Understanding Semi-conductors and Their Applications

Semiconductors are the building block of all electronics, and understanding how they operate is key for deeper insight into how your projects work, how to use them with the most effectiveness and why they work the way they do.

So, what are semiconductors?

• Time
• Desire to learn

To understand semiconductors, we first have to understand conductance.

Simply put, conductance is the material's ability to let electricity flow through it.

How?

This ability to let electricity flow through it is a chemical tendency. Materials are made of atoms, which carry electrons around it in orbits that are called "Shells." Electricity, loosely defined, is the flow of electrons. So, how good a material is in conducting is determined by how welling, or how good it is in losing its outer shell's electrons and gaining others. This process of continuously losing and gaining electrons is what allows electrons to transverse materials easily and for electricity to flow.

Let's get more scientific...

Conductance is the ratio of current intensity* to the potential difference** applied. In other terms, how much current is generated vs. how much force is used to move the electrons or the current. So the higher the conductance, the more efficient the material is at allowing electricity to pass. The opposite of conductance is resistance, which is the opposition to the flow of current.

Conductance is a spectrum.

To understand semiconductors, it's vital to think of conductance as a spectrum, a relative construct instead of absolute conductance or not. there are materials that can steadily and rapidly lose and gain electrons, like copper, these lie in the higher end of the conductance spectrum. There are also materials that have too many electrons at the outer shell or are tightly bound to their electrons that they can't lose and gain them as easily. These lie in the lower end of the conductance spectrum, also called insulators like rubber. But, there are materials which lie in the middle, and whose conductance may vary slightly towards the insulator or the conductor side, these are called insulators, like silicon.

*Current intensity: it is a measure of how many electrons flow through a given cross-section in a given time.

**Potential difference: A measure of the force driving electrons to move from one place to another, also known as voltage.

One of history's most famous scientists is credited with the observation of one of semiconductors' most famous property. In 1833, Michael Faraday noticed that certain materials— metal sulfides in general, silver in particular—conduct electricity better at higher temperatures, which was different than how they thought metals work. This was the first documented observation of semiconductors.

Later on, in 1914 the scientist "Johan Koenigsberger" decided to divide solid materials into the classification we know today, based on their conductivity: metals, insulators, and variable conductors.

Till 1948, most of the research and improvements of these semiconducting material weren't by any means important in the scientific field, due to a lack of applications. It was only when the 3 heroes of the bell labs: John Bardeen, Walter Brattain, and William Shockley, invented the first transistor that the world saw the goldmine that is semiconducting materials. And from then on, the number of technologies and devices dependent on semiconductors skyrocketed. From the 1950s to the 1970s, Germanium was the most used semiconducting material. It was easy to produce and refine into pure enough, functioning crystals. When high purity silicon was possible to manufacture, it began to replace germanium due to its superior qualities.

A list of significant semiconductors applications:

• Crystal radios — popular in WW2, these radios relied on germanium and contributed greatly to the race for developing semiconductors
• Transistors — whose invention is included in the electric and electronic engineering list of milestones, the manufacturing of transistors marked the dawn of electronics.
• Integrated Circuits— The cornerstone of today's devices. Rarely do we find an electric device that doesn't have an electronic circuit with ICs in it that do some functionality.

Semiconductors have 3 unique properties that made them gain this much popularity in electronics manufacturing:

1. Unique Structure
2. Their conductance being thermally sensitive
3. Conductance varying based on Impurities

But the most important property of all, and the one we can identify semiconductors with, is the flexibility of their properties.

Intro to atomic Structure:

Conductors, semiconductors, and insulators gain their properties from their atomic structure, specifically how the bond together to form molecules and more specifically, how their outer shell electrons behave near each other.

All atoms have electrons orbiting near them in shells, as we previously explained, but not all electrons influence electric conductivity or even participate. The electrons that matter to us, are those in the outermost shell, known as "Valence electrons". The maximum number of valence electrons is eight.

Conductors? Semiconductors?? Insulators???

So, what sets the three types of materials apart. Well, mainly it's the number of electrons in the outer shell. You see, atoms like to be stable, a property explained by quantum mechanics, but that's a story for another time. They can be stable through 2 ways, either losing all their outer electrons or completing the outer shell to be full (i.e have 8 electrons.)

For Insulators, that's achievable by holding tightly to their electrons and then attracting others to fill the gaps, as they often have 5+ electrons. So insulators tightly bind their electrons to them, making their flow almost impossible. Conductors have it the other way, they often have 3 or fewer electrons, so the easiest way to be stable is by letting their electrons be loose, or free. Conductor metals often bind in lattices, with their electrons moving freely amongst the atoms of the whole metallic structure, so when subjected to an electric field, a huge amount of electrons called the sea of electrons, flow these lattices.

Now. How about semiconductors? Semiconductors have a constant configuration that can be varied or influenced by outside conditions. What does that mean?!

Well, semiconductors bond together in covalent bonds, which means that each atom shares its electrons with its neighbors, such that no one atom has all 8, but the exchange them frequently. Semiconductors have 4 valence electrons which they share with their 4 surrounding neighbors, such that each covalent bond has 2 electrons shared, as seen in the figure. As we can guess by now, since semiconductors have exactly half the number of valence electrons required to be stable that means that they can either lose them all or gain another 4 with no apparent preference, which doesn't help us in determining whether their electrons are loose or rigidly tied to the atoms. So that's the constant configuration, but about the variable part?

Temperature, its effects, and semiconductors

The variable that affects semiconductors, and eventually helps us figure how loose or rigid their electrons ultimately are, is temperature, Or more accurately, the heat energy gained from temperature. Semiconductors are wildly affected by the ambient temperature. The higher their temperature is, the more energy their electrons gain, and the looser they get, and the more they act as conductors. By contrast, at a lower temperature, their electrons get rigid and they act as insulators. Semiconductors are at that sweet spot where the energy gained from ambient temperature affects their atomic structure, in contrast to conductors and insulators, who are far into their respective spectrums that the energy difference from temperature is negligible. And that's why they are called semiconductors.

One of semiconductors greatest properties is flexibility, and that property extends to much more than their atomic configuration. The fact the semiconductors produce greater conductive capabilities is semiconductors most useful property, and the reason multiple technologies were able to be developed.

Semiconductors have two types:

• Intrinsic semiconductors — which are pure semiconductor material.
• extrinsic semiconductors — which are semiconductor material, with added impurities

How are extrinsic semiconductors made?

The process of doping is to be thanked. Doping is the addition of impurities to semiconductor material on purpose to enhance its conductive material. Either trivalent atoms — atoms with 3 valence electrons — like boron or pentavalent — atoms with 5 valence electrons — ones, like phosphorous are added.

What does it do?

Once again, semiconductors are flexible; the effect of doping vary depending on the element added. If we add pentavalent elements, like phosphorus or arsenic, they become donor atoms in the semiconductor structure. Donor atoms donate extra, unneeded electrons to the other semiconductor structure. So, if we add one atom of phosphorus, for example, to silicon piece only 4 electrons will enter covalent bonds with the neighboring silicon atoms. This leaves one extra electron, which now becomes a free electron, able to move freely throughout the structure without any strong binding force attaching to it. This silicon piece is now called an n-type semiconductor.

The same thing occurs when we added trivalent dopants, like boron or gallium. This time they are called acceptor atoms, as they only have 3 electrons. However, acceptor atoms provide the semiconductor, not with extra electrons, but with extra holes. These holes then are able to flow freely, much like electrons do, and participate in the flow of electricity, and this silicon piece is now called a p-type semiconductor.

Holes, charge carriers & why doping is useful

Holes are theoretical particles, that participate in the flow of electricity. However, they indicate the absence of electrons, and by convention, we consider them to be positive particles and charge carriers. Now, for charge carriers. Electrons are negatively charged, this is rudimentary, basic knowledge. With the introduction of holes, which are positively charged, we modified how we described the flow of electricity, at least when considering semiconductors. Now, electricity passing through semiconductors is due to the flow of one or two charge carriers, negative electrons, and positive holes. The most basic application or usage of doping is the increase of semiconductors' conductance, by adding more charge carriers to the semiconductor, converting it either to p-type or n-type.

As always, humans found a way to innovate, and to produce more complex designs and objects from simple operations like doping, and this led to the invention of the PN junction

• PN junctions are formed by joining one P-type semiconductor side by side with an N-type semiconductor.
  • In the PN junction, carrier particles move in 3 ways: Random motion, Diffusion, and Drift.
• Due to the various types of carrier motion, PN junctions have three types of currents which their operation and usage depend on, each with their own laws:
• Diffusion current
  
• Drift current
• External current
• Pn junctions are useful due to their bias towards the electric current.

Soo, Types of motion?

First off, there is the random motion of particles. Since both types of carriers are particles, so they have kinetic energy that allows them to move freely around the crystal lattice of their substance — crystal lattices are chemical configurations of materials which enable charge carriers to move freely between atoms. However, they move randomly till they collide with each other or with some atom; therefore their motion isn’t guided by some force and is totally random and is considered a type of Brownian motion(which is the random motion of particles in a fluid due to their collision with each other). Brownian motion is too complex to predict, and therefore, we don’t use equations to predict it, but we do account for it when analyzing pn junctions.

*The second image shows Brownian motion of 5 particles among 800 others, by Lookang Author of computer model Francisco Esquembre, Fu-Kwun and lookang (this remix version) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1...

Diffusion motion; P-type semiconductors are rich in positively charged holes as we know, and N-type semiconductors are rich in negatively charged electrons. Naturally, both these charge carrier types exist in their own charge type material, where they are the majority carrier (P or N). However, when they are connected together a concentration gradient occurs, so particles in high concentration areas naturally diffuse or move to the areas with low concentration— a concentration gradient is when there is a difference between the concentration of the same particle at different parts. I.e, different parts have different concentrations. The movement of holes to the N-type region and vice versa due to a concentration gradient is called diffusion and The rate at which diffusion occurs is termed diffusivity, and it depends on the velocity of carriers and on the distance between scatterings and is measured in cm^2*s^-1. Diffusion produces an electric current, which by convention flows along the direction of holes, so the electric current flows from the p-type to the n-type material. Diffusion is a statistical process that has equations governing it, such as The derived equation for linear diffusion, visible in figure 3.

**Courtesy of Pv education.https://www.pveducation.org/pvcdrom/pn-junctions/diffusion

Now, for the type of motion that interests us, drift motion.

First, let's pick-up where we lift the diffusion motion. Holes diffuse from p-type to n-type, and the electrons diffuse in the opposite direction. If these were normal particles, they would diffuse till they are homogeneously distributed across the entire pn semiconductor, but they are not; they are charge carriers. Due to these particles inherently carrying charge, when they diffuse to someplace else they leave behind an ion— a positively or negatively charged atom —of the opposite charge type, fixed in its place, as atoms and ions don't move from their structure.

So, if holes diffuse from P to N, this leaves negative ions in the p-type semiconductor at the boundary between the two material types, and the same happens with electrons. Eventually, this phenomenon reaches a state of equilibrium —AKA an electrically neutral situation. We are left with an electric field, where there are positive ions on the n-type side and negative ions on the p-type side. Naturally, the placement of two types of differently charged particles this near to each other creates a potential difference, which flows from the n-type to the p-type side, opposite to the diffusion current. This electric field causes charge carriers to drift back to their charge majority material.

Drift current is defined as the current resultant from an electric field, which in this case, is the electric field that is produced from the diffusion of the particles in the first place. This is called the drift-diffusion model, and it is used to calculate the motion of charge carriers and the resultant electric current in pn junctions.

The potential difference occurring from diffusion acts to sweep, or repel any charge carrier trying to cross over from its majority type to the other side. This creates a "Potential barrier" that requires particles to have a certain amount, of relatively high, kinetic energy to cross over and. So, now, the boundary between both types become a "depletion region", as there are no charge carriers in this area, either they cross or remain in their majority type. This phenomenon is visible in figure 1.

***Courtesy of electronic tutorials, https://www.electronics-tutorials.ws/diode/diode_2...

The past phenomena all occur in the absence of an external electric field. This means there is no external potential difference affecting the diffusion current or potential barrier. This is where drift comes into play.

As we saw, there is a depletion region accompanied by a potential barrier that stops current from moving. But the most important thing to notice is that this depletion region is biased; its direction is from the n-type to the p-type. With a little human ingenuity, we discovered that if we applied an electromotive force or potential difference along the terminals of the pn junction we would get different results based on the polarity: a bias —polarity is the direction of the positive and negative terminals. There are 3 types of biases:

• Zero
• Forward
• Backward

Zero bias or the zero case is when the PN junction isn't connected to an external emf (electromotive force). In this case, there is a depletion zone, with a potential barrier of a standard strength that depends on the type of semiconductor and how doped both sides are.

Forward bias is when the positive terminal of the external source is connected to the p-type and vice versa with the negative terminal. This produces an emf in the opposite direction to the potential barrier, decreasing the size of the depletion region. If the emf is large enough it will overcome the potential barrier and will cause the junction to conduct electricity normally.

Backward bias is when we reverse the polarity of the terminals to be opposite to the forward bias case (positive with n-type). What this does is it produces an emf in the same direction as the potential barrier, strengthening it. The extra emf increases the depletion region, meaning that the stronger the external voltage source, the bigger the kinetic energy charge carriers need to overcome the potential barriers, making the semiconductor an insulator. Image four shows the relation between the current passed, the direction of voltage, and its amount.

The discovery of pn junctions biases made it a perfect candidate for making diodes— devices that conduct current in one direction but not the other.

Where do we see the principles of pn junctions or diodes?

1. LEDs, also known as Light-emitting diodes (They use the principles of diodes along a phenomenon known as recombination to efficiently produce heat and light)
2. AC current rectifiers. (If AC current passes through a diode it will pass when it's forward, and be blocked when it's backward, creating a sort of DC current)
3. Transistors. (The building blocks of transistor operation are the principles of diodes)
4. Modern ICs
5. Logic gates;and their logic are implemented using diodes or transistors, both based on pn junctions

Rarely has there been an invention that revolutionized human civilization like the transistor. Its idea and applications is deceptively simple, yet it had illuded scientists for years till they managed to find a method to create such a device.

*All of the figures used are courtesy of electronics tutorials at https://www.electronics-tutorials.ws/transistor/tr...

How did they manage such a feat?

Doped semiconductors.

The most basic type of transistor and the first to be manufactured is the BJT or Bipolar junction transistor. It's called bipolar because it uses two types of semiconductor material, n-type and p-type. It also operates on two types of charge carriers, unlike modern (field) transistors that operate on one

What is it made of?

*Use the above figure as visual aid

1. Semiconductor material:

Simply put, BJTs have 3 terminals, each connected to a unique piece of semiconductor material. There are two types of semiconductor configurations: PNP and NPN. The PNP BJT has one region of n-type material sandwiched between two p-type materials, and the NPN is the exact opposite. In almost all cases the middle section is lightly doped and smaller in area. to facilitate the loss of charge carriers to other regions. In any case, pnp and npn do not differ in applications or usage, and the only difference between them is the polarity of the external voltage source and the current.

2. Terminals:

As stated above, BJTs have 3 terminals which are connected to the circuit externally, and to the semiconductor regions internally. When referring to the semiconductor regions, we call them terminals, according to their function. The 3 terminals are called: Collector—because it collects electrons/charge carriers, Emitter— as it emits charge carriers and the Base, which is always the middle region. And so, while the base is differentiated from the rest of the terminals because it is in the middle, we differentiate the other two by the type of its connection. As all terminals need to be connected to external voltages for the transistor to function, we will find that there will always be a forward and backward connection. So, in the pnp BJT, we find that the emitter is forward biased while the collector is backward biased, and vice versa in the npn one.

When we draw transistor s circuit components, we differentiate the two types by drawing an arrow in the direction of the conventional current flow at the emitter terminal. *Refer to figure 1

Classification according to connection type :

Since the BJT has three terminals, this means that they could be connected as a transistor in 3 ways, each differing in how the three terminals are connected to the circuit. These configurations are based on which terminal shares a common node with the input and output voltages. These configurations are:

* Terminology: low/high input/output impedance: When talking about transistors, low impedance means that this particular terminal is forward biased, so it doesn't have a lot of impedance while high impedance means it operates in backward bias. For example, in a low input impedance circuit, the input signal passes through a forward-biased terminal connection.. and so on

• Common base

As shown in Figure 2. in this configuration, the base terminal is common to both input and output voltage. This connection significantly increases the input voltage, but at the same time, decreases the current, so it's is used to "attenuate" signals; it has low current gain, high voltage gain, and high resistance gain.

Based on these characteristics, including its good frequency response, common applications include:

1. Microphone pre-amplifiers, where they prepare the weak signal for amplification
2. Radiofrequency power amplifiers.
3. Voltage regulators— as it increases the voltage.

• Common Collector

As shown in figure 3, this one has the collector terminal common between both input and output voltages, whereas the input is given to the base, and the output is taken from the emitter. This configuration is known as emitter-follower or voltage follower because the emitter voltage (the output), follows the voltage of the base (The input). It has zero voltage gain, high current gain and it converts from high input Impedance* to low impedance; furthermore, the input and output voltages are in phase**.

*,** to know what impedance and in-phase voltage is, refer to our passive and active components instructable.

Applications:

1. Voltage buffer—voltage buffers are components that transfer the same voltage from a circuit with higher impedance to another circuit with a lower impedance without interfering with either circuits' operation.
2. Switching circuit
3. Amplifier
4. Impedance matching circuit— circuits that are match input and outer impedance to maximize power transfer.

• Common Emitter

This is the most common type of transistor configuration and is considered the default mode of connection. Why?

Because while CB configuration only provides voltage gain, and CC configuration provides only current gain; CE provides both, so it's used more widely as it gives the power gain required for most devices.

In the CE configuration, the emitter is common to both input and output signals, as shown in the 4th figure. As stated, the advantage pf this configuration is that it greatly increases the power gain, more so than any other configuration. This is due to its low input impedance, and high output impedance. The most significant property of this configuration is that the small input current (which comes from the base) controls the large current in the emitter-collector circuit. Thus any small change in the input current will result in a much greater change in the output current. This configuration has greater input impedance than other circuits, high current and power gain but lower voltage gain.

Common Applications:

1. Low-frequency voltage amplifiers— They don't respond well at higher frequencies
2. RF amplifiers
3. Audio amplifiers— generally low noise amplifiers
4. transistor amplifier circuit— an amplifier circuit with more than one transistor.
5. switch— they make great switches that need a voltage above a certain threshold (~0.7V) to turn on and allow current to flow