Bipolar Junction Transistor Circuits:Properties of the BJT

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Introduction

The bipolar junction transistor (BJT) was the workhorse of the electronics industry from the 1950s through the 1990s. This device was responsible for enabling the computer age as well as the modern era of communications. Although early systems that demonstrated the feasibility of electronic computers used the vacuum tube, this element was too unreliable for dependable, long-lasting computers. The invention of the point-contact transistor in 1947 [1] and the BJT shortly thereafter along with the rapid improvement in this device led to the development of highly reliable electronic computers and modern communication systems.

Integrated circuits (ICs), based on the BJT, became commercially available in the mid-1960s and further improved dependability of the computer and other electronic systems while reducing the size and cost of the overall system. Ultimately, the microprocessor chip was developed in the early 1970s and the age of small, capable, personal computers was ushered in. While the metal-oxide-semiconductor field- effect transistor (MOSFET) device is now more prominent than the BJT in the personal computer arena, the BJT is still important in larger high-speed computers, high-frequency communication systems, and power control systems.

Because of the continued improvement in BJT performance and the development of the heterojunction BJT, this device remains very important in the electronics field even as the metal oxide semiconductor (MOS) device becomes more significant.

Properties of the BJT

The present BJT technology is used to make both discrete component devices as well as IC chips. The basic construction techniques are similar in both cases with primary differences arising in size and packaging. The following description will be provided for the BJT constructed as an IC device on a silicon substrate. These devices are referred to as “junction isolated” devices. The cross-sectional view of a BJT is shown in Figure 10.1 [2]. Lower voltage devices fabricated on IC chips may occupy a surface area of around 100 mm2. There are three physical regions comprising the BJT. These are the emitter, the base, and the collector. The thickness of the base region between emitter and collector can be a small fraction of a micrometer, while the overall vertical dimension of a device may be a few micrometers.

Thousands of such devices can be fabricated within a silicon wafer. They may be interconnected on the wafer using metal deposition techniques to form a system such as a microprocessor chip or they may be separated into thousands of individual BJTs, each mounted in its own case. The photolithographic methods that make it possible to simultaneously construct thousands of BJTs have led to continually decreasing size and cost of the BJT.

Electronic devices, such as the BJT, are governed by current–voltage relationships that are typically nonlinear and rather complex. In general, it is difficult to analyze devices that obey nonlinear equations, much less developed design methods for circuits that include these devices. The basic concept of modeling an electronic device is to replace the device in the circuit with easy-to-analyze components that approximate the voltage–current characteristics of the device. A model can then be defined as a collection of

Bipolar Junction Transistor Circuits-0000

simple components or elements used to represent a more complex electronic device. Once the device is replaced in the circuit by the model, well-known circuit analysis methods can be applied.

There are generally several different models for a given device. One may be more accurate than others, another may be simpler than others, another may model the DC voltage–current characteristics of the device, while still another may model the AC characteristics of the device.

Models are developed to be used for manual analysis or to be used by a computer. In general, the models for manual analysis are simpler and less accurate while the computer models are more complex and more accurate. Essentially, all models for manual analysis and many models for computer analysis include only linear elements. Nonlinear elements are included in computer models, but increase the computation times involved in circuit simulation over the times involved in simulation of linear models.

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