Power semiconductor device
Power semiconductor devices are semiconductor devices used as switches or rectifiers in power electronic circuits (switch mode power supplies for example). They are also called power devices or when used in integrated circuits, called power ICs.
Some common power devices are the power diode, thyristor, power MOSFET and IGBT (insulated gate bipolar transistor). A power diode or MOSFET, for example, operates on similar principles as its low-power counterpart, but is able to carry a larger amount of current and typically is able to support a larger reverse-bias voltage in the off-state.
Structural changes are often made in power devices to accommodate the higher current density, higher power dissipation and/or higher reverse breakdown voltage. The vast majority of the discrete (i.e non integrated) power devices are built using a vertical structure, whereas small-signal devices employ a lateral structure. With the vertical structure, the current rating of the device is proportional to its area, and the voltage blocking capability is achieved in the height of the die. With this structure, one of the connections of the device is located on the bottom of the semiconductor [die].
Power semiconductor devices are only used in commutation mode (i.e they are either on or off), and are therefore optimised for this. Most of them shouldn't be used in linear operation.
History
Power semiconductor devices appeared with the introduction of the thyristor in 1957. They are able to withstand very high reverse breakdown voltage and are also capable of carrying high current. One disadvantage of the thyristor is that once it is 'latched-on' in the conducting state it cannot be turned off by external control. The thyristor turn-off is passive, i.e., the power must be disconnected from the device. This is a major disadvantage for switching circuits.
Although bipolar transistors where invented in 1948, the first devices with substantial power handling capabilities where introduced in the 1960s. These components overcome the limitations of the thyristors, as they can be turned on or off.
With the improvements of the Metal Oxide Semiconductor technology (initially developed to produce integrated circuits), power MOSFETs became available in the late 1970s. International Rectifier introduced a 25 A, 400 V power MOSFET in 1978. These devices allow operation at higher frequency than bipolar transistors, but are limited to the low voltage applications.
Developed in the 1980s, the Insulated Gate Bipolar Transistor (IGBT) became widely available in the 1990s. This component has the power handling capability of the bipolar transitor, with the advantages of the isolated gate drive of the power MOSFET. It has since almost completely replaced the bipolar transistor in power applications.
Common Power Semiconductor Devices
The realm of power devices is divided into two main categories (see figure 1):
- The two-terminal devices (diodes), whose state is completely dependent on the external power circuit they are connected to;
- The three-terminal devices, whose state is not only dependant on their external power circuit, but also on the signal on their driving terminal (gate or base). Transistors and thyristors belong to that category.
A second classification is less obvious, but has a strong influence on device performances: Some devices are majority carrier devices (Schottky diode, MOSFET), while the others are minority carrier devices (Thyristor, bipolar transistor, IGBT). The former use only one type of charge carriers, while the latter use both (i.e electrons and holes). The majority carrier devices are faster, but the charge injection of minority carrier devices allows for better On-state performances.
The Power MOSFET is currently the most common power device, particularly in lower than 1000 watt applications, such as switch-mode power supplies, motor drives and UPS. It has the best switching characteristics (due to its unipolar conduction), high input impedance (resulting in very low drive current and simple gate drive circuits), and ease of paralleling multiple devices to increase drive capability (because its positive thermal coefficient of resistance prevents current hogging caused by thermal runaway).
MOSFETs suffer from low transconductance and higher on-state voltage drop, compared to the BJT. A new device, incorporating features of both MOSFET and BJT, was proposed in 1980s. It is called insulated-gate bipolar transistor (IGBT). It has better current density than the MOSFET and better switching characteristics than the BJT but switches more slowly than the MOSFET. IGBTs are the primary choice in high-power (>10 kW), low to medium frequency (up to 30 kHz) applications. IGBT suffers from a typical 'current-tail' problem during turn-off. This is due to injection of minority carriers into its thick base region during conducting state by a mechanism called 'base conductivity modulation'.
A novel concept, called 'superjunction charge-compensation', has been used to minimize the on-state resistance of power MOSFET devices recently. Although the basic idea was known for some time, practical manufacturing difficulties prevented the realization of commercial device until recently. CoolMOS is the commercial name of this device from Infineon Technologies.
A device has recently been conceptualized and proposed. It is called an optically-triggered power transistor (OTPT). It features two electrical terminals–source and drain (similar to the MOSFET) and includes a third optical window instead of a gate terminal. By shining light of suitable wavelength and intensity on the transistor it may be possible to control the switching of the device. One great advantage of the device is that it is inherently suitable for III-V compound semiconductor material like gallium arsenide (GaAs) compared to silicon. This lends significant electrical performance enhancements because of better carrier dynamics and wider bandgap related properties of these III-V semiconductors compared to silicon. In short, OTPT is expected to have the potential of switching in the megahertz frequency range without sacrificing switching or conduction efficiency or breakdown voltage rating.
Parameters of power semiconductor devices
- Breakdown voltage. Often the trade-off is between breakdown voltage rating and on-resistance because increasing the breakdown voltage by incorporating a thicker and lower doped drift region leads to higher on-resistance.
- On-resistance. Higher current rating lowers the on-resistance due to greater numbers of parallel cells. This increases overall capacitance and slows down the speed.
- Rise and fall times for switching between on and off states.
- Safe-operating area (from thermal dissipation and "latch-up" consideration)