Transistors

Transistor Definitions

• MOS - Metal Oxide Semiconductor

• FET - Field Effect Transistor

• BJT - Bipolar Junction Transistor

MOSFET and BJT

n-channel MOSFET npn bipolar transistor

gate

source

body

drain collector

base

emitter

Basic MOSFET Construction

BJT Symbolscollector

base

emitter

collector

base

emitter

npn bipolar transistor pnp bipolar transistor

MOSFET Symbols

A circle is sometimes

used on the gate terminal

to show active low input

drain

gate body

source

drain

gatebody

source

drain

gatebody

source

drain

gate body

source

or or

A. n-channel MOSFET B. p-channel MOSFET

Basic MOSFET

Basic CMOS Logic Technology

• Based on the fundamental inverter circuit

• Transistors (two) are enhancement-mode MOSFETs

• N-channel with its source grounded

• P-channel with its source connected to +V

• Input: gates connected together

• Output: drains connected

CMOS Inverter

p

n

GND

VDD

A Y = A'

Since the gate is essentially an open circuit it draws no current, and the

output voltage will be equal to either ground or to the power supply

voltage, depending on which transistor is conducting.

When input A is grounded (logic 0), the N-channel MOSFET is

unbiased, and therefore has no channel enhanced within itself. It is an

open circuit, and therefore leaves the output line disconnected from

ground. At the same time, the P-channel MOSFET is forward biased,

so it has a channel enhanced within itself, connecting the output line to

the +Vsupply. This pulls the output up to +V (logic 1).

When input A is at +V (logic 1), the P-channel MOSFET is off and the

N-channel MOSFET is on, thus pulling the output down to ground

(logic 0). Thus, this circuit correctly performs logic inversion, and at

the same time provides active pull-up and pull-down, according to the

output state.

CMOS Inverter - Operation

CMOS 2-Input NOR

A

B

+V

Y = A + B

This basic CMOS inverter can be expanded into NOR and

NAND structures by combining inverters in a partially series,

partially parallel structure. A practical example of a CMOS 2-

input NOR gate is shown in the figure.

In this circuit, if both inputs are low, both P-channel MOSFETs

will be turned on, thus providing a connection to +V. Both N-

channel MOSFETs will be off, so there will be no ground

connection. However, if either input goes high, that P-channel

MOSFET will turn off and disconnect the output from +V, while

that N-channel MOSFET will turn on, thus grounding the output.

Note the two p-channel FETs in series.

CMOS 2-Input NOR - Operation

CMOS 2-Input NAND

A

B

+V

Y = A • B

+V

A two-input NAND gate: a logic 0 at either input will force the output to

logic 1; both inputs at logic 1 will force the output to go to logic 0.

Note the two n-channel FETs in series and the two p-channel FETs in

parallel.

The pull-up and pull-down resistances at the output are never the same,

and can change significantly as the inputs change state, even if the output

does not change logic states. The result is uneven and unpredictable rise

and fall times for the output signal. This problem was addressed, and was

solved with the buffered, or B-series CMOS gates.

CMOS 2-Input NAND - Operation

CMOS 2-Input NAND: Buffered

B

+V +V

Y = A • B

The technique here is to follow the actual NAND gate with a pair of inverters. Thus,

the output will always be driven by a single transistor, either P-channel or N-channel.

Since they are as closely matched as possible, the output resistance of the gate will

always be the same, and signal behavior is therefore more predictable. Typically, the p-

channel transistor is approximately twice as wide as the n-channel transistor, because of

the difference in conductivity between electronics and holes.

Note that we have not gone into all of the details of CMOS gate construction here. For

example, to avoid damage caused by static electricity, different manufacturers

developed a number of input protection circuits, to prevent input voltages from

becoming too high. However, these protection circuits do not affect the logical behavior

of the gates, so we will not go into the details here. This is not strictly true for most

CMOS devices for applications that are power-switched; special inputs are required for

power-off isolation between circuits.

CMOS 2-Input NAND: Buffered

Decoders

Decoder Fundamentals

• Route data to one specific output line.

• Selection of devices, resources

• Code conversions.

• Arbitrary switching functions

• implements the AND plane

• Asserts one-of-many signal; at most one output will be

asserted for any input combination

Encoding

Binary

Decimal Unencoded Encoded

0 0001 00

1 0010 01

2 0100 10

3 1000 11

Note: Finite state machines may be unencoded ("one-hot")

or binary encoded. If the all 0's state is used, then

one less bit is needed and it is called modified

one-hot coding.

Why Encode?

A Logarithmic Relationship

N

0 25 50 75 100 125 150

Log

2(N

)

0

1

2

3

4

5

6

7

8

2:4 Decoder

What happens when the inputs goes from 01 to 10?

1 1

1 0

0 1

00D 0

D 1

A

B

A

B

A

B

A

B

AND 2

AND 2 A

AND 2 A

AND 2 B

Y

Y

Y

Y

E Q 3

E Q 2

E Q 1

E Q 0

2:4 Decoder with Enable

1 1

1 0

0 1

00

1 1

1 0

0 1

00D 0

D 1

ENABLE

A

B

C

A

B

C

A

B

C

A

B

C

Y

Y

Y

Y

E Q 3

E Q 2

E Q 1

E Q 0

AND 3

AND 3 A

AND 3 A

AND 3 B

Static Hazards

Static Hazard

2:1 Mux implemented by

minimized Sum-of-Products

Idealized matched delays

A

S

B

A

B

A

B

Y X1

Y X2

A

BY Y

Static HazardIn real circuits, delays don't

exactly match; Added delay

for illustrationA

S

B

A

B

A

B

Y X1

Y X2

A

BY Y

A Y S DBUFF

AND 2

AND 2 A

OR 2

Static Hazard

We now have a "glitch."

Same waveform, zoomed in.

Static Hazard

S=0

S=1

0 0 0 1 1 1 1 0

A B

0 0 1 1

0 1 1 0

Illustrating the minimized function on a Karnaugh map.

Only two 2-input AND gates are needed for the product terms

Static Hazard

0

1

0 0 0 1 1 1 1 0

A B

0 0 1 1

0 1 1 0

The blue oval shows the redundant term used to cover the

transition between product terms.

S

Static Hazard

How can we verify the

presence and operation

of this gate?

Y S D

A

B

A

B

A

B

A

B

C

Y X1

Y X2

Y X3

A

S

B

Y Y

ABUFF

AND 2

AND 2 A

OR 3

AND 2

Static Hazard0000 0

0001 1

0010 2

0011 3

0100 4

0101 5

0110 6

0111 7

1000 8

1001 9

1010 10

1011 11

1100 12

1101 13

1110 14

1111 15

0000 16

Terminal count of

a 4-bit synchronous

counter.

CLDCK

ACLR

D Q

DFC1B

CLRCLK

D Q

DFC1B

CLRCLK

D Q

DFC1B

CLRCLK

D Q

DFC1B

CLRCLK

A

B

C

D

AND 4Y

TCNT

Static HazardFlight Design Example

TMR Triplet Majority Voter

High-skew buffer

D Q

DF1

CLK

D Q

DF1

CLK

D Q

DF1

CLK

D Q

DF1

CLK

A Y

VCC

Y

Y

Y

GND

D0

D1

D2

D3

S1 S0

Static HazardFlight Design Example

Care is needed when using TMR circuits. First,

the output of the voter may be susceptible to a

logic hazard “glitch.” This is not a problem if the

TMR is feeding the input of another synchronous

input. However, the TMR output should never

feed asynchronous inputs such as flip-flop

clocks, clears, sets, read/write inputs, etc.

“Design Techniques for Radiation-Hardened FPGAs”

Actel Corporation, September 1997

-- based on “SEU Hardening of Field Programmable Gate Arrays (FPGAs) for Space

Applications and Device Characterization,” R. Katz, R. Barto, et. al., IEEE Transactions

on Nuclear Science, Dec. 1994.

Static HazardWe have covered static hazards. There are also

dynamic hazards. An example of a dynamic

hazard would be when a circuit is supposed to

switch as follows:

0 1

But instead switches:

0 1 0 1

Any circuit that is static hazard free is also dynamic hazard free.

Common Output Stage

Definitions

• VOH - Output voltage when driving high

• VOL - Output voltage when driving low

• IOH - Output current when driving high

• IOL - Output current when driving low

• tT - Transition time, usually measured between 10% and

90% of the waveform (2.2)

VOH Test ConfigurationVCC

i

+

-

Output Stage

Programmable

Load

VOL Test Configuration

VCC

i

+

-

Output Stage

Programmable

Load

VCC

VOUT

0 1 2 3 4 5

I OU

T(A

)

0.000

0.010

0.020

0.030

0.040

0.050

S/N LAN3501

S/N LAN3502

S/N LAN3503

S/N LAN3504

A1460A TID (VOH) TestPost-Irradiation

Vcc-VOUT

0 1 2 3 4 5

I OU

T(A

)

0.000

0.020

0.040

0.060

0.080

0.100

S/N LAN3501

S/N LAN3502

S/N LAN3503

S/N LAN3504

A1460A TID (VOL) TestPost-Irradiation

VOUT

0 1 2 3

I OU

T(A

)

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

S/N LAN4403

S/N LAN4404

S/N LAN4405

S/N LAN4406

RT54SX32 TID (VOH) TestPost-Irradiation

Vcc-VOUT

0 1 2 3

I OU

T(A

)

0.000

0.020

0.040

0.060

0.080

S/N LAN4403

S/N LAN4404

S/N LAN4405

S/N LAN4406

RT54SX32 TID (VOL) TestPost-Irradiation

RT54SX16 Rise Time

RT54SX16 Fall Time

Common Interface Levels• TTL

• 5V CMOS

• 5V PCI

• 3.3V PCI

• LVDS

• LVTTL

TTL Voltage Specification

• VOH - 2.4 V

• VOL - 0.5 V

• VIH - 2.0 V

• VIL - 0.8 V

• '1' Noise margin = 400 mV

• '0' Noise margin = 300 mV

5V CMOS Voltages

• VOH - ~VDD (no DC load)

• VOL - ~GND (No DC load)

• VIH - 70% VDD

• VIL - 30% VDD

• '1' Noise margin = ~30% VDD

• '0' Noise margin =~30% VDD