EE314 IBM/Motorola Power PC620 IBM Power PC 601 Motorola MC68020 Field Effect Transistors

Chapter 12: Field Effect Transistors 1.Construction of MOS 2.NMOS and PMOS 3.Types of MOS 4.MOSFET Basic Operation 5.Characteristics

The MOS Transistor Polysilicon Aluminum JFET – Junction Field Effect Transistor MOSFET - Metal Oxide Semiconductor Field Effect Transistor n-channel MOSFET (nMOS) & p-channel MOSFET (pMOS)

The MOS Transistor n+ p-substrate Field-Oxide (SiO 2 ) p+ stopper Polysilicon Gate Oxide Drain Source Gate Bulk Contact CROSS-SECTION of NMOS Transistor

Switch Model of NMOS Transistor 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

Switch Model of PMOS Transistor Gate Source (of carriers) Drain (of carriers) | V GS | | V GS | > | V DD – | V T | || V GS | < | V DD – |V T | | Open (off) (Gate = ‘1’) Closed (on) (Gate = ‘0’) R on

MOS transistors Symbols D S G D S G G S DD S G NMOS Enhancement NMOS PMOS Depletion Enhancement B NMOS with Bulk Contact Channel

JFET and MOSFET Transistorsor L =  m W =  m SiO 2 Thickness =  m Device characteristics depend on L,W, Thickness, doping levels Symbol

MOSFET Transistor Fabrication Steps

Building A MOSFET Transistor Using Silicon

It is done. Now, how does it work?

n-channel MOSFET Basic Operation Operation in the Cutoff region Schematic pn junction: reverse bias i D =0 for v GS < V t0 When v GS =0 then i D =0 until v GS > V t0 (V t0 –threshold voltage)

n-channel MOSFET Basic Operation Operation in the Triode Region For v DS V t0 the NMOS is operating in the triode region Resistor like characteristic (R between S & D, Used as voltage controlled R) For small v DS, i D is proportional to the excess voltage v GS -V t0

n-channel MOSFET Basic Operation Operation in the Triode Region Device parameter KP for NMOSFET is 50  A/V 2

n-channel MOSFET Basic Operation Operation in the Saturation Region (v DS is increased) Tapering of the channel - increments of i D are smaller when v DS is larger When v GD =V t0 then the channel thickness is 0 and

n-channel MOSFET Basic Operation Example 12.1 An nMOS has W=160  m, L=2  m, KP= 50  A/V 2 and V to =2 V. Plot the drain current characteristic vs drain to source voltage for v GS =3 V.

n-channel MOSFET Basic Operation Characteristic Example 12.1 Channel length modulation i d depends on v DS in saturation region (approx: i D =const in saturation region)

p-channel MOSFET Basic Operation It is constructed by interchanging the n and p regions of n- channel MOSFET. Symbol Characteristic How does p-channel MOSFET operate? -voltage polarities -i D current -schematic