Breakdown characteristics of bipolar junction transistors (BJT) and switching speeds as switching applications

This article explores the breakdown characteristics of a bipolar junction transistor (BJT) and its performance as a switching element. Experimental results show that the BJT can switch in less than 20 nanoseconds, and under specific conditions like base-emitter short-circuit, the switching speed drops below 10 nanoseconds, making it an ideal choice for high-speed applications. A nanosecond Marx-type negative pulse generator was developed using BJT as the switch. The generated pulse has an amplitude of up to 2.3 kV, a width of less than 10 ns, and a falling edge as fast as 3 ns, demonstrating the potential of BJT in advanced pulse generation systems.

1. Introduction

To generate pulsed plasma for various applications, different types of pulse power generators have been developed, such as magnetic compression pulse generators (MPC), pulse forming circuits, and others. However, these systems are often large and complex, limiting their miniaturization and affecting the precision of signal edges. To address this, a Marx-type pulse generator was designed using capacitors for energy storage and BJT as the switching component. This system offers fast switching, adjustable output, and extremely short rising or falling edges, making it suitable for generating strong electric field pulses. Its compact size allows for easy portability and deployment in diverse environments.

2. Principle and Design

2.1 Marx Generator Principle

The Marx generator is a well-known high-power pulse generator, typically using spark gap switches as the main components. However, spark gaps are not ideal for high-frequency operations due to issues with lifespan and maintenance, which hinder miniaturization. In recent years, high-power switching devices like MOSFETs and IGBTs have replaced spark gaps in lower power settings. Here, BJT was selected for further miniaturization, longer life, and improved reliability. Despite having slower switching speeds compared to spark gaps, BJT can be optimized by utilizing its breakdown characteristics to reduce switching time from microseconds to nanoseconds, enabling high-speed operation.

The basic principle of the Marx generator is illustrated in Figure 1a. A DC voltage source charges capacitors in parallel through a resistor. Once fully charged, the first switch triggers conduction, followed by sequential activation of other switches, connecting the capacitors in series to discharge the load. The output voltage becomes n times the charging voltage, where n is the number of stages, as shown in Figure 1b.

Until the last stage switch is activated, the charging time is given by:
Tc = 2nRC (1)
The on-voltage range for each stage switch is Uc < Uon.

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