AN-7503 Datasheet, PDF (1/6 Pages) Fairchild Semiconductor – The Application Of Conductivity-Modulated

The Application Of Conductivity-Modulated
Field-Effect Transistors
Application Note
May 1992
AN-7503
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Summary
The development of conductivity-modulated field-effect
transistors, FETs, makes available to the system designer
another solid-state device that can be used to implement
power switching control. This paper reviews differences
between the standard and the newly developed FET. It
shows the significant advantages that the conductivity-
modulated FET has over the standard FET. Several
applications are presented to show that this new type of
device works well in practical situations. The relative
immaturity of the conductivity-modulated FET may limit its
initial utilization. But as the family grows and product
innovation and refinement takes place, this newest member
of the power semiconductor family will become a viable
alternative to the other members.
General Considerations
The development of the power field-effect transistor has
made available to the power-stage designer an entire new
family of power semiconductors. Over the past 5 to 6 years,
the breadth of product has grown to encompass the require-
ments of a large number of applications. A limiting factor that
has slowed the utilization of power FETs in the high-current,
high-voltage applications is the fact that the on-state
resistance (R_DS(ON)) in a standard FET is related to its
breakdown voltage (B_VDSS) by a nearly cubic power, i.e.,
R_DS(ON) ≈ B_VDSS 2.8. What this implies, as Figure 1 shows,
is that as the breakdown voltage increases, the on-state
resistance climbs even faster.
1
P-CHANNEL MOSFETs
N-CHANNEL MOSFETs
0.1
N-CHANNEL
CONDUCTIVITY
0.01
MODULATED FET
P-CHANNEL CONDUCTIVITY
MODULATED FET
0.001
10
100
1000
DRAIN-SOURCE VOLTAGE (V)
FIGURE 1. SPECIFIC ON-RESISTANCE OF P AND N-CNANNEL
MOSFETS AND CONDUCTIVITY-MODULATED
FETS vs FORWARD BLOCKING VOLTAGE.
The MOSFET on-state resistance is contributed to primarily
by three components of the transistor: the MOS channel,
the neck region, and the extended drain region. The
extended drain region contributes the most to the on-state
resistance in high-voltage MOSFETs. To achieve a lower on-
state resistance at a given blocking voltage, the usual
technique is simply to make the die larger. However,
increasing the die size has its limitations from a
©2002 Fairchild Semiconductor Corporation
manufacturing point of view, since MOSFETs, with their very
fine horizontal geometries, are highly defect-yield sensitive.
As die size increases, the likelihood of a defect resulting in a
nonfunctional part increases exponentially. This tendency,
combined with a smaller number of parts per wafer, limits the
availability of low-on-state-resistance, high-voltage MOS-
FETs.
A change in the horizontal geometry of the MOSFET can
lower the specific on-state resistance per unit area. By using
more channel width with smaller source cells placed closer
together, a reduction in on-state resistance can be achieved.
A limitation on how close these cells can be placed arises
from a possible localization of field concentrations that will
limit the voltage breakdown of the structure to less than the
theoretical rating due only to impurity concentrations.
Therefore, for a given breakdown voltage, there exists a
minimum spacing of the cell structure. Generally, the higher
the required breakdown voltage, the further apart the cells
must be placed.
As stated earlier, the extended drain region of the MOSFET
generally contributes the most to the on-state resistance in
high-voltage MOSFETs. As the required blocking voltage is
increased, this region must be made thicker and more lightly
doped to be able to support the desired voltage. It is this
region's contribution to on-state resistance that the conduc-
tivity-modulated field-effect transistor drastically reduces.
This reduction occurs as the result of the injection of minority
carriers from the substrate and, in specific on-state resis-
tance per unit area, is about 10 times less than in a standard
MOSFET at the 400V BV_DSS level, as shown in Figure 1.
Further analysis has shown that the specific on-state
resistance may be nearly independent of blocking-voltage
level. This finding implies that at a BV_DSS of 1000V, the
reduction in conductivity-modulated FETs over the standard
MOSFETs could be perhaps 50 to 1. These reductions in on-
state resistance per unit area that the conductivity-
modulated FETs can achieve present the possibility that
high-voltage high-current FET-type devices can become
more readily available because of the smaller die sizes
associated with conductivity-modulated FETs.
Comparison of Standard and Conductivity-Modulated
FETs
Standard and conductivity-modulated FETs share some
characteristics, but are substantially different in others.
Shown in Table 1 is a listing of the major characteristics that
make the conductivity-modulated FETs unique among power
semiconductor families. Foremost, it is a voltage-gated
device; its input characteristics are similar to standard power
MOSFETs of comparable chip size. Very little drive power is
required at low to moderate switching frequencies. The
device remains under the control of the gate within its normal
operating conditions. It exhibits the normal linear mode as
well as the fully saturated on-state of conventional power
MOSFETs. When the gate voltage is removed, the device
Application Note 7503 Rev. A1