Monolithic Planar Diode Configurations
We have seen that in the fabrication of
an IC the geometry and the doping of the various layers must be chosen
to optimize uncharacteristic of the transistor which is the most
important device. It is not economically feasible to provide extra
processing steps to fabricate diodes. Therefore diodes are generally
transistor adopted for this operation. There are basic five
configurations of transistor for diode operation as shown in the figure
below.
A base-collector diode is shown in
figure [a]. The emitter is floating and can be omitted. This diode has a
high breakdown voltage of around 50 V. However it has a relatively
long switching time of about 100 nano seconds due to the collector
access resistance Rcc, which is nothing but the resistance between the
collector terminal and the effective active region to which it is
connected. (This resistance is reduced by buried layer diffusion).
The switching time can be improved to
about 70 nano seconds by shorting the emitter and base to remove charge
stored at that junction, while retaining the high breakdown voltage, as
shown in figure [b].
Figure [c] shows the base-emitter
junction diode with collector open. The turn-off time, due mainly to
charge stored in the base collector junction is about 80 nano seconds
and it has a low breakdown voltage (associated with the high doped
emitter of around 5V.
In above case, the switching time can be
reduced to as low as 20 nano seconds, by shorting base and collector,
to remove minority stored charge as shown in figure [d]. The low
breakdown voltage is not affected in this diode.
Figure [e] shows the diode connection
where both emitter-base and base-collector junctions are in parallel. It
is obtained by shorting emitter and collector. This diode is not much
used due to high junction capacitance which causes low switching speed
of around 150 nano seconds, together with a poor breakdown voltage of
about 5V associated with the base-emitter junction.
From above discussion we conclude
that diodes shown in figure [b] and [d] are most useful, the former for
higher voltage applications, and the latter where switching speed is of
paramount importance. Supply voltage encountered and digital ICs rarely
exceed 5 or 6 V, hence the limitation of low breakdown voltage of diodes
shown in figure [d] is not a serious disadvantage. Further it has the
lowest series resistance and no parasitic p-n-p action to the substrate
(which occurs between the substrate and the p-type base, if the
collector-base region of the n-p-n were to become forward biased). Also
it has generally the lowest forward voltage drop for a given forward
current, lowest storage time and lowest reverse-bias capacitance. These
all favourable factors make the diode of figure [d] an ideal choice for
digital IC’s.
Avalanche Diode
The avalanche breakdown characteristic
in a reverse biased diode can be used for voltage reference or the dc
level-shift purposes in IC circuits. The base-emitter breakdown voltage
which falls within the 6 to 9 V range is the most commonly used
avalanche diode since its breakdown voltage incompatible with the
voltage levels available in analog circuits.
The breakdown voltage of base-emitter
junction of above diode exhibits a positive temperature coefficient,
typically in the range of +2 mV/°C to +5 /°C. By connecting a
forward-biased diode in series with avalanche diode, it is possible to
partially compensate the thermal drift of avalanche diode because the
thermal drift of forward voltage of the series connected diode is
negative. The composite connection is shown in the figure below which
has breakdown voltage of (VD + BVEB) with significantly reduced temperature coefficient. Here VD is forward drop of series diode and BVEB
is the breakdown voltage (BE junction) of avalanche diode. As the
figure shows, the composite connection consists of two transistors
back-to-back in diode connection. Since both transistors have their
collector and base regions in common, they can be designed as a single
transistor with two separate emitters.
Schottky Diode and Transistor
When a metal is placed in close contact
with an n-type semiconductor, a voltage barrier is created, which is
known as Schottky Barrier. ln such contact, there are many free
electrons in the metal, whereas the semiconductor contains relatively
low. With a positive voltage applied to the metal, the barrier is
overcome and the diode begins conducting. A negative bias enlarges the
barrier, thus the diode blocks conduction. Such diode differs from an
ordinary p-n junction as follows:
The barrier is only half as large as
that of a junction diode, at low current, a Schottky diode has a forward
voltage drop of only about 0.3 V to 0.5 V.
Only majority carriers are involved in
the conduction mechanism, which make the Schottky diode a very high
speed device with a recovery time less than 1 nano seconds.
The Schottky effect only takes place in
relatively high resistivity semiconductor material. When the
semiconductor is heavily doped, a tunnelling effect occurs which
provides a direct ohmic contact.
The figure below shows the cross-section
view of a typical Schottky diode. It is formed between the epitaxial
layer and the Al deposited for interconnections. The cathode connection
to the epitaxial layer is through a conventional n+ collector contact diffusion, to ensure a good ohmic contact, but at the anode connection the n+
diffusion is omitted due to direct ohmic contact provided by Schottky
junction. The Schottky barrier is formed between aluminium and the
n-type epitaxial silicon. The advantage of Schottky diode is that it can
be made with existing IC processes. No additional manufacturing steps
are required. This is important from yield point of view.
One important application of
Schottky diode is Schottky diode clamped transistor is shown in the
figure below. The figure [a] shows circuit symbol and [b] shows the
cross-sectional view of Schottky transistor. The Schottky transistor is
used in TTL logic circuits. The Schottky transistor provides very fast
speed operation. This is possible because the Schottky clamp prevents
the transistor from going into saturation. If an attempt is made to
saturate this transistor by increasing the base current, the collector
voltage drops, diode D conducts, and the base-to-collector voltage is
limited to about less than 0.5. V. Since the collector junction is
forward-biased by less than the cut-in voltage (0.5 V), the transistor
does not enter saturation.
Note that in the figure [b], the
aluminium metallization for base lead is allowed to make contact also
with the n-type collector region, but without an intervening n+
layer. This results in formation of metal semiconductor diode between
base and collector. Since the Schottky junction is formed during the
metallization process, the Schottky transistor requires the same number
of process steps as does an n-p-n transistor.
For practical Schottky diodes, the
dominant reverse current component is the edge leakage current which is
caused by the sharp edge around the periphery of the metal plate. To
eliminate this effect metal semiconductor diodes are fabricated with a
diffused guard ring as shown in the figure [b]. The guard ring is deep
p-type diffusion and the doping profile is tailored to give the p-n
junction a higher breakdown voltage than the metal semiconductor
contact, thus preventing premature breakdown and surface leakage.
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