Grace Xing---EE30357 (Semiconductors II: Devices) 1

EE 30357: Semiconductors II: DevicesLecture Note #19 (02/27/09)

MOS Field Effect Transistors Grace Xing

Outline:

1. Last class: Small signal equivalent circuit model & Short channel effects

2. Compound semiconductor based devices1. MESFET2. JFET3. HEMT

3. Quick revisit of MOS capacitors and FETs (read on your own how DRAM, CCD, flash memory etc. work, they are all based on MOS structures!) – next class

1. Flat band voltage2. Effects of oxide charges (interface charges, fixed charges)

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Schematic showing the structure of a GaAs MESFET.

Figure 8.26

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(a) Cross section of a MESFET at equilibrium indicating the depletion regions; (b) the energy band diagram

perpendicular to the gate. The channel thickness is t.

Figure 8.27

metal

SiO2 SiO2

Si

Similar to the SOI devices (ultra-scaled MOSFET), electrons are confined in the channel by large bandgap materials

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MESFET energy band diagram and depletion region (a) at equilibrium and (b) for an applied gate voltage

that depletes the channel. In the second case, the channel is still n type but it is empty of carriers because of

the increased barrier height.

Figure 8.28

Depletion region

Depletion region

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Cross section of a MESFET under small VDS bias and the corresponding energy band diagrams at the source

end and drain end of the gate.

Figure 8.29

1 V

1 eV

Mistakes in textbook(There are quite some mistakes on MESFET and HEMTs in Anderson. Use my slides instead.)

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The JFET. The shaded areas represent the depletion regions. (a) Cross-sectional diagram; (b) the energy band

diagram, (EC), for VDS = 0; (c) when VDS 0 (but not yet in saturation) the depletion region at the drain end

increases, narrowing the channel and increasing the channel resistance.

Figure 8.32

Substrate

gate

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Gradual channel approximation• Channel is long such that electric field L << T

• Gate depletion w is independent of L(y)

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• Gradual channel approximation• n(y) ~ ND’• long channel and constant low-field mobility lf

• No need to remember these equations

'

1/ 2

'

1/ 2

'

1/ 2

'

( )1

2( ) ( ( ))

( ( ))

, (0) , 0

2(0) ( )

( )1

D lf D ch

sbi GS ch

D

j bi GS ch

ch

sbi T

D

bi GS chD lf D

bi T

w yI dy qW N a dV

a

w y V V V yqN

whereV V V V y

At threshold w a V

w a V VqN

V V V yI dy qW N a

V V

'

3 / 2 3/ 2

2( )

3

ch

lf DD

DS bi GS bi GSDS bi T

bi T bi T

dV

current can be solved in sublinear regionqW N a

IL

V V V V VV V V

V V V V

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High electron mobility transistor (HEMT)(other names are Modulation Doped FET – MODFET and

Heterojunction FET - HFET)

Mobile electrons are separated from the ionized donors less scattering high mobility!

Error in A&A, Fig.8.24 (b) and description of how gate works are wrong.The other parts are reasonably correct.

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High speed IC – NTT 80 Gbits/s multiplexer

HEMTs can operate much faster than MOSFETs

high speed ICs