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

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This article delves into the breakdown characteristics of bipolar junction transistors (BJTs) and their performance as switching components. Based on experimental findings, the BJT demonstrates a switching speed of less than 20 nanoseconds, with even faster response times—under 10 nanoseconds—when the base-emitter junction is shorted. This makes it an excellent choice for high-speed switching applications. A nanosecond Marx-type negative pulse generator was designed using BJT as the switch. The results showed that the output pulse can reach up to 2.3 kV, with a pulse width below 10 ns and a falling edge as fast as 3 ns.

1. Introduction

To generate pulsed plasma for various applications, different types of pulse power generators exist, such as magnetic compression pulse generators (MPC), pulse forming circuits, and others. However, from a miniaturization perspective, these traditional systems face limitations in terms of signal compression, rise/fall time control, and reliability. To address this, a Marx-type pulse generator was developed using capacitors for energy storage and BJTs as switches. This design offers fast switching speeds, adjustable outputs, and extremely short pulse edges, enabling the generation of powerful electric field pulses. Its compact size makes it highly portable and easy to handle.

2. Principle and Design

2.1 Marx Generator Principle

The Marx generator is a well-known high-power pulse generator that typically uses spark gap switches as its main component. However, operating spark gaps at high frequencies presents challenges such as limited lifespan and increased maintenance needs, which hinder miniaturization efforts. In recent years, high-power semiconductor devices like MOSFETs and IGBTs have been used to replace spark gaps in lower-power environments, improving device size and efficiency. Here, we use BJT as the switch to further reduce size, extend life, and enhance reliability. Although BJTs are slower compared to spark gaps, their switching speed can be significantly improved by utilizing the breakdown time of the transistor, reducing the switching time from microseconds to nanoseconds, thus achieving high-speed switching functionality.

The basic principle of the Marx generator is illustrated in Figure 1a. A DC voltage source Udc charges capacitors in parallel through a resistor. Once fully charged, switch S1 is triggered to turn on, followed by sequential activation of switches S2 and S3. This allows the capacitors to discharge in series, resulting in an output voltage that is n times the charging voltage, where n represents the number of stages. As shown in Figure 1b, this configuration enables efficient voltage multiplication.

Until the final stage switch turns on, the charging time is given by:

Tc = 2nRC (1)

The range of the on-voltage Uon for each stage is Uc < Uon

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