Microelectronic Circuits Introduction to MOSFET MOSFET (Metal Oxide Semiconductor Field Effect Transistor) Slide 1
MOSFET Construction MOSFET (Metal Oxide Semiconductor Field Effect Transistor) Slide 2
MOSFET Operation n-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) Slide 3
MOSFET Operation Step 1: Apply Gate Voltage SiO 2 Insulator (Glass) Source Gate 5 volts Drain holes N N electrons P electrons to be transmitted Step 2: Excess electrons surface in channel, holes are repelled. Step 3: Channel becomes saturated with electrons. Electrons in source are able to flow across channel to Drain. Slide 4
MOSFET Slide 5
MOSFET Operation MOSFET is operating as a linear resistance device whose value is controlled by v gs infinite resistance at v gs v t The induced channel will be enhanced after gate voltage becomes more than the threshold voltage, hence the name enhanced type MOSFET Slide 6
MOSFET Operation When V DS is increased by keeping V GS constant > V t V DS appears as a voltage drop across the length of the channel. i.e when we traverse from the Source to Drain, the voltage drop increases from 0 to v DS Slide 7
MOSFET Operation Thus the voltage between gate and points along the channel decreases from v GS at the source and v GS -v DS at the drain end. Due to this channel depth is no longer uniform, being deepest at the source and shallowest at the drain end Slide 8
MOSFET Operation As the v DS is increased, the channel becomes more tapered and its resistance increases correspondingly. Thus i D ---v DS curve is not a straight line Slide 9
MOSFET Operation When the v DS is increased to a value such that, the voltage between the Gate and channel is reduced to V t (v DS = v GS - v t ) At this moment, the channel width at the drain end almost zero and the channel is said to be pinched off Increasing v DS beyond this value has no effect on the channel shape and current through the channel remains constant. (at v DS = v GS - v t ) Slide 10
MOSFET Operation Drain current saturates and MOSFET is said to be in the saturation region. v DS at which saturation occurs is denoted by v DSsat =v GS -v t The Device operates in a saturation region if v DS > v DSsat Slide 11
i D and v DS relationship i i D D k ' n 1 2 W L k ' n W L v GS V v v triode region 2 v V Saturation region GS t t DS 1 2 2 DS Where W= channel width, L = Channel length ' k n =Process transconductance parameter (Determines the value of MOSFET transconductance.) Slide 12
i D vs v DS characteristics Slide 13
NMOS Symbol Slide 14
Equivalent Circuit Slide 15
MOSFET as an Amplifier Slide 16
MOSFET as an Amplifier Slide 17
Biasing of MOSFET Two Supplies Method Slide 18
Biasing of MOSFET Voltage Divider Bias Method I D 1 nc 2 ox W L V V 2 GS t V G V GS I D R S Slide 19
Numerical based on Biasing It is required to design a circuit to establish a dc drain current ID = 0.5 ma. The MOSFET is specified to have Vt ' 2 = 1V and kn W / L 1mA / V. Use VDD = 15. calculate the percentage change in the value of ID obtained with the MOSFET threshold voltage Vt = 1.5 V Slide 20
Biasing Using Drain to Gate Feedback Biasing the MOSFET using a large drainto-gate feedback resistance, R G. V GS V DS V DD R D I D V DD V GS I D R D Slide 21