Digital Integrated Circuits: Definition, Types & Examples

Instructor: Shadi Aljendi

Shadi has a Ph.D. in Computer Science and more than 20 years experience in industry and higher education.

In this lesson, we will look at how we use logic functions to build complex digital logic systems. We will define two types of integrated circuits, digital and analog, with a focus on the functionalities and families of digital integrated circuits.

Inside the Microprocessor Package

What's inside a microprocessor? We know that a microprocessor is a digital circuit, mainly built using a combination of logic functions, but how are these functions built? What is really inside the plastic package that includes the microprocessor? In fact, the microprocessor package contains an integrated circuit, and this integrated circuit is the focus of this lesson.

What is an Integrated Circuit?

An Integrated Circuit (IC) is an electronic device that gathers (or integrates) a number of electronic components on a small semiconductor chip. Usually, an IC has a particular functionality. This functionality could be as specific as amplifying the voltage of a signal or applying a logic AND on 3 inputs and it could be broad as a microprocessor.

ICs can be digital or analog. Usually, analog ICs handle continuous signals, such as audio signals. Digital ICs handle discrete signals such as binary values.

What is a Digital Integrated Circuit?

Logic functions, such as AND, OR and NOT, are essential in building functionality for modern digital systems. So, how do you implement a logic function? Logic functions are implemented using transistors. For example, Figure 1 shows an internal transistor structure of a NAND gate.

Figure 1: One possible implementation of the NAND logic function

You can build this gate using electronic components, such as transistors. However, modern digital circuits might contain ten billion transistors. You can see how difficult it is to build such circuits by combining transistors. A practical approach is to use ICs that combine a small or large number of transistors and/or other electronic components on one tiny chip to achieve a particular functionality. This functionality can be as simple as a basic logic function, such as a NOT function, or as complicated as a microprocessor.

The semiconductor circuit is encapsulated in a plastic casing and it is connected to the outer space by metal pins as shown in Figure 2.

Figure 2: One shape of commercial ICs
Shape of integrated circuits

Figure 3 shows the internal structure of the 4011 NAND IC. ICs are distinguished by a combination of characters and/or numbers. For example, Intel has fabricated different microprocessors and named them using different schemes. Pentium is the name of one of these processors. In fact, it is the name of the plastic case that contains the semiconductor logic circuit that composes the processor. Previously, Intel used numbers to name its processors, such as 8086 or 80286 and sometimes it used a combination of digits and letters, for example, 80386 SX and 80386 DX.

Figure 3: Pin layout of the 4011 NAND gate
4011 NAND gate pinout

From Figure 3 we also know that the 4011 circuit contains 4 NAND gates and how they are connected to the external pins. We can always refer to datasheets to find this kind of information. Datasheets also provide more detailed information about the behavior of the IC.

Families of Digital Integrated Circuits

ICs are fabricated using different technologies. Earlier in this lesson, we saw the CMOS implementation of a NAND function. CMOS stands for complementary metal-oxide-semiconductor, which is a technology used to construct a certain type of transistor. Logic functions can be built using different technologies. For example, Figures 4 and 5 show the RTL (Resistor-Transistor Logic, which indicates that the electronic input of the circuit is a resistor and the output device is a transistor) and TTL (Transistor-Transistor Logic, which indicates that the input and the output of the circuit are transistors) structures of the NAND function.

Figure 4: RTL structure of the NAND function

Figure 5: TTL structure of the NAND function

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