AN4503 Application Note
AN4503
An Introduction To IGBT Operation
Application Note
Replaces September 2000 version, AN4503-4.0
AN4503-4.1 July 2002
can be grown and so this type of structure is limited to voltages
less than 1200V.
The power semiconductor devices available on the market can
be categorised into three groups viz.,
The NPT structure is fabricated by starting with a uniformly doped
(n-) silicon wafer. The emitter and MOSFET are formed by
diffusion on the top side of the wafer and the p+ collector is
formed by an implantation method on the other side of the wafer.
With the NPT structure it is currently possible to achieve forward
blocking voltages as high as 4.5kV. The static and dynamic
characteristics of the PT and the NPT IGBTs are different and
these will be discussed later.
1) The devices such as diodes which are turned on and off by
the action of the circuit;
2) Devices like thyristors and triacs which can be turned on by
the gate control but require separate circuit implementation to
turn them off.
3) Those devices such as bipolar transistors, gate turn-off
thyristors (GTOs) and power MOSFETs which can be turned on
and off by the gate signal.
The reverse breakdown voltage between emitter and collector
is characterised by the reverse breakdown of the un-terminated
collector to base junction (n+ in PT structure and n- in NPT
structure). This has a typical value of 10V. In many applications
an anti parallel diode is used with an IGBT switch and so it has
to withstand only the forward voltage drop of this diode in the
reverse breakdown mode. However the transient forward voltage
drop of a diode can be significantly higher than the steady state
value and it is likely that this junction is broken down transiently
by the diode’s transient forward voltage. This has no serious
detrimental effect as long as the duration is short and the
magnitude of the resultant transient power is within the device
avalanche power rating.
The final group of devices are preferred in power electronics as
they simplify circuitry, but they all have their advantages and
disadvantages. For example GTOs are available in high-voltage
and high current ratings but limited to lower frequencies (less
than a few kHz) and require high power gate control. Bipolar
junction transistors (BJTs) offer simpler driving than GTOs but
they are limited to lower voltages (<1500V), while MOSFETs
offer high speed operation (100kHz typical) and are very easy
to drive but are limited to lower voltages and currents.
Over the past decade a new group of power devices which
combines bipolar and mosfet technologies became commercially
viable. MOS controlled bipolar devices such as IGBTs (Insulated
Gate Bipolar Transistors) and MCTs (MOS Controlled Thyristors)
belong to this group. These types of devices offer the best
features of bipolar and MOSFETs devices. The aim of this note
is to give an introduction to IGBTs outlining the device structures,
mode of operation, ratings and characteristics so that the device
can be used optimally by the power circuit designer.
2. DEVICE OPERATION
In the normal mode of operation, the collector is made positive
with respect to emitter and if gate is at zero potential with respect
to emitter, no main current flows from collector to the emitter
(apart from blocking current). When gate potential is made
positive with respect to emitter, electrons are attracted in the p
region below the gate oxide and eventually inverting the polarity
of p type to n type. This inversion layer hence provides an n-
channel from the n+ layer to the n- layer. Electrons are injected
from the n+ emitter contact into the n- region thus lowering the
potential of this region and forward biasing the p+ n- junction
from the collector side. Hence holes are injected from the
collector into the n- layer (Fig.2).
1. IGBT STRUCTURES
All IGBTs on the market have either a punch-through structure
(PT) or non-punch-through structure (NPT).
Fig.1 shows the vertical cross section through one of the
elements of the PT and NPT IGBT structures. In practice an
IGBT chip consists of many such elements connected in parallel.
The NPT structure is the most basic one for an IGBT. It consists
of a four layer sandwich of n+pn-p+, very similar to a thyristor
structure except the gate consists of a polysilicon layer which is
separated by an oxide layer grown on the top surface of the
silicon wafer. The polysilicon layer is arranged such that it
overlaps the n+ and n- regions. On the top, the emitter contact
is made by aluminium which overlaps the n+ and p regions. On
the other side of the wafer the collector contact is made by
aluminium contact on the p+ region.
The excess holes and electrons in the n- region reduces the
resistivity of this region. This is known as conductivity modulation
which reduces the on-state resistance of the device. This is
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