Semiconductor BasicsChapter 1

Atomic StructureElements are made of atoms110 Elements; each has an atomic structureToday, quarks and leptons, and their antiparticles, are candidates for being the fundamental building blocks from which all else is made! Bohr ModelAtoms have planetary structure Atoms are made of nucleus (Protons (+) & Neutrons) and electrons (-)

110 th element is called Darmstadtium (Ds)

Atomic StructureAtoms go around the nucleolus in their orbits discrete distances Each orbit has some energy level The closer the orbit to the nucleus the less energy it has Group of orbits called shellElectrons on the same shell have similar energy levelValence shell is the outmost shell Valence shell has valence electrons ready to be freed Number of electrons (Ne) on each shell (n)

First shell has 2 electronsSecond shell has 8 electrons (not shown here)

Ne = 2n2

Valence ShellAtoms are made of valence shell and coreCore includes nucleus and other inner shellsFor a Carbon atom the atomic number is 6Core charge = 6 P + 2 e = (+6)+(-2)=(+4) Remember the first shell has 2 electrons

ElementsBasic categoriesConductorsExamples: Copper, silverOne valence electron , the e can easily be freedInsulatorsValence electrons are tightly bounded to the atomSemiconductorsSilicon, germanium (single element) Gallium arsenide, indium phosphide (compounds)They can act as conductors or insulators

Conduction band is where the electron leaves the valence shell and becomes freeValence band is where the outmost shell isAlways free electronsFree electrons

Semiconductors Remember the further away from the nucleus the less energy is required to free the electrons Germanium is less stableLess energy is required to make the electron to jump to the conduction band

When atoms combine to form a solid, they arrange themselves in a symmetrical patterns Semiconductor atoms (silicon) form crystalsIntrinsic crystals have no impurities

Conduction Electrons and HolesElectrons exist only within prescribed energy bands These bands are separated by energy gaps When an electron jumps to the conduction band it causes a hole When electron falls back to its initial valence recombination occurs Consequently there are two different types of currentsHole current (electrons are the minority carriers)Electron current (holes are the minority carriers)Remember: We are interested in electrical current!

DopingBy adding impurities to the intrinsic semiconductor we can change the conductivity of the material this is called doping N-type doping P-type dopingN-type: pentavalent (atom with 5 valence electrons) impurity atoms are added [Sb(Antimony) + Si] Negative charges (electrons) are generated N-type has lots of free electronsP-type: trivalent (atom with 3 valence electrons) impurity atoms are added [B(Boron) + Si] Positive charges (holes) are generatedP-type has lots of holes

DiodesN region has lots of free electrons P region has lots of holesAt equilibrium: total number positive and negative charges is the same (@ room temp)At the pn junction the electrons and holes with different charges form an electric fieldIn order to move electrons through the electric field (generate current) we need some force (voltage)This potential difference is called barrier voltageWhen enough voltage is applied such that electrons are moved then we are biasing the diodeTwo layers of positive and negative charges for depletion region the region near the pn-junction is depleted of charge carriers)

Biasing Types of a DiodeForward biasBias voltage VBias > barrier voltage VBar Reduction in + and ions smaller depletion region VBar Depends on material, doping, temp, etc. (e.g., for silicon it is 0.7 V)Reverse biasEssentially a condition that prevents electrons to pass through the diode Very small reverse break down currentLarger depletion region is generated

Connected to the negative side of the batteryConnected to the positive side of the batteryAK

Biasing Types of a Diode (Forward)AKMoving electronsSmall dynamic resistanceVBiasnpConventional Current FlowConventional Current FlowI (Forward)

Biasing Types of a Diode (Reverse)Very SmallMoving Electrons:Reverse Current)AKLarge resistanceVBiasnpConventional Current FlowHoles are left behind; large depletion region Instant pull of electrons

I-V Characteristic of a DiodeForward bias: current passes through The knee is where VBias=VBarAt point B VBias < VBar Very little currentNote that at the knee the current increases rapidly but V(forward) stays almost the same

Reveres bias: No current passes through When VBias < VBar Very little current (mu or nano Amp)At the knee, the reverse current increases rapidly but the reverse voltage remains almost the same Large reverse current can result in overheating and possibly damaging the diode (V=50V or higher typically)

Overheating results from high-speed electrons in the p-region knocking out electrons of atoms in n-region from their orbit to the conduction bandHence, we use limiting resistors

Electrons moving from n to p region

Modeling a Diode (Forward Biasing) Use rd (internal resistance)- Not linear!

Complete Modeling of a DiodeNote that IF is the actual direction of electron current Forward bias: VBias = VF + IF(RLIMIT+rd); rd is typically given, VF typically is 0.7 VReverse bias: VBias = VR + IR * RLIMIT; IR is typically given

VRVFShowing the Actual electron direction

ExampleFind the current through the diode and the voltage across the resistor.Assume rd = 10 ohm

Biasing?Forward bias: VBias = VF + IF(RLIMIT+rd)10 = 0.7 + IF(RLIMIT+10) IF=9.21 mAVF=0.7+IF*rd = 792 mVVRLIMIT = IF * RLIMIT = 9.21VVFForward biased

ExampleFind the current through the diode and the voltage across the resistor.Assume IR = I uA

Note: Reverse biasedVRReverse bias: VBias = VR + IR * RLIMITVRLIMIT = IR*VRLI MIT = 1mAVR=VBIAS-VRLIMIT=4.999 V

Forward BiasCalculate the voltage across the resistor.

Reverse BiasCalculate the voltage across the resistor.

Do this example on your own:Make sure you can calculate andfind all currents- Hint: find Vn, firstVnVni1i2i3Access this file from my web page