Budapest University of Technology and Economics Department of Electron Devices Microelectronics, BSc course Field effect transistors 2: The MOSFETs

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The abstraction level of our study: SYSTEM MODULE + GATE CIRCUIT n+ SD G DEVICE V out V in

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Field effect transistors 1 ► FET = Field Effect Transistor – the flow of charge carriers is influenced by electric field transversal field is used to control Channel JUNCTION FET: depletion layers of pn- junctions close the channel ► Unipolar device: current is conducted by majority carriers ► Power needed for controlling the device  0 Most important parameter: U 0 pinch-off voltage Flow depletion layer

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Field effect transistors 2 ► MOSFET: Metal-Oxide-Semiconductor FET First type: depletion mode device Most important parameter: U 0 pinch off voltage - Bulk Second type: enhancement mode device Most important parameter: V T threshold voltage Most frequently used today + depletion layer oxide inversion layer oxide

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Field effect transistors 3 ► Symbols: n channel p channel n channel enhancement mode p channel enhancement mode p channel depletion mode n channel depletion mode p channel depletion mode n channel enhancement mode

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET MOSFETs ► More realistic cross-sectional view of enhnacement mode MOSFETs: Gate oxide n+ SourceDrain p substrate Bulk contact p+ stopper Field-Oxide (SiO 2 ) n+ Polysilicon Gate

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The most modern MOSFETs: ► 2007/2008, Intel:

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET How is it manufactured?

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Poli-Si gate self-aligned device PSG metallization, contact window thin oxideSource/drain dopingpoli-Si gate Structure: Layout: L W

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Steps of the self-aligned poli-Si gate process 1) Open window for the active region M  photolitography, field oxide etching 2) Growth of thin oxide 3) Window for hidden contacts M  Contacts the poli-Si gate (yet to be deposited) with the active region (after doping). 3) Deposit poli-Si 4) Patterning of poli-SiM 5) Open window through the thin oxide (etching only)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Steps of the self-aligned poli-Si gate process 6) n+ doping: Form source and drain regions as well as wiring by diffusion lines. Through the hidden contact poli-Si gate will also be connected to diffused lines. 7) Deposit phosphor-silica glass (PSG) as insulator 8) Open contact windows through PSG-n M 9) Metallization 10) Patterning metallization layerM

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Further topics: ► Overview of operation of MOS transistors ► Characteristics ► Secondary effects ► Models

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Operation of MOSFETs ► The simplest (logic) model:  open (off) / short (on) Gate Source (of carriers) Drain (of carriers) | V GS | | V GS | < | V T | | V GS | > | V T | Open (off) (Gate = ‘0’) Closed (on) (Gate = ‘1’) R on open short enhancement mode device

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Operation of MOSFETs ► n-channel device:  electrons are flowing ► p-channel device:  holes are flowing  same operation, change of the signs ► Normally OFF device: at 0 gate (control) voltage the are "open" (enhancement mode device) ► Normally ON device: at 0 gate (control) voltage the are "short" (depletion mode device)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Overview of MOSFET types

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Overview of the operation  As a result of electrical field perpendicular to the gate surface positive charges accumulate at the metal (gate) in the p-type semiconductor –first the positive charges are "swept" out and a depletion layer is formed –further increasing the electric field, negative carriers are collected from the bulk under the metal –if the voltage at the surface exceeds a threshold value, the type of the semiconducter gets "inverted": an inversion layer is formed  V T threshold voltage – the minimal voltage needed to form the inversion layer; depends on: the energy levels of the semiconductor material the thickness and the dielectric constant of the oxide (SiO 2 ) the doping level and dielectric constant of the semiconductor (Si) ► The operation is based on the so called MOS capacitance:

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Overview of the operation ► Surface phenomena in case of the MOS capacitance Strong inversion: U F = 2  F Accumulation Depletion Inversion

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The MOS transistor ► MOS capacitance completed by two electrodes at its two sides: ► n-channel device: current conducted by electrons ► p-channel device: current conducted by holes

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Qualitative operation of the MOSFET ► If V GS > V T, inversion layer is formed  the electrons drifted there are all sank in the n+ region and the circuit is closed  the n+ region at the source can inject electrons into the inversion channel  the positive potential at the drain induces flow of electrons in the channel,  the positive potential of the drain reverse biases the pn junction formed there

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Qualitative operation of the MOSFET  the charge density in channel depends on the V GS voltage  there is a voltage drop in the channel, thus, the thickness of the inversion layer will deminish along the channel  at a given V DSsat saturation voltage the thickness will reach 0, this is the so called pinch-off V DSsat = V GS - V T After this voltage is reached, the MOSFET operates in saturation mode, the drain voltage does not influence the drain current any longer.

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Qualitative operation of the MOSFET In the pinch-off region the charge transort takes place by means of diffusion current.

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET I-V charactersitics output charactersitic: I D =f(U DS ), parameter: U GS input characteristc: I D =f(U GS ) Output characteristic: In saturation: The circuit designer can change the geometry only: the W width and the L length current constant

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Example Calculate the saturation current of a MOSFET for U GS =5V if V T =1V, and the geometry a) W= 5μm, L=0.4μm, b) W= 0.8μm, L=5μm ! a) b)b) By changing the W/L ratio the drain current can be changed by orders of magnitude

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET I-V charactersitics I D (A) V DS (V) X V GS = 1.0V V GS = 1.5V V GS = 2.0V V GS = 2.5V lineárisszaturáció V DSsat = V GS - V T Quadratic dependece nMOS tranzisztor, 0.25um, L d = 10um, W/L = 1.5, V DD = 2.5V, V T = 0.4V cut-off Voltage controlled current source voltage controlled resistor

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Overview of the physics: ► Charges and potentials at the surface ► The threshold voltage ► The charateristics ► Secondary effects

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Potentials of the MOS structure oxide semiconductor

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Potentials of the MOS structure

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The threshold voltage of the MOSFET Inversion

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The threshold voltage of the MOSFET

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The threshold voltage of the MOSFET Bulk constant: Flat-band potential: FBF T V   SBF U  2 2 P 

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Data of a MOS structure: N a = 4  /cm 3, dielectric constant of Si> 11,8, dielectric constant of oxide: 3,9, oxide thickness d ox = 0,03  m,  MS = 0,2 V, Q SS is neglected. Calculate the Fermi potential, the oxide capacitance, the bulk constants and the threshold voltage if U SB = 0 V! Problem

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The charactersitics of an enhnacement mode MOSFET Later we shall calculate these! inversion layer saturation triode region

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Derivation of the charactersitic U(0) = U GS, U(L) = U GD Q i (U) = Q i [U(x)] inversion layer

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Derivation of the charactersitic inversion layer

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Derivation of the charactersitic For all regions of operation! inversion layer

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The saturation region For all regions of operation! Saturation: U GD < V T inversion layer

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Overview of all types of MOSFETs

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Like an enhance mode MOSFET with a negative threshold voltage Depletion mode MOSFET

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Capacitances of the MOSFET Bulk S/D – B capacitance: reverse biased PN junction inversion layer

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET The gate capacitance: t ox n + n + Cross section L Gate oxide x d x d L d Polysilicon gate Top view Gate-bulk overlap Source n + Drain n + W

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Secondary effects ► Channel length reduction ► Narrow channel operation ► Temperature dependence ► Subthreshold current

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Dependence of threshold voltage on geometry Short channel: V T decreases Narrow channel: V T increases

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Velocity saturation ► Influences the operation of short channel devices In a L = 0.25  m channel device a few Volts of D-S voltage may already result in velocity saturation. Velocity saturation the speed of carriers (due to the collisions) becomes constant  (V/  m)  n (m/s)  sat =10 5 Constant velocity constant mobility (slope =  ) c= c= 5

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Velocity saturation ► In short channel device velocity saturation takes place sooner (at lower voltage) IDID Long channel devices Short channel devices V DSAT V GS -V T V GS = V DD VDSVDS

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Short channel charactersitics I D (A) V DS (V) X V GS = 1.0V V GS = 1.5V V GS = 2.0V V GS = 2.5V Linear dependence Early velocity saturation LinearSaturation nMOS transistor, 0.25um, L d = 10um, W/L = 1.5, V DD = 2.5V, V T = 0.4V

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Temperature dependence

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Subthreshold current ► Assuming a given V T is rough model; in reality the current vanishes exponentially with the gate voltage: I D (A) V GS (V) subthreshold, exponential region quadratic region linear region VTVT I D ~ I S e (qV GS /nkT) where n  1

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Subthreshold current ► Continuous transition between the ON and OFF states  Subthreshold is undesired: strong deviation from the switch model ► I 0, n – empirical parameters, n is typically 1.5 ► Slope factor: S = n (kT/q) ln (10) (tipically: mV/decade) – the smaller the better, depends on n értékétől függ. Can be reduced by SOI: SiO 2 Si Si substrate e.g. SIMOX process

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Subthreshold I D (V GS ) charactersitic V DS : V

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Subthreshold I D (V DS ) charactersitic V GS : V

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET MOS transistor models ► Neded for circuit simulators (SPICE, TRANZ-TRAN, ELDO, SABER, stb) ► Different levels of complexity:  level0, 1, 2,...n,  EKV,  BSIM3, BSIM4

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Examples for MOSFETs Micro-photograph by SEM Photograph by optical microscope S G D inversion layer

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Some more complex MOS circuits n- & p-channel devices : CMOS circuit, see later

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, The MOSFETs © András Poppe & Vladimír Székely, BME-EET Some more complex MOS circuits Designed by CAD tools