Semiconductor and diode theory and application.pptx

It introduces to semiconductors and diodes theory and application in electronics and electricity

1. Semiconductor Material and diode SYLLABUS: Semiconductors: Intrinsic, extrinsic n and p type, mobility of carriers, carrier transport in semiconductors; p-n junctions(diode). By Tesfay G .

2. Why semiconductors? SEMICONDUCTORS: They are here, there, and everywhere Computers, palm pilots, Silicon (Si) MOSFETs, ICs, CMOS laptops, anything “intelligent” Cell phones, pagers Si ICs, GaAs FETs, BJTs CD players AlGaAs and InGaP laser diodes, Si photodiodes TV remotes, mobile terminals Light emitting diodes (LEDs) Satellite dishes InGaAs MMICs (Monolithic Microwave ICs) Fiber networks InGaAsP laser diodes, pin photodiodes Traffic signals, car GaN LEDs ( green , blue ) taillights InGaAsP LEDs ( red , amber ) Air bags Si MEMs, Si ICs and, they are important, especially to Elec.Eng.& Computer Sciences

3. Introduction Semiconductors are materials whose electrical properties lie between Conductors and Insulators. Ex : Silicon and Germanium Give the examples of Conductors and Insulators! Difference in conductivity

4. Semiconductor Materials Elemental semiconductors – Si and Ge (column IV of periodic table) – compose of single species of atoms Compound semiconductors – combinations of atoms of column III and column V and some atoms from column II and VI . (combination of two atoms results in binary compounds) There are also three-element ( ternary ) compounds (GaAsP) and four-elements ( quaternary ) compounds such as InGaAsP.

5. Semiconductor materials

6. Semiconductor Materials The wide variety of electronic and optical properties of these semiconductors provides the device engineer with great flexibility in the design of electronic and opto -electronic functions. Ge was widely used in the early days of semiconductor development for transistors and diodes. Si is now used for the majority of rectifiers, transistors and integrated circuits. Compounds are widely used in high-speed devices and devices requiring the emission or absorption of light. The electronic and optical properties of semiconductors are strongly affected by impurities, which may be added in precisely controlled amounts (e.g. an impurity concentration of one part per million can change a sample of Si from a poor conductor to a good conductor of electric current). This process called doping .

7. Si 14 - - - - - - - - - - - - - - However, like all other elements it would prefer to have 8 electrons in its outer shell The Silicon Atomic Structure Silicon: our primary example and focus Atomic no. 14 14 electrons in three shells: 2 ) 8 ) 4 i.e., 4 electrons in the outer "bonding" shell Silicon forms strong covalent bonds with 4 neighbors

8. The Germanium Atomic Structure

9. A solid will have millions of atoms close together in a lattice so these energy levels will creates bands each separated by a gap. Conductors : If we have used up all the electrons available and a band is still only half filled, the solid is said to be a good conductor. The half filled band is known as the conduction band. Insulators : If, when we have used up all the electrons the highest band is full and the next one is empty with a large gap between the two bands, the material is said to be a good insulator. The highest filled band is known as the valence band while the empty next band is known as the conduction band. n=1 n=2 n=3 Conduction band, half filled with electrons Valence band, filled with electrons Empty conduction band Large energy gap Valence band, filled with electrons

10. Semiconductors: Some materials have a filled valence band just like insulators but a small gap to the conduction band. At zero Kelvin the material behave just like an insulator but at room temperature, it is possible for some electrons to acquire the energy to jump up to the conduction band. The electrons move easily through this conduction band under the application of an electric field. This is an intrinsic semiconductor . Top valence band now missing some electrons Conduction band, with some electrons At room temperature – some conduction Valence bands, filled with electrons Empty conduction band At zero Kelvin – no conduction Small energy gap So where are all these materials to be found in the periodic table ?

11. Possible Semiconductor Materials Carbon C 6 Very Expensive Band Gap Large: 6eV Difficult to produce without high contamination Silicon Si 14 Cheap Ultra High Purity Oxide is amazingly perfect for IC applications Germanium Ge 32 High Mobility High Purity Material Oxide is porous to water/hydrogen (problematic) Tin Sn 50 Only “White Tin” is semiconductor Converts to metallic form under moderate heat Lead Pb 82 Only “White Lead” is semiconductor Converts to metallic form under moderate heat

12. Si and Ge are tetravalent elements – each atom of Si (Ge) has 4 valence electrons in crystal matrix T=0 all electrons are bound in covalent bonds no carriers available for conduction. For T> 0 thermal fluctuations can break electrons free creating electron-hole pairs Both can move throughout the lattice and therefore conduct current. Electrons and Holes

13. Electrons and Holes Electron-hole pairs in a semiconductor. The top of the conduction band denotes as E c and the bottom of the valence band denotes as E v . For T>0 some electrons in the valence band receive enough thermal energy to be excited across the band gap to the conduction band. The result is a material with some electrons in an otherwise empty conduction band and some unoccupied states in an otherwise filled valence band. An empty state in the valence band is referred to as a hole . If the conduction band electron and the hole are created by the excitation of a valence band electron to the conduction band, they are called an electron-hole pair (EHP).

14. Silicon Lattice Structure Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Shares electrons with 4 neighbouring atoms  8 electrons in outer shell At 0K, all electrons are tightly shared with neighbours  no current flow Adding heat (even to room temperature) allows some bonds to break, and electrons can flow - Si - - - - Si - - - - - Free electron Vacancy left by electron. Overall charge on silicon is zero  this “hole” must be positive + +

15. Intrinsic Material A perfect semiconductor crystal with no impurities or lattice defects is called an intrinsic semiconductor. At T=0 K – No charge carriers Valence band is filled with electrons Conduction band is empty At T>0 Electron-hole pairs are generated EHPs are the only charge carriers in intrinsic material Since electron and holes are created in pairs – the electron concentration in conduction band, n (electron/cm 3 ) is equal to the concentration of holes in the valence band, p (holes/cm 3 ). Each of these intrinsic carrier concentrations is denoted n i. Thus for intrinsic materials n=p=n i Electron-hole pairs in the covalent bonding model in the Si crystal.

16. Increasing conductivity by temperature Therefore t he conductivity of a semiconductor is influenced by temperature As temperature increases, the number of free electrons and holes created increases exponentially.

17. The conductivity of the semiconductor material increases when the temperature increases. This is because the application of heat makes it possible for some electrons in the valence band to move to the conduction band. Obviously the more heat applied the higher the number of electrons that can gain the required energy to make the conduction band transition and become available as charge carriers. This is how temperature affects the carrier concentration. Another way to increase the number of charge carriers is to add them in from an external source. Doping or implant is the term given to a process whereby one element is injected with atoms of another element in order to change its properties. Semiconductors (Si or Ge) are typically doped with elements such as Boron, Arsenic and Phosphorous to change and enhance their electrical properties. Increasing conductivity

18. Trivalent Impurities: Aluminum (Al) Gallium (Ga) Boron (B) Indium (In) Pentavalent Impurites: Phosphorus (P) Arsenic (As) Antimony (Sb) Bismuth (Bi) Doping -the process of creating N and P type materials -by adding impurity atoms to intrinsic Si or Ge to imporove the conductivity of the semiconductor -Two types of doping – trivalent (3 valence e-) & pentavalent (5 valence e-) p -type material – a semiconductor that has added trivalent impurities n -type material – a semiconductor that has added pentavalent impurities N-types and P-types Semiconductors (Doping)

19. N -type semiconductor: - Pentavalent impurities are added to Si or Ge, the result is an increase the free electrons Extra electrons becomes a conduction electrons because it is not attached to any atom - No. of conduction electrons can be controlled by the no. of impurity atoms Pentavalent atom gives up an electron -call a donor atom Current carries in n-type are electrons – majority carries Holes – minority carries Pentavalent impurity atom in a Si crystal Sb impurity atom

20. P -type semiconductor: - Trivalent impurities are added to Si or Ge to create a deficiency of electrons or hole charges The holes created by doping process The no. of holes can be controlled by the no. of trivalent impurity atoms - The trivalent atom can take an electron- acceptor atom - Current carries in p-type are holes – majority carries - electrons – minority carries Trivalent impurity atom in a Si crystal B impurity atom

21. Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Inject Arsenic into the crystal with an implant step. Arsenic is Group5 element with 5 electrons in its outer shell, (one more than silicon). This introduces extra electrons into the lattice which can be released through the application of heat and so produces and electron current The result here is an N-type semiconductor (n for negative current carrier) Increasing conductivity by doping Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - - Si - - - - Si - - - - - + + + As - - - - - As - - - - - As - - - - - + - - -

22. Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Increasing conductivity by doping Si - - - - Inject Boron into the crystal with an implant step. Boron is Group3 element is has 3 electrons in its outer shell (one less than silicon) This introduces holes into the lattice which can be made mobile by applying heat. This gives us a hole current The result is a P-type semiconductor (p for positive current carrier) Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - Si - - - - - Si - - - - Si - - - - - + + B - - - + B - - - + B - - - + + + + - - - +

23. - n -type material & p -type material become extremely useful when joined together to form a pn junction – then diode is created - p region- holes (majority carriers), e- (minority carriers) - n region- e- (majority carriers), holes (minority carriers) -before the pn junction is formed -no net charge (neutral)

24. The Diode (The Depletion Region)

25. Si Si Si B Si Si B Si Si As Si Si Si B Si As Si B B Si Si B Si Si Si Si Si Si Si Si Si Si Si As As As As Si Si Si P-type Mostly B & free holes N-type Mostly As & free electrons Diffusion effects – The holes and electrons move from area of high concentration to areas of low concentration. Holes & electrons annihilate each other to form an area depleted of free charge. This is known as the depletion region and blocks any further flow of charge carriers across the junction

26. No electron move through the pn-junction at equilibrium state. Bias is a potential applied (dc voltage) to a pn junction to obtain a desired mode of operation – control the width of the depletion layer Two bias conditions : forward bias & reverse bias Depletion Layer Width Junction Resistance Junction Current Min Min Max Max Max Min The relationship between the width of depletion layer & the junction current Biasing The Diode (Bias)

27. Voltage source or bias connections are + to the p material and – to the n material Bias must be greater than barrier potential (0 .3 V for Germanium or 0.7 V for Silicon diodes) The depletion region narrows. R – limits the current to prevent damage for diode Diode connection Flow of majority carries and the voltage across the depletion region The negative side of the bias voltage push the free electrons in the n-region -> pn junction Also provide a continuous flow of electron through the external connection into n-region Bias voltage imparts energy to the free e- to move to p -region Electrons in p -region loss energy- positive side of bias voltage source attracts the e- left the p - region Holes in p -region act as medium or pathway for these e- to move through the p-region Biasing The Diode ( Forward Bias)

28. As more electrons flow into the depletion region, the no. of +ve ion is reduced. As more holes flow into the depletion region on the other side – the no. of –ve ions is reduced. Reduction in +ve & -ve ions – causes the depletion region to narrow Biasing The Diode ( The Effect of Forward Bias on the Depletion Region)

29. Electric field between +ve & -ve ions in depletion region creates “energy hill” -prevent free e- from diffusing at equilibrium state -> barrier potential When apply forward bias – free e- provided enough energy to climb the hill and cross the depletion region Electron got the same energy = barrier potential to cross the depletion region An add. small voltage drop occurs across the p and n regions due to internal resistance of material – called dynamic resistance – very small and can be neglected Biasing The Diode ( The Effect of the Barrier Potential during Forward Bias)

30. Condition that prevents current through the diode Voltage source or bias connections are – to the p material and + to the n material Current flow is negligible in most cases. The depletion region widens Diode connection Shot transition time immediately after reverse bias voltage is applied + side of bias pulls the free electrons in the n- region away from pn junction cause add. +ve ions are created , widening the depletion region In the p -region, e- from – side of the voltage source enter as valence electrons e- move from hole to hole toward the depletion region, then created add. –ve ions. As the depletion region widens, the availability of majority carriers decrease Biasing The Diode ( ReverseBias )

31. extremely small current exist – after the transition current dies out caused by the minority carries in n & p regions that are produced by thermally generated electron-hole pairs small number of free minority e- in p region are “pushed” toward the pn junction by the –ve bias voltage e- reach wide depletion region – they “fall down the energy hill” combine with minority holes in n -region as valence e- (flow towards the +ve bias voltage) – create small hole current the cond. band in p region is at higher energy level compare to cond. band in n- region e- easily pass through the depletion region Biasing The Diode ( Reverse Current)

32. PN Junction = Diode p N D BUT – PN is no ordinary capacitor, actually a diode +V -V Forward Bias : Shrink depletion region, current dragged through the barrier p N D Once the difficulty of getting through the depletion region has been overcome, current can rise with applied voltage (Ohm’s law) Reverse Bias : Grow depletion region, current finds it more and more difficult to get through the barrier p N D -V +V p N D Little current flows because barrier too high However increasing voltage further  high electric field Depletion region eventually breaks down  reverse current - + p N D

33. Modulators of conductivity Just reviewed how conductivity of a semiconductor is affected by: Temperature – Increasing temperature causes conductivity to increase Dopants – Increasing the number of dopant atoms (implant dose) cause conductivity to increase. Holes are slower than electrons therefore n-type material is more conductive than p-type material. These parameters are in addition to those normally affecting conducting material, Cross sectional area  Resistance  Length  Resistance 

34. Directional of current cathode anod Diode Models ( Diode structure and symbol)

35. Assume Forward current, by Ohm’s law Ideal model of diode- simple switch: Closed (on) switch -> FB Open (off) switch -> RB (1-2) Diode Models ( The ideal Diode model)

36. Combine-Forward bias & Reverse bias  Complete V-I characteristic curve Voltage-Current Characteristic of a Diode ( Complete V-I Characteristic curve)

37. Forward biased dioed : for a given value of For a given Barrier potential decrease as T increase Reverse current breakdown – small & can be neglected Voltage-Current Characteristic of a Diode ( Temperature effect on the diode V-I Characteristic)

38. P -materials are doped with trivalent impurities N -materials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction. A diode is nothing more than a PN junction. At the junction a depletion region is formed. This creates barrier which requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place. Diodes, transistors, and integrated circuits are all made of semiconductor material. Summary

39. When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage. There are three ways of analyzing a diode. These are ideal, practical , and complex . Typically we use a practical diode model. A diode conducts when forward biased and does not conduct when reverse biased Summary