半导体物理学半导体 (38).pdf
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1、Heterojunctions(异质结)Heterojunction materials01Energy band diagrams02Equilibrium electrostatics03Current-voltage characteristics04n The semiconductor material is homogeneous throughout the structure.This type of junction is called a homojunctionn Heterojunction:a junction formed by two different semi
2、conductor materials Heterojunction bipolar transistor:improving the injection efficiency of emitterHeterojunction FET:forming carrier transport channel of high mobility2Heterojunctionsn Heterojunction:the energy band on the surface of the heterojunction is discontinuous because the two materials of
3、the heterojunction have different band gap width.n Abrupt junction:the semiconductor changes abruptly from a narrow-bandgap material to a wide-bandgap material.n Graded heterojunction:for GaAs-AlxGa1-xAs system,x continuously varies.By changing the value of x,We can design the bandgap energy.Heteroj
4、unction materialsn Lattice match:the lattice constants of the two materials must be well matched.The lattice match is important because any lattice mismatch can introduce dislocations resulting in interface states.Lattice mismatchLattice mismatch=a1-a2/a1When lattice mismatch 5%,it is total match.Wh
5、en 5%lattice mismatch25%,it is total mismatch.The alignment of the bandgap energies is important in determining the characteristics of the junction.When the forbidden bandgap of the wide-gap material completely overlaps the bandgap of the narrow-gap material is called straddling.The other possibilit
6、ies are called staggered and broken gap.StraddlingEnergy band diagramsFigure 9.16 Relation between narrow-bandgap and wide-bandgap energies:(a)straddling,(b)staggered,and(c)broken gap.StaggeredBroken up6There are four basic types of heterojunction,where the capital letter indicates the larger-bandga
7、p material:Anisotype heterojunction:the dopant type changes nP or Np anisotype junctionsIsotype heterojunction:the dopant type remains nN and pP isotype heterojunctions Four basic types of heterojunctionThe electron affinity of the wide-bandgap material is less than that of the narrow-bandgap materi
8、al.The difference between the two conduction band and valence band energies are denoted by Ec,Ev,respectively.n-GeP-GaAs nP anisotype heterojunction Figure 9.17 Energy-band diagrams of a narrow-bandgap and a wide-bandgap material before contact.Figure 9.18 shows a typical ideal n-P heterojunction in
9、 a thermal equilibrium state.Forming space charge area near metallurgical junction The space charge width into the n-type region is xn and the space charge width into the P-type region is xpVacuum energyn-GeP-GaAs nP Anisotype n In the space charge region,there are potential differences on one side
10、of the n-type and p-type regions,which are equivalent to the built-in potential differences on both sides of the junction.n The built-in potential barrier is defined as the potential difference between the two ends of the vacuum level.The built-in potential barrier is the sum of the potential differ
11、ences of all space charge areas.Equilibrium ElectrostaticsFigure 9.18 Ideal energy-band diagram of an n-P heterojunction in thermal equilibrium.Assuming that n and p are of the same order of magnitude,the larger potential difference is across the lower-doped region.The ratio of the built-in potentia
12、l barriers can then be determined asThe electric potential of n region and p region can be found by integrating the electric field through the space charge region,respectively.Built-in potential of anisotype heterojunction The total built-in potential barrier:We can solve for depletion width:The tot
13、al depletion width is:?=?2?+?12 Built-in potential of anisotype heterojunctionA change in depletion width with a change in junction voltage yields a junction capacitance.For the n-P junction:A plot of(1/Cj)-2 versus VR yields a straight line.The extrapolation of this plot of(1/Cj)2=0 is used to find
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