固态电子器件重点第六章

固态电子器件重点Chapter 6 Field Effect Transistor1.how many forms are there in the field effect transistors?①JFET(junction FET) ②MESFET(Metal-semiconductor) ③MISFET(metal-insulator-semiconductor FET)2.Describe the differences between BJT and FET.①BJT is a biplor transistor whose action of both electrons and holes in thedevice ,but FET is a unipolar transistor.②FET are controlled by a voltage at the third terminal rather than by acurrent in the BJT .③The BJT uses the injection of minority carriers across a forward-biasedjunction. But the FET control of a junction de;letion width under reverse bias.3.What are the important circuit functions of transistors?amplification and switching4.Describe the operation of the junction FET(JFET).In a junction FET(JFET) the voltage-variable depletion region width of a junction is used to control the effective cross-sectional area of a conducting channel.5.Try to deduce the equation of the pinch-off voltage Vp and. of the draincurrent IdPinch-off occurs at the drain end of the channel when:6.What is the mutual transconductance of FET?7.Describe the structure and operation the metal-semiconductorFET(MESFET) with the GaAs MESFET.a.Substrate is Si-GaAsb.Channel n-GaAS (lightly-doped)c.Gate reverse biasing the schotlky gate, control the effectcross-sectional.d.Source and drain theohmicwrtact between mental and semiconductor.Operation: By reverse biasing the Schottky gate the channel can bedepleted to the semi-insulating substrate, and the resulting I-V character istic are similar to the JFET device.8.Describe the structure and operation of the high electron mobilitytransistor (MEMT).Electrons in the donor doped AlGaAs fall into the GaAs potential well and become trapped. As a result, theundopedGaAs becomes n-type with low lattice scattering (低晶格散射)and high mobility.9.Describe the two approximations of the electric fields for the short channeleffects.When ,at low electric fieldWhen,at high electric field10.Describe the structure and operation of a basic MOSFET.a.When ,the MOS transistor is normally off.b.If we give a positive gate voltage( ), negative charges areinduced in the underlying Si, by the formation of a depletion region ana thin surface region, containing mobile electrons.c.If the barrier is reduced by applying a gate voltage in excess of what isknown as the threshold voltage,VT, there is significant current flow from the source to thedrain ,the current become saturated.11.What is the threshold voltage VT of a MOSFET.The threshold voltage VT is the minimum gate voltage required to induce the channel.12.What are the modes of MOSFET? What is the difference between them?①enhancement-mode n channel ②enhancement-mode p channel ③depletion-mode n channel ④depletion-mode p channel①The p-channel device’s channel is p-type ,but the n-channel is n-typematerial.②depletion-mode devices have a channel already with zero gate voltage ,and in face a negative gate voltage is required to turn the device off.13.Briefly describe the processing steps of a MOSFET.a.An ultra-thin dry thermal silicon dioxide is grown on the p-typesubstrate.b.Covering it with LPCVD of polysilicon(多晶硅)c.The doped polysilicon layer is then patterned to form the gates.d.The gate itself is used as an implant mask for an n+ implant which formsthe source/drain junctions abutted to the gate edges, but is blocked(封闭的) from the channel region.e.The implanted dopants must be annealed.f.The MOFFETs have to be properly(适当的) interconnected according tothe circuit layout ,using metallization(镀金法).14.What are work functions（功函数）? Explain them with a band diagram.The work function be defined in terms of the energy required to move an electron from the Fermi level to outside. But in MOS we use a modified work function .The energy is measured from the Metal Fermi level to the conduction band of the oxide. is the modified work function at the semiconductor oxide interface(界面).15.How do the bands of semiconductor change if negative and positivevoltages are applied between the metal and the semiconductor respectively(各自的) in an ideal MOS capacitor? Explain them according to the distribution of carriers.a.Negative voltage causes hole accumulation in the p-typesemiconductor.b.Positive voltage depletes(使大大减少) holes from the semiconductorsurface.c. A larger positive voltage causes inversion-an “n-type”layer at thesemiconductor surface.If we continue to increase the positive voltage, the bends at the semiconductor surface bend down more strongly. In fact, a large voltage can bend Ei below Ef .since implies a large electron concentration in the conduction band.16.What is the inversion region(反型层)? Please draw a band diagram toexplain it according to the charge distribution.When ,we have depletion, and the bands at the surface are bent down such that Ei(x=0) lies below EF, and inversion is obtained.17.What is the practical criterion for inversion in an ideal MOS structure?The best criterion for strong inversion is that the surface should be as strongly n-type as the substrate is p-type.18.Please plot the space charge density per unit area Qs as a function of thesurface potential and describe the change of charge distribution.19.What is the equation of the threshold voltage VT in an ideal case? Describethe meanings of all quantities.a.Qd the charge per unit area in the depletion region.b.Ci is the insulator capacitance per unit area.c.Is the potential differene between the Fermi level antheintrrnsic levelof the bulk.20.Why do the departures from the ideal case strongly affect VT and otherproperties in real MOS devices?First ,there is a work function difference between the dopedpolysilicongate and substrate, which depends on the substrate doping .second ,there are inevitably(不可避免的) charges at the Si-SiO2 interface and within the oxide which must be taken into account.21.What is the work function difference? Please plot the work functiondifference vs. the impurity concentrations.22.Please write the equation of threshold voltage VT in real MOS devices anddescribe all terms in it.a.Qi is the effective positive charge at the interface.b.Ci is the insulator capacitance per unit area.23.What are the transfer characteristics? What is the difference between thetransfer characteristics and the output characteristics?The output characteristics plot the drain current as a function of the drain bias, with gate bias as a parameter . On the other hand, the transfer characteristics plot the output drain current as a function of the input gate bias, for fixed drain bias.24.What is the substrate bias(衬底偏置) effects of MOSFET, take n-channel forexample.With a reverse bias between the substrate and the source, the depletion region is widened and the threshold gate voltage required to achieve inversion must be increased to accommodate the large Qd. We called this substrate Bias Effects.。