CO2005: Electronics I (FET) Electronics I, Neamen 3th Ed. 1

CO2005: Electronics I

The Field-Effect Transistor The Field Effect Transistor

(FET)

Electronics I, Neamen 3th Ed. 1

 The metal-oxide-semiconductor field-effect transistor (MOSFET) becomes a practical

MOSFET

reality in the 1970s.

 The MOSFET, compared to BJTs, can be made very small, that is, it occupies a very small area in IC chip.

 In the MOSFET, the current is controlled by an electric field applied perpendicular to both the semiconductor surface and to the direction of current.

 The phenomenon applying an electric field perpendicular to the surface is called the field effect

effect.

 Basic MOS capacitor structure

The Physics of the MOS Capacitor

Gate

Electronics I, Neamen 3th Ed. 3

The Physics of the MOS Capacitor for N-type Semiconductor Substrate

 Enhancement mode: a voltage must be applied to the gate to create an inversion layer

 Enhancement mode: a voltage must be applied to the gate to create an inversion layer.

 P-type: a positive gate voltage must be applied to create the electron inversion layer

 N-type: a negative gate voltage must be applied to create the hole inversion layer

 Transistor Structure

NMOS

Large enough positive

 Transistor Operation

g g p

voltage induces an electron inversion layer.

Connection between D and S is created so that a

current can be generated

Electronics I, Neamen 3th Ed. 5

current can be generated.

 The threshold voltage of the n-channel MOSFET is denoted as and is defined as the

MOSFET Current-Voltage Characteristics

applied gate voltage needed to create an inversion charge.

 We can think of the threshold voltage as the gate voltage required to “turn on” the transistor.

 The versus characteristics for small values of

MOSFET Current-Voltage Characteristics

i

v

v

Electronics I, Neamen 3th Ed. 7

MOSFET Current-Voltage Characteristics

 Nonsaturation (triode) Region

Ideal MOSFET Current-Voltage Characteristics

TN GS

sat DS

DS

v v V

v 

 

) 2

( ]

) (

2

[ v V v v

K v

v v

K

i     

 Saturation Region

) 2

( ]

) (

2

[

n

D

K v V v v K v v v

i  

) also

( v V

v

v

 v

( also v

 V

)

v  

)

(

n

D

K v V

i  

Note: In the saturation region











o

v r 1 i /

Note: In the saturation region,

Electronics I, Neamen 3th Ed. 9

 Conduction Parameter

Conduction Parameter

2

ox n n

C L

K _ W _ 

(conduction parameter)

1 C



W

: oxide capacitance per unit area : electron mobility

: channel width

thickness oxide

: 1 ,

ox ox

ox

t

C  t

 The conduction parameter is a function of both electrical and geometric parameters.

W L

: channel width : channel length

 The conduction parameter is a function of both electrical and geometric parameters.

 Electrical Parameters: The oxide capacitance and carrier mobility are essentially constants for a given technology.

k W 

 Geometrical Parameters: The width-to-length ratio (W/L) is a variable in the design of

constant :

2

n

n

k k

L

K W  





g ( ) g

MOSFETs that is used to produce specific current-voltage characteritics in MOSFET

Electronics I, Neamen 3th Ed. 11

 In the p-channel enhancement-mode device, a negative gate-to-source voltage must be

PMOS

applied to create the inversion layer of holes that connects the source and drain regions.

 The threshold voltage, denoted an for the PMOS is negative for an enhancement- mode devices. The threshold voltage is positive for a depletion-mode device.^TP

V

 Holes flow from the source to the drain, the conventional current enters the source and leaves the drain.

 Nonsaturation (triode) Region

Ideal PMOS Current-Voltage Relationship

:

when v

 v

 v

 V

) 2

( ]

) (

2

[ v V v v

K v

v v

K

i     

 Saturation Region

) 2

( ]

) (

2

[

p

D

K v V v v K v v v

i   

: ) 0 also

(

when v  v ( also v  V  0 ) : _{i K (  V )}

when v

 v

v

 V

 _i

_{ K}

_{( v}

_{ V}

₎

Electronics I, Neamen 3th Ed. 13

Circuit Symbols

N channel enhancement N-channel enhancement- mode MOSFET

P-channel enhancement- P channel enhancement mode MOSFET

 Complement MOS (CMOS) technology uses both NMOS and PMOS in the same circuit.

CMOS

 To design electrically equivalent NMOS and PMOS devices, adjusting the W/L ratios of the transistors is required.

Electronics I, Neamen 3th Ed. 15

Summary of MOS Transistor Operation

NMOS Common-Source Circuit

Electronics I, Neamen 3th Ed. 17

PMOS Common-Source Circuit

Electronics I, Neamen 3th Ed. 19

Electronics I, Neamen 3th Ed. 21

 Load Line

Load Line

5

) 20 ( 5

D D DD

DS

V

I R

I V

V    

) mA 20 (

20

5

D

I   V

Electronics I, Neamen 3th Ed. 23

Electronics I, Neamen 3th Ed. 25

中央大學通訊系 張大中 Electronics I, Neamen 3th Ed. 27

 An enhancement-mode MOSFET is used as a nonlinear resistor.

Nonlinear Resistor

 The transistor is always biased in the saturation region and called a load device.

)

(_{Sat GS TN}

,

0

DS GS

DS

v v v V V

v     

2 2

) (

) (

)

(

n

D

K v V K v V

i    

Electronics I, Neamen 3th Ed. 29

Electronics I, Neamen 3th Ed. 31

CMOS Inverter

 If , the transistor is in cut-off.

NMOS Inverter

TN

I V

v 

DD O

D

V

v

i  0 

 If (and make ), the transistor is biased in the non-saturation region. v_I V_TN

2

DS TN

I

V v

v  

D D DD

O

O O

TN I

n D

R i v

v

v v

V v

K i









 [ 2 ( )

]

Electronics I, Neamen 3th Ed. 33

Digital Logic Gate

Electronics I, Neamen 3th Ed. 35

 We can establish a particular Q-point on the load line by designing the ratio of the bias

MOS Small-Signal Amplifier

resistors and .

R

R

Constant-Current Biasing

Electronics I, Neamen 3th Ed. 37

中央大學通訊系 張大中 Electronics I, Neamen 3th Ed. 39

Constant-Current Biasing

2 4 4

4 2

3 3

3

(

)

(

)

n

V V K V V

K   





 V V

V

4 3

3 4

3

/

n TN

TN GS

K K

V V

V  V  



4 3

3

1

/

GS

 K K

2 2 3

2

(

)

n

Q

K V V

I  

Electronics I, Neamen 3th Ed. 41

Multitransistor Circuit: Cascade Configuration

中央大學通訊系 張大中 Electronics I, Neamen 3th Ed. 43

Multitransistor Circuit: Cascode Configuration

中央大學通訊系 張大中 Electronics I, Neamen 3th Ed. 45