INTRODUCTION TO ELECTRONICS EHB 222E MOS Field Effect Transistors (MOSFETS) Asst. Prof. Onur Ferhanoğlu Asst. Prof. Onur Ferhanoğlu MOSFETS 1/ INTRODUCTION TO ELECTRONICS 1 MOSFETS: Metal oxide semiconductor field effect transistors • Transistors are 3-terminal devices. • Basic principle: Controlling the voltage between 2 terminals to control the current flowing in the 3rd terminal. • Transistors are used as amplifiers & switches. • Invention: 1959: Bell Lab`s (~10) years after BJT’s • Dissipate very low power • Compared to BJT’s, can be made smaller Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 2 MOSFET structure • n-channel enhancement type MOSFET: • Transistor is fabricated on a p-type substrate • 2 heavily doped n-regions (source & drain) • SiO2 is grown on the surface (1-10 nm thick) • Metal is deposited on oxide (Gate electrode) Metal contacts are also made on source & drain & body • Called `Metal oxide semiconductor` FET • Subtrate forms pn junction with source & drain • Voltage applied to the gate controls the current between source & drain Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 3 MOSFET: current flow & channel formation • Source & Drain grounded, positive voltage at gate: vGS : appears between G&S • Free holes (positive charge) are repelled under the gate and pushed towards the substrate • The channel is populated by the negative charge attracted from n+ drain and source regions. • When sufficient number of electrons are attracted, an n-channel is formed under the gate, connecting drain and source: • Vt : threshold voltage to form n-channel : ~0.3-1.0 V • Gate electrode and the channel form a capacitor: n-channel: - charge / gate: positive charge n-channel MOSFET • An electric-field is formed in vertical direction, which NMOS controls the charge / conductivity of the channel: FET: Field Effect Transistor Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 4 MOSFET: current flow & channel formation • vDS = 0 , voltage along the channel is 0 • vGS is the vertical voltage on oxide, btw gate and points of the channel • Effective voltage (overdrive voltage) = • Electron charge over the channel: Cox: capacitance over oxide, W,L: Width and Length of channel n-channel MOSFET NMOS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 5 MOSFET: Applying small vDS • vDS ~ 50mV • Current is carried with electrons, current from Drain -> source (~collector -> emitter) • vDS small -> voltage between gate and various points along the channel are still ~ constant • Charge in the channel is still equal to : • Charge per unit length: • Electric field along the channel: n-channel MOSFET • Drift: NMOS • iD = drift x charge/length from semiconductors lecture Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 6 Conductance: gDS MOSFET: Applying small vDS Determined by Transistor aspect process technology ratio Process transcundactance: MOSFET transconductance parameter -> Linear resistance,whose value is determined by vGS when vDS is small No current for vGS < Vt n-channel MOSFET NMOS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 7 MOSFET: increasing vDS • Voltage increases along the channel • Channel depth depends on voltage: channel is non-uniform, tapered deepest at the source end (voltage across channel at source : vGS – 0 > voltage across channel at drain: vGS - vDS) n-channel MOSFET NMOS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 8 MOSFET: increasing vDS • As vDS increases channel becomes even more tapered • Charge in the tapered channel is proportional to cross sectional area: (in the middle of the channel: Not-Tapered Tapered n-channel MOSFET NMOS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 9 MOSFET: increasing vDS Not-Tapered Tapered n-channel MOSFET NMOS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 10 MOSFET: increasing vDS beyond VOV • When vDS reaches VOV : channel is broken : channel pinch-off • Increasing vDS has no further effect on channel shape • Current through the channel remains constant • The drain current saturates Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 11 Example 1: a) b) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 12 Example 1 continued.. c) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 13 PMOS: p-channel MOSFET Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 14 CMOS: Complementary MOS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 15 MOSFET – current vs. voltage characteristics Simplified symbol NMOS NMOS PMOS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 16 MOSFET – iD vs vDS 3 regions: cut-off, triode, saturation Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 17 MOSFET – iD vs vGS At the saturation region, iD is independent of vDS, depends on vGS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 18 Exercise 2 a) b) VGS is kept constant -> ID decreases TRIODE region (check graph) In saturation 2 roots Needs to be smaller than V in a) Edge of saturation DS Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 19 Exercise 2 continued… c) Saturation region Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 20 Finite output resistance in saturation • Practically, vDS has an effect on channel’s shape & conductance channel-length modulation (Early effect equivalent) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 21 Large-signal equivalent (in saturation mode) Drain current without channel-length modulation Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 22 MOSFETs at DC – Exercise 3 VD > VG -> Saturation Gate is grounded -> Vs = -1.2 V Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 23 MOSFETs at DC – Exercise 4 VD = VG -> saturation mode (VD > VG – Vt) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 24 MOSFETs at DC – Exercise 5 Triode region -> (VD < VGS – Vt) VDS is small (assume linear relationship) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 25 MOSFETs at DC – Exercise 6 Gate current is 0 (can’t go through the insulator) Assume Saturation Quadratic equation -> 2 values for ID : 0.5, 0.89 A If ID = 0.89 A -> VS = 0.89 x 6 = 5.34 V > VG -> CUT OFF SATURATION as assumed Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 26 MOSFETs at DC – Exercise 7 PMOS: in saturation VS = 5V -> VG = 5 – 2 = 3V Voltage divider: choose -> (given in question) (saturation maintained till VD = VG + |Vtp|) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 27 MOSFETs at DC – Exercise 8 VGS = |2.5|V for both transistors, when vI = 0V. Circuit is symmetric (btw + and – 2.5 V) -> vo = 0V. -> SATURATION For vI = +2.5V -> PMOS cut-off: (VSG = 0) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 28 MOSFETs at DC – Exercise 8 continued… For vI = +2.5V -> PMOS cut-off: (VSG = 0) Vo < 0V -> VGD > Vtn -> NMOS in triode region 2 equations & 2 unknowns Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 29 MOSFETs at DC – Exercise 8 continued… For vI = -2.5V -> NMOS cut-off: (VSG = 0) (complement of the previous case) Asst. Prof. Onur Ferhanoğlu MOSFETS/ INTRODUCTION TO ELECTRONICS 30.