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Integrated injection logic (I²L) is a bipolar digital logic family in which a current injector and an inverting switching transistor are built together in one compact integrated structure. An input signal steers the injected current away from or into the base of the switching transistor, and that transistor’s collector provides the output. The approach was developed to pack dense logic onto bipolar silicon, and it was used in memories, processors, and custom chips before CMOS took over mainstream logic.
Other names for the same circuit family
You will see the family written as I²L, I2L, or IIL. Texas Instruments and some patent literature also call it merged-transistor logic (MTL), because the injector and switching transistor are physically merged. All of these names refer to the same bipolar technique, not to a separate product line.
How an I²L cell works
A simplified I²L cell has two transistors with distinct roles:
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- The injector is a PNP transistor that supplies current to the base of the next stage. It is the source of the drive current, and in a cell it is typically a lateral device.
- The switch is a vertical NPN transistor whose base receives the injected current. Its collector is the logic output.
The logic operation happens in the steering of that injected current. Depending on the input condition, the current is either diverted away from the switch’s base, leaving it off, or allowed to drive the base, turning it on so the collector pulls low. Because the switch can sink current but does not source a high level itself, the output is open-collector. A high state is established by an external or injected current path rather than by the cell’s own output transistor.
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Two features follow from this structure:
- Multiple outputs. A single injector can feed more than one collector region, so one cell can produce several outputs.
- Wired logic. Tying open-collector outputs together implements a wired function, the same idea used in other open-collector families.
The compact layout depends on closely integrating the lateral injector and the vertical switch in the same silicon region. This is the main reason the cell could be small.
I²L is often described loosely as “low power,” but that label needs context. A cell draws bias current while it is static. This differs from CMOS, where ideal static operation draws almost no current from the switching node. I²L should therefore not be explained as a voltage-mode CMOS gate. Its mechanism is injected-current steering in a bipolar structure.
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Why engineers used it
The design goal was large-scale integration with simple cells and a manageable power-delay product. In their 1973 review, C. M. Hart and A. Slob wrote that integrated injection logic “can lead to ‘bipolar’ integrated circuits whose performance can match that of MOST devices in many respects.” That is the authors’ assessment at the time. It is a historical judgment, not a current benchmark against CMOS.
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The 1972 foundational paper is Kees Hart and Arie Slob, “Integrated injection logic: a new approach to LSI,” IEEE Journal of Solid-State Circuits 7(5), 346–351, October 1972. The technical detail in this article comes from the 1973 review and from a 1987 engineering paper by Mark Grabosky, because the 1972 full text was not reviewed for this explanation.
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Where I²L was applied
Grabosky’s 1987 paper lists I²L use in:
- LSI memories
- microprocessors
- digital wristwatches
- A/D and D/A converters
- custom integrated circuits
A 1976 Texas Instruments patent, filed in 1975, shows the technique applied to a random-access memory cell. Its latch keeps current injection active during both read and write operations.
Reading the published performance figures
Grabosky’s 1987 paper reports propagation times as low as 63 ns and power consumption as low as 0.01 mW per gate. These values apply only to the specific developed processes discussed in that paper. They are not general specifications for I²L, and they should not be compared directly with figures from other processes or other years.
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Patent documents are useful for device structures and specific implementations. Their statements about advantages, however, are the applicant’s case for the invention, not neutral comparisons.
How to compare I²L with CMOS and other bipolar families
No controlled, same-generation benchmark across these families was available for this article, so a single “faster” or “lower power” verdict would be misleading. Compare them on these axes instead:
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- Integration density and the complexity of the cell and process.
- Switching speed and the power-delay tradeoff at a given process generation.
- Static bias current, which I²L draws and ideal CMOS does not.
- Output structure, fan-out, and the interface requirements of open-collector outputs.
- Whether the discussion concerns historical designs or a current product.
Where I²L stands today
I²L is a historical logic family. As CMOS improved in speed, power consumption, and cost, it displaced I²L in mainstream digital design, and the later patent background treats CMOS as the dominant approach. No current adoption figure or modern I²L product line was established for this article. If you encounter I²L in a current design, treat it as a legacy or specialized process that should be confirmed against the device’s own documentation.
Quick Recap
Key points to remember
- I²L is a bipolar logic family that pairs a current injector with a switching transistor.
- Logic comes from steering injected current at the switch’s base. The output is open-collector and can have multiple collectors.
- Its historical strength was dense integration. Speed and power depended on process and bias conditions.
- Static bias current is its main limitation, and CMOS scaling displaced it in mainstream logic.
- Published numbers must be read together with the paper and process that reported them.
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