This tutorial installment is: Power Semiconductor Switches, Classification. This topic answers the following questions:
- What are the main power semiconductor switch classifications?
- What are the important properties of the main classifications?
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Power Semiconductor Switch Classification
Power semiconductor devices can be classified in at least two ways:
- by type of charge carrier,
- by active vs. passive function.
Charge Carrier Classification
Power semiconductor devices can have two modes of charge carrier conduction:
- minority carrier, where charge is carried by electrons in a P-type semiconductor, or by holes in an N-type semiconductor, or
- majority carrier, where charge is carried by electrons in an N-type semiconductor, or by holes in a P-type semiconductor.
Understanding what type of charge carrier conduction a device has helps us to understand its most basic properties and suitability for different applications.
Minority Carrier Devices
Minority carrier devices excel for applications involving higher voltage levels. Minority carrier devices used for power semiconductors have a p-n– junction where the n– region is very lightly doped or intrinsic. Since, under reverse bias conditions, the n– region is nearly devoid of charge carriers and therefore has low conductivity, it can sustain a large breakdown voltage.
When the device is forward biased, minority carriers are injected into this same n– region, resulting in a great increase in conductivity, so that under forward bias conditions, the on-state voltage is relatively low. This change in the conductivity is referred as “conductivity modulation”. Comparably rated majority carrier devices for high voltage applications would actually have a higher on-state voltage due to the product of current and channel resistance.
However, minority carrier devices are slow to turn-on and turn-off due to a large stored minority carrier charge which must be inserted or removed prior to the device changing state. This effect limits the maximum practical switching frequency of these types of power devices to 50-100kHz.
Majority Carrier Devices
Majority carrier devices excel for applications involving lower voltage levels and for higher frequencies. Majority carrier devices have fast switching times because they do not store minority charge. They operate either by electrostatic control of conduction cross-sectional area or by employing a metal semiconductor junction. Neither method involves minority carriers.
Active vs. Passive Devices
Power semiconductor switches can further be classified according to whether they are passive responding devices or actively controlling devices.
Passive Responding Devices
Passive devices change conduction state based upon the external voltage and current conditions. They do not need to be controlled. For example, when the current through a diode attempts to change direction, the diode turns off. When the diode becomes forward biased, it turns on.
Active Controlling Devices
Active controlling devices are typically three terminal devices and change from a blocking state to a conducting state as commanded by a control input signal. Active devices control the operation of a switching power supply.
Next Topic
The next tutorial installment is: Power Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor. This topic answers the following questions:
- What are the minority carrier power semiconductor switches used today?
- What are the applications for these switches?
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Next topic: Power Semiconductor Switches | PIN Diode, BJT, IGBT, Thyristor
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References
[1] Robert W. Erickson, Dragan Maksimovic, Fundamentals of Power Electronics, 2nd Edition, Norwell, MA, Kluwer Academic Publishers, 2001.