Integrated Circuits and Logic Devices
The planar fabrication process was well suited to the creation of new semiconductor structures, for example, it enabled transistors with multiple emitters, sharing a common collector, to be integrated:
These are electrically equivalent to two BJT transistors connected in parallel, except for the emitters (not to be confused with conventional transistors with multiple emitter leads, which are used to reduce inductance to a single emitter). A current from either emitter injected into the base flows into the collector (minus a small base bias fraction). These structures have an important application in digital logic circuitry, where the emitters can serve as separate inputs to a logic function, as in Transistor-Transistor Logic (TTL), which was first introduced by Texas Instruments in 1962 as the "7400 series", remained popular until the 1990s, and still has important applications (often embedded within custom ICs).
Before discussing TTL further, I will briefly digress to mention their immediate predecessor technologies, which were first to be fully integrated.
In 1961, US President John F. Kennedy announced a programme for "landing a man on the Moon and returning him safely to the Earth", and the first commercial logic integrated circuits (ICs) were released to the market by Fairchild Camera and Instrument Corp. as the µL900 family, representatives of which are still available today:
Transistor Museum Fairchild uLogic® 923
These devices were an early type of logic gate known as resistor-transistor logic (RTL), which had developed out of circuits that could be conveniently made from discrete resistors and transistors. The first Apollo spacecraft guidance computer used approximately 4100 RTL logic gates.
RTL had poor noise immunity, and limited ability to drive multiple loads (fan-out), so buffer circuits such as this were needed to expand the number of drivable stages, provide isolation, or drive bus lines. Whereas the μL914 logic gate had a fan-out of 16, the μL900 buffer could increase the fan-out to 80.
The μL914 dual two input gate could be used as a NAND or NOR logic function. If either input 1 or 2 were driven high, output 7 would be driven low. When a high voltage represents a logic 1, this is NOR function (inverted version of OR, where the output would be high if either input went high). If negative logic is used, where a low voltage represents a logic 1, both inputs must be low for the output to be high (inverted version of AND, where the output would be low when both inputs are low).
The next intermediate stage between RTL and TTL was diode-transistor logic (DTL), which used diodes to perform logic functions (diodes require less space than resistors when integrated) and was somewhat faster.
Signetics launched a DTL logic range in 1962, and Fairchild launched their own in 1963, which was cheaper and outsold the Signetics devices.
Ferranti produced a logic IC range in Europe in 1961, marketed as Micronor I and aimed at computing for Naval applications, followed by Micronor II, a faster DTL range. DTL had the best speed and reliability characteristics for integrated logic, at least until TTL came along.
The following diagram shows an example of DTL from the Texas Instruments SN3900 / SN4500 Series:
The first integrated logic devices were very costly, so were limited to applications where the advantage of integration was essential, such as space exploration.
As integrated circuits began to be commercially successful, because of the great benefit of replacing discrete circuits with many transistors, they were often applied to uses well outside their intended application, such as is shown by the following circuit, which was an application of the Ferranti ZSS54A Micronor II triple inverter (a DTL device) as a signal injector for TV servicing, with one stage biased into a linear mode for use as a headphone amplifier. This circuit by A. J. McEvoy appeared in the August 1967 edition of "Practical Television":
Initially, only a few logic gates (up to about 20 gates) could be integrated, so the ability to create a logic function by building it in to the structure of a single transistor itself was a great advantage. One multi-emitter transistor could now do the job of multiple diodes. The multiple-emitter planar transistor structure when used as a logic input could be connected directly to the preceding transistor output driver stage and was faster than the earlier RTL and DTL families, because the resistor elements of the input stages were eliminated, so the capacitance of the input transistor and package capacitance could be charged directly by the driver transistors. This was the Transistor-Transistor Logic (TTL) logic family, introduced in 1964 by Texas Instruments as the 7400 (industrial) and 5400 (military) qualified series.
The next circuit shows the input stage of a TTL logic circuit:
If both input A and B are high (close to Vcc voltage), then both base-emitter junctions must be below the forward bias potential, and the base-collector junction of Q1 must be forward biased via the Vcc potential across R1, Q1, D1, R2 and the base-emitter junction of Q2, which is also forward biased. A current flows through this chain, and with Q2 biased into conduction, the voltage at X is low. The circuit is designed so that in this case, the operating point of Q2 brings the collector voltage down to Vce(sat), normally less than 1 Volt.
If an emitter input (A or B) is connected to a low input voltage, the Q1 base-emitter junction can become forward biased. The current from R1 into the base can now flow out through the emitter. The voltage at Q1's collector falls, as Q1 is now fully turned on and the collector-emitter voltage difference must be small, and as long as the emitter voltage is low enough, the base-emitter voltage at Q2 falls below forward bias levels, and current through R3 pulls the output X up to near Vcc voltage (as long as the output X is not loaded too heavily).
Thus, if A or B is low, or both are low, X is high, and if A and B are both high, then X is low. This is the function of a NAND gate: a logic gate which generates an inversion of the simpler AND function (where the output is high if both inputs are high).
Info
Zip file containing Circuit Scribe project files basicfab.zip [36KB]:
alloytr.cct - Diagram of alloy junction transistor
cosmos.cct - CMOS inverter and NOR gate mosdepl.cct - Diagrams of MOSFET depletion device
mosenh.cct - Diagrams of MOSFET enhancement device
mosfab.cct - Diagram of MOSFET fabrication
planardtl.cct - DTL logic gates
planarfab.cct - Diagram of planar fabrication process
planarttl.cct - Diagram of multiple-emitter transistor and TTL circuit
transistors.cct - Diagrams of point-contact transistor
ul900914.cct - uL900 buffer and uL914 dual two-input gate
For Circuit Scribe software, see: Software
Catalog: Fairchild-uL900-914 (Industrial RTL Micrologic Integrated Circuits (uL900, uL914))
[1] Kahng D, Atalla MM. "Silicon-silicon dioxide field induced surface devices", IRE-AIEEE Solid-State Device Research Conference, Carnegie Institute of Technology, Pittsburgh, PA, 1960.
