MOSFETs in comparison with BJTs
BJT – lower on-state losses (UCEss > 300 V)
MOSFET – considerable simplification of drive circuits
BJT
MOSFET
VD MOS
IGBT
IGBT (Insulating Gate Bipolar Tranzistor)
Although the IGBT structure may appear very
similar to that of the VDMOS transistor, its
function is rather different
As with a MOSFET, the application to the
gate of a positive voltage, VGE> VGE(th),
creates a conducting channel between the N+
and N regions. This forward biases junction,
J1, and holes are injected from the p+-emitter
of the P+NP transistor into the N-base.
The hole current which crosses junction J2
(the collector current of the PNP transistor)
 α pnp 
 I n
I p = 
1α

pnp 
where In is the electron current passing
through the channel of the MOS transistor
If gate-emitter voltage VGE > VGE(th) is high enough
In =
As far as
IE = Ip + In,
IE = IC =
In
z
µeff Cox (VGE − VGE(th) )Vch
l
1- α pnp
the collector current of the IGBT can be expressed as
IC =
z µeff Cox
(VGE − VGE(th) )Vch
l (1 − α pnp )
When the channel voltage exceeds (VGE - VGE(th)), the
collector current saturates
IC =
1 z µ eff Cox
(VGE − VGE(th) )2
2α l (1 − α pnp )
The transconductance of the IGBT
g fs =
The transconductance of the IGBT in
saturation is
g fs =
∂ IC
∂ VGE
VCE =const
µeff Cox
(VGE − VGE(th) )
α l (1 − α pnp )
z
The on-state characteristics of the IGBT
The main conduction path through an IGBT can be
modelled as a diode in series with an MOS
transistor. The diode current IF = (1 - αPNP)IC
E
C
G
JF =
The current density JF flowing through the diode area Sd
Forward voltage drop VF across
the diode
VF = K 0 + K1 ln J F + K 2 J F
Vch =
The voltage drop across the
MOS channel
µeff
ox
GE
Sd
m
(1 − α )I l
C z (V − V
pnp
1
C
GE(th)
(1 − α PNP )I C
VF = VT 0 + rT I F
)
VCE(on) (I C ) = Vch (IC ) + VF (I C )
The total on-state voltage drop
Using a linear approximation of the I-V characteristic
VCE ( on ) = VTO +
(1 − α ) l
C z (V − V
pnp
µeff
ox
GE
GE(th)
)
I C + rT (1 − α pnp )I C = VTO + rT* I C
The maximum blocking voltage is limited by the breakdown voltage, VCEO, of
the PNP+ transistor. Two basic construction are used:
NPT IGBT
The problem is similar to the one
described in detail for thyristors, i.e., the
base thickness and donor concentration
in the base have to be optimised
PT IGBT
A compressed field structure obtained
by forming an N+ layer of about 10 µm
thickness between the P+ collector layer
and the N-base is often used to
combine a high blocking voltage with a
low on-state voltage
IGBT Switching Characteristics
The beginning of the IGBT turn-on process is closely
related to the turn-on process of a power MOSFET
td(on) =


VGE

+ RG (CGE + CGC )ln


2 Dp
 VGE − VGE(th) 
2
wpnp
IGBTs have a current handling capacity that is
approximately an order of magnitude higher than that
of an equivalent VDMOS transistor. Therefore, the area
of silicon wafer necessary for an IGBT with the same
nominal current is a similar magnitude lower. However,
this means that CGE and CGC are lower in the same
ratio for devices having the same nominal current
rating..
Simultaneous solution of equations, taking
account of the variation of the capacitance, CGE,
and the current gain of the bipolar transistor, is
very complicated. Numerical solution is required.
IGBT Turn-off Process
Part of the total on-state electron current that passes through
the MOS channel is controlled by the gate charge. For this to
be interrupted, the gate capacitance has to be discharged
As soon as VGE decreases below VGE(th), the MOS
channel is turned-off.
(
)
∆I C = I n = 1 − α pnp I C
The collector-emitter voltage then rises so that
its rate of rise is given by
dVCE g fsVGE(th) + IC
=
dt
g fs RG CGC
Tail current
 −t

 τ eff 
I C ( t ) = α pnp I C ( 0) exp 
Turn-off losses
Woff ≈ VC0 ICα pnpτ eff
Trench IGBT
IGBT
RC IGBT
RB IGBT
Different vertical IGBT designs
a) PT IGBT
b) NPT IGBT
c) SPT IGBT