Semiconductor Physics: Atoms and Electrons

• An atom is comprised of a nucleus with orbiting electrons in shells or orbits
• Electrons are held in orbit by the electrostatic force between electrons and the nucleus
• Electrons are negatively charged, while protons in the nucleus are positively charged
• An electrically neutral atom has an equal number of protons and orbiting electrons
• An atom that loses an electron becomes a positively charged ion
• An atom that gains an electron becomes a negatively charged ion
• Electrons orbit the nucleus in different shells
• Each shell has a definite number of electrons
• The first, second, and third shells hold 2, 8, and 18 electrons, respectively
• The number of outermost shell electrons determines the electrical property of a material, called valence electrons
• The outermost shell is either completely or partially filled
• The outermost orbit requires eight electrons, and if it lacks electrons, these are vacancies or holes.

Semiconductor Materials: Silicon and Germanium

• Silicon has 14 electrons in three orbits, distributed as 2, 8, 4, and its nucleus contains 14 positively charged protons, making it electrically neutral
• Germanium has 32 electrons in four shells, distributed as 2, 8, 18, 4, and its nucleus contains 32 protons, making it electrically neutral
• Silicon and germanium have four valence electrons and four holes in their outermost orbits.
• The binding force on electrons closer to the nucleus is stronger
• Electrons in the outermost orbit are loosely bound and require little energy to be freed, becoming free electrons when they leave their orbit

Binding Forces: Covalent Bonding

• Semiconductor atom with four valence electrons/holes requires four more electrons to fill the outermost orbit
• Atoms in a crystal arrange so that electrons orbit in valence shells of two atoms where each valence shell fills its neighbor's hole
• Atoms of silicon share electrons with neighboring atoms to satisfy the need for eight electrons in the valence shell in covalent bonding
• Covalent bonding fills valence shells to eight electrons so at absolute zero temperature, there are no free electrons in the crystal
• A rise in temperature breaks some covalent bonds, freeing electrons
• Intrinsic semiconductors like silicon or germanium, where electrons are bound and cannot conduct current, also exist
• External heat raises teh temperature and provides energy to valence electrons, enabling them to break free and become free electrons
• An electron that leaves creates a vacant space called a hole where a free electron there will be a corresponding hole, called an electron-hole pair
• Temperature rises in the semiconductor produce many electron-hole pairs where when electrons become free, they attract and fall into another electron's created hole in a process called recombination
• The creation and fall of a free electron into a hole occurs quickly, in nanoseconds
• Pure silicon or germanium is not very useful in electronics, except in heat- or light-sensitive resistors
• Conductivity increases by adding materials with three or five valence electrons, which is called doping
• Doping creates extrinsic semiconductors

Extrinsic Semiconductors

• Doping adds either a pentavalent or trivalent element to a pure semiconductor
• The doping material is called an impurity material
• Pentavalent doping material create n-type semiconductors and trivalent create p-type semiconductors
• Joining these two materials creates a p-n junction and is the basis of all electronic devices

n-Type Semiconductor Material

• N-type semiconductors are formed by doping pure silicon or germanium with a material that has five valence electrons
• Pentavalent materials are antimony, arsenic, and phosphorus
• If antimony is added to a silicon crystal, four of its five valence electrons form covalent bonds with silicon atoms, leaving one electron free
• Each antimony atom contributes one free electron, making available a significant amount of free electrons in the n-type semiconductor
• These free electrons are loosely bound to their parent atom, are not part of covalent bonds, and can conduct electricity
• Each antimony atom makes covalent bonds with four neighboring germanium atoms.
• Each bond has one electron from germanium and one from antimony.
• Every antimony atom forms four covalent bonds with four germanium atoms.
• Sharing electrons fulfills valence shell of all atoms
• N-type semiconductors are electrically neutral because the total number of electrons, including free electrons, equals the total number of protons in the nuclei
• Impurity material donates one free electron per atom to the extrinsic semiconductor, calling them donor atoms.
• Donor atoms form free electrons, which form the majority charge carrier responsible for current flow in n-type materials
• Temperature rise above absolute zero creates free electrons/holes breaking covalent bonds, however a certain amount of holes are also formed
• When electrons leave their positions, creating holes, electron movement associates with hole movement
• Holes form charge carriers and minority charge carriers in n-type semiconductors
• Majority charge carriers are electrons, and minority charge carriers are thermally generated holes.

P-Type Semiconductor Material

• Formed when silicon/germanium crystal is doped with a material like boron, gallium, or indium
• Boron forms covalent bonds with silicon, which creates a shortage of one electron
• There will be one hole for each impurity atom to form covalent. This makes seven out of eight of the positions being filled
• One vacant position remains which is called a hole and it is electrically neutral
• Holes in p-type material are the majority charge carriers
• Raising temperature creates free electrons/holes where the thermally generated electrons will be minority carriers. Holes constitute charge carriers.

The p-n Junction

• P-type semiconductors have holes as the majority charge carriers. Trivalent impurities that produce a p-type semiconductor are called acceptor impurities

• A free electron occupying a hole in a p-type material creates negative ions on the p-side, creating one more electron than protons with positive charge on the nucleus

• P-type semiconductors have negative acceptor ions and holes as majority carriers

• N-type semiconductors have donor impurities because they give one free electron to the semiconductor crystal, which create positive ions.

• Small circles represent the holes as the majority carriers in p-type material

• Black dots represent the free electrons as the majority charge carriers in n-type materials

• Electrons are negative charge carriers, and holes are positive charge carriers

• An atom becomes a positively charged immobile ion when an electron moves out

• Addition of an electron in a hole makes an atom a negatively charged immobile ion

• When a p-type semiconductor is joined with an n-type semiconductor, a junction known as a p–n junction is formed

• Free electrons from the n-type diffuse into the p-side and combine with holes nearest to the junction.

• Free electrons leaving leave behind immobile ions on the n-side where the elecrtons occupy the holes in the p-type and cause atoms to become negatively charged, immobile ions

• Atoms accepting the negative charge become negatively charged ions which were neutral before where this creates an accumulation of negative ions on one side, and the other side creates positive ions

• Negative and positice ions close to the junction will cause each side to acquire voltage

• When a p-n junction is developed, a barrier voltage is created at the junction and will stop the diffusion of charge creating the manufacturing process

• The barrier voltage depends upon the amount of doping, charge carriers, and the junction temperature

• The barrier voltage for germanium is 0.3 V, and for silicon it is 0.7 V at room temperature (25°C)

• This potential creates a potential difference, which will prevent movement of ions

• There are no free electrons in the region near the p-n junction, which is referred to as the depletion region

• It may be noted that the thickness of the depletion region has been expanded. Actually this layer is very thin, an order of micrometer

Biasing of p-n Junction

• A p-n junction becomes forward biased if the p-side is connected to the positive terminal, and n-side to the negative terminal of a supply.
• This application will narrow the depletion layer on forward voltage
• The majority charge carriers current is what is established in a forward-biased transition
• Barrier potential for germanium is 0.3 V, silicon is 0.7 V at room temp. These are voltage across the p-n junctions as current flows
• A reverse bias is when the p-side is connected to the negative terminal, and the n-side connects to the positive terminal
• An increase in voltage will increase or widen the depletion layer
• A minute current can flow through the reverse-biased current due to a certain amount of minority charge carriers
• The resistance in current flow will be smaller with forward-biasing vs reverse

P-n Junction Semiconductor Diode

• A semiconductor diode is a p-n junction that offers low resistance when forward-biased and high resistance when reverse-biased

• Diodes are available in different current ratings for different power levels

• Low-current diodes use switch circuits allowing current to flow one way

• Connecting leads on either side a p-n juncition forms a diode

• The p-side has a positive and a negative terminal for forward bias called anode/cathode respectively

• Anodes have a high amount tolerance for forward current vs very high resistance to damage in reverse voltage

• High-currentd power sources have a large amount of tolerance for reverse voltage and forward currents

• With forward bias, the diode is gradual. If the relationship gives you a forward V-I characteristic, then you got it in such a way

• The connection diagram can be determined using the V-I, or voltage/current diagram

• When you gradually increase the voltage, the current is small

• The voltage increases suddenly to 0.3V where there is much more power

• This constant voltage is cut in the diode

• The reverse is found by switching directions. Ideally, there should be none

• Minority curretns increase when the diodes is reverse bias to reach a small microamp

• This leakage will saturate and will not increase with added biasing voltage

• Break down is sudden and reversed. Current continues here

• Forward Voltage drop, VF

• Reverse Breakdown Voltage, VRB

• Reverse saturation current, IR

• Dynamic resistance, rd

• Maximum forward current, IFM

• The values of these parameters are normally provided by the manufacturers in their specification sheet. Diodes are available in low-, medium-, and high-current ratings. Diodes of low-current ratings are used in electronic switching circuits, i.e., they work as switches. Their forward current ranges from a few mA to a maximum of 100 mA.

• The safe reverse bias that can be applied is around 75 V. The reverse saturation current is very small, usually less than a micro-ampere.

• Medium current diodes have a maximum current rating of 400 mA and reverse voltage of about 200 V. High-current diodes are also called power diodes. They are rated for high current and high reverse voltage ratings. Metal heat sinks are used for dissipation of heat produced in a diode when it is conducting.

Zener Diode

• Only a small satturated current flows backward when reversed
• The reverse can break down with high volage and release a large current. It needs resistance so heat does not burn it
• The diod can operated under its carrying capacity to do work at operating capacity under breakdown
• I is made to drop with under reverse-biasing
• Diods operating under breakdown have constant voltage
• Zener diodes will drop with voltage under the saturation
• The thickness will break down once reverse volage is applied past the limit
• Electrons will break a little when the electrons are close enough
• To narrow, we put it in reverse a higher voltage
• When the reverse voltage increases in a zener- this causes more to gain for electrons
• This ionization is avanlanched
• Zener is under 5 and avalanche over 5v

The forward V–I characteristic is a zener diode similar to an ordinary diode As shown in operation the reverse voltage is applied to the zener to have current

• A resistance is present so nothing goes over capacity, no current. VR is available, types are IN-746- 759

Rectifiers

• The circuits convert AC to DC with diodes

• AC power is halved and fullaved

• The voltage needs adjusting with a transformer step. A singular diode can halfwave rectify

• They utilize a resistor transformer, diode transformer and transformer and load diode in a half wave

• A halfwave has transformer resistance d

• The bias is positive and negative

• Voltage rises at from Z to pie. Anode does things and then cuurent flows to the resistor

• The transformer will work on reverse volage as needed

• A diode must be steady to work for ac

• Flucutating de on average as is.

Rectifer circuits stop the negative. This makes the positive. Work a switch If you put into load there will be full wave

Full-wave Rectifiers

• By adding a center diode tap transformer or by four tap diodes it helps Here, the load voltage is tap secondary or central
• There are diodes, transformers and resistance circuits

The positive diode one will transfer positive to one.

• With the negative, diode 2 makes things negative
• There is a voltage in the center
• The form is a series
• Current flcutates but only in one direction.

Bipolar Junction Transistors

• Transistors are used in many electrical circuits and ability to amplify electrical signals
• “transfer resistor." transfers the signal from low resistance to high resistance as named
• The three layers arrange in sequences by sequence n–p–n or a p-n-p.
• N-type materials are surrounded by the material base and vice versa

General Transistors

• has junctions connected at base, the emitter, the collector

• The operation requires that only a small base to let current through through

• can amplify current and voltage

• functions as a duo ofdidos

• Applying polairty with correct voltage

• EB required to forward, CB in reverse

• Lower voltage is with EB junction and get reduced

• CBs get larger width

• will go up too because of this

• There has to be a small number fo holes (holes come in pairs now)

• 2 percent make it through

• Expansion is through collection layers and holes . The electrons will gather on the cbs

• 98 goes through

• The amoutn is set by voltage

• emitter has currents set by charge

• Emitter requires different current directions- flow with eb and cb direction and collector by the emitter

• It cannot work if it is operating. But is satruated

• The ratio needs collector current, or else th etransistor

Transitor configerations

• Three terminals are connected to output,one is most
• Base makes things command at 2 connecito
• 3 types, one is base-emitter, collector
• A transistor needs to have the most advantage. These are fig based

The comman has high voltage