IGBT or MOSFET: Choose Wisely
by Carl Blake and Chris Bull, International Rectifier
With the proliferation of choices between MOSFETs and IGBTs, it is becoming increasingly difficult for today’s
designer to select the best device for their application. Here are a few basic guidelines that will help this decision-
making process.
Device Evolution: Bipolar Transistors, MOSFETs and IGBTs
The bipolar transistor was the only “real” power transistor until the MOSFET came along in the 1970’s. The bipolar
transistor requires a high base current to turn on, has relatively slow turn-off characteristics (known as current tail), and is
liable for thermal runaway due to a negative temperature co-efficient. In addition, the lowest attainable on-state voltage or
conduction loss is governed by the collector-emitter saturation voltage V
CE(SAT)
.
The MOSFET, however, is a device that is voltage- and not current-controlled. MOSFETs have a positive temperature co-
efficient, stopping thermal runaway. The on-state-resistance has no theoretical limit, hence on-state losses can be far
lower. The MOSFET also has a body-drain diode, which is particularly useful in dealing with limited free wheeling
currents.
All these advantages and the comparative elimination of the current tail soon meant that the MOSFET became the device
of choice for power switch designs.
Then in the 1980s the IGBT came along. The IGBT is a cross between the bipolar and MOSFET transistors (see figure 1).
The IGBT has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a
MOSFET. In general, this means it has the advantages of high-current handling capability of a bipolar with the ease of
control of a MOSFET. However, the IGBT still has the disadvantages of a comparatively large current tail and no body
drain diode.
Early versions of the IGBT are also prone to latch up, but nowadays, this is pretty well eliminated. Another potential
problem with some IGBT types is the negative temperature co-efficient, which could lead to thermal runaway and makes
the paralleling of devices hard to effectively achieve. This problem is now being addressed in the latest generations of
IGBTs that are based on “non-punch through” (NPT) technology. This technology has the same basic IGBT structure
(see Figure 1) but is based on bulk-diffused silicon, rather than the epitaxial material that both IGBTs and MOSFETs have
historically used.
GATEGATE
p+
- - - -
Channel
- - - -
Channel
r'
b
N - Epi
Transparent Anode (Collector)
+ + + + + + + + + + + +
n+ n+
p+
METAL (Emitter)
Figure 1. NPT IGBT cross section
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