半导体器件基础3课件

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices第二章

双极结型晶体管基本特性Chapter2

BasicProperties

ofBipolarJunctionTransistorsChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesBJT（BipolarJunctionTransistors）是一种最重要的分立固体器件，也是绝大多数半导体有源器件和集成电路的基础器件。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基本结构

BasicStructures晶体管由两个

pn

结：发射结和集电结，将晶体管划分为三个区：发射区、基区及集电区。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesPNP基区发射区集电区E发射极C集电极B基极发射结集电结Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesB基极C集电极E发射极PNP晶体管的图示和电路符号Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesNPN基区发射区集电区E发射极C集电极B基极发射结集电结Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶体管的制造工艺大体上分为获得均匀基区杂质分布和不均匀基区杂质分布两类。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices掺Sb的n型GeIn球In球p型In和Ge合金集电区p型In和Ge合金发射区n型基区Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices后者的典型例子是平面扩散结晶体管，结构示意如图Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices双极型NPN晶体管的纵向结构N+型Si衬底N-型Si外延层P型基区N+型发射区SiO2基极基极发射极集电极Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices双极型

NPN晶体管制造过程1、在N

+型衬底上外延N–层2、在N–外延层中扩散P型杂质3、在P型扩散区中再扩散N+

型杂质4、在磷氧化层上开出基区和发射区接触孔Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices5、蒸发金属6、光刻金属，引出及区、发射区引线7、切片、封装Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesP型衬底N+埋层N+埋层N–外延层N-外延层集电区集电区P+隔离区P+隔离区P+隔离区P型基区P型基区N+发射区N+发射区集成晶体管结构示意图Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesn+

发射区扩散接触孔腐蚀金属淀积及腐蚀钝化和开启键合窗孔Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶体管正常工作时，发射结正偏，集电结反偏，其能带如下图UCECEVUBXCXBφn

φp

φn

φp

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶体管内部载流子传输及电流放大系数TransistorinternalcarrierstransferringandthecurrentamplifyfactorsChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices发射结处于正偏，大量电子从发射区注入到基区，同时也有空穴从基区注入到发射区。其中，一部分电子流与空穴流复合，如图中

2、3。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基区大部分注入电子凭扩散及漂移运动到达集电结边界，被反偏集电结强电场扫入集电区，从集电极流出，即图中1所示。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices实际电路中晶体管有三种连接法：共基(CommonBaseConfiguration)共射(CommonEmitterConfiguration)共集(CommonCollectorConfiguration)Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesUBIBEBICIEnpnUBUCRLRECEBC共基极Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesBUBIBCBEICUCRLREIECEnpn共发射极Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesECBIBUBUCRLREIEICnpnEBC共集电极Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices设晶体管处于线性放大区，三种接法，发射结均为正偏，集电结均为反偏。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices电子线路中最常用的是共射极接法，它具有较高的电流放大倍数和功率放大倍数；共集电极接法用得较少；共基接法物理上意义直观。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesIB典型共基连接如图所示：

npnUBUCRLREIEICBCEChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices直流共基极电流放大系数定义为按照输运过程，α由以下三个因子组成：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中γ为发射效率，表示注入到基区的电子电流与发射极总电流之比，也称注入比：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesJE是发射极电流密度，

JpE、JnE分别是发射极空穴电流密度和电子电流密度。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices就γ而言，对放大作用有贡献的是注入到基区的电子电流JnE，JpE并无贡献，γ要接近于1，JnE应尽量大、JpE尽量小或JpE／JnE

尽可能小。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesβ*

称基区输运系数，表示到达集电结的电子电流与注入到基区的电子电流之比，即Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中

JnC表示到达集电结的电子电流密度。根据β*

的定义，它反映了载流子在基区中的复合损失，为使β*

接近于

1，要求基区中复合损失越小越好。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesα*

称集电区倍增因子，表示集电极总电流与到达集电结的电子电流之比，即Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices一般，α*近似等于

1，仅在集电区杂质浓度很低的情况下，α*

才可能大于

1。

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices根据上述定义，有

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考虑到集电结势垒区雪崩倍增效应将使集电极电流迅速增大，因此电流放大系数还应乘以雪崩倍增因子M：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices以上讨论及定义，尽管只针对npn晶体管，但将电子和空穴对换，也适用于pnp晶体管。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices典型共射连接如图所示：

ICBUBIBCBEUCRLREIECEnpnChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices共发射极电流放大系数定义为集电极电流IC

与基极电流

IB之比：

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices为导出β与α的关系，考察上图，并把

IB＝IE–IC

代入上式，得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices或者Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices均匀基区晶体管的直流特性和电流增益DCCharacteristicsandCurrentGainofTransistorswithUniformBase

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices合金晶体管是典型的均匀基区晶体管。假定发射结及集电结是理想突变结，发射区、基区及集电区杂质均匀分布，浓度分别为Ne、Nb

和Nc。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices室温下杂质已全部电离，各区域平衡时的多数载流子密度等于该区域的杂质浓度：

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶体管两个

pn

结上末加偏压时各区域的多数载流子及少数载流子分布均匀如图所示：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesN+PNEBC-x1x20Wbn0nep0pbp0pcn0pbn0ncxn、p0p0neChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices发射结正偏，电子在正偏电压作用下，从发射区注入到基区，引起基区靠发射结边界处电子积累；空穴从基区注入到发射区，发射区靠发射结边界处有空穴积累。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假定势垒区中无复合，载流子服从玻尔兹曼分布，则发射结势垒两侧的少数载流子密度可分别表示为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中

UE为发射结外加偏压，p0ne为平衡时发射区的空穴密度，n0pb为平衡时基区的电子密度。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices集电结反偏，且

|UC|>>kT/q势垒区两侧的少数载流子密度几乎为零，分别表示为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中

UC为集电结外加偏压，p0nc为集电区中平衡时的空穴密度。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices0Wbn0nep0pbp0pcn0pbn0ncp0ne-x1x2Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考察基区中非平衡少数载流子(电子)的分布：在

x＝0

处，非平衡电子密度为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices在

x＝W

处，非平衡电子密度为

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices实际晶体管的基区宽度

Wb比基区少子(电子)扩散长度

Lnb小得多，因此非平衡电子的分布可近似看作线性，即

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices集电结反偏电压引起非平衡电子在基区的分布为

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基区中非平衡电子的分布应为

Δn’pb

与

Δn’’pb

的叠加，即Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices从稳态连续性方程(扩散方程)出发：利用边界条件解得基区中非平衡电子的分布函数Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考虑到

Wb<<Lnb

及

|UC|>>kT/q，上式可简化为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假定发射区宽度比空穴扩散长度大很多，故发射极接触在数学上近似为-∞，故发射区中空穴分布的边界条件为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices同样近似认为发射区中非平衡空穴为线性分布，且在

x＝-(Lpe+x1)

处，非平衡空穴密度为零，于是发射区非平衡空穴的密度分布可写为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices如从扩散方程出发，利用边界条件，可得到Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices展开式中的指数项，只取一次幂，可得同样结果。如从扩散方程出发，利用边界条件，得到Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices近似认为集电区中非平衡空穴亦是线性分布，且在

x＝x2

+LpC

处非平衡空穴密度为零，利用边界条件Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是，集电区中非平衡空穴的分布为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考虑到

|UC|>>kT/q，集电区中空穴的分布可进一步简化。类似地，从扩散方程出发考虑边界条件，解得集电区中非平衡空穴的分布为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices展开括号外面的指数项，只取一次幂，即得一样的解。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假定晶体管的发射区、基区及集电区中不存在电场也就是说各区域的电流只考虑扩散电流。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是基区中电子的扩散电流密度为即基区中电子电流密度为常数，与x无关。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考虑到UE>>kT/q，且为正值，为形式上与以后公式一致，将上式改写为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices如从下式出发，Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices得基区电子的扩散电流密度为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices分别求

JnB(0)

及

JnB(Wb)，可以发现

x＝0

处的电子电流密度比

x＝Wb

处大，表明基区是存在复合的。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考虑到

Wb/Lnb<<1，式中双曲函数可按泰勒级数展开，只取一次幂，同时考虑

|UC|>>kT/q，上式可写为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices由Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices求得发射区中空穴电流密度分布为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices取

x＝-x1，即发射结势垒区靠发射区一边的边界，上式可写为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices此即发射结发射区一边的空穴电流密度Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices发射极电流密度应等于发射区任一位置处电子电流密度和空穴电流密度之和在x＝-x1处求JnE及JpE最为方便。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices不计发射结势垒区的复合即

JnE(-x1)＝JnB(0)于是得发射极电流密度为

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices或者有

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices由Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices得集电区的空穴电流密度为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices据Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices则集电区的空穴电流密度分布为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices随着x增加，集电区空穴电流密度不断减小，即集电区的空穴电流不断转化为电子电流。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices集电极电流密度也应等于集电区内电子和空穴的电流密度之和。忽略集电结势垒区中的复合，可得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices或者

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices按照上述方程可以得到均匀基区晶体管内各区域的电流密度分布-x1x20WbJEJnEJpEJnBJCJnCJpCChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices

JE和

JC的方程描述了均匀基区晶体管的直流伏安特性。它们是均匀基区晶体管最基本的方程。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices综上所述，伏安特性表达式导出时，作了如下假设(1)

发射结和集电结是理想突变结，杂质在发射区基区和集电区都是均匀分布的；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(2)一维晶体管，发射结和集电结平行，且面积相等；(3)

外电压都降在势垒区，势垒区以外没有电压降；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(4)

发射区和集电区宽度比少数载流子扩散长度大得多，因此两区端侧的少数载流子密度等于其平衡值；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(5)

势垒区宽度比少数载流子扩散长度小得多，势垒区中的复合作用可忽略，也就是电流通过势垒区数值不变；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(6)

注入基区的少数载流子比基区的多数载流子少得多，即不考虑大注入效应。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices均匀基区晶体管的短路电流放大系数ShortCircuitCurrentamplifyFactorofTransistorwithuniformBaseChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices发射效率

EmittingEfficiency

根据

npn晶体管发射效率

γ的定义

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices把已求得的

JpE

、

JnE

代入上式，有Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices根据严格计算的结果，上式成立有两个条件：

集电结短路

(UC=0)；基区宽度

Wb比基区中电子的扩散长度

Lnb

小得多Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices类似地，在后面导出

γ、

β*、α*

等量时，均要求

UC=0。我们称由UC=0

条件导出的电流放大系数为

短路电流放大系数。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices得

平衡时发射区及基区中的少数载流子密度可表示为Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假设发射区和基区中电子和空穴迁移率相等，利用爱因斯坦关系，则有Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices代到γ的表式中，得

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices定义发射区和基区方块电阻Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesγ可写成如下形式：

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices可见要提高γ，必须提高

Ne/Nb，即降低

R□e/R□b。但降低

Nb

，晶体管基极电阻增大，功率增益下降噪声上升，大电流特性变坏。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices通常都采用提高

Ne来增大发射效率γ，使之接近于

1。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基区输运系数

BaseTransferFactor

根据npn晶体管基区输运系数β*

的定义，取

x＝Wb

及

x＝0

处的值

Jnb(Wb)

和

Jnb(0)

代入，考虑集电结短路，即UC=0，可得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices展开双曲函数，并取二次幂及前面的项，得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices根据β*

的物理意义，由于少数载流子通过基区必然发生复合，故β*

值永远小于1。设基区的复合电流密度为

JrB，则Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是

为求

JrB，我们考察基区中非平衡少数载流子——电子的分布。

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices设电子的分布为线性分布如图所示0WbΔnpb(0)xΔnpbChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices单位时间内，单位基区面积上少子复合的数量等于非平衡少子的总数除以少子寿命τnb，即图中阴影的面积除以τnb：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是而Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices上式与下式一起代入Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices可知

β*

表达式中的第二项就是基区中非平衡少子复合引起的。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices为提高β*，必须选用良好的单晶体使少子寿命τ较长，同时也使Dnb较大；且使

Wb小。当前，平面型晶体管的

Wb已达

1μm

以下，β*

往往在0.98以上。

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices实际晶体管从发射区进入基区的少子不可能仅沿一维方向运动，在发射区的边缘，少子运动逐渐散开一部分显然会流至表面而被复合。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices复合对β*的影响NCollectorPBaseNemitterChapter2BasicPropertiesofBipolarJunctionTransistorsFundamen