Analog_Electronics_E_Contents semiconductor
- 1. Semiconductors, diodes, transistors (Horst Wahl, QuarkNet presentation, June 2001) ● Electrical conductivity ! Energy bands in solids ! Band structure and conductivity ● Semiconductors ! Intrinsic semiconductors ! Doped semiconductors " n-type materials " p-type materials ● Diodes and transistors ! p-n junction ! depletion region ! forward biased p-n junction ! reverse biased p-n junction ! diode ! bipolar transistor ! operation of bipolar pnp transistor ! FET
- 2. ELECTRICAL CONDUCTIVITY ● in order of conductivity: superconductors, conductors, semiconductors, insulators ! conductors: material capable of carrying electric current, i.e. material which has “mobile charge carriers” (e.g. electrons, ions,..) e.g. metals, liquids with ions (water, molten ionic compounds), plasma ! insulators: materials with no or very few free charge carriers; e.g. quartz, most covalent and ionic solids, plastics ! semiconductors: materials with conductivity between that of conductors and insulators; e.g. germanium Ge, silicon Si, GaAs, GaP, InP ! superconductors: certain materials have zero resistivity at very low temperature. ● some representative resistivities (ρ): ! R = ρL/A, R = resistance, L = length, A = cross section area; resistivity at 20o C resistivity in Ω m resistance(in Ω)(L=1m, diam =1mm) " aluminum 2.8x10-8 3.6x10-2 " brass ≈8x10-8 10.1x10-2 " copper 1.7x10-8 2.2x10-2 " platinum 10x10-8 12.7x10-2 " silver 1.6x10-8 2.1x10-2 " carbon 3.5x10-5 44.5 " germanium 0.45 5.7x105 " silicon ≈ 640 ≈ 6x108 " porcelain 1010 - 1012 1016 - 1018 " teflon 1014 1020 " blood 1.5 1.9x106 " fat 24 3x107
- 3. ENERGY BANDS IN SOLIDS: ! In solid materials, electron energy levels form bands of allowed energies, separated by forbidden bands ! valence band = outermost (highest) band filled with electrons (“filled” = all states occupied) ! conduction band = next highest band to valence band (empty or partly filled) ! “gap” = energy difference between valence and conduction bands, = width of the forbidden band ! Note: " electrons in a completely filled band cannot move, since all states occupied (Pauli principle); only way to move would be to “jump” into next higher band - needs energy; " electrons in partly filled band can move, since there are free states to move to. ! Classification of solids into three types, according to their band structure: " insulators: gap = forbidden region between highest filled band (valence band) and lowest empty or partly filled band (conduction band) is very wide, about 3 to 6 eV; " semiconductors: gap is small - about 0.1 to 1 eV; " conductors: valence band only partially filled, or (if it is filled), the next allowed empty band overlaps with it
- 4. Band structure and conductivity
- 5. INTRINSIC SEMICONDUCTORS ! semiconductor = material for which gap between valence band and conduction band is small; (gap width in Si is 1.1 eV, in Ge 0.7 eV). ! at T = 0, there are no electrons in the conduction band, and the semiconductor does not conduct (lack of free charge carriers); ! at T > 0, some fraction of electrons have sufficient thermal kinetic energy to overcome the gap and jump to the conduction band; fraction rises with temperature; e.g. at 20o C (293 K), Si has 0.9x1010 conduction electrons per cubic centimeter; at 50o C (323 K) there are 7.4x1010 . ! electrons moving to conduction band leave “hole” (covalent bond with missing electron) behind; under influence of applied electric field, neighboring electrons can jump into the hole, thus creating a new hole, etc. ⇒ holes can move under the influence of an applied electric field, just like electrons; both contribute to conduction. ! in pure Si and Ge, there are equally many holes (“p- type charge carriers”) as there are conduction electrons (“n-type charge carriers”); ! pure semiconductors also called “intrinsic semiconductors”.
- 6. ● Intrinsic silicon: ● DOPED SEMICONDUCTORS: ! “doped semiconductor”: (also “impure”, “extrinsic”) = semiconductor with small admixture of trivalent or pentavalent atoms;
- 7. n-type material ! donor (n-type) impurities: " dopant with 5 valence electrons (e.g. P, As, Sb) " 4 electrons used for covalent bonds with surrounding Si atoms, one electron “left over”; " left over electron is only loosely bound⇒ only small amount of energy needed to lift it into conduction band (0.05 eV in Si) " ⇒ “n-type semiconductor”, has conduction electrons, no holes (apart from the few intrinsic holes) " example: doping fraction of 10-8 Sb in Si yields about 5x1016 conduction electrons per cubic centimeter at room temperature, i.e. gain of 5x106 over intrinsic Si.
- 8. p-type material ! acceptor (p-type) impurities: " dopant with 3 valence electrons (e.g. B, Al, Ga, In) ⇒ only 3 of the 4 covalent bonds filled ⇒ vacancy in the fourth covalent bond ⇒ hole " “p-type semiconductor”, has mobile holes, very few mobile electrons (only the intrinsic ones). ! advantages of doped semiconductors: " can”tune” conductivity by choice of doping fraction " can choose “majority carrier” (electron or hole) " can vary doping fraction and/or majority carrier within piece of semiconductor " can make “p-n junctions” (diodes) and “transistors”
- 9. DIODES AND TRANSISTORS ! p-n JUNCTION: " p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ; " “diffusion” = movement due to difference in concentration, from higher to lower concentration; " in absence of electric field across the junction, holes “diffuse” towards and across boundary into n- type and capture electrons; " electrons diffuse across boundary, fall into holes (“recombination of majority carriers”); ⇒ formation of a “depletion region” (= region without free charge carriers) around the boundary; " charged ions are left behind (cannot move): # negative ions left on p-side ⇒ net negative charge on p-side of the junction; # positive ions left on n-side ⇒ net positive charge on n-side of the junction # ⇒ electric field across junction which prevents further diffusion.
- 10. Pn junction ● Formation of depletion region in pn-junction:
- 11. DIODE ! diode = “biased p-n junction”, i.e. p-n junction with voltage applied across it ! “forward biased”: p-side more positive than n-side; ! “reverse biased”: n-side more positive than p-side; ! forward biased diode: " the direction of the electric field is from p-side towards n-side " ⇒ p-type charge carriers (positive holes) in p- side are pushed towards and across the p-n boundary, " n-type carriers (negative electrons) in n-side are pushed towards and across n-p boundary ⇒ current flows across p-n boundary
- 12. Forward biased pn-junction ● Depletion region and potential barrier reduced
- 13. Reverse biased diode ! reverse biased diode: applied voltage makes n-side more positive than p-side ⇒ electric field direction is from n-side towards p-side ⇒ pushes charge carriers away from the p-n boundary ⇒ depletion region widens, and no current flows ! diode only conducts when positive voltage applied to p-side and negative voltage to n-side ! diodes used in “rectifiers”, to convert ac voltage to dc.
- 14. Reverse biased diode ● Depletion region becomes wider, barrier potential higher
- 15. TRANSISTORS ! (bipolar) transistor = combination of two diodes that share middle portion, called “base” of transistor; other two sections: “emitter'' and “collector”; ! usually, base is very thin and lightly doped. ! two kinds of bipolar transistors: pnp and npn transistors ! “pnp” means emitter is p-type, base is n-type, and collector is p-type material; ! in “normal operation of pnp transistor, apply positive voltage to emitter, negative voltage to collector;
- 16. operation of pnp transistor: ! if emitter-base junction is forward biased, “holes flow” from battery into emitter, move into base; ! some holes annihilate with electrons in n-type base, but base thin and lightly doped ⇒ most holes make it through base into collector, ! holes move through collector into negative terminal of battery; i.e. “collector current” flows whose size depends on how many holes have been captured by electrons in the base; ! this depends on the number of n-type carriers in the base which can be controlled by the size of the current (the “base current”) that is allowed to flow from the base to the emitter; the base current is usually very small; small changes in the base current can cause a big difference in the collector current;
- 17. Transistor operation ! transistor acts as amplifier of base current, since small changes in base current cause big changes in collector current. ! transistor as switch: if voltage applied to base is such that emitter-base junction is reverse-biased, no current flows through transistor -- transistor is “off” ! therefore, a transistor can be used as a voltage- controlled switch; computers use transistors in this way. ● “field-effect transistor” (FET) ! in a pnp FET, current flowing through a thin channel of n-type material is controlled by the voltage (electric field) applied to two pieces of p-type material on either side of the channel (current depends on electric field). ! Many different kinds of FETs ! FETs are the kind of transistor most commonly used in computers.