Debre Markos University Debre Markos Institute of Technology School of Electrical and Computer Engineering Power Electronics and Electric Drives

Debre Markos University
Debre Markos Institute of Technology

School of Electrical and Computer Engineering
Power Electronics and Electric Drives
Chapter assignment
1. What is the difference between a normal semiconductor device and a power
semiconductor device? [using structural of device]
2. Compare the power semiconductor devices by performance characteristic factors? (Using
switching loss, conduction loss, efficiency, and thermal performances etc.)
3. Write and list new power semiconductor technology for future trend power electronic
converter?
What is the difference between a normal semiconductor device and a power semiconductor
device?
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Power semiconductor devices are capable of handling large currents and can block large voltages which
their normal semiconductor counterparts, used in analog electronic circuits, would never be capable of.
While both are made up of semiconductor materials, there are various techniques used while fabricating
power semiconductor devices which impart large power handling capabilities.
Power semiconductor devices are predominantly used as switches in a power electronics circuit. So, in
their off state, they behave like an open circuit and block the current through them. The maximum
amount of voltage a power semiconductor device can withstand in its off state is called its voltage rating
and is specified on a device’s datasheet under the Absolute Maximum Ratings.
Further they are required to conduct current without offering much resistance in their on state, just like
a short circuit. The current rating of a power semiconductor device is the amount of current that can
flow through the device without the device getting permanently damaged by excessive heating due to
the power dissipation.
In an analog device, the silicon wafer is doped with acceptor impurities to form the p layer and doped
with donor impurities to form the n layer. The usual impurity doping concentration in an analog device is
on the order 1014 atoms/cm3 to 1017 atoms/cm3.
However, the concentration of impurities is of the order 10 19 atoms/cm3 in a power semiconductor

device simply because more doping generates more charge carriers - hence greater current rating. The
high concentration doping of the n and the p layer is denoted by adding a ‘+’ as a superscript denoting
them as n+ layer and p+ layer.
The current density in a power device is proportional to the total area available for the flow of charges.
To increase the current density in a power semiconductor device, the cross section of the device is
increased. This is achieved by using a vertical structure instead of the planar or lateral structure like in
an analog semiconductor device. Increasing the current density automatically increases the current
rating of the device. So, power semiconductor devices are, in general, vertical in nature.
For example, the current rating of a diode is in the order of mA typically. Following the two techniques
mentioned above - of increasing the doping and increasing the cross section, the current rating of a
power diode is increased in the range of one amp to several thousand amps.
When a power semiconductor device being used as a switch is in its off state, it blocks the current and
disconnects the load from the voltage across it. However, if the voltage exceeds a certain limit, the
device breaks down and starts conducting current and may also get permanently damaged in the
process. This value of voltage which when applied across the device causes it to break down and
conduct is termed as the breakdown voltage. The higher the breakdown voltage of a device, the higher
is its voltage rating. To understand how the voltage rating of the power semiconductor device is
increased, we will need to look at the expression of breakdown voltage first. Whenever a pn junction is
formed in a semiconductor, there is a breakdown voltage associated with it, denoted as VBD.
Let’s get into the math of how to find this breakdown voltage, showing its relationship with the
semiconductor materials as well. Let’s start where VBD is given by:
“Good Luck”

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However, the concentration of impurities is of the order 10 19 atoms/cm3 in a power semiconductor

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