2.5 The Bipolar Junction Transistor (BJT)

A Bipolar Junction Transistor (BJT) is a three-terminal active semiconductor device used to amplify or switch electronic signals. It consists of two p-n junctions configured in either NPN or PNP formation. Current flowing into one terminal (the base) controls the current flowing between the other two terminals (collector and emitter), making it a current-controlled device.

The following circuit shows a BJT in a common-emitter configuration. The xschem simulation file can be found here.

Figure 2.5.1: Simple BJT amplifier circuit.

Characteristics

BJT operation is governed by the relationship between the base-emitter and collector-emitter voltages. In the active region, the collector current \(I_C\) is given by:

\[I_C = \beta I_B\]

where:

• \(I_C\) is the collector current,
• \(I_B\) is the base current,
• \(\beta\)(or \(h_{FE}\)) is the DC current gain of the transistor.

BJTs have three operating regions:

• Cut-off: Both junctions reverse biased → transistor OFF
• Active: Base-emitter forward biased, base-collector reverse biased → transistor amplifies
• Saturation: Both junctions forward biased → transistor ON (switching)

Characteristics curve

The output characteristics of a BJT (i.e., \(I_C\) vs \(V_{CE}\) for different \(I_B\) values) show how the collector current varies with collector-emitter voltage:

Figure 2.5.2: BJT output characteristics.

IC level Implementation

At the IC level, BJTs are commonly implemented using bipolar processes, such as the BiCMOS (Bipolar CMOS) process, which combines bipolar and CMOS devices on a single chip. In standard bipolar or Bi-CMOS processes, the BJT is built vertically through the silicon substrate. The vertical NPN transistor, which is more common, is constructed with:

• Emitter: A shallow n+ diffusion,
• Base: A p-type region beneath the emitter,
• Collector: A deep n-type region reaching into the substrate.

This vertical structure provides high current density and better performance at high frequencies.

BJTs in ICs are often used for:

• Analog signal amplification,
• Bandgap references,
• High-speed digital drivers,
• Temperature sensors.

Although CMOS has largely replaced BJTs in most digital logic, BJTs remain essential in analog and mixed-signal IC design where precision and speed are critical.

Figure 2.5.3: IC level implementation of a BJT (cross-sectional view) [https://www.circuitstoday.com/monolithic-ic] .

BJT layout

Figure 2.5.4: IC level layout of a BJT. [https://ihp-open-pdk-docs.readthedocs.io/en/latest/]

The following image shows a scanned cross section view of a BJT.

Figure 2.5.5: scaned cross section view of a BJT.