TTL Circuits

The abbreviation TTL stands for transistor-transistor-logic and has been the well known standard for integrated logic circuits for decades. The 74xx series moved on to modern technologies, but still the chips inherit their functionality from the late 1960's TTL devices.

1. Technology
2. Function
3. Application

1. Technology  

The basic components of integrated TTL circuits are resistors and NPN transistors. These structures are formed on typically moderately p-type doped silicon wafers with an epitaxial n-type doped layer on top. Before the n-layer is grown, a highly p-type doped surface layer is formed on the substrate to serve as channel stop. A buried highly n-type layer underneath each transistor is also formed by ion implantation.
The base area and the current path for resistors is then formed by p-type (boron) ion implantation overcompensating the n-type doping in the epitaxial layer. The emitter area is formed by another n-type doping, partly overcompensating the p-type base implantation.
A schematic cross-section of both, a transistor and a resistor are shown below. The value of the resistor - typically some hundred ohms to some ten kilo ohms is determined by the width-to-length ratio of the implanted p-type current path. All integrated devices on a chip are isolated against each other by silicon dioxide regions.

Cross section of the basic parts of an integrated TTL circuit: an NPN transistor and a resistor.

2. Function  

The schematic of a type 7400 2-input NAND gate shows the totem-pole output stage as well as the emitter coupled inputs. The input transistors are integrated with a common-base, common-collector, multi-emitter transistor and only drawn here separately for clearance and simulation purposes.

Schematic of a 2 input TTL NAND gate of the 7400 type.
(R1=4k, R2=1.6k, R3=1k, R4=130)

Logic response and power consumption of a 2 input TTL NAND gate. (pspice simulation)

Transfer function of a TTL inverter. The output voltage is ploted as a function of the input voltage. (pspice simulation)

3. Application  

The 74xx series and its 54xx counterpart with militaric specifications (wider operating temperature and voltage range) defined todays standard for microelectronic devices with low densities of gates per chip. The highest integrated family members count approximately a few hundred transistors and as many resistors on one chip. The available functions in the TTL family range from single gates to counters with on-chip decoders and drivers. TTL chips are not of wide use anymore because of their comparably poor performance - they are slow and power consuming. Pin compatible devices were fabricated by almost every manufacturer. They have been replaced by advanced technologies like Low-Power Schottky or CMOS.
   As of today (march 2000) TTL circuits are still available from Texas Instruments (www.ti.com) and Fairchild (www.fairchildsemi.com)

Responsible for these pages: U. Zimmermann