EE210 Digital Electronics

Class Lecture 6

May 08, 2008

2

MOS Field-Effect Transistors (MOSFETs)MOS Field-Effect Transistors (MOSFETs)

Today We Will Discuss Following Topics:

4.1 Device Structure and Physical Operation4.2 Current-Voltage Characteristics4.3 MOSFET Circuits at DC

IntroductionIntroduction• Three Terminal Device – (as seen in BJT) the

Control Signal used on Two terminals can cause Current to Change on Third from Zero to Hi value (switch) – Similar concept for MOSFET

• Switch is Basis for Logic Inverter – basic Element of Digital Ckts/Electronics

• MOSFET can be made Smaller than BJT, and Manf. Process is simple and Require Lower P

• > 200 Mil MOSFETs on Single Chip• Can be used as Amp and Dig Logic Inverter

Typically L = 0.1 to 3 m, W = 0.2 to 100 m, and thickness of the oxide layer (tox) = 2 to 50 nm.

4.1.2 No Gate Voltage Operation4.1.2 No Gate Voltage Operation

• Two Back-to-Back Diodes in Series Between Drain and Source

• Prevent Current Flow between Drain and Source when vDS is Applied

• The path between Drain and Source has very High Resistance in the Order of 1012 Ω

4.1.3 Creating a Channel for Current Flow4.1.3 Creating a Channel for Current Flow Positive Voltage to gate Repels (Pushes) Holes Down in Substrate and Attracts Electron. When sufficient number of electrons gather under Gate an n-region is created thereby an n channel is INDUCED under the gate. Now if vDS is applied current will flow in n-region (n-Channel)

Note …Note …• Because of n-channel This MOSFET is called NMOS

Transistor• n-Channel (or Inversion Layer) created by inverting the p-

type sub to n-type with application of Gate Voltage (Field)

• The value of vGS at which conducting channel is formed is called Vt (Positive for n)

• Vt is controlled during fabrication process and typically range from 0.5 to 1 V

• Gate and Channel form parallel plate capacitor (oxide as dielectric) creating the E filed in Vertical direction

• E field controls the Charge in Channel and thus determines the conductivity of channel

4.1.4 Applying a Small v4.1.4 Applying a Small vDSDS

When a Small vDS (50 mV) is Applied along with vGS > Vt

NMOS acts as a resistor whose value is determined by vGS Specifically, the channel conductance is proportional to Excess Gate Voltage (vGS – Vt), and thus iD is proportional to (vGS – Vt) vDS. (Depletion region not shown)

The The iiDD––vvDSDS characteristics of the MOSFET characteristics of the MOSFET When

voltage applied between drain and source, vDS, is kept small, device operates as a linear resistor whose value is controlled by vGS. The increase of vGS above Vt Enhances the Channel, Hence Enhancement-mode or Enhancement-type MOSFET

4.1.5 Operation as 4.1.5 Operation as vvDS DS is increasedis increased. vGS is kept

constant at a value > Vt vDS appears as voltage drop across the channel L (voltage increases from 0 to vDS from S to D). Thus voltage between gate and points along channel decreases from vGS to vGS – vDS. Thus induced channel acquires a tapered shape, and its resistance increases as vDS is increased. And iD - vDS curve no longer straight line but bends – Next Figure

When vDS increases to value so that voltage between gate and drain side of channel reaches Vt – vGD = Vt or vGS – vDS = Vt or vDS = vGS – Vt the channel depth at drain end is almost zero (or channel is pinched off) Increasing vDS beyond this has no effect on channel shape and iD stays constant

Eventually, as vDS reaches vGS – Vt’ the channel is pinched off at the drain end. Increasing vDS above vGS – Vt has no effect on the channel’s shape and current saturates and MOSFET has entered Saturation Region. That is, vDSsat = vGS – Vt

Note That … For every value of vGS > Vt there is corresponding vDSsat. Device in Saturation if vDS ≥ vDSsat

4.1.6 Derivation of the 4.1.6 Derivation of the iiDD––vvDSDS Relationship Relationship

In Next Class

We Will Continue to Discuss:

Chap 4 MOS Field-Effect Transistors

How Boolean Logic WorksHow Boolean Logic Workshttp://computer.howstuffworks.com/boolean.htmHave you ever wondered how a computer can do something like balance a check book, or play chess, or spell-check a document? These are things that, just a few decades ago, only humans could do. Now computers do them with apparent ease. How can a "chip" made up of silicon and wires do something that seems like it requires human thought? If you want to understand the answer to this question down at the very core, the first thing you need to understand is something called Boolean logic.

Boolean logic, originally developed by George Boole in the mid 1800s, allows quite a few unexpected things to be mapped into bits and bytes. The great thing about Boolean logic is that, once you get the hang of things, Boolean logic (or at least the parts you need in order to understand the operations of computers) is outrageously simple. In this edition of HowStuffWorks we will first discuss simple logic "gates," and then see how to combine them into something useful