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1、 UniversitySoutheastNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunctionPresented by:Lei WangSoutheast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction2junctions p-n junctions within a single semiconductor(homojunction)junctions between a met
2、al and a semiconductor ?junctions between two semiconductors with different band gaps.电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction3Homojunction A junction between a metal and a semiconductor电子器件11Heteroju课件Southeast UniversityNanjing
3、ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction4What will be the band diagram of a heterojunction?(1) different materials, and different doped types(2) different materials, but one doped types电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of Ed
4、ucationHeterojunction5(1) different materials, and different doped type电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction6电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction7x0 x2x1
5、Barrier Width W, Charges in the depletion region, Possions equations:电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction8x0 x2x1电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction9x0
6、 x2x1At equilibrium, the potential contact V0,电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction10 x0 x2x1电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction11x0 x2x1电子器件11Heteroju课
7、件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction12电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction13电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of Educ
8、ationHeterojunction14(2) Different material,but one doped type (Example 5-7,P237) These electrons are confined in a narrow potential well in the GaAs conduction band. If we construct a device in which conduction occurs parallel to the interface, the electrons in the potential well form a two-dimensi
9、on electron gas with very high mobility.电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of EducationHeterojunction15Some examples : Heterojunctions applications HEMT HBJ HL(D)电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of Ed
10、ucation6.3 The metal-semiconductor FET (MESFET)16The channel region is generally doped by n-type dopants for practical applications. The reason for this is that the electron mobility is as high as 8500cm2/Vs while the hole mobility is as low as 400cm2/Vs . At the same electric field, therefore, the
11、average velocity of carriers will be large if their mobility is high.Ohmic contactsSchottky contact=108cm电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of Education6.3.2 The High Electron Mobility Transistor (HEMT)17To improve the performances of a transistor, it
12、 shouldmaintain high transconductanceincrease the operation speed(or high mobility)To maintain the high transconductance, the channel conductivity must be as high as possible. This can be completed by increasing the doping in the channel and thus carrier concentration.However, increased doping also
13、causes increased scattering by the ionized impurities, which leads to a degradation of mobility.(see P105, Fig.3-23)电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of Education18It is also called a modulation doped FET (MODFET)(调调制掺杂场掺杂场 效应应晶体管 )or high electron m
14、obility transistor(HEMT) Eg=1.85eV (AlGaAs)Eg=1.43eV(GaAs)Undoped GaAsn-doped Heterojunction!What is needed is a way of creating a high electron concentration in the channel of a MESFET by some means other than doping.(page 262)电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS
15、of Ministry of Education7.9 Heterojunction Bipolar Transistor (HBT)19 For bipolar transistor amplification, we hope emitter injection efficiency can be as high as possible , and therefore, and . From (7-25), high value of can be maintained by using lightly doped material for the base region and heav
16、ily doped material for the emitter. 电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS of Ministry of Education7.9 Heterojunction Bipolar Transistor (HBT)20 But that will result in high base resistance and slight shrink of EgShrink of EgThe emitter injection efficiency decreasing rb, rc, re :RC charging times, the series resistance must be designed to be as small as possible for high frequency transistor.电子器件11Heteroju课件Southeast UniversityNanjing ChinaKey Laboratory of MEMS
