Semiconductor Electronics

Step 1. Electric Field

From a physics perspective, an electric field is a form of matter existing around charged bodies. Terms like matter, body, and charge won't be discussed here, as they border on philosophical debate. The key understanding is that these relate to electrical phenomena, where "charge" means electrical charge. For clarity, consider two objects — spheres — each possessing a characteristic: charge. This can be positive or negative. These spheres interact through an electric field surrounding them. This interaction reveals the essence of electric fields: they arise from electrical forces between charged bodies. These forces operate such that like charges repel while opposite charges attract.

To characterize electric fields conveniently, physicists use a special quantity: field strength (or intensity). This allows comparison of "stronger" versus "weaker" fields. The specific calculation method and measurement units matter less than understanding that greater field strength exerts stronger influence on charged bodies. This characteristic also has direction, making it a vector quantity. Direction matters for determining how the field "pushes" a charged object.

The simplest example of a real charged body is the electron. Though a purely quantum object following "magical" quantum laws, it can be approximated as a sphere following the above reasoning. Since electrons carry negative charge, they move against the field direction.

Beyond field strength, another characteristic exists: potential. Potential represents the energy aspect of fields. While energy in physics presents interesting philosophical questions, its mathematical properties make it highly useful. Potential describes energetic properties at each point where a field exists; generally, it differs at each location. Potential difference deserves special interest. A relationship exists between potential difference and field strength. Potential difference is typically called voltage (distinct from field strength). It's not exactly the voltage measured in circuits with voltmeters, though some connection exists.

In physics, any directional movement of charged bodies constitutes current — in this case, electrical current. Movement can occur not solely from field action. Due to directional specificity, current itself has direction. Historically, for negatively charged particle movement, current direction is considered opposite.

Practically, physicists use current magnitude (force of current), measured in Amperes. This quantity describes how many charged bodies pass through a material cross-section per unit time. Ohm's Law defines the relationship between current and voltage. A coefficient called resistance connects these quantities, characterizing the material and physical phenomena accompanying current flow. Usually resistance is simply a number, though complex cases exist. Sometimes conductivity — the inverse concept — is used instead. Greater resistance means lower conductivity.

Another remarkable electrical characteristic: when opposite charges accumulate, an electric field (voltage) develops between them, indicating the system stores electrical field energy. These systems are characterized numerically by electrical capacitance — the principle underlying capacitors. We won't detail this further.

Additionally, electrical phenomena represent a special case of electromagnetism manifestation.

Summary: Electronics fundamentally rests on various electrical phenomena. The most important concepts are charged particle electric fields and electrical current. Electric fields are characterized by field strength and potential distribution. Potential difference existence indicates field presence and vice versa. Various forces, including electric fields, produce electrical current — ordered charge movement. Opposite charge accumulations store electrical field energy, characterized by electrical capacitance (capacitors).

Step 2. Diffusion

Energy possesses one remarkable property: it constantly seeks the minimal possible value in any situation. This is a law of nature. Everything occurring around us essentially follows this principle, sometimes called the least energy principle. This explains why we fall downward after jumping, why coffee cools, why wind blows. This same principle explains diffusion. For a visual understanding, imagine placing a sugar cube in a glass of water without stirring.

Initially dissolved, sugar particles concentrate in one location — say, the bottom of the glass. Eventually, the entire water becomes sweet, meaning sugar particles have distributed throughout the entire volume. This behavior of substance particles is called diffusion. Since all objects consist of molecules and atoms, when regions exist where particles concentrate more densely than elsewhere, transfer occurs from higher to lower concentration areas. Nature strives to equalize everything, minimizing energy and reaching equilibrium.

Summary: Due to natural laws, substance particles, when concentrated somewhere, always tend toward areas of lower concentration. This phenomenon is called diffusion.

Step 3. Solid Bodies

Understanding how different materials are structured is extremely complex. Various physics branches investigate different substance-related questions. For semiconductor electronics understanding, solid bodies hold primary interest. Various details appear in solid-state physics or materials science textbooks. The basic idea: all solid bodies can be represented as lattices — specific structures comprising atoms.

Material properties depend on lattice types, atom varieties, and other factors. Any substance structure discussion ultimately leads to quantum physics questions with their "magical" phenomena.

Band theory of solid bodies excellently describes material "internals." This theory inseparably connects to the energy concept. Understanding energy's role emerges from a standard atomic model. A crude atomic model shows a layered structure: the nucleus at center, surrounded by electron clouds (the electrons themselves). This electron representation better approximates reality than spherical representations from the quantum physics perspective. Electronic layers are called electron levels. Electrons at each level possess specific energy — farther from the nucleus means higher energy. This structure conveniently appears as an energy diagram, frequently encountered in physics textbooks. Energy is denoted by "E." The topmost energy level is called the valence level.

When atoms "connect," forming lattices and creating materials, their energy levels change due to mutual interaction. Among energy levels, a specific organization appears: levels divide into so-called bands — hence "band theory." Below are the possible band distribution cases. These characterize the entire material.

The lower band is called the valence band. Electrons there belong to atoms. Electrons in the upper band are "shared." These electrons strongly respond to electric fields, directly producing electrical current in substances — determining electrical properties. This zone is called the conduction band. Between them lies a forbidden band. The images show how forbidden band width affects material properties and substance classification. Metals show overlapping zones, enabling electrons to become "shared" easily; thus many electrons occupy the conduction band and substantial current flows. Semiconductors position zones at some distance but not excessively — electrons can "jump" across if receiving sufficient external energy. Dielectrics have wider zones, making electron transfer to conduction bands difficult, resulting in very small currents. Generally, dielectrics don't conduct current.

Summary: Band theory describes solid body properties. The theory's essence: within materials exist electrons that can participate in electrical current formation, while others belong to atoms. Applying energy to materials (light, heat, electric fields) can "detach" electrons from atoms for current conduction. Some materials enable this easily; others find it difficult. Semiconductors occupy the "golden middle" — relative ease with manageable difficulty. Quantum physics ultimately describes all substance properties.

Step 4. Semiconductors

What makes semiconductors so exceptional that they revolutionized technology? The secret lies in their sufficiently narrow forbidden band. Providing electrons with energy allows regulating material conductivity. This energy might be electrical fields, light (electromagnetic waves), temperature, etc. This explains semiconductor devices' extraordinarily wide application.

Detailed consideration requires introducing some theoretical terms. First, "holes." Here's why: when an electron "flies away" from the valence band, a vacant space remains. Another electron can "arrive" at this spot. Since electrons carry negative charge, departing from atoms, they remove this charge portion, making atoms slightly more positive. While reasoning is crude, it suggests representing the electron's departure as a "hole" carrying positive charge. This idea is so useful that semiconductor theory actually treats these "holes" as particles, though they're really just a conceptual trick — convenient for theory, so we'll use this terminology. Theory calls "holes" and electrons "p" and "n" particles, respectively (positive and negative).

With new terminology: when energy sufficient for electron "jumping" applies to semiconductors, a p-n pair forms. At temperatures above absolute zero (-273 degrees C), some energy always applies to semiconductors from temperature. Therefore, taking any semiconductor piece, one can confidently say that p-n pairs constantly form and disappear (recombine). This process continuously occurs; such material always contains some uncombined electrons and holes. Equal quantities exist since they form in pairs. However, this is insufficient for practical applications — specially created materials with predetermined parameters are required. Additional substances are introduced into pure semiconductors, increasing either electrons or "holes." Such semiconductors are called doped (because something was added). Some are called n-type semiconductors, others p-type. We won't address physical details; supplementary literature covers this.

Since semiconductors contain two charge-carrying particle types capable of ordered movement, two current types exist: electronic and hole current. Additionally, particles move either through electric fields or diffusion. Accordingly, currents are either diffusion currents or drift currents.

Currently popular semiconductor materials include silicon, germanium, and gallium arsenide.

The most complex and important semiconductor construction is the contact between p-type and n-type semiconductors — the so-called p-n junction.

Summary: Semiconductor materials possess relatively narrow forbidden bands, making them flexible for electrical applications. Theory employs the "hole" concept, interpreted as a particle creating electrical current. A "hole" represents a vacant electron position possessing positive charge. Pure semiconductors contain equal electrons and "holes." Technical applications use specially created materials with abundant "holes" (p-type) or electrons (n-type).

Step 5. P-N Junction

Taking two semiconductor materials and connecting them as shown below creates a p-n junction. Actually, this doesn't occur practically because achieving a unified crystal lattice by simply placing materials together is impossible, but this model suffices for general understanding.

Recall that pure semiconductors received additives creating elevated charge concentration — in one case "holes," in another electrons.

Once materials "connect," forming a unified substance, diffusion phenomena emerge, striving to equalize particle concentration throughout. The most interesting developments occur at the boundary where electrons and "holes" recombine (annihilate). Eventually, a thin layer forms near the two-material boundary containing no "holes" or electrons — all charge compensates. However, the added impurities contribute here. These impurities are ions (atoms) possessing charge. This layer prevents further recombination-layer growth. Between oppositely charged regions, an electric field forms, with intensity directed from positive to negative regions. Eventually, equilibrium develops: at a specific layer thickness, the field becomes strong enough to redirect charged particles (p and n) attempting diffusion-driven transfer to other regions. Electric field and diffusion opposition create the p-n junction.

Squares denote ions; circles denote particles (p and n). Note that n-type conductors contain not only electrons but also many fewer holes — visible in the diagram. The field will accelerate scarce particles in either region, allowing passage. This represents the minority carrier current. With untouched p-n junctions, these currents equalize and total current equals zero.

Most interesting p-n junction properties manifest when applying energy, such as electrical energy.

Situation 1 — Forward Bias: Wires connecting material ends with p-n junctions create potential differences, allowing external energy to generate specific field strength. When the region of contact lacks primary charge carriers (all recombined), it functions as a high-resistance area — greater than the remaining semiconductor volume. Additionally, the present field creates a potential difference, usually called contact potential. This forms a potential barrier.

Notably, external and internal fields have different directions. The external field overcomes the internal field, eliminating its influence. This upsets equilibrium. The diffusion current no longer faces suppression; instead, drift current (minority carriers) faces suppression. Consequently, connecting per this diagram produces semiconductor current, directed from p to n regions. This situation is called p-n junction "forward bias."

Situation 2 — Reverse Bias: Same construction but reversed potential at material ends. The opposite direction of external and internal fields increases minority carrier drift current. Since minority carriers are scarce, this current remains small. This situation is called p-n junction "reverse bias."

Situation 3 — Photovoltaic Effect: Same construction but unwired, simply connected together. The external energy source is radiation — light. When light strikes a p-n junction, electron-"hole" pairs form. The present internal field "ejects" particles apart, preventing destruction. Examining connecting wires shows current flow. Note that not all radiation creates such pairs — specific light properties matter: wavelength or frequency. Shining blue versus red light on the same junction might not produce current in both cases. Solar panels operate on this principle. Better efficiency occurs by combining this with reverse p-n junction bias, allowing light to control significant external circuit current.

Situation 4 — Light Emission: Energy possesses a property: it never appears from nowhere nor disappears anywhere — only changes form. When electrons recombine with holes (remembering band theory, the electron returns to its valence band position, decreasing its energy), this energy doesn't vanish but transforms. Constructing systems where this energy radiates as light enables p-n junctions as emitters. Impurity type significantly affects radiation frequency (wavelength). Mixing different impurities in semiconductors allows managing radiation color. More complex multiple-junction constructions exist — which junction emits depends on current. Thus, multicolor emitters are possible.

Engineering and scientific practice typically displays complex structure properties through special diagrams showing various quantity relationships. Electronics frequently uses current-voltage relationships. For p-n junctions, voltage-current dependencies prove convenient. Such dependencies appear below — called volt-ampere characteristics (VAC).

VAC knowledge allows avoiding holding all material physics processes mentally; instead, designers rely on such relationships. Notice that increasing reverse-bias voltage beyond certain values causes current to dramatically increase. Physically, this represents electron behavior resembling avalanches — called avalanche breakdown. Usually afterward, p-n junctions no longer function as p-n junctions. Note only that sometimes controlled avalanche breakdown proves useful.

Another p-n junction peculiarity: the depleted layer at two-conductor contact creating internal fields greatly resembles a capacitor. Therefore, the junction contains capacitance — called barrier capacitance, emphasizing its formation method. Another capacitance appears during forward junction biasing, explained by semiconductor dynamics — called diffusion capacitance. Capacitances require time for charging and discharging, so quickly alternating junction voltage causes non-instantaneous transition between modes.

Junction capacitance is of significant practical interest. Under reverse biasing, the depleted layer thickness (and thus charge quantity on both sides) depends on voltage. Therefore, voltage changes junction capacitance.

Summary: The most popular semiconductor construction is the p-n junction — a contact between two different-type (p and n) semiconductors. This junction possesses unique properties determining wide application. The primary characteristic: electrical current flows only in one direction. Additionally, p-n junctions generate and respond to light.

Step 6. Semiconductor-Metal Junction

Brief mention deserves another contact type: between metal and semiconductor. Under certain conditions, such contacts develop potential barriers. The name "Schottky barrier" frequently appears (named after the inventor). For clarity, observe a metal-to-n-type semiconductor connection. Reasoning for p-type semiconductors is similar.

At such contacts, primary charge carriers (electrons) from semiconductors transfer to metal. This creates a situation similar to p-n junctions. A field-containing region emerges, slowing further depleted layer growth. Internal processes differ from p-n junctions but hold some similarities. The VAC possesses similar structure.

The primary distinction from p-n junctions: this junction operates faster because only electrons carry current (electron mobility exceeds "hole" mobility). Additionally, junction capacitance is much smaller. Also, less voltage drops across this junction than at two-semiconductor contacts.

Summary: Beyond p-n junctions, other connection types exist. Practically interesting is the metal-semiconductor contact possessing properties similar to p-n junctions.

Step 7. Diode

The simplest semiconductor device is the diode. The name comes from two terminals (input and output). All diodes fundamentally use p-n junction properties, emphasizing particular parameters to create different diode types.

Brief descriptions follow for other diode types. Schottky diodes use Schottky barrier properties; varicaps use p-n junction capacitance effects; Zener diodes use reverse-branch VAC characteristics for voltage stabilization; tunnel diodes find use in generator design; photodiodes and light-emitting diodes apply to light detection and emission (modifications enable coherent light emission — lasers).

A three-terminal device class functions closer to diodes functionally. These thyristors frequently apply to high-power electrical applications. Working principles distantly compare to another complex but more popular device — transistors. In thyristors, one p-n junction controls another. Besides, numerous other types exist: avalanche photodiodes (highly sensitive, used in quantum computing and cryptography experiments), PIN diodes, optocouplers, impulse diodes, etc.

This vast diode variety indicates enormous application areas. More academically popular uses include signal rectification (AC to DC), voltage stabilization, and frequency tuning (through capacitance changes) in oscillatory circuits.

Summary: Semiconductor diodes are the simplest final devices utilizing p-n junctions. Various diode modifications employ different p-n junction parameters. Semiconductor diodes are extremely widespread.

Step 8. Transistor

Equally popular and significant semiconductor devices are transistors. These typically three-terminal devices divide into two major classes: bipolar and field-effect.

A field-effect transistor represents a semiconductor structure with clearly defined p- or n-type regions. Such regions are called channels. These channels possess specific conductivity (resistance). Field-effect transistor operation fundamentally involves additional semiconductor constructions as p-n junctions allowing channel conductivity (resistance) management. In other words, field-effect transistors function as variable resistances — useful in various circuits. The transistor term itself essentially means "variable resistance."

The channel possesses two specially designated terminals: source and drain. These connect to circuit sections requiring variable resistance. The third transistor terminal is the control terminal — called the gate. Applied signals modify channel conductivity (resistance). Various control-effect organization methods divide field-effect transistors into further subclasses.

The image shows field-effect transistors with controlling p-n junctions. Operating principles rely on previously discussed p-n junction properties. Here, the p-n junction connects in reverse direction; an external electric field causes junction enlargement or reduction. Thus, channel area is regulated. Smaller cross-sectional material area (carrying current) means lower current strength (all else equal). An analogy exists with pipe valves operating similarly.

The following image presents another field-effect transistor subclass — transistors with isolated gates. Based on their structure, they're called MOS (metal-oxide-semiconductor), emphasizing metallic gate isolation through oxide from semiconductor channels. Isolation enables avoiding complex contact structures, as only electric fields are needed, created by gates (between gate and other contacts — substrates, like capacitors).

Applying gate potential exceeding substrate potential directs field intensity toward substrates, "attracting" the few electrons existing in p-semiconductors to the gate. Over time, a thin electron layer forms, creating a "bridge" between drain and source, functioning as a channel. The channel is said to be induced (formed). Therefore, these transistors are called "induced-channel" types. Slightly different modifications exist where extremely thin channels are initially created, enabling conduction without gate potential application — called built-in-channel types.

As noticeable, current strength varies from two parameters: conductor cross-sectional area and charge quantity "flowing" through this section. Field-effect transistors manage current using both parameters — in one case regulating charge concentration (isolated gate), in another regulating cross-sectional area (controlling p-n junction).

Another transistor class comprises bipolar transistors, which unlike field-effect transistors, consist of two p-n junctions where one junction's current management is accomplished through another.

Main bipolar transistor parts are identifiable: base, emitter, and collector. The image shows equal sizing, though bases are actually made much narrower.

Connecting wires to two transistor ends (emitter and collector) and applying voltage produces no current whatsoever in any direction. However, applying appropriate base-contact voltage increases current. The secret: applied voltage causes one junction to be forward biased and another reverse biased. The first junction's forward bias creates excellent "hole" flow from emitter to base, where they recombine with enormous electron quantities. Sufficient current causes some "holes" to avoid recombination, accumulating. Diffusion drives them toward collector junctions; there, reverse-bias junctions become accelerating for minority carriers — the junction field "ejects" approaching "holes," creating collector current. Such bipolar transistor operating mode is called active (one junction open, another closed).

Both field-effect and bipolar transistors connect to electrical circuits differently; the primary difference is how control signals are managed (more precisely: which output is common for control and controlled circuits).

To avoid confusion regarding transistor symbol drawing and arrows, remember: the arrow, like a compass, points to N (n-type semiconductor). This applies to both bipolar and field-effect transistors.

Summary: Transistors are the most interesting semiconductor devices. Transistors enable various digital and analog devices. Two classes exist: bipolar (current-controlled) and field-effect (field-controlled) transistors.

Step 9. Transistor as Amplifier

When discussing device amplification properties, graphs typically appear showing how output signals change with input signals.

Examining the image, one sees the simplest transistor connection scheme (bipolar here), enabling signal amplification. The most difficult aspect involves "shifting" input signals (by input characteristics) enabling amplification without distortion. This requires positioning signal "centers" at points where curves change identically on both sides. Such points are called working points. A special resistor in the scheme provides constant input signal offset. Graph analysis shows signal amplification. Amplification occurs in current; consequently, voltage amplification follows. Therefore, special coefficients characterizing amplification properties show how many times output current, voltage, or power exceeds input. The h21 parameter, mentioned earlier, approximately equals the current-amplification coefficient.

This deserves attention because amplifier and amplification-cascade questions are complex, representing entire separate electronics branches. Semiconductor amplifier perfection reaches special cascades called operational amplifiers, now representing complex semiconductor analog devices with sophisticated architecture.

Summary: Analog electronics' most popular transistor application involves amplification cascades. Amplification occurs from transistor operating properties. Appropriate bias voltage selection enables signal amplification without distortion.

Step 10. Transistor as Switch

Another transistor application direction, equally popular and initiating modern digital electronics, involves switch applications. Switches are simple two-state devices.

Creating transistor switches requires selecting two transistor states for switching. Usually, states involve large or small current conduction; transistors are "open" or "closed." From digital perspectives, this corresponds to "1" and "0."

The basic idea: specific electrical circuits have one fixed resistance and one variable — the transistor. When transistor resistance greatly exceeds fixed resistance, all voltage drops across it; fixed resistance retains the remainder. When transistor resistance greatly falls below fixed resistance, the opposite occurs. The combined voltage across transistor and fixed resistance always equals supply voltage (Kirchhoff's second rule). From this, clear ideas emerge: applying signals (current, voltage) to transistor inputs opens them, and voltage drops across fixed resistance. Removing signals closes them; voltage drops across transistors. Measuring transistor voltage shows an input-output relationship resembling logical inversion: "1" on input produces "0" on output and vice versa.

Currently popular schemes use complementary pairs. Simply applying identical signals to different-channel-type transistors ensures only one conducts. This reduces circuit power consumption.

Transistor digital switches enable complex logical schemes capable of sophisticated computations.

Summary: Digital electronics' most popular transistor application involves switches. Transistors switch between two states, manifesting "switch" effects. Real switches represent complex cascades with additional elements improving parameters. Logical elements form on transistor switches.

Step 11. Logic Elements

Transistor switches examined above primarily found digital electronics application, essentially forming it. Using only single switches can create all necessary constructions for logical function calculations. Mathematics proves that implementing all possible functions is unnecessary — several basic functions (bases) suffice, from which all others can be expressed. Examples include inversion and conjunction (NAND) or exclusive OR and inversion. Simple switches independently implement inversion. Constructions shown below enable NAND operation (such cascades are called gates).

The image shows two schemes: one for bipolar transistors, another for field-effect transistors, where field-effect transistors use complementary pairs. Enormous quantities of different connection modifications exist. These represent the basics. CMOS logic currently dominates, though not in pure form. Technological innovations remain manufacturer trade secrets.

Examining TTL (transistor-transistor logic) as an example: suppose a 5-volt supply voltage with "0" signals on A and B inputs — approximately 0.1 volts. This indicates forward-biased emitter junctions. With silicon devices, approximately 0.7 volts drop across such junctions. Then, using Kirchhoff's second law, the base voltage between the entrance transistor and ground remains insufficient — electric current requires circuit flow needing at least 0.7 + 0.7 volts, as the circuit contains two p-n junctions. Thus, the output transistor closes; output shows "1." When A and B differ ("1" and "0" or "0" and "1"), nothing changes. However, applying "1" and "1" (5 volts each) reverses the situation: bases reach nearly 5 volts. This allows current flow, opening the output transistor, producing "0" outputs. This A-B-Y relationship is called NAND — logical multiplication followed by inversion.

Summary: Transistor switches form universal logical cascades — gates enabling, through cascade connections, more complex logical function formation. All modern digital devices comprise such gates. Contemporary microchip design rarely uses transistor-scale reasoning; everything reduces to ready-made logical module connections (typically using HDL languages).

Step 12. Transistors in Processors

Modern semiconductor devices — processors and other microchips — prominently exemplify contemporary semiconductor engineering. All these devices build on p-n junctions. Millions of transistors form switches, which in turn form logical elements implementing diverse operations: addition, subtraction, multiplication, division, etc. Regarding how NAND elements accomplish all this, consult supplementary literature on Boolean algebra.

Modern complex machines compress enormous transistor quantities into tiny sizes through so-called planar technology. This method involves taking pure semiconductor pieces (thin plates) and layering different impurity levels through special masks positioning impurities precisely. This sequentially creates complex connections. This represents genuinely high technology, continuously advancing. Currently, operations far exceed descriptions here; however, principles remain unchanged.

Summary: This article briefly discussed extremely important electronics areas. Covered topics truly rank as key for comprehending semiconductor device functioning principles. Semiconductor electronics will undoubtedly continue setting the scientific-technical development pace for a long time to come.

Step 13. Further Reading

1. Kittel C. Introduction to Solid State Physics.
2. Landau L. D., Lifshitz E. M. The Classical Theory of Fields.
3. Feynman R., Leighton R., Sands M. The Feynman Lectures on Physics: Mainly Electromagnetism and Radiation.
4. Feynman R., Leighton R., Sands M. The Feynman Lectures on Physics: Mechanics of Continua.
5. Paul Horowitz and Winfield Hill. The Art of Electronics.
6. Peter Y. Yu, Manuel Cardona. Fundamentals of Semiconductors: Physics and Materials Properties.
7. Sheng S. Li. Semiconductor Physical Electronics.
8. S. A. Gavrilov. Semiconductor Circuits: Developer Secrets.
9. Tolmachyov V. V., Skripnik F. V. Physical Foundations of Electronics.