BipolarJunction Transistors

Key Takeaways

• The three terminals and four operating modes of a BJT.

• Performance efficiency measures of BJTs and building block topologies.

• Why BJTs excel in high-speed designs and continued technological advancements.

The TO-220 is the iconic package for bipolar junction transistors, but SMT models are available.

Back in its heyday, the bipolar junction transistor was the premier power element, before the maturation of MOSFET technology yielded the CMOS. Today, the power and size advantages offered by CMOS are simply too great to ignore in most applications, yet bipolar junction transistors still see appreciable usage in high-voltage/current switches and high-speed analog circuits.

Comparison of BJT and MOSFET Technology

Bipolar Junction Transistor Structure and Operation

The BJT is an active current-controlled element in circuits, able to take an input current on the base terminal and effectively multiply it for switching and amplification purposes. Until the maturation of MOSFET technology in the 1970s, BJTs were the primary active element in electronics and are still seen to this day in certain high-frequency applications where CMOS exhibit large switching losses. The bipolar junction is built of three alternating doped regions that are either electron-rich (n-type) or rich in the absence of electrons from available positions (p-type). Together, these two regions are alternated to make a PNP or NPN BJT. 

The BJT is comprised of three terminals:

• Base - The control terminal of the BJT that is lightly doped opposite to the surrounding collector and emitter regions. A biasing voltage placed on the base will have the current run either from collector to emitter (typically the preferred direction for maximum current gain) or emitter to collector.

• Collector - The largest semiconductor region of the BJT. It surrounds the base and emitter to prevent the escape of injected electrons. 

• Emitter - A heavily doped region that enables high current gain by injecting the bulk of charge carriers into the base-emitter junction.

The major difference between these two BJT styles is the direction of current flow: emitter to base for NPN and base to emitter for PNP. As BJTs require a biasing voltage to operate, the device is technically a voltage-controlled current source (like MOSFETs). However, it is often considered and treated as a current-controlled current source due to the low impedance of the base terminal. 

Depending on the biasing direction between terminals and the relative voltage on the pins, there are four distinct operational modes for BJTs:

• Forward - The standard amplification mode where the base-emitter junction is forward-biased and the base-collector junction is reverse-biased. Almost all BJTs provide the greatest common-emitter current gain in the forward direction.

• Reverse - A seldom used mode where the base-emitter junction is reverse-biased and the base-collector junction is forward-biased. Traditionally, the amplification is much less effective than in the forward direction, as the heavily doped region of the emitter cannot be optimized in this orientation.

• Saturation - A forward bias at both base-emitter and base-collector junctions acts as a closed switch for the BJT, effectively a logical high state.

• Cut-off - A reverse bias at both base-emitter and base-collector junctions acts as an open switch for the BJT, effectively a logical low state.

Designing Amplification Around Topology

As the BJT can be utilized in several configurations, multiple metrics measure its performance. The transistor characteristics relay the efficiency of the BJT due to its uneven doped region. In an NPN BJT, the heavier doped n-type emitter region can provide more electrons than the lightly-doped p-type base can provide holes. This construction prevents a loss of current due to recombination and the greater mobility of the electrons improves the diffusion rate to the collector across the base. The carrier proportions are defined as:

• α (alpha)  - Represents the common-base gain or the ratio of emitter terminal DC to collector terminal DC in the forward active mode. Ideally 1, but practically the value approaches 1 from the left due to electron and electron-hole recombination.

• β (beta) - Represents the common-emitter gain, or the ratio of collector terminal DC to base terminal DC in the forward active mode. Standard signal amplification may be in the range of 50, but this value can be less for high-power usage.

These gains are representative of the BJT topology used to fashion the particular transistor circuit. These topologies aren’t simple rearrangements of circuit elements, but rather complete changes to the functionality of the transistor based upon which pins of the BJT are tied to I/O or ground/power:

• Common-base - I/O on the emitter and collector (respectively) while the base is grounded. The separation between the collector and emitter reduces output feedback and improves signal stability. It can be used as a voltage amplifier or current follower and typically sees greater use in high-frequency applications, as the bandwidth of input signals remains constant.

• Common-collector - I/O on the base and emitter (respectively) with the collector tied to the ground or a power rail. It is often used as a voltage buffer, as its high input impedance and low output impedance allow the BJT to multiply currents and drive larger electrical loads than the input. 

• Common-emitter - I/O on the base and collector (respectively) with the emitter tied to the ground or a power rail. Application-wise it is similar to a common collector, but with a higher current gain and lower input impedance. Bandwidth suffers from parasitic capacitance formed between the base and collector, but this can be somewhat alleviated with an increase in input impedance (at the cost of gain), reducing the output impedance, or with differential amplification.

These single-stage transistors represent the building blocks of larger transistor networks and can be modified and combined to improve performance or create multi-stage transistor amplification.

Cadence Solutions Provide Robust PDN Simulation

As mentioned, the BJT has been largely supplanted by CMOS technology, but there are still areas where the former is preferred. The BJT offers two stark advantages over CMOS technology, with a high output impedance and a large transconductance (output current over input voltage) due to the high current gain present in many topologies. Further specialization of BJTs including heterogenous junction transformers that use different semiconductor materials for the base and emitter and integrated BJT and CMOS technology as BiCMOS. The prevailing usage of BJT or its sub-types is for high-speed signal switching in ultra/very high-frequency ranges and beyond.

Despite the march of technology, the bipolar junction transistor still holds relevance today and will continue to do so with high frequencies powering an increase in the transfer rate of modern communication protocols. Climbing speeds in design will also necessitate greater scrutiny of stack-ups and conditions leading to EMI. 

Fortunately, Cadence offers a comprehensive PCB Design and Analysis Software toolset that can fold SPICE simulation results directly into design files for an optimized workflow.  And, with the speed and functionality of OrCAD PCB Designer, it’s even easier to make the jump from design documents to prototype to full production.

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