Multi-Emitter Transistors

Multi‒Emitter Transistors

• Component density of an integrated circuit (IC) is enhanced by the efficient utilization of the chip area.

• Merged transistors, that is two or more devices which share one or more common regions, provide one method of space saving on a chip. The most widely used merged transistor is the multiple‒emitter transistor.

• Fig. 4.5.1 (a) shows the cross‒section of multi‒emitter bipolar npn transistor. As shown in Fig. 4.5.1 (a), multi‒emitters are normally fabricated within the same base region. Each emitter stripe can be considered as the emitter of a separate transistor.

• Fig. 4.5.1 (b) shows the top view of multiple emitter transistor.

• Fig. 4.5.2 shows three transistors with common collector and base terminals and its single multiple emitter transistor equivalent. In this construction each transistor shares a common base and collector.

• Multi‒emitter transistor is the basis of of the Transistor‒Transistor Logic (TTL).

• Multiple‒emitter transistors have been fabricated with more than 60 emitter stripes.

Applications of Multi‒Emitter Transistors

1. Primary application ‒ Transistor‒Transistor Logic (TTL)

• The most common and important application of a multi‒emitter transistor is in the input stage of TTL logic gates, especially in the NAND gate configuration. Each emitter acts as an independent input line.

• Function: The transistor performs the NAND logic function efficiently. If any one emitter receives a LOW signal, it pulls the base low, cutting off collector current and driving the output LOW.

• Advantage: This design eliminates the need for multiple input diodes (as used in earlier Diode‒Transistor Logic, DTL), providing faster switching speed, lower propagation delay and better circuit compactness. Hence, multi‒emitter transistors are the key elements that make TTL logic families faster and more efficient.

2. In Digital Integrated Circuits (ICs)

• Multi‒emitter transistors are extensively used in digital IC design due to their compact and multi‒input structure. By integrating multiple inputs into a single transistor, the total component count, chip area, and interconnection delay are significantly reduced.

• This leads to high‒speed and reliable operation in devices such as counters, registers, microcontrollers, and processors.

3. In Input Interface Circuits

• In microprocessors, microcontrollers, and programmable logic devices, multi‒emitter transistors are used in input interface sections.

• They enable simultaneous sensing and processing of several incoming digital control or data signals, ensuring efficient communication and signal handling between different digital modules.

4. In Specialized Circuits

• Although less common, multi‒emitter transistors can also be used in specialized high‒frequency or high‒power circuits.

• In such cases, multiple emitters may be connected in parallel to distribute the emitter current evenly, improving current‒handling capability and high‒speed performance.

• They are also used in certain bipolar semiconductor memory architectures for addressing and control functions, where multiple input lines must be managed by a single transistor device.

Review Questions

1. Write a note on multi‒emitter transistor.

2. What is the main advantage of multi‒emitter transistor?

3. What is multiple emitter transistor? Draw the symbol of that.

4. State the applications of multi‒emitter transistor.