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EENG 3520: Electronics II
Lecture 3
Oluwayomi Adamo
MOS Field-Effect Transistors
Physical Structure and Physical Operation
Figure 4.1 Physical structure of the enhancement-type NMOS transistor: (a) perspective view; (b) cross-section.
Typically L = 0.1 to 3 mm, W = 0.2 to 100 mm, and the thickness of the oxide layer (tox) is in the range of 2 to 50 nm.
MOS Field-Effect Transistors
Physical Structure and Physical Operation (cont.)
Tri-Gate Transistor
MOS Field-Effect Transistors
Physical Structure and Physical Operation (cont.)
10
Feature Size (mm)
10
6
3
1.5
1
1
0.8
0.6
0.35
0.25
0.18
0.13
0.09
0.1
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Bipolar Junction Transistor
Physical Structure and Physical Operation
Bipolar Junction Transistor
Physical Structure and Physical Operation (cont.)
BJT Modes of Operation
Mode
EBJ
CBJ
Cutoff
Reverse
Reverse
Active
Forward
Reverse Active
Reverse
Forward
Saturation
Forward
Forward
> 0.5 V
Reverse
Bipolar Junction Transistor
Physical Structure and Physical Operation (cont.)
iE  iC  iB
iB 
iC

iE 

IS

e vBE /VT
 1
 1 v
iC 
ISe


iC   iE


 1


1
BE
/ VT
 ( I S /  )evBE /VT
Bipolar Junction Transistor
Graphical Representation of Transistor Characteristic
iC  I S e vBE / VT
Figure 5.16 The iC –vBE characteristic for an npn transistor.
Figure 5.18 The iC–vCB characteristics of an npn transistor.
Bipolar Junction Transistor
The BJT as an Amplifier and as a Switch
vO  vCE  VCC  RC iC
Grounded-emitter, Common-emitter (CE)
Voltage-controlled current source
Figure 5.26 (a) Basic common-emitter amplifier circuit. (b) Transfer characteristic of the circuit in (a). The amplifier is biased at a point Q, and a
small voltage signal vi is superimposed on the dc bias voltage VBE. The resulting output signal vo appears superimposed on the dc collector voltage
VCE. The amplitude of vo is larger than that of vi by the voltage gain Av.
Bipolar Junction Transistor
BJT Circuits at DC
Assumption:
• Only DC voltage are applied
• |VBE| = 0.7 V (Active mode)
• |VBE| = 0.7 V, |VCE| = 0.2 V (Saturation mode)
Analysis Method:
In which mode is the transistor operating?
 Assume one mode
 Determine voltage and current
 Check for consistency.
Assume active mode, check vCB: > -0.4V (npn) < 0.4V (pnp)
Assume saturation, IC/IB < , or forced < 
Bipolar Junction Transistor
Example :
To determine the voltages at all nodes and the currents
through all branches. Assume that the transistor  is
specified to be at least 50.
1. Assume active: VBE = 0.7V, VE = 6 – 0.7 = 5.3 V
2. IE = 5.3 / 3.3 = 1.6mA
3. IC = IE = (50/51)IE  1.6 mA
4. VC = 10 – 1.6 x 4.7 = 2.48 V < VB
1. Assume saturation: VBE = 0.7V, VE = 6 – 0.7 = 5.3 V
2. IE = 5.3 / 3.3 = 1.6mA
3. VCE = 0.2 V, VC = VE + VCE = 5.3 + 0.2 = 5.5 V
4. IC = (10 – 5.5) / 4.7 = 0.96 mA
5. IB = IE – IC = 1.6 – 0.96 = 0.64 mA
6.  forced = IC / IB = 0.96 / 0.64 = 1.5 < 50
Bipolar Junction Transistor
Biasing in BJT Amplifier Circuits
Goals: To establish a constant dc current IC in the collector of the
BJT
• Insensitive to variations in temperature and to the large
variations in the value of 
• To allow for maximum output signal swing
Two obvious Examples:
I B  (VCC  0.7) / RB
IC  I S e
VBE / VT
Small VBE, Large IC
IC depends on 
Bipolar Junction Transistor
Biasing in BJT Amplifier Circuits (cont.)
I E  VE  VBE  I E 
Bipolar Junction Transistor
Biasing in BJT Amplifier Circuits (cont.)
VCC  VBE
IE 
RC  RB /(1   )
VCC  VBE
RB
RC 
1 
Swing range is determined by VCB
VCB  I B RB  I E
RB
1 
Bipolar Junction Transistor
Biasing in BJT Amplifier Circuits (cont.)
CURRENT MIRROR
I REF 
VCC  (VEE )  VBE
R
SAME VBE
I  I REF 
VCC  VEE  VBE
R
Figure 5.47 (a) A BJT biased using a constant-current source I. (b) Circuit for implementing the current source I.
Bipolar Junction Transistor
Small Signal Models
Bipolar Junction Transistor
Small Signal Models (cont.)
Bipolar Junction Transistor
Application of the Small-Signal Equivalent Circuits
Systematic process for the analysis of BJT
1. Determine the dc operating point of the BJT and in particular the
dc collector current IC
2. Calculate the values of the small-signal model parameters: gm, r, re
3. Eliminate the dc sources by replacing each dc voltage source with
a short circuit and each dc current source with an open circuit
4. Replace the BJT with one of its small-signal equivalent circuit
models.
5. Analyze the resulting circuit to determine the required quantities
(voltage gain, input resistance)
Bipolar Junction Transistor
Application of the Small-Signal Equivalent Circuits
Known
Unknown
Homework
6.59, 6.95, 5.9, 5.26, 5.76