Power semiconductor device: Difference between revisions

Power semiconductor devices are semiconductor devices used as switches or rectifiers in high-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. For example, the common small-signal MOSFET employs a lateral structure where drain and source of the device reside at the top surface of the semiconductor die. In the case of the power MOSFET the source is located on the top surface and the drain is located at the bottom of the device so a vertical structure results. Naturally, different and extra fabrication steps are needed to achieve this. One type of power MOSFET in use today is fabricated using a double-diffusion process (diffusing dopants twice in the semiconductor bulk), hence their name DMOS.

Parameters of power semiconductor devices

1. 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.
2. 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.
3. Rise and fall times for switching between on and off states.
4. Safe-operating area (from thermal dissipation and "latch-up" consideration)

Common Power Semiconductor Devices

Perhaps the true revolution in power electronics started with the discovery and wide-spread use of the thyristor. 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.

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 untill 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.