1、 2.5 Generation and Recombination Processes 載流子產(chǎn)生與復(fù)合過(guò)程 3. Characteristics of Diodes二極管特性 3.1 Introduction介紹 Pn junction Alloying (合金） Epitaxy（外延） Diffusion（擴(kuò)散） Ion implantation（離子 注入） Wheneverthethermal-equilibriumconditionisdisturbed(i.e.,pn ni2), processes exist to restore the syetem to equilibriu

2、m(i.e.pn=ni2).In the case of the injection of excess carriers, the mechanism that restores equilibrium is recombination of the injected minority carrier with the majority carriers. Depending on the nature of the recombination process, the released energy that results from the recombination process c

3、an be emitted as a photon or dissipated as heat to the lattice. When a photon is emitted , the process is called radiative recombination ;otherwise, it is called nonradiative recombination. 當(dāng)熱平衡條件受到擾亂時(shí)（即pnni2），會(huì)出現(xiàn)一些使系統(tǒng)回復(fù)平衡的機(jī)制（即 pn=ni2）。在超量載流子注入的情形下，回復(fù)平衡的機(jī)制是將注入的少數(shù)載流子 與多數(shù)載流子復(fù)合。視復(fù)合過(guò)程的本征而定，復(fù)合過(guò)程所釋放出的能量，一

4、般以 光子形式輻射出或是對(duì)晶格產(chǎn)生熱而消耗掉。一個(gè)光子被輻射出，此過(guò)程稱為輻 射復(fù)合，反之則稱為非輻射復(fù)合。 2.5 Generation and Recombination Processes 載流子產(chǎn)生與復(fù)合過(guò)程 Recombinationphenomenacan be classified asdirect and indirect processes . Direct recombination , also called band-to-band recombination , usually dominates in direct-bandgap semiconductors ,

5、such as gallium arsenide ; while indirect recombination via bandgap recombination centers dominates in indirect bandgap semiconductors , such as silicon. 復(fù)合現(xiàn)象可分為直接和間接過(guò)程。直接復(fù)合，又稱為帶至帶復(fù)合，通常在直 接禁帶的半導(dǎo)體中較為顯著，如砷化鎵；而通過(guò)禁帶復(fù)合中心的間接復(fù)合則 在間接禁帶的半導(dǎo)體中較為顯著，如硅晶。 Consider a direct-bandgap semicond uctor in thermal equili

6、brium . The continuous thermal vibration of lattice atoms causes some bonds between neighboring atoms to be broken . When a bond is broken ,an electron-hole pair is generated. In terms of the band diagram, the thermal energy enables a valence electron to make an upward transition to the conduction b

7、and leaving a hole in the valence band. This process is called carrier generation and is represented by the generation rate Gth(number of electron-hole pairs generated per cm3per second) in Fig.2.11(a). 考慮一個(gè)在熱平衡狀態(tài)下的直接禁帶半導(dǎo)體。 晶格原子連續(xù)的熱擾動(dòng)造成鄰近原子間的鍵 斷裂。當(dāng)一個(gè)鍵斷裂，一對(duì)電子空穴對(duì)就產(chǎn) 生了。以能帶圖的觀點(diǎn)而言，熱能使得一個(gè) 價(jià)電子向上移到導(dǎo)帶，而留下一個(gè)空

8、穴在價(jià) 帶。這個(gè)過(guò)程稱為載流子產(chǎn)生，并以產(chǎn)生速 率Gth（每立方厘米每秒產(chǎn)生的電子-空穴對(duì) 數(shù)目）表示，如圖2.11（a）所示。 When an electron makes a transition downward from the conduction band to the valence band, an electron-hole pair is annihilated. This reverse process is called recombination; it is represented by the recombination rate Rthin Fig.2.11(a

9、). Under thermal equilibrium conditions, the generation rate Gthmust equal the recombination rate Rth, so that the carrier concentrations remain constant and the condition pn=ni2is maintained. 當(dāng)電子從導(dǎo)帶向下移到價(jià)帶， 一個(gè)電子-空穴對(duì)消失。這 種反向過(guò)程稱為復(fù)合，并以 復(fù)合率Rth表示，如圖2.11 （a）所示。在熱平衡狀態(tài) 下，產(chǎn)生速率Gth必定等于 復(fù)合率 Rth，所以載流子濃 度維持常數(shù)，且維持

10、pn=ni2 的狀況。 當(dāng)超量載流子被導(dǎo)入一個(gè)直接禁帶半導(dǎo)體時(shí)，電子與空穴直接復(fù)合的幾率 較高，這是因?yàn)閷?dǎo)帶的底部與價(jià)帶的頂端位于同一線上，因此在禁帶間躍 遷時(shí)，無(wú)需額外的動(dòng)量。直接復(fù)合率R應(yīng)正比于導(dǎo)帶中含有的電子數(shù)目及 價(jià)帶中含有的空穴數(shù)目。也就是R=np。其中為比例常數(shù)。 When excess carries are introduced to a direct-bandgap semiconductor, theprobabilityishighthatelectronsandholeswillrecombine directly ,because the bottom of the

11、conduction band and the top of the valence band are lined up and no additional crystal momentum is required for the transition across the bandgap . The rate of the direct recombination R is expected to be proportional to the number of holes availableinthevalenceband;thatisR=npwhereisthe proportional

12、ity constant. As discussed previously, in thermal equilibrium the recombination rate must be balanced by the generation rate . Therefore , for an n-type semiconductor, we have Gth=Rth=n nopno where nnoand pnorepresent electron and hole densities in an n-type semiconductor at thermal equilibrium. Whe

13、n we shine a light on the semiconductor to produce electron-hole pairs at a rate GL(Fig.2.11(b), the carrier concentrations are above their equilibrium values. 如前面所討論的，在熱平衡下 復(fù)合率必定與產(chǎn)生速率保持平 衡，因此，對(duì)以n型半導(dǎo)體而言， 我們可以得到G th=Rth=nno p no 。nno及pno分別表示在熱 平衡下，n型半導(dǎo)體中的電子及 空穴濃度。我們?cè)诎雽?dǎo)體上照 光，使它以GL的速率產(chǎn)生電子- 空穴對(duì)（如圖2.11（b

14、），載 流子濃度將大于平衡時(shí)的值。 Therefore , the net recombination rate is proportional to the excess minority carrier concentration .The proportionality constant 1/ nnois called the lifetime Tpof the excess minority carriers . The physical meaning of lifetime can best be illustrated by the transient response of a

15、 device after the sudden removal of the light source . Consider an n-type sample, as shown in Fid.2.12(a) ,that is illuminated with light and in which the electron-hole pairs are generated uniformly throughout the sample with a generation rate GL. Fig.1.12(b) shows the variation of pnwith time. The

16、minority carriers recombine with majority carriers and decay exponentially with a time constant Tpwhich corresponds to the lifetime. 因此，凈復(fù)合率正比于超量少數(shù)載流子濃 度。比例常數(shù)1/ nno稱為超量少數(shù)載流子 的壽命Tp。壽命的物理意義可通過(guò)器件在 瞬間移去光源后的暫態(tài)響應(yīng)作最好的說(shuō)明。 考慮一個(gè)n型樣品，如圖2.12（a），光照 射其上且整個(gè)樣品中以一個(gè)產(chǎn)生速率GL 均勻地產(chǎn)生電子-空穴對(duì)。圖2.12（b）顯 示pn隨時(shí)間的變化。少數(shù)載流子與多數(shù)載 流子復(fù)

17、合，且壽命Tp成指數(shù)衰減。 The above case illustrates the main idea of measuring the carrier lifetime using photoconductivity method . Fig.2.12(c) shows a schematic setup .The excess carriers ,generated uniformly throughout the sample by the light pulse , cause a momentary increase in the conductivity . The incr

18、ease in conductivity manifests itself by a drop in voltage across the sample when a constant current is passed through it . The decay of the conductivity can be observed on an oscilloserved on an oscilloscope and is a measure of the lifetime of the excess minority carriers. 以上情形說(shuō)明使用光電導(dǎo)方法來(lái)測(cè)量 載流子壽命的主要

19、概念。如圖2.12（c） 顯示一個(gè)圖示的裝置。通過(guò)光脈沖照 射，整個(gè)樣品中均勻產(chǎn)生超量載流子， 因而造成電導(dǎo)率瞬間增加。而電導(dǎo)率 的增加，可由將一定電流通過(guò)樣品使 樣品兩端產(chǎn)生一小電壓而顯示出來(lái)。 電導(dǎo)率的衰減可由示波器上觀察得知， 它同時(shí)又是測(cè)量超量少數(shù)載流子壽命 的一種方法。 3.1 Introduction Most electronic devices depend on the electrical characteristics of junctions between different materials. Such junctions are two-terminal dev

20、ices and are referred to as diodes. When n-type and p-type silicon crystals are joined together ,a pn junction is formed . In practice ,a pn junction is formed by adding accepter impurities to an n-type wafer or donors to a p-type wafer . There are a variety of methods for junction formation. 大多數(shù)的電子

21、裝置依靠的是不同材料之間的結(jié)的電特性。這樣的結(jié)有兩 個(gè)終端裝置就像以前提到的二極管。當(dāng)一個(gè)n型硅晶和一個(gè)p型硅晶結(jié)合 在一起的時(shí)候，一個(gè)pn結(jié)就產(chǎn)生了。通常，一個(gè)pn結(jié)的產(chǎn)生是通過(guò)在一 塊n型硅片上添加受主雜質(zhì)或在一塊p型硅片上添加施主雜質(zhì)。結(jié)的產(chǎn)生 有多種方式。 The techniques of alloying , epitaxy,diffusion,andion implantation have reported in some referencesto produce the pn junction. 在一些參考文獻(xiàn)上記載的生產(chǎn) pn結(jié)的技術(shù)有合金，外延， 擴(kuò)散和離子注入。 In

22、alloying technique , a thin aluminum film is evaporated onto a clean n-type silicon wafer ,called a substrate ,inside a vacuum chamber. The silicon is then placed in a furnace set at about 600C for 30 min , Al and Si constitute a eutectic system . When the semiconductor is cooled down ,the silicon f

23、rom the liquid alloy will form a recrystallized layer which contains a significant amount of aluminum atoms . Since aluminum is a p-type impurity in silicon , the recrystallized region is a p region. Consequently ,a pn junction is produced . The alloying process is simple and inexpensive in making a

24、 single pn junction , but it does not produce a uniform and smooth junction .And the control of aluminum concentration is also difficult. For these reasons , the alloyed junction is seldom used in practical devices . 合金工藝，在真空室中將一個(gè)薄鋁膜蒸發(fā)到一個(gè)干凈的n型硅片上作為襯底。然 后將這個(gè)硅片放入溫度在600度的熔爐中，30分鐘后，鋁和硅構(gòu)成一個(gè)共晶體結(jié) 構(gòu)。當(dāng)半導(dǎo)體冷卻后

25、，液體合金中的硅將形成一個(gè)包含大量鋁原子的再結(jié)晶層。 此后，在硅中鋁稱為p型雜質(zhì)，在再結(jié)晶區(qū)域稱為p型區(qū)域。因此，一個(gè)pn結(jié)就產(chǎn) 生了。合金過(guò)程用來(lái)制造單一的pn結(jié)既簡(jiǎn)單又低成本，但是它不能生產(chǎn)不變且平 滑的結(jié)。而且鋁濃度的控制非常困難。由于這些原因，合金結(jié)在實(shí)際生產(chǎn)中很少 用到。 Epitaxy describes the growth technique of arranging atoms in single-crystal fashion upon a crystalline substrate so that the lattice structure of the newly gr

26、own film duplicates that of the substrate .An important reason for using this growth technique is the flexibility of impurity control in the epitaxial film .The dopant in the film may be n or p type and independent of the substrate doping .Therefore ,epitaxial growth can be used to form a lightly do

27、ped layer on a heavily doped substrate ,or a pn junction between the epitaxial film and the substrate .Three different methods are available to produce epitaxial films :vapor-phase epitaxy (VPE),liquid-phase epitaxy(LPE),and molecular- beam epitaxy(MBE). 外延描述的是在單晶襯底上制成原子排列的單晶層以便它與襯底晶向相同的 生長(zhǎng)技術(shù)。使用這種生長(zhǎng)

28、技術(shù)的最重要的原因是在外延膜上能夠靈活地控制 雜質(zhì)濃度。在膜上的摻雜劑可能是n型或者p型，而且襯底摻雜是獨(dú)立的。因 此，外延生長(zhǎng)經(jīng)常用于在一個(gè)重?fù)诫s襯底中形成薄摻雜的雜質(zhì)層，或者在外 延膜和襯底之間形成pn結(jié)。以下是三種不同的用于生產(chǎn)外延膜的方式：氣相 外延，液相外延和分子束外延。 外延的設(shè)備 LPE3061 NUFLARE EGV30GX 外延爐 The most widely used technique in forming a pn junction is solid-state impurity diffusion .The diffusion of impurity is ,in

29、principle , the same as carrier diffusion except that it occurs at very high temperature .In a typical diffusion system, a wafer is placed in a gaseous atmosphere containing impurity atoms inside a furnace .These impurity atoms may come from slices of boron nitride inserted between silicon wafers .T

30、he temperature of the furnace usually ranges between 900 and 1200. The number of impurity atoms taken in by the solid is limited by the solid solubility ,which is the maximum impurity concentration that the solid can accommodate at a given temperature .The values of solid solubility for B,P, and As

31、in silicon in the normal range of diffusion temperatures are approximately 6(10201021),and 21021cm-3, respectively. 形成pn結(jié)的最廣泛的技術(shù)是在固態(tài)下的雜質(zhì)擴(kuò)散。原則上，雜質(zhì)擴(kuò)散除了是要 發(fā)生在很高的溫度下，其他的都與載流子擴(kuò)散相同。在一個(gè)典型的擴(kuò)散系統(tǒng)中， 硅片放置在包含了雜質(zhì)原子的氣體環(huán)境的火爐中。這些雜質(zhì)原子可能來(lái)自于插入 硅片間的氮化硼的薄片?；馉t中的溫度通常在900-1200度之間。固體上收納的 雜質(zhì)原子的數(shù)目受固溶度的限制，即在給定溫度下固體所能容納的最多的雜質(zhì)集 中

32、。在正常的擴(kuò)散溫度范圍內(nèi)，對(duì)于硼、磷、砷在硅中的固溶度標(biāo)準(zhǔn)大約分別是 6(10201021),和21021cm-3。 擴(kuò)散爐 Ion implantation is a primary method for introducing dopants into a semiconductor and making pn junction .The ion-implantation process is attractive because it can be performed at low temperature with negligible impurity diffusion .In ad

33、dition ,the impurity concentration introduced is better controlled than witn standard diffusion techniques .A beam of dopant ions is accelerated through a desired energy potential ranging between 30 and 500 keV .The ion beam is aimed at the semiconductor target so that the high-energy ions penetrate

34、 the semiconductor surface. The energetic ions will lose their energy through collosions with the target nuclei and electrons so that the ions will finally come to rest. The distance traveled by the ions ,i.e. the penetration depth, is called the range . The range is a function of the kinetic energy

35、 of the ions and the semiconductors structural properties , e.g., lattice spacing and mass ofatoms.Atypicalimpurity-rangedistributioninanamorphoustargetis approximately Gaussian. 離子注入是向半導(dǎo)體中引入摻雜劑制造pn結(jié)的一項(xiàng)主要方法。離子注入過(guò)程是很有 吸引力的，因?yàn)樗軌蛟跇O低的雜質(zhì)擴(kuò)散，很低的溫度下運(yùn)行。另外，雜質(zhì)的集中 引入比需要標(biāo)準(zhǔn)的擴(kuò)散技術(shù)好控制。一束摻雜離子過(guò)早的穿過(guò)了一個(gè)在30-500kev 之間的想得

36、到能力的可能存在的距離修正。這個(gè)離子束目的在于半導(dǎo)體靶以能夠?qū)?現(xiàn)高能力離子穿透半導(dǎo)體表面。這些非?；钴S的離子將會(huì)失去能力和電子通過(guò)撞擊 靶心這樣離子最后將會(huì)靜止。離子的旅行距離，即穿透深度，叫做修正。修正是離 子動(dòng)力的機(jī)能和半導(dǎo)體結(jié)構(gòu)上的特性，例如，晶格空隙、原子團(tuán)。典型的雜質(zhì)修正 的分配是無(wú)固定形狀靶中，大概如高斯分布。 EATON10-80(離子注入機(jī)） VARIAN 350D(離子注入機(jī)） Before an incident ion loses its kinetic energy ,it collides with lattice atoms and the host atoms

37、are dislodged .As a result ,the crystalline region is turned into a disordered or amorphous layer .For this reason ,it is necessary to anneal the semiconductor after implantation to reestablish the crystalline structure .In the annealing step , the semiconductor is placed in a high-temperature oven

38、set at a temperature between 200 and 800.The annealing temperatures are typicality well below those used in solid-state diffusion . A typical doping profile for ion implantation is shown schematically in Fig.3.1(a).The doping in the p type substrate (NA) is assumed constant throught the crystal . Wh

39、erever there are more donors than acceptors (NDNA), the semiconductor is n type in that region . It is p type wherever NAND. 在一個(gè)小的離子丟掉它的動(dòng)力能量之前，它隨著晶格原子撞 擊，大量原子強(qiáng)行離開(kāi)原位。結(jié)果，晶體區(qū)域變成一個(gè)混亂 的或者無(wú)固定形狀的層。因?yàn)檫@些原因，它需要在注入以使 晶體結(jié)構(gòu)復(fù)位之后對(duì)半導(dǎo)體進(jìn)行退火。在退火步驟，半導(dǎo)體 放置在溫度在200-800度的高溫爐中。退火溫度在典型的用于 固態(tài)擴(kuò)散的溫度之下。一個(gè)典型的離子摻雜側(cè)面如圖3.1（a） 圖表所示。在

40、晶格各處p型底層（NA）的摻雜假定是固定不變 的。在這有施主濃度大于受主濃度（NDNA），半導(dǎo)體在此 區(qū)域是n型的。在NAND處是p型的。 The metallurgical junction is at x0,the point at which ND=NA, or the net doping is zero . It is the net doping profile that determines the energy band diagram ,so in Fig.3.1(b),ND=ND- NAand is plotted as a function of position. To

41、 solve for the electrostatic properties of the junction , the NDprofile must be known ,but it is typically not a simple mathematical function . Thus ,an approximation to the ND-NAprofile is often used . The most common of these is the step approximation ,in which the net doping is assumed to be a st

42、ep function ,as shown by the dashed line in Fig.3.1(b). 在x0處的冶金結(jié)，頂部在ND=NA處，或者說(shuō)凈摻雜為0。這是一個(gè)確定能帶圖表 的凈摻雜的側(cè)面，所以如圖3.1（b），ND=ND-NA，作為一個(gè)位置的機(jī)制被繪制 出。為了解決結(jié)的靜電特性，ND側(cè)面必須是已知的，但是它是一個(gè)典型性的不 是簡(jiǎn)單的數(shù)學(xué)上的機(jī)制。因此，一個(gè)ND-NA側(cè)面的近似值經(jīng)常被使用。它們的最 大共同點(diǎn)是等級(jí)近似，在這凈摻雜是假定成一個(gè)等級(jí)機(jī)制，在圖3.1（b）中用撞 擊線表示。 A pn homojunction(often called simply a pn junction )consists of a single crystal of a given semiconductor in which the doping level changes from p type to n type at some boundary .The term homojunction implies that the junction is between two regions of the same material(e.g., silicon),as opposed to the term heterojun