analog bicmos design practices and pitfalls phần 3

Thành phần phù hợp và bảo vệ từ phóng tĩnh điện được thiết kế thực hành quan trọng. Chính xác phù hợp với thành phần làm giảm chi phí và cải thiện chức năng mạch. Bảo vệ khỏi phóng tĩnh điện là một đề phòng cần thiết cho độ tin cậy.
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analog bicmos design practices and pitfalls phần 3Figure 1.21 Figure for problem 9. • What is the position of the Fermi level relative to the intrinsic level on the p-side of the junction? 7. For the pn junction in problem 6, the junction area is 10 microns by 10 microns. What is the saturation current Is . Use mobility vs doping curves (Figure 1.2). 8. Consider a pn junction with the P-side doped with NA = 1020 cm−3 . Approximately, what is the required doping on the N-side to obtain a breakdown of 20 V? Use the one-sided step junction approxima- tion. 9. A 10 K resistor is in series with an NMOS transistor as shown in Figure 1.21: [W/L]µn Cox = 10−5 . The threshold voltage is one volt. What is the output voltage, Vo?References [1] S. M. Sze and J. C. Irvin, Resistivity, Mobility and Impurity Levels in GaAs, Ge, and Si at 300◦ K, Solid-State Electronics, Vol 11, pp. 599-602, 1968. [2] S. M. Sze, Physics of Semiconductor Devices, Wiley-Interscience, New York, 1969. [3] Edward S. Yang, Microelectronic Devices, McGraw-Hill, New York, 1988. [4] P.R. Gray and R.G. Meyer, Analysis and Design of Analog Inte- grated Circuits, 2nd edition, Wiley, New York, c. 1984, pp. 1-5. [5] R.S. Muller and T.I. Kamins, Device Electronics for Integrated Cir- cuits, 2nd edition, Wiley, New York, c. 1986, pp. 15-27, 173-188, 235-244.[6] K. Lee, M. Shur, T.A. Fjeldy and T. Ytterdal, Semiconductor De- vice Modeling for VLSI, Prentice Hall, Englewood Cliffs, NJ, c. 1993, p. 63.[7] Shelby Raymond, private communication, January 1999.chapter 2Device Models2.1 IntroductionModels are mathematical descriptions that predict performance. Theycan be physical or empirical. Physical models are based on device physicsand can be related to physical properties. Empirical models fit measure-ments to mathematical descriptions that do not necessarily correspondto device physics. Physical models are easier to adapt when parameterssuch as doping levels or device dimensions change. Modeling is a tradeoff between accuracy and utility. Exact modelstend to be more complex than approximate ones. The model to use isthe simplist one that provides the required accuracy. Models for handcalculation, where computational power is limited, should be simple.Even with high speed computers, complex models can make the simula-tions of large systems prohibitive.2.2 Bipolar Transistors2.2.1 Early EffectIncreasing the voltage across the transistor VCE results in an increasein transistor current IC . The physical cause, is a decrease in the widthof the base. As VCE is increased, the reverse voltage on the collector-base pn junction increases. The collector-base depletion region extendsfurther into the base, effectively reducing the base width. Since collectorcurrent varies inversely with base width, collector current increases. Theslope of IC vs. VCE in the normal operation range is modeled by theEarly voltage as shown in Figure 2.1.2.2.2 High Level InjectionThe simple model we used in Section 1.5.2 for β breaks down at highand low current levels. At high current levels, high level injection effectsFigure 2.1 The dependence of IC on VCE is described by the Early voltageVA .cause collector current to be less than predicted by Equation 1.52. As VBE is increased, large numbers of electrons are injected into thebase from the emitter. High level injection is defined to be when thedensity of electrons in the base approaches the density of acceptor atomsin the base. Extra positive voltage has to be applied to the base in orderto maintain the negative charge density in the base which is produced bythe high level injection of electrons from the emitter. VBE is distributedbetween the junction and across the base region containing the high levelof injected electrons. Only a portion of the voltage applied to the baseand emitter terminals, VBE , appears across the base emitter junction.Therefore, VBE is not as effective in increasing injection across the base-emitter junction. The result is IC proportional to exp(VBE /2VT ). Thatis, collector current does not increase as fast with increases in VBE as itdoes in low level injection. The reduction in collector current results ina reduction in β . At low current levels, the component of base currentdue to spontaneous generation of electron hole pairs in the base emitterdepletion region becomes significant. This component of base currentvaries as eVBE /2VT . It represents base current that does not contributeto collector current. This results in a decrease in β at low current levelsas shown in Figure 2.2.2.2.3 Gummel-Poon ModelThe Gummel-Poon model, like the Ebers-Moll, is not limited to positivebase-emitter and positive collector-emitter voltages, but is valid for bothpositive and negative applied voltag ...