Microelectronics Circuit Analysis and Design Donald A. Neamen Chapter 3 The Field Effect Transistor In this chapter, we will: Study and understand the operation and characteristics of the various types of MOSFETs. Understand and become familiar with the dc analysis and design techniques of MOSFET circuits. Examine three applications of MOSFET circuits. Investigate current source biasing of MOSFET circuits, such as those used in integrated circuits. Neamen Microelectronics, 4e Chapter 3-1 Neamen Microelectronics, 4e Chapter 3- Basic Structure of MOS Capacitor MOS Capacitor Under Bias: Electric Field and Charge Parallel plate capacitor Negative gate bias: Holes attracted to gate Positive gate bias: Electrons attracted to gate Neamen Microelectronics, 4e Chapter 3-3 Neamen Microelectronics, 4e Chapter 3-4 1
Schematic of n-channel Enhancement Mode MOSFET Basic Transistor Operation Before electron inversion layer is formed After electron inversion layer is formed Neamen Microelectronics, 4e Chapter 3-5 Neamen Microelectronics, 4e Chapter 3-6 Basic Transistor Operation Current Versus Voltage Characteristics: Enhancement-Mode nmosfet Neamen Microelectronics, 4e Chapter 3-7 Neamen Microelectronics, 4e Chapter 3-8
Family of i D Versus v DS Curves: Enhancement-Mode nmosfet p-channel Enhancement-Mode MOSFET Neamen Microelectronics, 4e Chapter 3-9 Neamen Microelectronics, 4e Chapter 3-10 Symbols for n-channel Enhancement-Mode MOSFET Symbols for p-channel Enhancement-Mode MOSFET Neamen Microelectronics, 4e Chapter 3-11 Neamen Microelectronics, 4e Chapter 3-1 3
n-channel Depletion-Mode MOSFET Family of i D Versus v DS Curves: Depletion-Mode nmosfet Symbols Neamen Microelectronics, 4e Chapter 3-13 Neamen Microelectronics, 4e Chapter 3-14 p-channel Depletion- Mode MOSFET Cross-Section of nmosfet and pmosfet Symbols Neamen Microelectronics, 4e Chapter 3-15 Both transistors are used in the fabrication of CMOS circuitry. Neamen Microelectronics, 4e Chapter 3-16 4
Summary of I-V Relationships Region NMOS PMOS Nonsaturation v DS <v DS (sat) v SD <v SD (sat) i = K [( v V ) v v ] i = K [( v + V ) v v ] D n GS TN DS DS D p SG TP SD SD NMOSFET Conduction Parameters K n Wµ nc = L ox = k ' n W L Saturation v DS >v DS (sat) v SD >v SD (sat) i D = K n[ vgs VTN ] i = K [ v + VTP ] D p SG PMOSFET K p Wµ pc = L ox = k ' p W L Transition Pt. v DS (sat) = v GS - V TN v SD (sat) = v SG + V TP Enhancement Mode Depletion Mode V TN > 0V V TN < 0V V TP < 0V V TP > 0V where: C ox = ε o t ox Neamen Microelectronics, 4e Chapter 3-17 Neamen Microelectronics, 4e Chapter 3-18 Channel Length Modulation: Early Voltage Body Effect Causes an increase in the required threshold voltage This is a small effect for a single device and will be ignored Neamen Microelectronics, 4e Chapter 3-19 Neamen Microelectronics, 4e Chapter 3-0 5
Subthreshold Condition Small current flows for voltages threshold voltage. This is a small effect for a single device and will be ignored. Neamen Microelectronics, 4e Chapter 3-1 Neamen Microelectronics, 4e Chapter 3- Problem-Solving Technique: NMOSFET DC Analysis NMOS Common-Source Circuit 1. Assume the transistor is in saturation. a. V GS > V TN, I D > 0, & V DS V DS (sat). Analyze circuit using saturation I-V relations. 3. Evaluate resulting bias condition of transistor. a. If V GS < V TN, transistor is likely in cutoff b. If V DS < V DS (sat), transistor is likely in nonsaturation region 4. If initial assumption is proven incorrect, make new assumption and repeat Steps and 3. Neamen Microelectronics, 4e Chapter 3-3 Neamen Microelectronics, 4e Chapter 3-4 6
Neamen Microelectronics, 4e Chapter 3-5 Neamen Microelectronics, 4e Chapter 3-6 PMOS Common-Source Circuit Neamen Microelectronics, 4e Chapter 3-7 Neamen Microelectronics, 4e Chapter 3-8 7
Use Non Sat. EQN. Neamen Microelectronics, 4e Chapter 3-9 Neamen Microelectronics, 4e Chapter 3-30 Load Line and Modes of Operation: NMOS Common-Source Circuit Neamen Microelectronics, 4e Chapter 3-31 Neamen Microelectronics, 4e Chapter 3-3 8
Enhancement Load Device -Input NMOS NOR Logic Gate K n = 1mA/V V TN = 1V V 1 (V) V (V) V O (V) 0 0 High 5 0 Low 0 5 Low 5 5 Low Neamen Microelectronics, 4e Chapter 3-33 Neamen Microelectronics, 4e Chapter 3-34 MOS Small-Signal Amplifier Cross Section of n-channel Junction Field Effect Transistor (JFET) Neamen Microelectronics, 4e Chapter 3-35 Neamen Microelectronics, 4e Chapter 3-36 9