Mosfet Operation 211l18

MOSFET Operation

Dr. Arti Noor, M. Tech Division, CDAC Noida. Email : [email protected]

29-9-2009

BVDLecture-4

 

• •

•

MOSFET

Now add source and drain to MOS capacitor. The new structure is called MOS transistor. Two types of MOS transistors : NMOS and PMOS

The distance between S and D is called channel length L and lateral extent of channel is width W. Both W and L are very important parameters. The channel is formed through applied gate voltage between S and D.

BVDLecture-4

MOSFET Types

 

BVDLecture-4

N-Channel MOSFET Operation

BVDLecture-4

N-Channel MOSFET Operation

• •

When Vgs > Vto is applied on gate inversion layer is formed. Source and drain are connected through a n-type channel. Gate Voltage is applied with respect to source then Vto is same as in case of MOS Capacitor. BVDLecture-4

Threshold Voltage in MOSFET Case

Qox QB −QBo Vt = QGC − 2φ F − − − Cox Cox Cox QBo

= Vto −

QB −QBo Cox BVDLecture-4

Threshold Voltage in MOSFET Case

QB − QB o Co x

= −

2 q⋅ N A⋅ε si • Co x

(

BVDLecture-4

− 2 φ F + VS B −

− 2φ F

)

BVDLecture-4

N-Channel MOSFET Operation : I-V Curve (1) NMOS in Linear Region : • if small drain voltage is applied, drain current flows through the conducting channel. • As drain voltage increases, drain current also increases linearly with voltage. The channel region acts as voltage controlled resistor. • This operation mode is called linear mode.

BVDLecture-4

N-Channel MOSFET Operation : I-V Curve (2) NMOS in saturation Region : • As drain voltage increases again, the gate voltage is not sufficient to maintain channel below the gate. • The charge and channel depth start to decrease at drain end. At Vds =Vdsat the charge and depth at drain end become zero. • This point is called pinch off point. Beyond pinch-off depletion region forms adjacent to drain and grows to source with increasing Vds . • This operation mode is called saturation mode. Near pinch-off point, high field region forms between channel-end and drain. Electrons arriving at this end injected to drain region and accelerated to drain.

BVDLecture-4

N-Channel MOSFET Operation : I-V Curve (3)

• 3-D analysis of this system is very complex to establish the I-V relation of NMOS. • Several approximations be made to simplify the problem. • The Gradual-Channel Approximation (GCA) is used which converts 3D problem into 1-D for analytic derivation of I-V. BVDLecture-4

Gradual Channel Approximation (GCA)

BVDLecture-4

Gradual Channel Approximation (GCA)(2)

• •

This means entire channel region is inverted. Assume QI(y) is the charge in inversion layer which is function of Vgs and Vcs (y) and can be defined as

•

Inversion layer thickness varies from S to D. Now consider incremental resistance dR of differential channel region and assume constant surface mobility μn of electrons in channel.

BVDLecture-4

Gradual Channel Approximation (GCA)(3) • •

Now assume current density is uniform across the segment. Voltage drop along the incremental segment in y-direction is

Arrange and integrate along the channel : 0
L

∫I 0

D

dy = Wµn COX

VDS

∫ (V

GS

0

BVDLecture-4

− VCS − VTO )dVCS

Gradual Channel Approximation (GCA)(4)

BVDLecture-4

Current – Voltage Characteristic •

Given : W =20μm, L=2μm, Vt0=1.0V, k=0.42 mA / V2

BVDLecture-4

Current – Voltage Characteristic •

Saturation region current equation.

BVDLecture-4

Current – Voltage Characteristic •

Saturation region current equation.

BVDLecture-4

Channel Length Modulation

For Vds > Vdsat

BVDLecture-4

Channel Length Modulation •

Electrons now have to travel L’ length. One can write current equation again replacing L by L’. GCA still holds.

Simplify the expression

BVDLecture-4

Channel Length Modulation

BVDLecture-4

Substrate Bias Effects

BVDLecture-4

Current-Voltage Summary

BVDLecture-4

Current-Voltage Summary

BVDLecture-4

Assignment

1. Explain accumulation, depletion and inversion conditions for PMOS transistor with the help of cross section view of transistor. 2. Exercise 3.7 of Chapter-3 from Kang’s book.

BVDLecture-4

Next Class Topic

MOSFET Scaling and Small Geometry Effects.

BVDLecture-4