Integrated Injection Logic (IIL or I²L) Circuit:

It is the latest entry into the bipolar saturated logic field. It uses no biasing and loading resistors at all. Resistors require lot of power and space on an IC chip. Hence, their elimination results in higher density circuits operating at much reduced power. Because of its high speed and less power dissipation, it is used in large computers. Another feature of Integrated Injection Logic (IIL or I²L) Circuit is that it is easy to fabricate.

The circuit, as depicted in Fig. 47.7, consists of multiple collectors with a common base and emitter. The base current is provided by another transistor built in the chip.

Q₁ and Q₂ are multiple collector transistors and Q₃ and Q₄ act as current sources instead of resistors as in RTL. The base of Q₃ and emitter of Q₁ are tied, so also the collector and base, so that it makes for small area on the chip.

When supply voltage V is connected to emitters of Q₃ and Q₄, current is supplied to base of Q₁ and Q₂. The working of the Integrated Injection Logic (IIL or I²L) gate is explained below.

Case I : Both A and B LOW : A low at A will bypass the injected current from Q₃ and make Q₁ off. The input source acts as a sink for the current I₁. Similarly Q₂ will be off because input B is also low. Voltage at Z will be high.

Case II : Either A or B HIGH or Both HIGH : Current I₁ or I₂ or both are diverted to BE junctions of transistors Q₁ and Q₂ and they are turned on. The voltage at Z will be low being saturated voltage of 0.2 V. Thus a NOR output is obtained.

It may be noted that the current injection transistors Q₃ and Q₄ are PNP transistors whereas Q₁ and Q₂ are NPN type. This makes ease of manufacture and high packing density. The collector is relatively lightly doped, the base doped more heavily and emitter doped very heavily.