Prof. Dr. Thorsten Jungmann (last updated 2026-10-11)
1 What does it look like?
A logic gate (German: Logikgatter) is usually not a component of its own but a small part of an integrated circuit. As a single component, gates come in integrated circuits of a logic family, several gates per package: the 74HC00, for example, contains four NAND gates in a package with fourteen pins [11].
Integrated circuit with four gates
The 74HC132 contains four NAND gates with two inputs each.
2 What can it do?
A logic gate carries out one of the basic operations of the binary system [5, p. 43]: the basic operations NOT, AND and OR or the derived operations NAND, NOR, XOR and XNOR. Its inputs and its output take only the values 0 and 1.
3 How does it work?
A logic gate is built from transistors that work as switches. In complementary metal oxide semiconductor (CMOS) technology, p-channel MOSFETs connect the output to the supply voltage, and n-channel MOSFETs connect it to ground; the two never conduct at the same time. CMOS builds the NOT, NAND and NOR gates directly; an AND gate is a NAND gate followed by an inverter, an OR gate a NOR gate followed by an inverter [5, p. 64]. A real gate needs a few nanoseconds until its output follows a change at its inputs, the propagation delay [5, pp. 56–57].
4 What does it look like inside?
Inside, there are a few transistors and their connections on a chip of semiconductor: two for an inverter and four for a NAND gate with two inputs [5, pp. 64–66].
5 What role does it play in teaching?
In Digital Electronics, the logic gates are the building blocks of every circuit. DET-02 introduces each of them in seven forms of representation: schematic symbol, contact logic, truth table, logic expression, VHDL code, Karnaugh map and timing diagram. In the logic simulator, every gate can be tried out.
6 What role does it play in practice?
Processors and microcontrollers consist of billions of transistors that form logic gates [5, p. 62]. Integrated circuits with a few gates, such as those of the 74HC family, solve small tasks on circuit boards and are used in laboratory exercises. In industry, the logic of a machine is often programmed on a programmable logic controller instead of being built from gates.
Symbols
- Rectangle according to IEC 60617 with a qualifying symbol: 1 for the inverter, & for AND, ≥1 for OR, =1 for XOR; a circle at the output negates it
Calculations
- Propagation delay , and (DET-02, 3.5)
- Delay of a circuit as the delay of its longest path (DET-02, 6.3)