Device Physics II

Summary

This lecture delves into the physics of thyristors, key components in power electronics and control. It covers the thyristor's history, structure, and operation, highlighting its role in high-power applications since the 1960s. The discussion includes the device's forward and reverse blocking capabilities, the significance of the gate for triggering, and the impact of design on switching speed. Technical specifications from data sheets are reviewed, emphasizing the importance of understanding these for practical applications, with a look ahead to modern devices like IGBT and IGCT.

Takeaways

• 📚 The lecture is part of a series on device physics and power electronics, focusing on the thyristor.
• 💡 The thyristor was commercialized in 1960 and replaced thermionic-based switches like vacuum tubes due to its high power handling capabilities.
• 🔍 Thyristors are essentially two diodes connected in series, with a PNP-NPN transistor model used for analysis.
• 🛠️ The gate of a thyristor can be used to trigger the device from a forward-blocking state to a forward-conducting mode.
• ⚠️ It's crucial not to trigger a thyristor when the cathode voltage is greater than the anode voltage, as this could damage the device.
• 🔑 The thyristor consists of a four-layer p1n1p2n2 structure, with an anode, cathode, and gate as the main terminals.
• 🌡️ High voltage thyristors use a highly resistive n-base region to support large forward voltages when in the off state.
• 🔋 The thickness of the n-base region affects the thyristor's switching speed; a thicker region provides higher voltage blocking but slower switching.
• 🔗 Thyristors have different appearances and are often mounted on heat sinks due to their high power handling capabilities.
• 📈 The characteristics of thyristors include forward blocking, gate-triggered conduction, and a specific set of switching times like delay time (td), rise time, and gate turn-on time (tgt).
• 📉 The trade-off between forward blocking voltage rating and forward voltage drop during conduction is essential for selecting the right thyristor for an application.

Q & A

What is a thyristor and when was it first commercialized?

A thyristor is a four-layer semiconductor device composed of alternating PN type materials, with three terminals: anode, cathode, and gate. It was first commercialized in the 1960s and has since replaced thermionic-based emission switches like vacuum tubes due to its high power handling capabilities.

Why is the thyristor normally off?

The thyristor is normally off because when a voltage is applied such that VA>VK (anode voltage greater than cathode voltage), junction J2 is reverse biased, allowing only a small leakage current to flow, thus blocking the forward current.

How can a thyristor be turned on?

A thyristor can be turned on by applying a voltage to the gate terminal that is sufficient to break down the junctional barrier, allowing current to flow and putting the device into forward conduction mode.

What is the significance of the n-base region in a thyristor?

The n-base region in a thyristor is a highly resistive area that provides the device with its blocking capability. It is crucial for supporting the large forward voltage when the switch is off or in the forward blocking state.

How does the thickness of the n-base region affect the thyristor's performance?

The thickness of the n-base region affects the thyristor's forward blocking voltage and switching speed. A thicker n-base region can support a higher forward blocking voltage but will result in slower turn-on and turn-off times.

What is the role of the gate in a thyristor's operation?

The gate in a thyristor serves as a control terminal. It can be used to inject a small current that, when applied, can trigger the thyristor to switch from the off state to the on state, even when the anode-cathode voltage is below the breakdown voltage.

What are the two transistor models that represent a thyristor?

A thyristor can be represented by two transistor models: a PNP transistor and an NPN transistor, which are connected in such a way that the base-emitter junction of one transistor is connected to the base-collector junction of the other.

What is the forward breakdown voltage (VBO) of a thyristor and how is it determined?

The forward breakdown voltage (VBO) of a thyristor is the voltage at which the thyristor transitions from the forward blocking state to the forward conduction state. It is typically determined through destructive testing and is specified in the device's data sheet.

What is the difference between latching current and holding current in a thyristor?

The latching current is the minimum anode current required to maintain the thyristor in the on state immediately after it has been turned on and the gate signal is removed. The holding current is the minimum current needed to keep the thyristor in the on state without the gate signal.

Why is it not advisable to use thyristors with high dv/dt waveforms?

Using thyristors with high dv/dt waveforms is not advisable because the rapid change in voltage can falsely trigger the thyristor, leading to unintended conduction. This is due to the high rate of change of voltage causing a breakdown across the anode-cathode junction.