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Preparation, Orientation: [You may skip this part if you are already
familiar with the buttons and components in this applet.]
Maximize your Browser window (a 17" monitor should show the entire applet
The top-bar in the applet has the "Schottky-clamped" checkbox, "Ckt Param"
button, "BJT Param" button, and the choice for images at the lower-right
part of applet screen.
The bottom-bar of the applet has four buttons to control animation. Click
on the "Pause" button (the third button in the bottom-bar) to suspend the
animation thread. This will suspend the 'eye-disturbing' effect of animation
while you are settling in.
Also note that the horizontal spacing of grids in the waveform graphs is
roughly 5 ns. The horizontal axes represent the elapsed time and the waveforms
travel from left to right. The horizontal axes are the same in all four
Note that the "elapsed time=", shown in the circuit diagram in the upper-left
corner of applet screen, represents the elapsed time since the last 'switching'
action in the input signal voltage, Vi.
The main screen view is composed of five sections:
The four waveform graphs are drawn versus the elapsed time:
i) top left part is the basic RTL circuit with user's switching ability
using a mouse click in the input voltage Vi area;
ii) below the RTL circuit there are four graphs, Vi (input voltage), iB
(Base current), Qb (total excess minority carrier charge in Base), and
iC (Collector current);
iii) top-center part shows a BJT device schematic;
iv) top-right part of screen is the excess minority charge profile in Base;
v) Choice box on top-bar has three (3) items: "Planar BJT", "EB Junction"
and "Delay Times". The corresponding image appears at the lower-right part
of applet screen.
Vi = input voltage waveform,
iB = Base current waveform,
Qb = total excess minority carrier charge in the Base. Here Qeos (the red
horizontal line) is the excess Base charge at the edge-of-saturation (i.e.,
the boundary between forward-active region and saturation region of BJT,
iC = Collector current waveform.
Delay Times for Conventional BJT (without Schottky-diode
clamp between C and B).
In the Choice box on top-bar (fourth item) select "Delay Times". This will
display five different delay times in the lower-right part of the screen.
Write down the values on a scratch paper to refer to it when palying the
applet. These are explained individually below.
Turn-on delay consists of td1, td2and
tr. Turn-off delay consists ofts
Turn-On Delay: td1 + td2 + tr
td1 = Emitter junction charging time :
td2 = Time for first minority carrier to reach the Collector
junction. For modern day BJTs, this delay time is negligibly small. You
may skip this one and move on to the rise time tr, or you can follow the
steps below to see how small it is for a fT = 1GHz device.
Select "EB Junction" in the Choice box on top-bar. You should see a magnified
view of the Emitter-Base junction in the lower-right part of the screen.
Click on the Vi (input voltage) area to set Vi at the lower voltage (0.2
V). Let the applet animate and wait until the BJT is in "Cutoff" state
(look at the top-right part of applet screen: in cutoff state, the excess
minority carrier in Base is zero -- empty in the Base charge profile).
While watching the enlarged EB Junction image at bottom-right part of applet
screen, click on the Vi to switch it to input High (3.7 V) if you are in
animation mode. Or, click on the "StepForward" button to move forward if
you are in the "Pause" mode.
Watch that the thickness of depletion region (white region) in the EB Junction
is decreasing (from about xn + xp = 100nm corresponding to Vbe = 0.2V to
about xn + xp = 59 nm corresponding to Vbe = 0.7V.) This is the charging
process of EB junction, from the depletion width of the initial cutoff
region (or 'nearly' reverse biased PN junction) to that of forward-active
The time elapsed in this process is td1 (1.54 ns with
default parameter values.)
One can verify this by using the "StepForward" button at the bottom-bar.
The enlarged EB Junction image is on screen. To do this, first drive BJT
into the Cutoff state. Second, click on the "Pause" button. Third, click
on Vi area to set Vi = High (3.7 V). Last, keep clicking the "StepForward"
button until the depletion edge first reaches the minimum value (xp = 58
nm). The "elapsed time" in the RTL circuit area should show 1.54 ns.
tr = Collector current iC rise time.
Continuing on from the last step above, the EB junction is charged to the
forward-bias value (say 0.7V). The applet is still in the "Pause" mode,
with the last two buttons in the bottom-bar displaying "Resume" and "StepForward".
[At this point, you may wish to click twice on the Vi area of the circuit
diagram to reset the "elapsed time" to zero. ( the EB junction capacitance
has been charged to 0.7V and Vi = High.)]
By first or second click of the "StepForward" button the "Cutoff" state
will be changed to "Forward Active". At this point, the first minority
carriers have already arrived at the Collector junction. The "elapsed time"
in the RTL circuit area 'may' indicate 5.31E-11 s = 0.053 ns ! Check if
this value agrees with the td2 value in the "Delay Times"
image. As I told you, this delay time can be safely ignored.
Approximately, this value equals 1 / (6*PI * fT). The higher
the cutoff frequency fT of BJT the shorter it will take for
the first minority carriers to arrive at the Collector junction. The default
value is set to fT = 1 GHz in this applet. [Try changing the
fT value and see its effect on td2 if you like.] Modern day transistors
with much higher cutoff (unity current gaiun) frequency fT,
the td2 may be ignored.
Turn-Off Dealy: ts + tf
Set the applet in "Pause" mode. The Base (or BJT) has just entered the
"Forward Active" state, which is the last step above.
Click twice on the Vi area to reset the "elapsed time" to zero second.
Click as many times as necessary on the "StepForward" button until the
BJT enters "Saturation" for the first time. Read the "elapsed time" value.
This value should be a bit larger than the rise time tr which
you read from the "Delay Times" image because tr is calulated
for iCto rise from 10% of Isat to 90% of Isat,
instead of 0 to Isat.
Note that the minority carrier concentration profile in the Base has now
reached the maximum possible slope. This also measns that the Collector
current has reached its maximum value as iC is proportional
to this slope (diffusion current).
The rise time tr has something to do with how fast the
base charge increases from zero toward the edge-of-saturation value,
Therefore, tr is a function of the BJT internal capacitances,
Cje and Cjc.
Further driving the transistor into saturation does not increase slope
of the concentration profile. See it yourself by keep clicking on the "StepForward"
button, or by clicking the "Resume" button to animate. The Base charge
continues to increase, but the iC has reached its maximum,
as the Qb and iC waveform graphs show.
ts = stored delay time or saturation delay time.
tf = fall time.
When Vi is switched to low (0.2V), iCdoes
not respond but remains constant for a time ts. This
is the time required to remove the saturation charge from the Base and
Qb reaches Qeos from the maximum ts
* IB2 where IB2 = (V2 - Vbe)/RB
= (3.7V - 0.7V)/10k.
During this time ts, the EB junction remains at Forward
bias Vbe = 0.7V and Vi = 0.2 V. A reverse base current IB1 =
(V1 - Vbe)/RB = (0.2V - 0.7V)/10k flows
which helps to 'discharge' the Base. If this reverse IB1
is not present, the saturation charge will be removed entirely by the recombination
process alone, which will take a much longer time to remove Qb.
The stored delay time, ts, is given by
ts = ts*
ln[ (IB2 - IB1)/(IBsat - IB1)
where IBsat = Isat/Beta.
In order to 'feel' the ts, follow the following steps.
Set Vi = High = (3.7 V), and animate the applet until Qb no longer increases.
Click on "Pause" button to suspend the animation. Click on Vi area to set
Vi = Low = (0.2V) and reset the "elapsed time" to 0 s.
Click "Resume" button to animate. As the "Saturation" state first turns
to "Forward Active" state, click on the "Pause" button to suspend the animation.
At this point, Qb should have just reached the edge-of-saturation value,
Qeos; iC still remains at its maximum value, while the Base charge profile
shows the same slope.
The elapsed time should read a little over 2.0E-8 s = 20 ns. This is the
stored delay time that effects a very slow turnoff time for saturated BJT
As Qb falls from Qeos toward zero, iC falls exponentionally with a time
constant that depends on the junction capacitances, Cje and
Cjc. [From the network theorem known as Miller's theorem, the
Collector junction capacitance Cjc is augmented by the voltage
gain Av. But in this applet the Miller effect was not considered
and thus the fall time may be somewhat smaller that actual value.]
The slope of minority charge profile in Base decreases toward zero, and
thus iC decreases from its maximum value, Isat, toward zero.
Finally, the EB junction capacitance charges up to the reverse bias value
(or the turnoff value, which is V1 = 0.2V in this applet). You
may see this in the EB Junction image. However, this EB Junction charging
does not contribute to the turnoff delay time because the iC
already has reached zero before the EBJ started to charge.
Delay Times for Schottky-clamped BJT.
Turn-On delay time: td1 + td2 + tr = 1.54
+ 0.05 + 5.70 = 7.3 ns.
Turn-Off delay time: ts + tf = 20.5 + 3.21 = 23.7
These numbers should be taken as a rough estimate only.
Compare these numbers to the gate delay times of less than 1.5 ns of modern
day TTL and the gate delay time of less than 1 ns for ECL even in the SSI
In order to eliminate the saturation charge in Base, which is responsible
for the long turnoff time (ts is due to the saturation
charge in the tturnoff = ts + tf),
a Schottky diode is put in parallel to the CB junction.
Check the "Schottky-clamp" checkbox.
Notice the changes in the RTL circuit, in the BJT device (top-middle image
in screen) and in the planar structure of BJT (select the "Planar BJT"
in the choice box on top-bar.).
In the Planar BJT cross-section, the Aluminium contact metal to the Base
is extended all the way to the Si surface of the Collector region. The
Al in contact with p-type Base is an Ohmic contact, whereas the Al in contact
with n--type Collector is a rectifying Schottky contact. This
diode has the current flow direction from Base into Collector. Thus the
Base-Collector PN diode now has a Al-Si Schottky diode connected in parallel,
as can be seen in the RTL circuit diagram.
The consequence is: when the CB junction is forward biased (in the saturation
mode), the Schottky diode is also forward biased and conducts current in
parallel with the CB pn junction. At this point, the user may want to check
out the forward voltage drop across a Si Schottky diode and the forward
voltage drop in a Si pn diode when the current is the same (or comparable)
using this applet. [In IV applet,
select Si PN diode, set V = 0.7V, and read the current I. Then select Schottky,
the applet will show you a new voltage drop at the same current level.
The Schottky diode shows much smaller V drop than a PN diode at the same
current flow level.]
Because the current almost entirely pass through this Schottky diode and
nearly nothing through the CB pn junction, and because the voltage drop
is now 0.5 V (with a forward-biased Schottky diode) rather than 0.7 V (forward-biased
silicon pn diode alone), the excess charge at the CB junction is exp[(0.7-0.5)/kT)
= exp(0.2/0.0259) = 2,257 times smaller with
the Schottky diode
clamp than without the Schottky diode clamp.
The net result is that the CB junction has a negligibly small excess charge
at the highest possible Base current, thus keeping the BJT in "Forward
Active" mode at all times, and eliminating the "Saturation" mode altogether.
Verify the above points using the applet by
first, check the "Schottky-clamped" checkbox.
drive the BJT into cutoff by animating with Vi = LOW.
then click on Vi area to switch to Vi = HIGH.
Note that the Base charge NEVER increases into the "Saturation" region.
The maximum Base charge is now 'clamped' at the edge-of-saturation value
not affected. same as the conventional BJT. Quiz (why ?)
Verify this using the applet yourself.
Drive the BJT to 'ON' state by setting Vi=HIGH and animating the applet
for long enough.
Now switch Vi to Vi=LOW by mouse click.
Observe that the Qb returns to zero and the iC returns to zero in just
time from the third and fourth graphs.
The turnoff time is now equal to tf. instead of
+ tf in the saturated logic.
The Schottky-diode clamp eliminates the storage delay time altogether.
tturnoff = tf = 3.2 ns. Compare this to tturnoff
= ts + tf = 23.7 ns for the conventional BJT.
the Schottky diode clamp reduces the turnoff time very much.
you could 'feel' the slow removal of saturated charge from the Base using
this decreases the gate delay time substantially.
all high speed BJT digital circuits need to avoid the saturation of BJT.