半導(dǎo)體器件基礎(chǔ)3課件

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices第二章

雙極結(jié)型晶體管基本特性Chapter2

BasicProperties

ofBipolarJunctionTransistorsChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesBJT（BipolarJunctionTransistors）是一種最重要的分立固體器件，也是絕大多數(shù)半導(dǎo)體有源器件和集成電路的基礎(chǔ)器件。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基本結(jié)構(gòu)

BasicStructures晶體管由兩個

pn

結(jié)：發(fā)射結(jié)和集電結(jié)，將晶體管劃分為三個區(qū)：發(fā)射區(qū)、基區(qū)及集電區(qū)。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesPNP基區(qū)發(fā)射區(qū)集電區(qū)E發(fā)射極C集電極B基極發(fā)射結(jié)集電結(jié)Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesB基極C集電極E發(fā)射極PNP晶體管的圖示和電路符號Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesNPN基區(qū)發(fā)射區(qū)集電區(qū)E發(fā)射極C集電極B基極發(fā)射結(jié)集電結(jié)Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶體管的制造工藝大體上分為獲得均勻基區(qū)雜質(zhì)分布和不均勻基區(qū)雜質(zhì)分布兩類。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices摻Sb的n型GeIn球In球p型In和Ge合金集電區(qū)p型In和Ge合金發(fā)射區(qū)n型基區(qū)Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices后者的典型例子是平面擴(kuò)散結(jié)晶體管，結(jié)構(gòu)示意如圖Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices雙極型NPN晶體管的縱向結(jié)構(gòu)N+型Si襯底N-型Si外延層P型基區(qū)N+型發(fā)射區(qū)SiO2基極基極發(fā)射極集電極Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices雙極型

NPN晶體管制造過程1、在N

+型襯底上外延N–層2、在N–外延層中擴(kuò)散P型雜質(zhì)3、在P型擴(kuò)散區(qū)中再擴(kuò)散N+

型雜質(zhì)4、在磷氧化層上開出基區(qū)和發(fā)射區(qū)接觸孔Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices5、蒸發(fā)金屬6、光刻金屬，引出及區(qū)、發(fā)射區(qū)引線7、切片、封裝Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesP型襯底N+埋層N+埋層N–外延層N-外延層集電區(qū)集電區(qū)P+隔離區(qū)P+隔離區(qū)P+隔離區(qū)P型基區(qū)P型基區(qū)N+發(fā)射區(qū)N+發(fā)射區(qū)集成晶體管結(jié)構(gòu)示意圖Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesn+

發(fā)射區(qū)擴(kuò)散接觸孔腐蝕金屬淀積及腐蝕鈍化和開啟鍵合窗孔Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶體管正常工作時，發(fā)射結(jié)正偏，集電結(jié)反偏，其能帶如下圖UCECEVUBXCXBφn

φp

φn

φp

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶體管內(nèi)部載流子傳輸及電流放大系數(shù)TransistorinternalcarrierstransferringandthecurrentamplifyfactorsChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices發(fā)射結(jié)處于正偏，大量電子從發(fā)射區(qū)注入到基區(qū)，同時也有空穴從基區(qū)注入到發(fā)射區(qū)。其中，一部分電子流與空穴流復(fù)合，如圖中

2、3。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基區(qū)大部分注入電子憑擴(kuò)散及漂移運(yùn)動到達(dá)集電結(jié)邊界，被反偏集電結(jié)強(qiáng)電場掃入集電區(qū)，從集電極流出，即圖中1所示。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices實(shí)際電路中晶體管有三種連接法：共基(CommonBaseConfiguration)共射(CommonEmitterConfiguration)共集(CommonCollectorConfiguration)Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesUBIBEBICIEnpnUBUCRLRECEBC共基極Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesBUBIBCBEICUCRLREIECEnpn共發(fā)射極Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesECBIBUBUCRLREIEICnpnEBC共集電極Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices設(shè)晶體管處于線性放大區(qū)，三種接法，發(fā)射結(jié)均為正偏，集電結(jié)均為反偏。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices電子線路中最常用的是共射極接法，它具有較高的電流放大倍數(shù)和功率放大倍數(shù)；共集電極接法用得較少；共基接法物理上意義直觀。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesIB典型共基連接如圖所示：

npnUBUCRLREIEICBCEChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices直流共基極電流放大系數(shù)定義為按照輸運(yùn)過程，α由以下三個因子組成：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中γ為發(fā)射效率，表示注入到基區(qū)的電子電流與發(fā)射極總電流之比，也稱注入比：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesJE是發(fā)射極電流密度，

JpE、JnE分別是發(fā)射極空穴電流密度和電子電流密度。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices就γ而言，對放大作用有貢獻(xiàn)的是注入到基區(qū)的電子電流JnE，JpE并無貢獻(xiàn)，γ要接近于1，JnE應(yīng)盡量大、JpE盡量小或JpE／JnE

盡可能小。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesβ*

稱基區(qū)輸運(yùn)系數(shù)，表示到達(dá)集電結(jié)的電子電流與注入到基區(qū)的電子電流之比，即Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中

JnC表示到達(dá)集電結(jié)的電子電流密度。根據(jù)β*

的定義，它反映了載流子在基區(qū)中的復(fù)合損失，為使β*

接近于

1，要求基區(qū)中復(fù)合損失越小越好。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesα*

稱集電區(qū)倍增因子，表示集電極總電流與到達(dá)集電結(jié)的電子電流之比，即Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices一般，α*近似等于

1，僅在集電區(qū)雜質(zhì)濃度很低的情況下，α*

才可能大于

1。

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices根據(jù)上述定義，有

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考慮到集電結(jié)勢壘區(qū)雪崩倍增效應(yīng)將使集電極電流迅速增大，因此電流放大系數(shù)還應(yīng)乘以雪崩倍增因子M：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices以上討論及定義，盡管只針對npn晶體管，但將電子和空穴對換，也適用于pnp晶體管。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices典型共射連接如圖所示：

ICBUBIBCBEUCRLREIECEnpnChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices共發(fā)射極電流放大系數(shù)定義為集電極電流IC

與基極電流

IB之比：

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices為導(dǎo)出β與α的關(guān)系，考察上圖，并把

IB＝IE–IC

代入上式，得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices或者Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices均勻基區(qū)晶體管的直流特性和電流增益DCCharacteristicsandCurrentGainofTransistorswithUniformBase

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices合金晶體管是典型的均勻基區(qū)晶體管。假定發(fā)射結(jié)及集電結(jié)是理想突變結(jié)，發(fā)射區(qū)、基區(qū)及集電區(qū)雜質(zhì)均勻分布，濃度分別為Ne、Nb

和Nc。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices室溫下雜質(zhì)已全部電離，各區(qū)域平衡時的多數(shù)載流子密度等于該區(qū)域的雜質(zhì)濃度：

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices晶體管兩個

pn

結(jié)上末加偏壓時各區(qū)域的多數(shù)載流子及少數(shù)載流子分布均勻如圖所示：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesN+PNEBC-x1x20Wbn0nep0pbp0pcn0pbn0ncxn、p0p0neChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices發(fā)射結(jié)正偏，電子在正偏電壓作用下，從發(fā)射區(qū)注入到基區(qū)，引起基區(qū)靠發(fā)射結(jié)邊界處電子積累；空穴從基區(qū)注入到發(fā)射區(qū)，發(fā)射區(qū)靠發(fā)射結(jié)邊界處有空穴積累。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假定勢壘區(qū)中無復(fù)合，載流子服從玻爾茲曼分布，則發(fā)射結(jié)勢壘兩側(cè)的少數(shù)載流子密度可分別表示為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中

UE為發(fā)射結(jié)外加偏壓，p0ne為平衡時發(fā)射區(qū)的空穴密度，n0pb為平衡時基區(qū)的電子密度。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices集電結(jié)反偏，且

|UC|>>kT/q勢壘區(qū)兩側(cè)的少數(shù)載流子密度幾乎為零，分別表示為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices式中

UC為集電結(jié)外加偏壓，p0nc為集電區(qū)中平衡時的空穴密度。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices0Wbn0nep0pbp0pcn0pbn0ncp0ne-x1x2Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考察基區(qū)中非平衡少數(shù)載流子(電子)的分布：在

x＝0

處，非平衡電子密度為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices在

x＝W

處，非平衡電子密度為

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices實(shí)際晶體管的基區(qū)寬度

Wb比基區(qū)少子(電子)擴(kuò)散長度

Lnb小得多，因此非平衡電子的分布可近似看作線性，即

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices集電結(jié)反偏電壓引起非平衡電子在基區(qū)的分布為

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基區(qū)中非平衡電子的分布應(yīng)為

Δn’pb

與

Δn’’pb

的疊加，即Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices從穩(wěn)態(tài)連續(xù)性方程(擴(kuò)散方程)出發(fā)：利用邊界條件解得基區(qū)中非平衡電子的分布函數(shù)Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考慮到

Wb<<Lnb

及

|UC|>>kT/q，上式可簡化為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假定發(fā)射區(qū)寬度比空穴擴(kuò)散長度大很多，故發(fā)射極接觸在數(shù)學(xué)上近似為-∞，故發(fā)射區(qū)中空穴分布的邊界條件為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices同樣近似認(rèn)為發(fā)射區(qū)中非平衡空穴為線性分布，且在

x＝-(Lpe+x1)

處，非平衡空穴密度為零，于是發(fā)射區(qū)非平衡空穴的密度分布可寫為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices如從擴(kuò)散方程出發(fā)，利用邊界條件，可得到Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices展開式中的指數(shù)項，只取一次冪，可得同樣結(jié)果。如從擴(kuò)散方程出發(fā)，利用邊界條件，得到Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices近似認(rèn)為集電區(qū)中非平衡空穴亦是線性分布，且在

x＝x2

+LpC

處非平衡空穴密度為零，利用邊界條件Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是，集電區(qū)中非平衡空穴的分布為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考慮到

|UC|>>kT/q，集電區(qū)中空穴的分布可進(jìn)一步簡化。類似地，從擴(kuò)散方程出發(fā)考慮邊界條件，解得集電區(qū)中非平衡空穴的分布為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices展開括號外面的指數(shù)項，只取一次冪，即得一樣的解。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假定晶體管的發(fā)射區(qū)、基區(qū)及集電區(qū)中不存在電場也就是說各區(qū)域的電流只考慮擴(kuò)散電流。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是基區(qū)中電子的擴(kuò)散電流密度為即基區(qū)中電子電流密度為常數(shù)，與x無關(guān)。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考慮到UE>>kT/q，且為正值，為形式上與以后公式一致，將上式改寫為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices如從下式出發(fā)，Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices得基區(qū)電子的擴(kuò)散電流密度為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices分別求

JnB(0)

及

JnB(Wb)，可以發(fā)現(xiàn)

x＝0

處的電子電流密度比

x＝Wb

處大，表明基區(qū)是存在復(fù)合的。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices考慮到

Wb/Lnb<<1，式中雙曲函數(shù)可按泰勒級數(shù)展開，只取一次冪，同時考慮

|UC|>>kT/q，上式可寫為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices由Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices求得發(fā)射區(qū)中空穴電流密度分布為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices取

x＝-x1，即發(fā)射結(jié)勢壘區(qū)靠發(fā)射區(qū)一邊的邊界，上式可寫為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices此即發(fā)射結(jié)發(fā)射區(qū)一邊的空穴電流密度Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices發(fā)射極電流密度應(yīng)等于發(fā)射區(qū)任一位置處電子電流密度和空穴電流密度之和在x＝-x1處求JnE及JpE最為方便。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices不計發(fā)射結(jié)勢壘區(qū)的復(fù)合即

JnE(-x1)＝JnB(0)于是得發(fā)射極電流密度為

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices或者有

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices由Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices得集電區(qū)的空穴電流密度為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices據(jù)Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices則集電區(qū)的空穴電流密度分布為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices隨著x增加，集電區(qū)空穴電流密度不斷減小，即集電區(qū)的空穴電流不斷轉(zhuǎn)化為電子電流。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices集電極電流密度也應(yīng)等于集電區(qū)內(nèi)電子和空穴的電流密度之和。忽略集電結(jié)勢壘區(qū)中的復(fù)合，可得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices或者

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices按照上述方程可以得到均勻基區(qū)晶體管內(nèi)各區(qū)域的電流密度分布-x1x20WbJEJnEJpEJnBJCJnCJpCChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices

JE和

JC的方程描述了均勻基區(qū)晶體管的直流伏安特性。它們是均勻基區(qū)晶體管最基本的方程。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices綜上所述，伏安特性表達(dá)式導(dǎo)出時，作了如下假設(shè)(1)

發(fā)射結(jié)和集電結(jié)是理想突變結(jié)，雜質(zhì)在發(fā)射區(qū)基區(qū)和集電區(qū)都是均勻分布的；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(2)一維晶體管，發(fā)射結(jié)和集電結(jié)平行，且面積相等；(3)

外電壓都降在勢壘區(qū)，勢壘區(qū)以外沒有電壓降；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(4)

發(fā)射區(qū)和集電區(qū)寬度比少數(shù)載流子擴(kuò)散長度大得多，因此兩區(qū)端側(cè)的少數(shù)載流子密度等于其平衡值；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(5)

勢壘區(qū)寬度比少數(shù)載流子擴(kuò)散長度小得多，勢壘區(qū)中的復(fù)合作用可忽略，也就是電流通過勢壘區(qū)數(shù)值不變；Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices(6)

注入基區(qū)的少數(shù)載流子比基區(qū)的多數(shù)載流子少得多，即不考慮大注入效應(yīng)。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices均勻基區(qū)晶體管的短路電流放大系數(shù)ShortCircuitCurrentamplifyFactorofTransistorwithuniformBaseChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices發(fā)射效率

EmittingEfficiency

根據(jù)

npn晶體管發(fā)射效率

γ的定義

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices把已求得的

JpE

、

JnE

代入上式，有Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices根據(jù)嚴(yán)格計算的結(jié)果，上式成立有兩個條件：

集電結(jié)短路

(UC=0)；基區(qū)寬度

Wb比基區(qū)中電子的擴(kuò)散長度

Lnb

小得多Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices類似地，在后面導(dǎo)出

γ、

β*、α*

等量時，均要求

UC=0。我們稱由UC=0

條件導(dǎo)出的電流放大系數(shù)為

短路電流放大系數(shù)。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices得

平衡時發(fā)射區(qū)及基區(qū)中的少數(shù)載流子密度可表示為Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices假設(shè)發(fā)射區(qū)和基區(qū)中電子和空穴遷移率相等，利用愛因斯坦關(guān)系，則有Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices代到γ的表式中，得

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices定義發(fā)射區(qū)和基區(qū)方塊電阻Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesγ可寫成如下形式：

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices可見要提高γ，必須提高

Ne/Nb，即降低

R□e/R□b。但降低

Nb

，晶體管基極電阻增大，功率增益下降噪聲上升，大電流特性變壞。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices通常都采用提高

Ne來增大發(fā)射效率γ，使之接近于

1。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices基區(qū)輸運(yùn)系數(shù)

BaseTransferFactor

根據(jù)npn晶體管基區(qū)輸運(yùn)系數(shù)β*

的定義，取

x＝Wb

及

x＝0

處的值

Jnb(Wb)

和

Jnb(0)

代入，考慮集電結(jié)短路，即UC=0，可得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices展開雙曲函數(shù)，并取二次冪及前面的項，得Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices根據(jù)β*

的物理意義，由于少數(shù)載流子通過基區(qū)必然發(fā)生復(fù)合，故β*

值永遠(yuǎn)小于1。設(shè)基區(qū)的復(fù)合電流密度為

JrB，則Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是

為求

JrB，我們考察基區(qū)中非平衡少數(shù)載流子——電子的分布。

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices設(shè)電子的分布為線性分布如圖所示0WbΔnpb(0)xΔnpbChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices單位時間內(nèi)，單位基區(qū)面積上少子復(fù)合的數(shù)量等于非平衡少子的總數(shù)除以少子壽命τnb，即圖中陰影的面積除以τnb：Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices于是而Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices上式與下式一起代入Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevicesChapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices可知

β*

表達(dá)式中的第二項就是基區(qū)中非平衡少子復(fù)合引起的。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices為提高β*，必須選用良好的單晶體使少子壽命τ較長，同時也使Dnb較大；且使

Wb小。當(dāng)前，平面型晶體管的

Wb已達(dá)

1μm

以下，β*

往往在0.98以上。

Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices實(shí)際晶體管從發(fā)射區(qū)進(jìn)入基區(qū)的少子不可能僅沿一維方向運(yùn)動，在發(fā)射區(qū)的邊緣，少子運(yùn)動逐漸散開一部分顯然會流至表面而被復(fù)合。Chapter2BasicPropertiesofBipolarJunctionTransistorsFundamentalofSemiconductorDevices復(fù)合對β*的影響NCollectorPBaseNemitterChapter2BasicPropertiesofBipolarJunctionTransistorsFundamen