COMSATS Institute of Information Technology Virtual campus Islamabad
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 - EEE 231 Fall Semester – 2012
Current -Voltage Characteristics I-V Characteristics Lecture No. 29 Ø Contents: Ø Qualitative theory of operation Ø Quantitative ID-versus-VDS characteristics Ø Large-signal equivalent circuits. 2
Lecture No. 29 Current-Voltage Characteristics Reference: Chapter-4. 2 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar. 3
Circuit Symbol (NMOS) Enhancement-Type: D ID = I S G B IG = 0 IS S G-Gate D-Drain S-Source B-Substrate or Body 4
Circuit Symbol (NMOS) Enhancement-Type Ø The spacing between the two vertical lines that represent the gate and the channel, indicates the fact the gate electrode is insulated from the body of the device. Ø The drain is always positive relative to the source in an nchannel FET. 5
Qualitative Theory of Operation Modes of MOSFET Operation 6
Modes of MOSFET Operation MOSFET can be categorized into three modes of operation, depending on VGS: v. VGS < Vt: The cut-off Mode v. VGS > Vt and VDS < (VGS − Vt): The Linear Region v. VGS > Vt and VDS > VGS − Vt: The Saturation Mode Nasim Zafar. 7
MOSFET-Structure Enhancement Type-NMOSFET Body (bulk or B substrate) Source S y Gate: metal or heavily doped poly-Si G Drain IG=0 D ID=IS IS metal n+ oxide p n+ x L W 8
VGS<0 n+p n+ Structure ID ~ 0 body B Source S Gate G - + Drain D VD=Vs n++ n+ oxide p L n+ W 9
VGS < Vt The Cut-off Mode: n+-depletion-n+ structure ID ~ 0 body B source S gate G - + drain D VD=Vs +++ n++ oxide n+ p L n+ W 10
VGS > VT The Linear Mode of Operation: n+-n-n+ structure inversion body B source S VGS > VT n+ gate G - + +++ +++ n++ oxide ----p L drain D VD=Vs n+ W 11
Quantitative ID-versus-VDS Relationships 12
Quantitative ID-VDS Relationships S G (VG) D (VDS) QN = inversion layer charge For VG < VT, Inversion layer charge is zero (Slide 11). For VG > VT, Qn(y) = QG = Cox (VG V VT) (Slide 12) 13
Quantitative ID-VDS Relationships Ø In the MOSFET, the gate and the channel region form a parallel-plate capacitor for which the oxide layer serves as a dielectric. Ø If the capacitance per unit gate area is denoted Cox and the thickness of the oxide layer is tox, then Ø Cox=εox/ tox (4. 2) Where εox is the permittivity of the silicon oxide Ø ε= 3. 9 ε 0= 3. 9× 8. 854× 10 -12= 3. 45× 10 -11 F/m Nasim Zafar. 14
Quantitative ID-VDS Relationships v Current and Current Density: Ø In general, Jn= q n n E , for the drift current Ø Here, current ID is the same everywhere, but Jn (current density) can vary from position to position. since Let “ ” be the potential along the channel 15
Quantitative ID-VDS Relationships v. Current and Current Density: To find current, we have to multiply the above with area, but Jny, n, etc. are functions of x and z. Hence, Integrating the above equation, and noting that ID is constant, we get Since we know expression for Qn(y) in terms of , we can integrate this to get ID 16
Quantitative ID-VDS Relationships v. Current and Current Density: ; ID will increase as VDS is increased, but when VG – VDS = VT, pinchoff of channel occurs, and current saturates when VDS is increased further. This value of VDS is called VDS, sat. i. e. , VDS, sat = VG – VT and the current when VDS= VDS, sat is called IDS, sat. ; Here, Cox is the oxide capacitance per unit area, Cox = ox / xox 17
Current-Voltage Characteristics 18
Current-Voltage Characteristics IDS B C D A VDS
The i. D-VDS Characteristics v Figure 4. 11(a) shows an n-channel enhancement-type MOSFET with voltages VGS and VDS applied and with the normal directions of current flow indicated. Fig. 4. 11 (a): An n-channel enhancement type MOSFET 20
The i. D-VDS Characteristics v Figure 4. 11 (b) shows a typical set of i. D-VDS Characteristics. The i. D–v. DS Characteristics for a MOSFET Device with k’n(W/L) = 1. 0 m. A/V 2. 21
The i. D-VDS Characteristics v Current-Voltage characteristics of Fig. 4. 11 (b) show that there are three distinct regions of operation: Ø The Cutoff Region, Ø The Triode Region, and Ø The Saturation Region. 22
The i. D-VDS Characteristics The i. D–v. DS Characteristics for a MOSFET Device.
The i. D-VDS Characteristics v Saturation Region: Ø The saturation region is used if the MOSFET is to operate as an amplifier. v Cutoff and Triode Regions: Ø For operation as a switch, the cut-off and triode regions are utilized. 24
Operation in the Triode Region v To operate the MOSFET in the triode region we must first induce a channel: v VGS≧Vt (Induced channel) v VDS<VGS – Vt (Continuous Channel) v The n-channel enhancement-type MOSFET operates in the triode region when VGS is greater than Vt and the drain voltage is lower than the gate voltage by at least Vt volts. 25
The i. D-VDS Characteristics v The Triode Mode: In the triode region, the i. D-VDS characteristics can be described by the following equation: ID = kn’(W/L)[(VGS-VT)VDS - 1/2 VDS 2] (4. 11) v Where kn’= μn. Cox is the process transcondctance parameter, its value is determined by the fabrication technology 26
The i. D-VDS Characteristics v The Triode Mode: • If VDS is sufficiently small • ID = kn’(W/L)[(VGS-VT)VDS] (4. 12) v This linear relationship represents the operation of the MOSFET as a linear resistance r. DS whose value is controlled by VGS. 27
Operation in the Saturation Region Ø To operate the MOSFET in the Saturation Region we must first induce a channel. v v. GS≧ Vt (Induced channel) v v. GD≦ Vt (Pinched-off channel) v v. DS≧ v. GS-Vt (Pinched-off channel) (4. 16) (4. 17) (4. 18) Ø The n-channel enhancement-type MOSFET operates in the saturation region when v. GS is greater than Vt and the drain voltage does not fall below the gate voltage by more than Vt. Ø The boundary between the triode region and the saturation region is characterized by v v. DS= v. GS-Vt (Boundary) (4. 19) 28
The i. D-VDS Relationship v Saturation Mode In the Saturation region, the i. D-VDS characteristics can be described by eq. (4. 20): Nasim Zafar. 29
The i. D–v. GS characteristic The i. D–v. GS Characteristic for an NMOS Transistor in Saturation 30
Summary: MOSFET I-V Equations v The Cut-off Region: VGS< VT ID = I S = 0 v The Triode Region: VGS>VT and VDS < VGS-VT ID = kn’(W/L)[(VGS-VT)VDS - 1/2 VDS 2] v The Saturation Region: VGS>VT and VDS > VGS-VT ID = 1/2 kn’(W/L)(VGS-VT)2
Output Characteristics of MOSFET 32
Large-Signal Equivalent-Circuit Model Ø In saturate mode, MOSFET provides a drain current whose value is independent of the drain-voltage VDS and is determined by the gate-voltage VGS Ø Thus, the Saturated MOSFET behaves as an ideal current source whose value is controlled by VGS according to the nonlinear relationship in Eq. (4. 20). Ø Figure 4. 13 shows a circuit representation of this view of MOSFET operation in the saturation region. Note that this is a large-signal equivalent-circuit model. 33
Large-signal equivalent-circuit model of an n-channel MOSFET operating in the saturation region.
MOSFET Summary 35
I-V Characteristics of MOSFET 36
MOSFET: Summary Ø A majority-carrier device: fast switching speed Ø Typical switching frequencies: tens and hundreds of k. Hz Ø On-resistance increases rapidly with rated blocking voltage Ø The device of choice for blocking voltages less than 500 V Ø 1000 V devices are available, but are useful only at low power levels (100 W)
MOSFET Summary v. Importance for LSI/VLSI – Low fabrication cost – Small size – Low power consumption v Applications – Microprocessors – Memories – Power Devices v Basic Properties – Unipolar device – Very high input impedance – Capable of power gain – 3/4 terminal device, G, S, D, B – Two possible channel types: n-channel; p-channel 38
MOSFET: Merits/ Demerits v Advantages • • Voltage controlled device Low gate losses Parameters are less sensitive to junction temperature No need for negative voltage during turnoff v Limitations • One disadvantage of MOSFET devices is their extreme sensitivity to electrostatic discharge (ESD) due to their insulated gate-source regions. • The Si. O 2 insulating layer is extremely thin and can be easily punctured by an electrostatic discharge. • High-on-state drop as high as 10 V • Lower off-state voltage capability • Unipolar voltage device. 39
- Slides: 39