Note: Descriptions are shown in the official language in which they were submitted.
36-SP-1082
This invention relates in general to semiconduc-tor
devices and more particularly to three-element transistor
semiconductor devices having faster turn-off time and
higher reverse second breakdown characteristics than has
been heretofore found. As power semiconductor devices
become more and more widely employed in a variety of
applications and, especially, in applications where high
speed, small size, low weight, high efficiency, and the like,
are demanded, the utmost in device performance is required.
A particularly important characteristic in devices which are
employed as solid state switches is that the speed of
switching be as rapid as possible. This is due to the
greatly increased power dissipation which is the result of
low switching speed. ~ncreased power dissipation results
in poor device performance, the requirement for large heat
sinks, and the use of larger and more expensive devices for
the same application than would be required were higher
switching speed provided to reduce power dissipation to a
minimum.
In general, ~.he greatest contribution ko power
dissipation in switching semiconductor devices is made during
the turn-off rather than the turn-on period. This is due to
the fact that, during turn off, device current typically
remains at its quiescent value during at least a portion of
the time that devlce voltage is increasing from a low
saturation state value to a high blocking state value.
During this period, substantial power may be dissipated by
the device. It is advantageous, therefore, to provide a
switching transistor in which the current through device
begins to decrease rapidly as soon as possible after the
application of a turn-off signal to the base~ To this end,
it has recently been the practice to provide an
- 1 -
~r~5~3G 36~SP-lOg2
interdigitated structure for power semiconductor devices
and particularly for transistor semiconductor devices which
structure provides a longer base-emitter,,turn-off line .,
than is achievable with noninterdigitated structures, other
considerations being the same. While the interdigitated
structure provides many advantages over noninterdigitated
devices, it is nevertheless not the ultimate attainable
transistor device, at least with respect to switching speed.
During turn o~f of the power transistor of the type
including an interdigitated emitter including a spine portion
for ready connection of a wire lead capable of sustaining
the high current flow to the control by the device and a
plurality of finger portions extending from the spine portion,
which finger portions are interdigitated with like finger
portions of the base of the device, turn off generally
proceeds by first removi~g carriers from the edges of the
fingers which are most closely proximate the base of the
device followed by restrickion of current flow to an area
closer and closer to the center of the finger until ultimately
complete turn off ~s achieved. It is inherent in this
turn-off mechanism that as turn oif proceeds and the current
flow is increasingly restricted to the center portion of the
. emitter fingers, turn off becomes increasingly difficult.
This is due to the fact that the current density becomes
higher and the distance from the base of the device becomes
,~ greater. This results in both a time delay preceding the
, : onset of current reduction during turn off, and, a reduction
in the turn-off speed during the time when current is
:~ rapidly decreasing. The not-insubstantial increase in
3Q current density duxing th.e turn-o~f pxocess not only ~:
increases the difficulty of turn off of the device and
decreases the speed thereof but, in addition, produces
36-SP-l,O g2
..3~
yet another phenomenon, reverse second breakdown. As
the current density increases, the electric field ln the
device becomes more and more dominated by the effect of
mobile charge carriers rather than by background charge
levels. As w.ill be demonstrated below, the increase in
mobile charge carriers during turn off results in greatly
increased electric fields which in extreme cases cause
the device to undergo reverse second breakdown.
It is an object of this invention to provide a ~ :
semiconductor device having improved switching speed over
prior art devices.
It is another object of this invention to provide
a power transistor capable of controlling substantial amounts
of current at high voltages which is not only aster than
has been heretofore possible but which, also, exhibits
improved reverse second breakdown characteristics.
It is still another object of this inventi.on to
provide an improved power transistor of the type discussed
which is not substantially more expensive to manufacture than
prior art devices.
Briefly stated and in accordance with a presently
: pxeferred embodiment of this invention, a new-and improved,
~ high-speed,,switching tranststor is provided haYing a
collector re~ion o~ a first conducti~ity type a base region
: of a second conductivity type opposite said first
~ conductiYity type and forming a first p-n junction ~ith said
collecior xegion, and an emitter region interdigitated with
; said base regi.on and includin~ a plurality of relatively
. narrow finger portions extending rom a relati~ely w.ide spine
- 30 portion to which a h.igh current capacity lead may he
conveniently connected. The base layer, interposed between
the emitter layer and the collector layer exhibits a first
- 3 -
~3~ 36-SP-1082
sheet resistance under most o~ the emitter layer and a
second, lower, sheet resistance under certain portions of
the emitter layer for promoting rapid turn o~f of those
portions. In accordance with a specific preferred embodiment
of this invention, the sheet resistance of the base layer
underlying the center portion of each of the emitter fingers
is selected to be relatively lower than the sheet resistance
underlying the other portions of the fingers. In accordance
with another preferred embodimen-t of this invention, the
sheet resistance of the base layer underlying thé
relatively wider spine portion of the emitter region and
especially the center of the spine portion is selected to ke
lower than the sheet resistance underlying the periphery of
the spine portion of the emitter~ In accordance with a
presently preferred embodiment of this invention, this
decxease in base-layer sheet resis-tance is provided by
forming an emitter which has a first thickness, as measured
from the surface of the device, in the area o~ the periphery
of the em:itter fingers and in the area of the periphery of
the spine portion of the emitter layer, and a second,
reIati~ely shallower thickness in the center portion o~ the
emitter fingers and of the spine portion of the emitter layer.
In accordance with an alternative embodiment of
this in~ention, an emitter electrode is pro~ided which is
spaced from the emitter layer of the device as, fox example,
by an intermediate oxide layer, essentially only in the
center portion of the emitter finger and in the center
portion o~ the spine region of the emitter layer. In this
way, self~debiasing occurs in the emitter underlyiny said
spaced poxtions o~ the emitter electrodes, reduciny the yain
of the device in these areas and consequently xeduciny
current cro~ding.
-- 4 --
~ ~ 36 ~P-1082
In accordance with each of these embodirnents of
this invention, high current densit;.es hereto~ore associated
with the restriction of current during turn off to the
center portion of the interdigitated emitter fingers is
alleviated; the electric fields associated therewith are
reduced substantially and reverse, second breakdown
characteristics of the device are enhanced.
The features of the invention which are believed
to be novel are pointed out with particularity in the .
a~pended claims. The invention itself, however, both as to
its organization and method of operation together ~ith
further objects and advantages thereof may best be understood
: by reference to the following description taken in
connection with the accompanying drawings in which: :
Figure 1 is a graphical representation of the
current and voltage signals of a transistor during turn off;
Figure 2 is a cross sectional view of a portion of
a transistor ~n accordance wi-th the prior art;
.Figure 3 is a view similar to that of Figure 2
~ 20 including a schematic representation o:~ the turn-off p.rocess;
:~ Figure 4 is a graphical representa-tion of the
-~ electric fields in transistors for various current densities;
Figure 5 is a cross sectional view of a portion of
a transisto~ according to this invention;
. ~igu~e 6 is a cross section view of a portion of. .
a transistor according to another embodiment of this
invention;
Figure 7 i,s a cross section view of a portion of
a transistor accoxdin~ to another embodiment of this
. 30 invention;
Figures 8 and 9 are top ~iews o~ a poxtion of a
transistor in accordance with two embodlments of this
.' '
.
-- 5 --
.
~ ~tj5~ 36-SP-10~2
invention.
Figure 1 is a graphical representa-tion of the
collector current, collector-emitter voltage and base
current in a transistor of the type to which this invention
relates, during the turn-off portion of a device switching
cycle, an inductive load being presumed. Each of the turn-
off parameters is separately plotted on the same time scale
so that -the interrelationship between the base drive signal
and the collector current and voltage may be readily observed.
The wave forms of ~igure 1 are understood to be exemplary,
but are, in fact, typical of the wave forms which might be
observed in switching circuits as would be expected to be
found in inverter circuitsO Several portions of the turn-
off current wave form are of interest. Between the onset of
the fall of base current and the beginning of the change in
collector current and voltage, a time delay is observed which
- is conventionally designated tsv. During this period,
relatively little power is dissipated in the device since,
although the collector current remains high, the collector-
emitter voltaye remains low. ~fter this time, the collector-
emitter voltage, VcE, starts to rise at a rapid rate while
the collector current, Ic, remains above about 90~ of its l;
quiescent value. This period of relatively high IC and
rising VcE is referred to as trV. During this period,
substantial amounts of power must be dissipated by the
device. Immediately after trV has elapsed, IC and VCE are
both near their maximum values and during this period the
maximum amount of power dissipation occurs. Shortly ater
the collector-emitter voltage has reached its blocking
3Q level, IC begins to fall rapidly during the period
designated tfi. During this time, substantial amounts of
power, although less than during the time when IC and VCE
~ 6 --
'`' ~ 36-SP-10~
are both high, ,are dissi~ated. 'rhe overall pe~iod ~rom the
time when VcE increases to about 10% of its maximum value
and the time when IC decreases to 10% of its maximum value
is designated 'tc and is a time period during which
substantially all of the power dissipation which occurs
during turn off occurs. This time period may be as long as
several microseconds even in switching transistors such as
interdigitated switching transistors which are designed for
high speed.
Figure 2 is a cross section view of a conventional
interdigitated switching transistor ~t in accordance ~ith the
prior art. The section of Figure 2 is taken through an
- emitter finger 26 and shows the current densities under that
finger during turn on. As the device turns on,,the ef~ect
of the base current signal in debiasing the base emitter
junction 25 is most pronounced near the center of the emitter
,' fingers closest to the base electrode. This causes a
non-uniform current distribution under the emitter fln~er
with relatively larger current flowing near the edges and
less current in the center portion oE the fingers. As the
de~ice turns on, charge is stored both in the base 24 and
collector 22 regions 'of the device. The greater the
magnitude of the ~ase drive signal, the more stored charges
accumulate, especially,,in the collector region.
Figure 3 is a cross-section view of the same
~ structure as illustrated in ~igure 2 showing the turn-off~
'~ mechanism in a transistor of the type to which this invention
is addressed~ Like elements of Figures,~ and 2 are
designated with'like reference numerals. During turn off,
minorit~ carriers under emitter Z6 are ~irst swept from
beneath the ed~es o~ the emitter closest to base electrodes
28 by the application of a turn-of current signal to base
;
-- 7 --
~ 5~ 36-SP-1082
electrodes 28. As the deviee ceases to conduet under the
outer edges of emitter 26~ the current density underlying
the center of emitter 26, where stored eharge is the most
diffieult to remove, inereases as the emitter starts to
inject more heavily in the center region, thus making up for
the injection whieh has eeased to oceur at the emitter
edges. In addition, the base turn-off signal tends to debias
the edges of the formerly forward-biased base emitter
junction, thus further eausing restrietion of eurrent flow
to the eenter portion of the emitter.
As the collector current is restricted or pinched
toward the center region of the emitter, the eurrent
density increases dramatieally insofar as the eollector
current remains relatively eonstant during the initial phases
of turn off. This inerease in eurrent density manifests
` itself in -two phenomenis: the inereasing diffieulty in
achieving eomplete turn off, and, in extreme eases, reverse
second breakdown. Reverse seeond breakdown may be readily
understood by referriny now to Figure ~ wherein the eleetric
field proile for the eolleetor region and the two junctions
adjaeent the eollector region of a transistor of the type
illustrated in Figure 3 are shown. The maynitude of the
eleetric field may be obtained from the relationship of
dE _ q rYc - m~x
dx ~ s qVl
L _
wherein the magnitude of the electric field is seen to
increase rapidly as the current dansity Jmax increases
while the baekground eharye level Ne remains constant.
Figure 4 illustrates the eleetric field profiles in a
colleetor for three values of eolleetor eurrent: eurve 30
where the eurrent density equals zero, corresponding to the
~,.,. '
. . .
36-SP-10~2
cutofE state; curve 32 where the current equals the ~uiescent
`~ O~ ~ 5-f" f~'
s~ collector current; and the curve 34 where the
current density is greater than the steady state value, J0,
and wherein the increase in electric ~ields at the n -n+
junction is readily observed.
It is a feature of this invention that not only is
turn o~f facilitated by the improved emitter design but,
in addition r reverse second breakdown is substantially
eli~inated.
Referring now to Fi~ure 5, a cross sectional view
similar to that of Figures 2 and 3 is shown including an ~:~
e~itter according to the instant invéntion. Emitter finger
40 includes relati~ely thicker portions 42 and 44 w,hich
surround relati~ely thinner portion 46. The thickness of
~ase layer 48, ,collector layer 50, and collector contact
layer 52 are relatively unchanged ~ith respect to the ~: ,
corresponding layers in Figures 2 and 3. Figure 5 indicates,
by arrows r ,the current distribution duxing the initial pha~e
of turn o~ as current ~lows from base electrodes 54 and 56
~hich aXe understood to be connected in the conventional
manner ~or an inte~digitated s~itching transistor. For
, completeness, emitter electrode 58 and collector electrode
; 60 are also shown. ~s has been hereinabove described, the
current during turn off is squeezed towards the center of
` emitter 40. The relati~ely shallower portion 46 of emitter
40 h.as lower injection ef~iciency and lower transport factor,
resulting in lo~er ~ain,,and there~ore during turn off,
essenti~ no current flows in that region as the current
density is zero or quite low. Therefore, whereas in the
3~ pxiox art transistoX, injection increased towards the
center o the de~ice as current was squeezed during turn
o~f, in accoxdance ~ith thi:s inYention, the current density
_ g _ '
36-SP~lOS2
in the center of the emitter is lo~.
It is preferred that the thickness o~ region 46
be as low as possible. Where the emitter is formed by
diffusion in t~o steps, it is recognized that arbitrarily
thin regions are difficult to form and may lead to short~ng
where the dif~usion is not completely uni~orm. It has
been found that where the nominal emitter diffusion depth
is to 10 to 20 microns a depth in region 46 of one to two
micxons provides satisfactory resul-ts.
It will be recognized that pro~iding a relatively
thinner emitter inner portion 46 is but one way of achieving
that decreased injection efficiency which limits the amount
- of current flo~ under the emitter of a transistor in
accordance ~ith thiS invention. Other methods which reduce
the base sheet resistance ln the center portion o~ the
emitter are equally e~fectual to provide the'desired function.
Referring now to Figure 6, an alternate embodiment
of this inVention is illustra-ted wherein emitter electrode
; 64 i~ spaced ~xom emi,tter 66 by oxide layer 68 in the center
poxtion o~ the emitter. In this way, an emitter may he
~ormed ~ithout the necessit~ for a two-step dif~u$ion or
similax process inso~ar as the emitter itsel~ is o$ unifoxm ~ , '
thickness, the decrease in injecti4n e~iciency bei,ng
achieved by physicall~ spacing the eIectrode from the emitter
and electrically isolatiny it at least in the center portion
of the emitter. In all other respects,,the structure of
Fiyure 6 is ident~cal to that o~ ~iguxe 5~
Referxing now to ~i~uXe 7, still another
' alternative embodiment of this invention is illustr~ted
; 30 ~herein the'inne~ portion o~ emitter 74 is o~ zexo
thickness,,base la~er 70 te~minating at sur~ace 72. The
device o~ Figure 7 represents the limitln~ case o~ the
-- 1 0
_J ~ t~
_1!"~t~ O,~
36 SP-lOg2
embodiment of th.is lnvention illustr~ted at Fi~uxe 5 and
requires the addition of oxide layer 76 to preVent shorting
of the base emitter junction by eIectrode 7~. It ~ill be
appreciated that although emitter layer 74 appears in the
section vie~ of Figure 7 as -t~o discrete regions the two
outer portions of the emitter are connected not only by
electrode 78 but are joined at the spine portion of the
comb-shaped emitter structure as well as at the ends of the :
individual ingers. The base layex sheet resistance under
the center portion of emitter 74 will be appreciated to be :
much less than under the edge portion of the emitter and
further it will be readily obser~ed that the injection :
efficiency in the center portion where the emi,tter layer is
completely absent will be essentially zero. Therefore, no
current will flow in this center portion either durin~ turn
on or turn o~
Figure 8 is a top view of a portion o~ a transistor
in accordance with this in~ention wherein the emitter 80
includes a xegion 82 o~ xeIatively lowe:r injection
ef~iciency essentially only under the spine portion of the
comb-shaped emitter structure. It is under this relatiYely
wide spine portion (w;th respect to the emitter fingers~
that tuxn o~f is most difficult and that, -therefore,,is
primarily responsible for poor turn-off speed characteristics.
It has been ~ound, ,howevex, that with the further addition
of regions 84 under the finger portions of the emitter as
illustrated in ~igure 9, eVen fuxther improvement in
swi.tchin~ speed and, therefore,,in device dissipation may
be obtained.
Th.e improvements achieved by the new emitter
structuxe o~ this invention are substantial. In a controlled
device, the .fall time, tfi~ was foun.d to be as lony as
36-SP-1082
.4 u/sec~, while in a device haviny the emitter structure
o~ this invention, the fall time o the collector current
was on the order o~ 0.1 u/sec. The improvement in storage
time, tsi, is likewise improved with the new emitter
structure. During the storage time, carrier removal is,
at least in part, aided by minority carrier recombination
During the period when current is falling, the ra-te of
minority carrier removal is almost entirely dependent
upon base current and is therefore greatly improved in
accordance with the new structure.
By far, the most important measure of device
performance is power dissipation during switching. A
readily observed parameter indicative of switching loss is
the change in temperature o the device during s~itching.
Devices in accordance with the emitter construction of this
invention have been found -to exhibit improvements in power
dissipation of at least three times over prior art devices
and improvements in reverse bias safe operatiny areas in
excess o~ lO~. The following table compares the
characteristias o~ a prior art control device and a device
according to the instant invention.
_ _ . ~ _ ~ ~ ~ ~ ~ ~ ~ _ ~ B V~Eo
Device lHA~e~V ' 5HA~5eV lOA~V ~ 2~ 250V ( c) (Volts)
, _ _
Control 35 28 15.8 2.9 .4 rlsing 551
~ New Emitter
.. Structure 29 25 15 2.2 .1 33 578
~; While this invention has been described in
; accordance with several preferred embodiments therebf, it
~¦ will be appreciated b~ those skilled in the art that many
;` 30 modi~ications and chan~es ma~ be made herein ~lithout
~; deviating ~rom the true spirit and scope of the invention.
~or example, ~hile several methods for forming a two-level
~ 12 -
~'t ~ SP-~082
emitter has been described ~herein di~usion is employed,
it will be appreciated that other methods are applicable,
For example~ the thicker portions of the emitter may be
formed by diffusion while the center portions are formed
by ion implanation or the like. Accordingly, it is
intended that the scope of the invention be limited only by
the appended claims.
;
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