Electronic Circuits for Mechatronics ELCT609 Lecture 7: MOS-FET Transistor Assistant Professor Office: C3.315 E-mail: eman.azab@guc.edu.eg 1
Introduction Why we call it Transistor? The name came as an abbreviation of the device function Transfer Resistor Input Voltage Scaled with a predefined Trans-conductance gain Output Current MOS-FET: Metal Oxide Semiconductor- Field Effect Transistor 2
MOS-FET Transistor Physical Structure 3
Physical Structure MOS: Metal-Oxide-Semiconductor FourTerminal Device: Drain, Source, Gate and Bulk (Body). Drain and Source are of the same Semiconductor type (N or P), while the Body is made of the opposite type (P or N) Gate is a metal plate with Silicon dioxide beneath. N-MOS (Source and Drain are N-Type) 4 Sedra/Smith Copyright 2010 by Oxford University Press, Inc.
Physical Structure MOS: Metal-Oxide-Semiconductor (Cont.) The Gate current is always ZERO (Capacitance) Current flows only from Drain to Source as the vertical PN junctions (Body-Drain and Body-Source) are ALWAYS reverse biased In order for a current to flow from Drain to source, A Channel must be formed (resistance). Applying PositiveV GS creates this channel Enhancement NMOS is ON V GS >? 5 Sedra/Smith Copyright 2010 by Oxford University Press, Inc.
Physical Structure MOS: Metal-Oxide-Semiconductor (Cont.) Applying positive V GS attracts the electrons in the P-Substrate: which causes inversion in the semi-conductor type beneath the gate An N-channel is created and the electrons can move from Drain to Source The minimum voltage required to create this N-Channel is called: Threshold VoltageV T Enhancement NMOS is ON V GS >V T (Threshold Voltage) I DS 0 6 Sedra/Smith Copyright 2010 by Oxford University Press, Inc.
MOS-FET Transistor I-V Characteristics and Modes of Operation 7
N-MOS I-V Characteristics N-MOS: Drain and Source are N-type Semiconductor Body (Bulk) and Source are connected together Gate current is always ZERO (Behaves as Ideal Capacitor) Enhancement N-MOS Circuit Symbol (Bulk and Source are connected together) 8
N-MOS I-V Characteristics For V GS >V T : N-channel is created between the Drain and Source In presence of a voltage difference between the Drain and Source V DS >0,current will flow Current will flow from the Drain to the Source of the MOS transistor, we call it I DS (I DS >0) 9
N-MOS I-V Characteristics Increasing V GS changes the N-Channel resistivity and thus increases the charge carrier concentration (I DS increases) Increasing V DS changes the charge carrier distribution in the channel and thus I DS increases, we call this Linear Mode (Triode) Further increase of V DS won t change the current as the channel is pinched off, Saturation Mode 10
Enhancement N-MOS I-V Characteristics I DS versus V GS I DS versus V DS Assuming V DS >0 Assuming V GS is Varying 11 Figures from Lecture notes of Dr. Hassan Moustafa and Dr. Soliman Awad
N-MOS Modes of Operation Mode Gate-to- Source Voltage Drain Current Drain-to-Source Voltage Cutoff/OFF V GS < V T I DS 0 ItsValue is irrelevant Triode/Linear V GS > V T I DS = K V GS V T V DS 2 V DS 2 V DS < V GS V T V GD > V T Saturation I DS = K 2 V GS V T 2 V DS > V GS V T V GD < V T K = k n W L k n : Process Trans-conductance Parameter [ma/v 2 ] W: Channel Width L: Channel Length W/L: Aspect Ratio 12
MOS-FET Transistor Types Physical Structure, I-V Characteristics and Modes of Operation 13
Depletion N-MOS Physical Structure Artificial Channel (Transistor is normally ON) 14 Figure from Lecture notes of Dr. Hassan Mostafa (Semiconductor Technology) Depletion N-MOS Circuit Symbol (Bulk and Source are connected together)
Depletion N-MOS I-V Characteristics I DS versus V DS and V GS Assuming V DS >0 Figure from Lecture notes of Dr. Hassan Mostafa (Semiconductor Technology) 15
Enhancement P-MOS Physical Structure P-MOS: Drain and Source are P-type Semiconductor Bulk and Source are connected together Current flows from Source to Drain Enhancement P-MOS Circuit Symbol (Bulk and Source are connected together) 16
Enhancement P-MOS I-V Characteristics I SD versus V SD andv SG Figure from Lecture notes of Prof. Soliman Mahmoud 17
Depletion P-MOS Physical Structure Artificial Channel (Transistor is normally ON) Figure from Dr. Hassan Mostafa Lecture (Semiconductor Technology) Depletion P-MOS Circuit Symbol (Bulk and Source are connected together) 18
Depletion P-MOS I-V Characteristics I SD versus V SD andv SG Figure from Lecture notes of Dr. Hassan Mostafa (Semiconductor Technology) 19
P-MOS Modes of Operation Mode Source-to- Gate Voltage Drain Current Source-to-Drain Voltage Cutoff/OFF V SG < V TP I SD 0 ItsValue is irrelevant Triode/Linear V SG > V TP I SD = K V SG V TP V SD V SD 2 2 V SD < V SG V TP V DG > V TP Saturation I SD = K 2 V SG V TP 2 V SD > V SG V TP V DG < V TP K = k p W L k p : Process Trans-conductance Parameter [ma/v 2 ] W: Channel Width L: Channel Length W/L: Aspect Ratio 20
Calculating MOS DC Operating Point? Rules and Exercises 21
Example Find the DC Operating Point for the MOS? Given: V DD = 10V, V T = 1 V and K =1 ma/v 2 Solution: V DD 10M R=5K M 10M 6 K (I DS =0.5mA, V DS =4.5V, V GS =2 V, Sat. mode) 22
Example Steps of Solution: Write KVL in Input Loop (V GS ) Write KVL in Output Loop (V DS ) Assume MOS mode of operation Use the I-V Characteristics of the mode you have assumed Solve the three equations simultaneously Verify your Assumption!!! 23
Example Find V out for V in = 1V, 4V, and 8V for the shown circuit? Assuming V DD = 10V, V T = 2 V and K=0.5mA/V 2 Solution: V DD R=5K V out Vin M (Ans. 10 V, 5 V, 0.659 V) 24