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1、JFET and MESFET,Buried channel device I-V characteristics Velocity-Electric field relationship Microwave performance Device structures,JFET and MESFET,Buried channel device Recall device structure for MOSFET Difference between MOSFET and JFET -Bias can not approach the flatband situation for JFET or
2、 MESFET -Double channel formed when bias high enough for MOSFET (buried channel),JFET and MESFET,Advantages -Hot carrier injection -interface charge trapping Disadvantages -Gate voltage limitation,JFET and MESFET,Comparison between JFET and MESFET MESFET: -Simpler process: low temperature formation
3、of MS; -Diffusion and implantation for PN junction; -Low gate resistance and IR drop along the channel width are factors for high frequency applications -MS has a better process control in fabrication of short channel length; -MS is a heat sink for power device applications -Easy defining gate for s
4、hort channel,JFET and MESFET,JFET: -Robust junction, higher breakdown and power capability -high build-in potential for possible enhancement mode device application -Higher build-in potential for preventing gate current -Difficulty on formation of good Schottky barrier on some p-type material (MESFE
5、T) -More possible gate configurations: heterojunction or buffered-layer gate,JFET and MESFET,Device structure - Drain-channel-Source - Gate junction P-N junction for JFET Schottky junction for MESFET - Working mechanism Voltage-controlled resistor,JFET and MESFET,Normally-on device Vg=Vd=0 Vg=0; Vd
6、positive ?VgVt; Vd positive VgVt; Vd positive,JFET and MESFET,I-V characteristics -Linear region Id proportional to Vd -Nonlinear region -Saturation region Current saturated,JFET and MESFET,Deriving I-V characteristics Assumptions: - Uniform doping channel GCA Abrupt depletion layer Negligible gate
7、current,JFET and MESFET,One dimensional Poissons Equation,JFET and MESFET,One sided abrupt junction depletion approximation,where bi is the build-in potential,JFET and MESFET,For JFET, bi is given by (p+ -n junction),For MESFET, bi is given by shottky junction,whereBn is the Schottky barrier,JFET an
8、d MESFET,and,The depletion widths at the source and drain ends:,JFET and MESFET,When Vg = bi , WDS =0. -Flat-band condition This is not achievable due to the excessive forward gate current in practice,JFET and MESFET,Maximum WDd =a The band bending for the situation is called pinch-off potential, de
9、fined as,JFET and MESFET,The channel charge density,The channel current,JFET and MESFET,The total channel current,JFET and MESFET,Assuming mobility is a constant, v= uE and Ex =di/dx. Integrating,We have,JFET and MESFET,In linear region VDVG and VD bi,We can observe the ohmic characteristics. If do
10、a Talors expansion around VG =Vt,JFET and MESFET,Vt is the threshold voltage at which the device is turned on or turned off.,With the drain voltage increasing, VD approaches VDsat, at which the channel is at the pinch-off situation,JFET and MESFET,Pinch-off point (Recall MOSFET) WDd =a Current decre
11、asing?,JFET and MESFET,Input VDsat :,We have,JFET and MESFET,The maximum IDsat is Gip/3. However it is not approachable The transconductance is,JFET and MESFET,When VD is larger than VDsat , the pinch-off point shifts to the source. The potential of the pinch-off point is maintained at VDsat , indep
12、endent on VD . the current saturation is not only depending on VD due to the reduction of effective channel length. The effective channel length is defined as the distance between source and pinch-off point.,JFET and MESFET,If do a Talors expansion around VG =Vt,JFET and MESFET,Observations: Only at
13、 VG =Vt , we have similar I D expressions as MOSFET Id Origin comes from the gate difference: -MESFET and JFET: Gate capacitance is related to the gate bias -MOSFET: Gate capacitance is constant; inversion charge density is proportional to gate bias,JFET and MESFET,Velocity-field relationship -Long
14、channel device Mobility is a constant: linear relationship -Short channel device Strong electric field in device,JFET and MESFET,Velocity-field relationship (long channel and short channel) Empirical formula Experimental curve The critical field Ec,JFET and MESFET,Output I-V characteristics Velocity
15、 saturation,JFET and MESFET,Improvement of velocity Material property :GaAs Transferred-electron effect Drift velocity at moderate field Vs -ballistic effect Ultra-short channel device,JFET and MESFET,JFET and MESFET,Breakdown - increasing Vd Vdsat -Avalanche breakdown -Occur at gate edge (high fiel
16、d) -Drain current increases sharply -Mechanism (impact ionization),JFET and MESFET,Si JFET GaAs MESFET,Breakdown characteristics for Si JFET and GaAs MESFET Gate and Drain similar to a junction Gate-Drain breakdown is like a junction BD,JFET and MESFET,Tunneling current of GaAs MESFET -related to channel current and junction temperature -earlier impact ionization GaAs material property Higher current than that for Si Temperature ef
