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GVSU EGR 214 - NMOS Transistors

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EGR 315 – Fall 2005 1 Grand Valley State University Prof. Bogdan Adamczyk Field Effect Transistors 5.1 NMOS Transistors In this chapter we study the metal-oxide field-effect transistor (MOSFET). MOSFET is a three-terminal device; the three terminals are called Gate (G), drain (D) and source (S). The circuit symbol for MOSFET is shown in Fig. 5.1 D G S Figure 5.1 Circuit symbol for n-channel enhancement MOSFET The simplest explanation of the MOSFET’s behavior is that the voltage from Gate to Source, VGS, controls the value of a resistance from the Drain to the Source (RDS), as shown in Fig. 5.2 D G RDS = f(VGS) + VGS − S Figure 5.2 MOSFET as a potentiometer There is no current flowing into the Gate terminal. The gate terminal can be modeled as an open circuit; that is only the voltage at this node VGS controls the drain to source resistance RDS.EGR 315 – Fall 2005 2 Grand Valley State University Prof. Bogdan Adamczyk Field Effect Transistors 5.2 Operation in the Cutoff Region Consider the situation shown in Fig. 5.3. Suppose that a positive voltage is applied to the drain relative to the source and that we start with 0=GSv . G D S iD --++ vDS vGS Figure 5.3 NMOS in cutoff region With the path between the drain and the source has a very high resistance (of the order of 100=GSv12 Ω) and virtually no current flows into the drain terminal. This is called the cutoff region of operation. As vGS increases, the device remains in cutoff until vGS reaches a particular value called the threshold voltage Vto. Typically, the threshold voltage is one to a few volts. Thus, in a cutoff mode, we have toGSDVvfori ≤= 0 (5.1)EGR 315 – Fall 2005 3 Grand Valley State University Prof. Bogdan Adamczyk Field Effect Transistors 5.3 Operation in the Triode Region Now consider the situation in which and the voltage vTOGSVv >DS, between drain and source is applied. Applying a Small vDS We now apply a positive vDS between drain and source. We first consider the case where vDS is small (say 0.1 or 0.2V). The voltage vDS causes a current iD to flow from drain to source. For small values of vDS , the drain current is proportional to vDS . Furthermore, for a given (small) value of vDS, the drain current is also proportional to the excess gate voltage, , also known as effective voltage. Fig. 5.4 shows a sketch of itoGSVv −D versus vDS for various values of vGS. iD [mA] vDS [mV] toGSVv≤ increasing vGS decreasing vDSVVvtoGS2+= VVvtoGS1+= VVvtoGS3+= 0.3 0.2 0.1 100 200 Figure 5.4 iD – vDS characteristic of NMOS We observe that the MOSFET is operating as a liner resistance whose value is controlled by vGS (voltage-controlled resistor). The resistance is infinite for toGSVv<, and the resistance value decreases as vGS increases. The current that enters drain terminal iD is equal to the current that leaves the source terminal iS, and the gate current 0=Gi .EGR 315 – Fall 2005 4 Grand Valley State University Prof. Bogdan Adamczyk Field Effect Transistors Operation as vDS is Increased We next consider the situation when vDS is increased. For this purpose let vGS be held constant at a value greater than Vto. As we increase vDS , the iD – vDS curve does not continue as a straight line but bends as shown in Fig. 5.5. increasing vGS decreasing vDSTriode toGSDSVvv −< Saturation 23 GSGSvv >12 GSGSvv >GSGSvv >1Cut-off toGSVv <iD vDStoGSDSVvv−> toGSDSsatVvv−= Figure 5.5 Characteristic curve for NMOS Eventually, vDS is increased to the value toGSDSVvv−=. Increasing vDS beyond this value has little effect on the current iD , the current iD remains constant at the value reached for . The MOSFET is said to have entered the saturation region of operation. toGSDSVvv −= The voltage vDS at which the saturation occurs is denoted vDSsat . toGSDSsatVvv −= (5.2) Obviously, for every value of , there is a corresponding value of . The device operates in the saturation region if . The region of the characteristic obtained for is called triode region, a carryover from the days of vacuum tube devices whose operation FET resembles. toGSVv ≥DSsatvDSsatDSVv ≥DSsatDSVv <EGR 315 – Fall 2005 5 Grand Valley State University Prof. Bogdan Adamczyk Field Effect Transistors In triode region, we have ()[]22DSDStoGSnDvvVvKi −−= (5.3) and in the saturation region we have (2toGSnDVvKi −=) (5.4) Where Kn is a constant determined by the process technology used to fabricate the MOS transistor. It is known as the process transconductance parameter and has dimensions of A/V2. ♦ Referring back to Fig. 5.8, we not that the boundary between the triode and saturation regions is shown as the broken line curve. Since this curve is characterized by , its equation can be found by substituting for toGSDSVvv −=toGSVv− by in either the triode equation (5.3) or the saturation-region equation (5.4). The result is DSv 2DSnDvKi = (5.5) NMOS Inverter MOSFETs may be used as switches, perform digital logic functions; and amplify small time-varying signals. In this section, we will examine the switching properties of an NMOS transistor. The MOSFET can be used as a switch in a wide variety of electronic applications. The transistor switch provides an advantage over mechanical switches in both speed and reliability. The transistor switch considered in this section is also called an inverter.EGR 315 – Fall 2005 6 Grand Valley State University Prof. Bogdan Adamczyk Field Effect Transistors Figure 5.9 shows the n-type MOSFET inverter circuit. VDD − D S vDS+ G RD iD vo vI + vGS − Figure 5.9 NMOS inverter circuit If , the transistor is in cutoff and toIVv <0=Di. There is no voltage drop across RD, and the output voltage is . Also, since DDoVv =0=Di, no power is dissipated in the transistor. If , the transistor is on and initially in the saturation region, since toIVv >toGSDSVvv−> . As the input voltage increases, the drain-to-source voltage decreases, and the transistor eventually enters the triode region. When DDIVv=, the transistor is in the triode region, reaches minimum value, and the drain current reaches maximum value. ov The current and voltage are given by ()[]22ootoInDvvVvKi −−= (5.6) And DDDDoiRVv −= (5.7)EGR 315 – Fall 2005 7 Grand Valley State University Prof. Bogdan Adamczyk


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