From analog to digital circuits
A phenomenological overview
Bogdan Roman
Outline
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Insulators, conductors and semiconductors
Semiconductor diodes: the p-n junction
The Field Effect Transistor (FET):
– The Junction FET (JFET)
– The Metal Oxide Semiconductor FET (MOSFET)
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The MOS Inverter
– Resistive and same MOS type
– Complementary MOS (CMOS) technology
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Elementary gates
Flip-flops
Examples
Loosely based on the IA and 3B Engineering dept. courses
(Linear Circuits and Devices, Digital Circuits,
Information Processing, Integrated Digital Electronics)
Insulators and conductors
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Current I given by: I  ne u A
V
ne A
I
V
L
L
  conductivity  ne
u  E  
R  resistance 
Mean velocity of electron:
u  E
where  = mobility
L
A
Ohm's Law: I 
V
R
Semiconductors: Intrinsic silicon
At low temperatures, silicon is an
insulator since there is not enough
thermal energy to break the covalent
bonds.
At room temperature, the thermal energy kT
~ 1/40 eV is enough to break a few covalent
bonds to produce free electrons. This also
leaves holes (i.e. positive net charges left by
the broken covalent bond).
Both electrons and holes contribute to
current flow.
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Semiconductors: Extrinsic silicon
electron concentration n
electron mobility n
hole concentration p
hole mobility  p
n-type conductivity  n  nen
p-type conductivity  p  pe p
when n  p then  n  p
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The p-n junction: The diode
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Reverse biased diode:
Forward biased diode:
The Diode (contd.)
Reverse biased diode:
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Forward biased diode:
The Junction Field Effect Transistor (JFET)
- Proposed by Shockley in 1951
- First made by Teszner in 1958 in
France
JFET Interactive (opens browser)
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Metal Oxide Semiconductor FET (MOSFET)
- First made in 1960 at Bell Laboratories in
the USA by Atalla and Kahng.
- Offers extremely high component density
in integrated circuits.
- Very high input resistance, low noise,
simpler fabrication than bipolar transistors.
MOSFET Interactive (opens browser)
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The NMOS inverter
Resistive load:
-When input is low (0) then T1 is off,
hence output goes high (1) (i.e. VDD)
-When VIN = high (1) then T1
conducts (linear region) and brings
the output low (0), depending on RL
-High RL = low logic zero and low
power consumption but large area on
silicon and slow switching =>
compromise
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NMOS load:
- T2 has the gate tied to its drain and
is always on (and in saturation). Acts
as a pseudo-resistor load.
- Similar operation to the resistive
load inverter
- Smaller area on silicon (so easier to
manufacture) and faster switching but
has a lower high logic voltage (VDD –
VT), and high power consumption
when input high.
Complementary MOS (CMOS) inverter
In CMOS technology, the output is
clamped to one of the power rails by a
conductive (on) device, while the other
device serves as a load of effectively
infinite resistance (off). This leads to
static properties that approximate those
of the ideal inverter.
- The PMOS devices is slower (lower
mobility of holes) so it has to be larger
to compensate. It is also more complex
to manufacture.
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NOR and NAND gates
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+V
+V
A
B
A+B
B
NOR Gate
The 74HC02 IC has
four 2-input NOR gates
A+B
A
B
A
A+B
A
A
B
B
+V
+V
A·B
NAND Gate
A
B
A·B
The 74HC00 IC has four
2-input NAND gates
A·B
B
B
A
A
Flip-flops and clocked flip-flops
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S

=
Q
S
Ck
R
Master
Slave
Q
Q
E
R
Q
Multiplexer
A crucial circuit, vital for implementing functions:
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Binary Counter
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Current state
(ABCD)
Next state
(ABCD)+
0000
0001
0001
0010
0010
0011
….
….
1111
0000
Real stuff
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