order to still reduce the electrical field, as described above. The doping concentration of
the FS layer is approximately 10
. The concentration of the buffer layer
of aPT IGBT is higher by approximately oneorder ofmagnitude.
Layer sequenceandelectrical field distribution of anFS IGBT (not to scale)
The setting of the n
-base and FS layer is critical to the switching behaviour of an FS
IGBT, i.e. at which blocking voltage the electrical field penetrates the FS area. At lower
voltages, the electrical field is already diminished in the base and enough charge
carriers remain in the semiconductor, and these contribute to the tail current. The higher
the voltage to be turned off becomes, the further the electrical field will extend into the
base and, eventually, the FS layer. Therefore, fewer and fewer charge carriers are
available at the moment of turn-off to form the tail current. Turn-off losses are usually
lower, due to the reduced tail current. However, from a critical voltage onwards there is
almost nomore tail current visible and the IGBT shows snappy turn-off behaviour.
Just likeNPT IGBTs, FS IGBTs have apositive temperature coefficient.
1.5.4 Trench IGBTs
So far, all IGBT designs have one thing in common: A planar gate structure. Because of
the geometry of the gate structure, this causes JFET behaviour between the p-regions
on the emitter side, as described in a previous section
, and a lower
conductance modulation in the emitter region. In IGBTs with a planar gate, the charge
carrier concentration reduces from the collector to the emitter. The goal of a new
generation of IGBTs was therefore to maintain a homogenous or even rising charge
carrier profile over the entire range, which would further reduce the conduction losses,
without influencing the tail current and turn-off losses. This led to the development of a
trench structure for IGBTgates.
left) shows the principal structure and distribution of the electrical field in the
blocking state for a Trench IGBT based on a PT IGBT. Also i
(right) a close-
up view of the trench gate structure in a scanning electron microscope image can be
found. The main difference between this and a planar gate is that, when the IGBT is
turned on, the inversion channel in the p-emitter region is vertical rather than horizontal.
This means that there is no JFET effect at all. It also means that the conductance
modulation near the emitter can be executedmore effectively through the injection of a
greater number of electrons.