analog electronic - transistors, Schemes and Mind Maps of Electronics

Download analog electronic - transistors and more Schemes and Mind Maps Electronics in PDF only on Docsity!

1

Chapter 6 Physics of MOS Transistors

 6.1 Structure of MOSFET  6.2 Operation of MOSFET  6.3 MOS Device Models  6.4 PMOS Transistor  6.5 CMOS Technology  6.6 Comparison of Bipolar and CMOS Devices

Chapter Outline

Structure and Symbol of MOSFET

 This device is symmetric, so either of the n+ regions can be source or drain.

State of the Art MOSFET Structure

 The gate is formed by polysilicon, and the insulator by Silicon dioxide.

Voltage-Dependent Resistor

 The inversion channel of a MOSFET can be seen as a resistor.  Since the charge density inside the channel depends on the gate voltage, this resistance is also voltage-dependent.

Voltage-Controlled Attenuator

 As the gate voltage decreases, the output drops because the channel resistance increases.  This type of gain control finds application in cell phones to avoid saturation near base stations.

L and t

ox

Dependence

 Small gate length and oxide thickness yield low channel resistance, which will increase the drain current.

Effect of W

 As the gate width increases, the current increases due to a decrease in resistance. However, gate capacitance also increases thus, limiting the speed of the circuit.  An increase in W can be seen as two devices in parallel.

Channel Pinch-Off

 As the potential difference between drain and gate becomes more positive, the inversion layer beneath the interface starts to pinch off around drain.  When VD – VG = Vth, the channel at drain totally pinches off, and when VD – VG > Vth, the channel length starts to decrease.

Channel Charge Density

 The channel charge density is equal to the gate capacitance times the gate voltage in excess of the threshold voltage. ( ) ox GS TH Q  WC V  V

Charge Density and Current

 The current that flows from source to drain (electrons) is related to the charge density in the channel by the charge velocity. I  Q  v

Drain Current

2

D n ox GS TH DS DS D ox GS TH n n

V V V V

L

W

I C

dx

dV x

I WC V V x V

dx

dV

v

I

D

• V DS

for Different Values of V

GS   2 D , max GS TH

I  V  V

Linear Resistance

 At small VDS, the transistor can be viewed as a resistor, with the resistance depending on the gate voltage.  It finds application as an electronic switch.   n ox GS TH on

V V

L

W

C

R

