Power semiconductor devices are fundamental constituents of modern power electronics that are used for the manipulation of high voltages and currents, as well as the conversion and regulation of electrical energy. These devices are of utmost importance in a variety of power electronics applications including power supplies, motor drives, renewable energy systems, and electric vehicles.

This chapter delves into the properties and uses of power diodes, thyristors, bipolar junction transistors (BJTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), and insulated-gate bipolar transistors (IGBTs).

Power Diode

Diode Basics

In electronics applications, diodes function as simple switches that solely permit the flow of current in one direction. Power diodes possess greater power, voltage, and current handling capabilities. They are extensively employed in power electronic circuits for a range of purposes, including rectification, voltage regulation, and safeguarding.

Diode Structure

A diode is a semiconductor device that comprises a p-n junction formed by the combination of p-type and n-type semiconductor materials. The p-type material contains an excess of holes or a deficiency of electrons, while the n-type material contains an excess of electrons, which restricts the flow of current through the junction in one direction only. The two terminals of a diode are the anode, which is connected to the p-type layer, and the cathode, which is connected to the n-type layer, as depicted in Figure 1.

The depletion region, which is the interface between the p-type and n-type materials, is devoid of charge carriers and plays a critical role in the diode's operation.

Figure 1: Diode pn-junction

Figure 2: Diode Symbol

The common symbol used for diodes is shown in Figure 2.

Diode Operation

The operation of a diode can be understood by examining the behavior of the p-n junction under different voltage conditions. When a positive voltage is applied to the anode with respect to the cathode (forward bias), the depletion region narrows, allowing current to flow through the diode. In the forward-biased state, the diode exhibits low resistance to current flow and allows current to pass through it with minimal voltage drop.

On the contrary, when a negative voltage is applied to the anode with respect to the cathode (reverse bias), the depletion region widens, effectively blocking current flow through the diode.

In the reverse-biased state, the diode exhibits very high resistance to current flow, allowing only negligible leakage current to flow (in the range of micro- or milliamperes). However, when the reverse voltage exceeds the diode's breakdown voltage, the diode starts conducting current in the reverse direction. This may permanently damage the device if the current is not properly limited. Sometimes, requirements in certain applications require higher voltage and current ratings, which can be catered to by a single diode. It is possible to increase the voltage rating by connecting multiple diodes in series. Similarly, to increase the current rating, diodes can be connected in parallel. However, it is important to choose diodes with similar characteristics to avoid any damage to the deployed diodes.

Diode Characteristics

Understanding diode characteristics is crucial for engineering power electronic circuits. The primary characteristics of diodes include the following:

Forward Voltage Drop (Vf)

The forward voltage drop (Vf) refers to the voltage drop across the diode terminals at a defined current level when it is forward-biased (when the anode has a higher potential compared to the cathode). When a diode is forward-biased, the applied voltage must overcome the built-in potential barrier at the p-n junction for the current to flow. The forward voltage drop is typically between 0.6V and 0.7V for silicon diodes and around 0.2V to 0.3V for Schottky diodes.

Reverse Breakdown Voltage (Vbr)

The reverse breakdown voltage (Vbr) is a crucial parameter that characterizes the maximum reverse voltage that a diode can endure before it initiates the conduction of current to a specified level when reverse biased. In the reverse-bias mode, the diode permits only a minimal leakage current until the reverse voltage attains the reverse breakdown voltage, at which point it enters the breakdown region, allowing a substantial current to flow.

The V-I characteristics of a typical power diode are illustrated in Figure 3.

Figure 3. V-I characteristics of diode

Other Diode Characteristics

Other characteristics include: junction capacitance, temperature coefficients, and reverse recovery time.

Figure 4. Diode reverse recovery characteristics

The reverse recovery characteristics of a diode, specifically the reverse recovery time (trr), are depicted in Figure 4.

The trr is the sum of two time intervals, ta and tb. Ta represents the time between the zero crossing of the diode current and when it reaches the value of IR, while tb refers to the time interval between the maximum reverse recovery current and when it reaches approximately 0.25 of IR. The time constant ta is attributed to the storage of charges in the depletion region of the junction, while tb is due to the storage of charges in the bulk semiconductor material. A small trr is desirable, particularly for high-frequency applications, and ideally, it should approach zero. However, this can lead to increased manufacturing expenses. In contrast, the forward recovery time of a diode is characterized by the turn-on time, which is the duration it takes for a diode to switch on and allow all majority carriers to contribute to the current flow after being forward-biased from a reverse-biased state.

Diode Types

Power diodes can be classified into three major categories based on their reverse recovery time and manufacturing techniques namely:

1. General-purpose diodes: These types of diodes usually have a high reverse recovery time, usually in the range of tens of microseconds, like 25 µs. They are appropriate for low-frequency applications, like rectifiers and converters with low input frequencies. The voltage ratings of these diodes usually vary from 50V to approximately 5kV. Additionally, current ratings usually range from less than 1A to thousands of amperes.
2. Fast-recovery diodes: Diodes of this type typically have a reverse recovery time lesser than 5 µs, making them suitable for use in DC-DC and DC-AC converter circuits where recovery time is a critical factor. The voltage range of these diodes is typically between 50 V and approximately 3 kV, while the current range is usually from less than 1 A to thousands of amperes.
3. Schottky diodes: Schottky diodes exhibit a low forward voltage drop ranging from 0.15-0.45 V and a short reverse recovery time of approximately ten nanoseconds. The reduced capacitance present in the diode, which is solely the junction capacitance, accounts for the low reverse recovery time. However, Schottky diodes have limitations such as low reverse voltage ratings and increased leakage current. The maximum allowable voltage for this type of diode is typically limited to 100V, and the current rating varies from 1 to 400A. Schottky diodes are suitable for high-current and low-voltage DC power supplies, as well as low-current power supplies to enhance efficiency.

Diode Applications

Diodes have a wide range of uses in electronic circuits due to their capacity to conduct current in a single direction. Here are the following applications:

Rectification

Diodes are commonly used in rectification circuits to convert AC inputs into DC outputs. This process involves the prevention of current flow in one direction, allowing only half or the full cycle of an AC waveform to pass through. Two primary types of rectification exist:

• Half-wave
• Full-wave rectification

In half-wave rectification, a single diode blocks either the positive or negative half of the AC waveform, resulting in a pulsating DC output with a frequency equivalent to that of the input AC signal. Conversely, full-wave rectification uses a diode bridge consisting of four diodes arranged in a specific configuration to rectify both halves of the AC waveform. This results in a pulsating DC output with a frequency twice that of the input AC signal, and is a more efficient process that produces lower ripple content compared to half-wave rectification. As such, full-wave rectification is a more suitable option for various applications, including power supplies, battery chargers, and DC motor drives.

Clipping and Clamping

In half-wave rectification, a single diode blocks either the positive or negative half of the AC waveform, resulting in a pulsating DC output with a frequency equivalent to that of the input AC signal. Conversely, full-wave rectification uses a diode bridge consisting of four diodes arranged in a specific configuration to rectify both halves of the AC waveform. This results in a pulsating DC output with a frequency twice that of the input AC signal, and is a more efficient process that produces lower ripple content compared to half-wave rectification. As such, full-wave rectification is a more suitable option for various applications, including power supplies, battery chargers, and DC motor drives.

Other applications

Diodes possess the capability to function as voltage multipliers, whereby their arrangement can result in the doubling, tripling, or quadrupling of an AC signal. Also, diodes are employed in power converters, power controllers, and snubber circuits for protection purposes.