Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

Called complementary mos cmos technology the above

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Called “complementary MOS” (CMOS) technology, the above structure requires more com- plex processing than simple NMOS or PMOS devices. In fact, the first few generations of MOS technology contained only NMOS transistors, and the higher cost of CMOS processes seemed prohibitive. However, many significant advantages of complementary devices eventually made CMOS technology dominant and NMOS technology obsolete. The first Intel microprocessor, the 4004, was realized in NMOS technology.
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 311 (1) Sec. 6.6 Comparison of Bipolar and MOS Devices 311 6.6 Comparison of Bipolar and MOS Devices Having studied the physics and operation of bipolar and MOS transistors, we can now compare their properties. Table 6.2 shows some of the important aspects of each device. Note that the exponential - dependence of bipolar devices accords them a higher transconductance for a given bias current. Bipolar Transistor MOSFET Exponential Characteristic Quadratic Characteristic Active: V CB > 0 V CB Saturation: < 0 Finite Base Current Early Effect Diffusion Current Saturation: V TH V GS V < DS V TH V GS V Triode: DS > Zero Gate Current Channel Length Modulation Drift Current Voltage Dependent Resistor Table 6.2 Comparison of bipolar and MOS transistors. 6.7 Chapter Summary A voltage-dependent current source can form an amplifier along with a load resistor. MOS- FETs are electronic devices that can operate as voltage-dependent current sources. A MOSFET consists of a conductive plate (the “gate”) atop a semiconductor substrate and two junctions (“source” and “drain”) in the substrate. The gate controls the current flow from the source to the drain. The gate draws nearly zero current because an insulating layer separates it from the substrate. As the gate voltage rises, a depletion region is formed in the substrate under the gate area. Beyond a certain gate-source voltage (the “threshold voltage”), mobile carriers are attracted to the oxide-silicon interface and a channel is formed. If the drain-source voltage is small, the device operates a voltage-dependent resistor. As the drain voltage rises, the charge density near the drain falls. If the drain voltage reaches one threshold below the gate voltage, the channel ceases to exist near the drain, leading to “pinch-off.” MOSFETs operate in the “triode” region if the drain voltage is more than one threshold below the gate voltage. In this region, the drain current is a function of and . The current is also proportional to the device aspect ratio, . tem MOSFETs enter the “saturation region” if channel pinch-off occurs, i.e., the drain voltage is less than one threshold below the gate volatge. In this region, the drain current is proportional to .
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