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1、The basic working principle of HEMTAs frequency needs,power capacity,and low noise performance requirements，the high electron mobility transistor(HEMT)fabricated from heterojunctions.The degradation in mobility and achieve high doping by separating themajority carriers from the ionized impurities,Th
2、is separation can be achievedin a heterostructure that has an abrupt discontinuity in conduction and valencebands.Conduction-band edges for N-AlGaAsintrinsic GaAs abrupt heterojunction.Quantum Well StructuresA thin spacer layer of undoped AlGaAs can be placed between the doped AlGaAsand the undoped
3、GaAs to increase the separation of the electrons and ionizeddonor impurities,and decreased the coulomb attraction,further to improve thecarrier mobility.The molecular beam epitaxial process allows thegrowth of very thin layers of specific semiconductormaterials with specific dopings.In particular,am
4、ultilayer modulationdoped heterostructure can beformed.This structure would be equivalent toincreasing the channel electron density,which wouldincrease the current capability of the FET.Quantum Well StructuresMultilayer modulationdoped heterostructure.Transistor PerformanceA typical HEMT structure i
5、s shown in Figure.A“normal”AlGaAs GaAs HEMT.An“inverted”GaAsAlGaAs HEMT(Lower yield).With zero bias,the conduction-band edge in the GaAs is below the Fermi level,implying a large density of the two-dimensional electron gas.With a negative voltage applied to the gate,the conduction-band edge in the G
6、aAs is above the Fermi level,implying that the density of the two-dimensional electron gas is very small and the current in an FET would be essentially zero.Transistor PerformanceEnergy-band diagram of a normal HEMT(a)with zero gate bias and(b)with a negative gate bias.Transistor PerformanceTwo-dime
7、nsional electron gas in a normal structureIf a positive gate voltage is applied,the density of the two-dimensional electron gas will increase.At this point the gate loses control over the electron gas since a parallel conduction path in the AlGaAs has been formed.Energy-band diagram of an enhancemen
8、t mode HEMT(a)with a slight forward gate voltage,and(b)with a larger forward gate voltage that creates a conduction channel in the AlGaAs.Multiple heterojunction layers HEMT：The device current capacity is increased,and power performance is improved.Transistor PerformanceCurrentvoltage characteristics of an enhancement mode HEMT,in which solid curves are numerical calculations and dots are measured points.A multilayer HEMT.