Semiconductors (Chất1bán1dẫn)

Transcription

1 To describe the properties of n-type and p-type semiconductors and how a pn jun formed To study a diode and the characteristics of diode 1-1 Atomic Structure Atomic1Structure An atom is the smallest particle of an element that contains three basic particles: (positive charge) and neutrons (uncharged particles) that make up the nucleus (core Atom1(nguyên1tử):1the1smallest1particle1of1an1element1that1 atom and electrons (negative charge) that orbit around the nucleus. Semiconductors (Chất1bán1dẫn) contains1three1basis1particles protons1(positive1charge) neutrons1(uncharged1particles) electrons1(negative1charge):1 orbit1around1the1nucleus. insulators in its ability to conduct electrical current. They are 2 characterized by atoms with four valence electrons. E.g.: silicon, germanium, and carbon! Energy bands Semiconductors,1Conductors,1and1Insulators When an electron acquires enough additional energy, leaves the valence shell, The difference in energy between the valence band and the conduction band is called become a free electron, and exists in conduction band. Semiconductors,1Conductors,1and1Insulators an energy gap. This is the amount of energy that a valence electron must have in Materials1can1be1classified1into131groups: Energy1bands: order to jump from the valence band to the conduction band Conductors:1have-one-valence-electron-(điện-tử-hóa-trị)-that1 loosely1bound1to1the1atom.1this1valence1electron1can1easily1 break1away1from1their1atoms1and1become1free1electron.1 E.g.:1copper,1silver,1aluminum,1gold Energy1gap:1amount1of1energy1that1a1valence1electron1must1 figure below shows the energy diagrams for insulators, semiconductors, and The have1in1order1to1jump1from1the1valence1band1(miền1hóa1trị)1to1 conductors. the1conduction1band1(miền1dẫn). Insulators:1material1that1does-not-conduct-electrical-current1 under1normal1conditions.1the1valence1electrons1are1tightly1 bound1to1the1atoms. Semiconductors:1material1that1is1between1conductors1and1 insulators1in1its1ability1to1conduct1electrical1current.1they1are1 characterized1by1atoms1with1four-valence-electrons. 3 4 Energy diagrams for the three types of materials a) insulators have a very wide energy gap

2 Atomic1Structure1of1Semiconductors Covalent1Bond1(liên1kết1hóa1trị) 8 (2)5/6 The Pure Silicon Crystal Each atom exchanges an electron with a neighbor to form a covalent bond. Each atom has four neighbors to bond with. Covalent Bond 1-4 Conduction in Semiconductors 1-3 Covalent Bonds When a valence electron has sufficient 5energy to jump to the conduction band, becoming free or conduction electron. When this happen, a vacancy is left in the valence band within the Figures below show how each silicon atom positions itself with four adjacent silicon crystal and this vacancy is known as hole. This creating an electron-hole pairs as shown below: atoms to form a silicon crystal. A silicon atom with its four valence electron shares an electron Conduction1in1Semiconductors with each of its four neighbors. This creates eight valence electron for each atom and produces a state of chemical stability and covalent bonds that hold the atoms together. 6 When a voltage is applied across the intrinsic silicon, there will be a movement of free Conduction1in1Semiconductors electrons, which are easily attracted toward the positive end. And this movement of free electron is called electron current. Whereas electrons that remain in the valence band are still attached to the atom and are not Two1current1types: free to move randomly as free electron. However, it can move into nearby hole thus leaving! An intrinsic crystal is one that has no impurities another where it comes from. It is seems like the hole has moved from one place to another Electron1current:1the1movement1of1free1electrons. hole in the crystal structure and is called as hole current Hole1current:1the1movement1of1holes1that1move1from1one1place1 to1another1in1the1crystal1structure. At1higher1temperatures,1thermal1energy1can1cause1an1electron1 to1leave1its1covalent1bond 1Pure1silicon1crystal1at1high1temperature1 1some1electrons1 are1freed1 1leave1behind1a1hole1 1conduction1current1may1 occur.occurs when a conduction- band electron loses energy and falls back into a Recombination hole in the valence band. There will be an equal number of holes in the valence band created 7 when these electrons jump into the conduction band. 8

3 Intrinsic1semiconductor N[type1and1P[type1semiconductors Intrinsic1semiconductor1(bán1dẫn1thuần1[1bán1dẫn1nội1tại):1 N[type1semiconductor undoped1semiconductor1that1has1no1impurities. 1Pentavalent1impurity1atom1 In1intrinsic1semiconductor:1 covalent1bonds1with1four1 adjacent1silicon1atoms1 1leave1 Whereas in the p region, one1extra1electron1without1 bonds. minimum1energy1required1to1free1an1electron1(bandgap1energy) the1absolute1temperature covalent1bonds1with1four1 adjacent1silicon1atoms1 1result1 as the electrons from n region one1hole1since1four1electrons1 required. move a loses holes as the electron and holes in p region combine. T 1The1pentavalent1atom1gives1up1 1The1trivalent1atom1can1take1an1 Eg ni2 = kt 3 exp pentavalent To increase the number of conduction band electrons in intrinsic silicon, kt an1electron1 1donor1atom.ions) charges (trivalent impurity atom is added. When a pentavalent atom (five valence electron) impurity such as 1Majority1carriers:1electrons. electron. 1Minority1carriers:1holes. arsenic (As), phosphorus (P), bismuth (Bi) and antimony (Sb) are added, each pentavalent atom forms covalent bonds with four adjacent silicon atoms. And this will leave one extra electron without bonds and will become a conduction electron because 9 it is not attached to chất1nhóm13)1with1three1valence1 electrons1is1added. 1Each1pentavalent1atom1forms1 1Each1trivalent1atom1forms1 n = p = ni! N-Type semiconductor 1Trivalent1impurity1atom1(tạp1 (tạp1chất1nhóm15)1with1five1 valence1electrons1is1added. density1of1free1electrons1n1=1density1of1free1holes1p ni1depends1on: P[type1semiconductor electron1 1acceptor1atom. near the junction since the trivalent ato 1Majority1carriers:1holes. 1Minority1carriers:1electrons. 10 any atom. Because the pentavalent atom gives up an electron, it is called a donor atom This doping process, silicon or germanium doped with pentavalent atom will give n-type three of the boron atom are used in covalent bonds. Because the trivalent atom can take semiconductor because most of current carriers are electrons which are called the majority an electron, it is referred as acceptor atom. carriers and a few holes that are created in n-type materials are called minority carriers. N[type1and1P[type1semiconductors Since most of the current carriers are holes, it is called a p-type semiconductor and because of the absence of electrons, gives a net positive charge on the atom. And the majority carriers are holes and electrons in p-type are the minority carriers. These two layers of positive and negative charges form the deple quickly and is very thin compared to the n region and p region. When1N[type1and1P[type1dopants1are1introduced1side[by[side1 in1a1semiconductor,1a1pn1junction1is1formed.! P-Type semiconductors And in order to increase the number of holes in intrinsic silicon, trivalent atoms are added. When trivalent (three valence electrons) impurity atoms are added such as boron (B), indium (In), and gallium (Ga), each trivalent electron forms covalent bonds with The Diode four adjacent silicon atoms. This will result one hole since four electrons required and all Diode is created when we combine together a piece of N-type and P-type. For both types, 12

4 Whereas in the p region, as the electrons from n region move across the junction, the p region loses holes as the electron and holes in p region combine. This creates a layer of negative charges (trivalent ions) near the junction since the trivalent atoms in p region have gained an electron. In1the1n[region,1free1electrons1near1the1junction1begin1to1 diffuse1across1the1junction1into1p[region1where1they1combine1 These two layers of positive and negative charges form the depletion region that is formed very with1holes1near1the1junction1 1create1a1layer1of1positive1 quickly and is very thin compared to the n region and p region. charges. In1the1p[region,1as1the1electrons1from1n1region1move1across1the1 junction,1the1holes1in1the1p1region1are1lost1 1create1a1layer1of1 negative1charges. Electrons1continue1to1diffuse1to1holes1across1the1junction1 1create1 more1positive1and1negative1charges1near1the1depletion1region. Many1positive1and1negative1charges1on1opposite1sides1of1the1p[n1 junction1 1form1an1electric-field-(điện-trường-tiếp-xúc). Two1current1types: These1two1layers1form1 the1depletion-region (miền-nghèo).! At the point of there are no electrons left in n-region conduction band with enough energy to 13 Minority[carrier 1drift1current1(dòng1trôi):1caused1by1minority1 carriers1sweeping1across1the1junction1due1to1the1electric1field. The1potential1difference1of1the1electric1field1across1the1depletion1 region1is1called1barrier-potential-(hàng-rào-điện-thế). Equilibrium1is1established1when1there s1no1further1diffusion1of1 electrons Biasing The Diode Majority[carrier 1diffusion1current1(dòng1khuếch1tán):1caused1by1 majority1carriers1diffusing1into1other1region. across the junction, the junction is at equilibrium, where the depletion region is complete because diffusion has ceased As no electrons move through the pn junction at equilibrium, we need to bias the diode. Bias refers to the use of dc voltage to establish certain operating conditions for an electronic device. There are two bias conditions: As electrons continue to diffuse to condition holes across the junction, this will creates more positive Forward-biasIs the that allows current through the p-n junction. Figure below and Forward1bias:1the1condition1that1allows1current1through1the1 negative chargesshows near athe depletion region is formed. A point is direction reachedtowhere totalbias. negative dc voltage source connected across a diode in the producethe forward pn1junction. The external bias repels voltage any is designated VBIAS. The sideinto of VBIAS is connected to charge in the depletion region further as diffusion ofnegative electrons p-region and the The1negative1side1of1V the BIAS1is1connected1to1the1n1region. n region of the diode and positive side is connected to the p region. The bias voltage, V must be greater than the barrier potential. The resistor, R, limits the current to a value The1positive1side1of1V is1connected1to1the1p1region.1 BIAS11 will not damage the pn structure. The1bias1voltage1V that BIAS1must1be1greater1than1the1barrier1 diffusion stops, where equilibrium is established. BIAS As many positive and negative charges on opposite sides of the p-n junction, this will form a potential. field of forces called an electric field. This electric field is a barrier to the free electrons in the n-region and external energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region. The potential difference of the electric field across the depletion region is the amount of voltage required to move the electrons through the electric The1negative1side1of1the1bias[voltage1source1pushes1the1free1electrons1 in1n1region1toward1the1junction1and1provides1a1continuous1flow1of1 electrons1through1the1external1connection1(conductor)1into1the1n1 region. The bias-voltage gives sufficient energy to the free electrons to overcome the barrier 10 The1positive1side1of1the1bias1voltage1source1anracts1the1valence1 potential electrons1toward1the1left1end1of1the1p1region.1the1holes1in1the1p1 of the depletion region and move on into the p region. Once in the p region, these conduction electrons have lost energy and combine with holes in the valence band region1move1to1the1right1toward1the1junction. 15 Refers the figure below, as the same charges repel, the negative side of the bias-voltage source pushes the free electrons in n region toward the junction. This flow of free electrons is 16 And as unlike charges attract, the positive side of the bias voltage source attracts the valence electrons toward the left end of the p region. The electrons move from one hole to the next

5 Reverse1Bias1(phân1cực1ngược) Reverse1Bias:1the1condition1that1essentially1prevents1current1! Reverse Bias-through1the1diode.1 is the condition that essentially prevents current through the diode. The external bias VBIAS, is connectedbias that1is1connected1to1the1n1region the positive side of VBIAS is connected to the n The1positive1side1of1V voltage BIAS1 The1depletion1region1is1much1wider1than1in1forward1bias region of the The1negative1side1of1V diode and the negative side is connected to the p region. The depletion region is is1connected1to1the1p1region.1 much wider than in forward bias 17! Reverse Bias- is the condition that essentially prevents current through the diode. The 18 As unlike charges attract, the positive side of the bias-voltage source pulls the free external bias voltage VBIAS, is connected that the positive side of VBIAS is electrons connected to nthe n away from the p-n junction. As the electrons flow toward the positive in the region, of the voltage region of the diode and the negative side is connected to the p region. Theside depletion regionsource, is additional positive ions are created. It also happen in the p region, Voltage[Current1Characteristics1 (Đặc1tuyến1Vol[Ampe) much wider than in forward bias electrons from the negative side of the voltage source enter as valence electrons and move from 14 he condition that essentially prevents current through the diode. The ge VBIAS, is connected that the positive side of VBIAS is connected to the n e and the negative side is connected to the p region. The depletion region is n forward bias As unlike charges attract, the positive side of the bias-voltage source pulls the free electrons in the n region, away from the p-n junction. As the electrons flow toward the positive side of the voltage source, additional positive ions are created. It also happen in the p region, 19 electrons from the negative side of the voltage source enter as valence electrons and move from For both forward and reverse currents, as the temperature is increased, both currents will also increase.