (for silicon)
& 2 (for germanium).

All other diodes like the Zener Diode, LED, etc are derived from the basic semiconductor diode with a few changes in their design & functions.

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In the previous post, we looked at p & n type semiconductors. These are more useful when combined together to form something called the P-N junction.

The p-doped region has holes as its majority charge carriers & the n-doped region has free electrons as its mobile charge carriers. Hence, the holes & free electrons attract & eliminate each other. This process is called recombination.

Thus, due to the diffusion of the charge carriers, a potential difference gets established in the region of recombination. This potential is called the barrier potential or the space charge potential & the region is called the depletion region.

The device resulting from the p-n junction is called a p-n junction diode or simply, a diode. P-N junctions are also used in transistors & rectifiers.

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We looked at pure or intrinsic semiconductors in the previous post on Analog Electronics.

We can however, change the electrical properties of the pure semiconductors by adding certain impurities to their structure, a process called doping.

When doping semiconductors of groups 3 & 4, these impurities are usually elements of group 3(acceptors) or 5(donors).

This gives rise to two kinds of extrinsic semiconductors : ones having free electrons as their majority charge carriers(called n-type) & those which have holes as their majority charge carriers(called p-type).

Extrinsic semiconductors are used in many electrical devices. A more useful version of doped semiconductors is the p-n junction.

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We’ve often heard of terms like the Silicon Valley & the Silicon Economy. What do they really refer to?

All modern day electronics are build using a special class of materials called semiconductors. These materials have an electrical resistivity between a conductor & an insulator.

They are the foundations of all electronics which are computerized(computers, ipods, etc) & ones which use radio waves(radio, cell phones, etc), silicon being the heart of all these devices.

The elements like Silicon & Germanium having 4 valence electrons are elemental semiconductors. The 4 valence electrons can easily bond with 4 neighbouring electrons to give rise to a lattice structure with no free electrons(at zero temperature).

Since, there are no free electrons at zero temperature, Intrinsic(pure/elemental) Semiconductors behave as insulators at zero temperature.

Then how do they differ from insulators? Well, the difference is in terms of the energy gap between the valence & conduction bands.

This energy gap is zero in case of conductors, very high for insulators & very small for semi conductors(about 1 eV)

Hence, on increasing the temperature, the electrons in the valence band of the semiconductor gain energy & some of them get sufficient energy to move to the conduction band.

This is what happens physically inside the lattice. In terms of the energy bands, we could show this as follows…

These electrons leave behind empty spaces called holes. The holes appear to move in a direction opposite to that of the electron & hence, are the positive charge carriers of the semiconductor.

Hence, a semiconductor conducts only at high temperatures & the conduction is due to both electrons & holes, also, the electrons & holes are equal in number.

However, the conductivity of the semiconductors can be changed drastically by adding certain impurities to the semiconductor materials. This process is called doping & is explained in the next post.

Semiconductors find their major application in manufacturing transistors. The first transistor was made of Germanium. Germanium, in fact, would have more free electrons at a particular temperature than silicon. But Silicon is preferable as it can be used at extremely high temperatures.

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