Microsoft Word - Digital Logic Design v_4_6a

1.3. Digital Design Overview (from Transistor to Super Computer)

 All digital systems from the smallest to largest run on a 2-valued system (also called Binary system).
So a mechanism is needed to represent the two values. This is typically accomplished with a switch
that can be on or off. In the early days, mechanical switches were used, followed by vacuum tubes
as switches. Today we use transistors that can be configured to approximate the switch on and off
modes. Transistors are fast, inexpensive and small.

 Transistor overview (the invention that makes today’s automation possible)

The transistor, invented by three scientists at the Bell Laboratories in 1947, rapidly replaced the
vacuum tube as an electronic signal regulator.

 Transistors are the basic elements in integrated circuits (ICs). An IC consists of a very large

number of transistors interconnected with circuitry and packaged into a single silicon microchip or

"chip." A typical processor chip has many millions of transistors.

 A transistor is developed based on semiconductor material characteristic. Semiconductor

material used basically as a switch as shown below:

 Semiconductor material is given special properties by a chemical process called doping. The

doping results in a material that either adds extra electrons to the material (which is then called N-

type for the extra negative charge carriers) or creates "holes" in the material's crystal structure

(which is then called P-type because it results in more positive charge carriers).

Today's computers use circuitry made with complementary metal oxide semiconductor (CMOS)

technology. CMOS uses two complementary transistors per gate (one with N-type material; the

other with P-type material). When one transistor is maintaining a logic state, it requires almost no

power when not switching.

 Semiconductor Integration scaling

 Small-Scale Integration, SSI (Basic gates: OR, NOR, NOT, AND)

 Example: Inverter (NOT) is a common SSI element used in Digital Design (Vendors provide

usage information and specifications in the form of a data sheet)

Base

Emitter (E)

Collector (C)

NPN Transistor Example

”A small current at the base causes the CE connection to change from open to a short”

P-Type

N-Type

Base

Collector

N-Type

Emitter