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Fabrication and Characterization of
Schottky Diode
Arnab Dhabal
Page 2 of 22
Acknowledgements
I would like to express my greatest gratitude to the people who have helped and supported
me in this project. I wish to thank Prof. Damodaran and Prof. Anjan K. Gupta for helping
throughout right from giving invaluable suggestions and comments to making arrangements
for obtaining the samples.
I also thank Prof. Y.N.Mohapatra, for giving advice on our method of fabrication and providing
me with the silicon sample.
Special thanks go to Mr. Ramesh, whose vast experience with the Vacuum Deposition Unit was
of great help, and Mr. Upendra, for helping me in getting the project done smoothly and in
time.
The data acquisition and characterization part would have been unsatisfactory without the
assistance of Mr. Indranuj Dey, who not only helped set up the electric instruments, but also
took interest in discussing the results, thereby providing deeper insight as I worked on the
project. I am extremely grateful to him.
Page 3 of 22
Contents
Introduction ………………………………………………………………………………………………..…… 4
Fabrication of a Schottky Diode ………………………………………………………………………. 4
Choice of Material 4
High Vacuum System Overview 5
Steps of Operation 5
Electrical Characterization …………………………………………………………………………..…… 8
Current-Voltage Characteristics 8
Current Variation with Time due to Heating 12
Voltage Characteristics on Application of Square Wave Pulses 13
Discussion and Scopes of Improvement …………………………………………..…………….. 21
References ..…………………………………………………………………………………….………………. 22
Page 4 of 22
Introduction
A Schottky diode is a special type of diode with a very low forward-voltage drop and a very
fast switching action. When current flows through a diode, there is a small voltage drop across
the diode terminals. A normal diode has between 0.7-1.7 volt drops, while a Schottky diode
voltage drop is between approximately 0.15-0.45 – this lower voltage drop translates into
higher system efficiency.
Chemically, a Schottky diode has a Schottky contact between a semiconductor and some
appropriate metal. The other end of the semiconductor has an Ohmic contact, with a metal. To
ensure that the two ends of the semiconductor form different junctions, a gradient in the
doping concentration is required within the semiconductor, such that the end with the Ohmic
contact has more carriers than the Schottky contact.
Fabrication of a Schottky Diode
Choice of material
SiC performs best as a semiconductor for Schottky diodes. However, only Si wafers were
available within IIT Kanpur. On reading a few papers, it was understood that n-typed Si works
better as a diode than p-type Si. Also a Schottky contact is formed on Si by depositing Au, while
the Ohmic contact could be of any metal, like Aluminium. The initial plan was to take a glass plate
as the base, and to sputter deposit the following components as 2mm x 5mm strips in the order
given:
• Aluminium
• Heavily doped n- Si (1018 /cc)
• Low doped n- Si (1016 /cc)
• Gold
This had to be done in such a way that later the wires can be soldered to the Gold and
Aluminium ends.
However this plan was rejected due to two major reasons:
i. Unavailability of n-Si of two different types of doping concentration.
ii. The possibility that on sputter depositing the Si, the doping and the Si might not
evaporate and get deposited in the same ratio as in the original wafer sample.
So it was decided that I should start with a Si sample that already has a gradient in the
doping concentration. Au was to be sputter deposited onto the lowly doped (shiny) Si surface.
Page 5 of 22
The Ohmic contact was to be made of Indium solder, while the wire to be drawn from the Gold
side was to be attached to it by Silver paste.
High Vacuum System Overview
The HINDIVAC system at IIT Kanpur Modern Physics Lab utilizes 3 pumping devices in
stages:
i. The rotating Mechanical Pump
- Primary source for creating vacuum. Can reach only up to 10-3 mbar
ii. The Diffusion Pump
- It uses hot oil and has the advantage of reaching up to 10-7 mbar but
must be backed by a rotary pump.
iii. The cold trap
- It reduces pressure by condensing, or freezing out condensable
vapours that may exist in the system. It also prevents oil vapour in
diffusion pump from back streaming into the system. Liquid N2 is
used for the purpose.
Other system components include valves and baffles to aid the control of action of these
pumps. The valves allow Roughing and Backing modes of operation. In the roughing mode,
only a rough vacuum is obtained by the rotary pump. The Foreline valve and the Hi-Vac valve
isolates the diffusion pump, and the cold trap, from the chamber. After completion of roughing
(Pressure < 0.005 mbar), the Foreline Valve is opened to start the Backing mode.
Within the Vacuum chamber there are electrodes on which the material to be deposited
are kept in boats, and high Power is given to them continuously until the material starts
evaporating. The substrate which is placed above the electrodes gets coated. The system also
has a Digital Thickness Monitor, which should be placed in the vacuum chamber at the same
distance from the material being sputtered as the substrate. The Acoustic Impedance and
Density of material are given as input. It displays the thickness of material deposited in
Angstroms, and the rate of Deposition.
Steps of Operation
Initially the vacuum chamber is tested for leaks. This is done by just running the Rotary
pump, with the Combination Valve in Roughing. Possible sources of leaks are identified and
attended to.
The substrate is prepared by cleaning it using acetone, and placing it appropriately on a
plate, using cleaned blades for support. The Aluminium boat is placed on the electrode with
the gold after rinsing it in acetone as well.
Page 6 of 22
After closing the chamber properly, the vacuuming process is started:
i. The power supply and the main MCB is switched on. All valves (High-Vacuum
valve - HV and Combination Valve- CV) are closed.
ii. The Rotary Pump is switched on.
iii. CV is put to roughing
iv. When vacuum reaches ~0.005 mbar in the roughing line, CV is changed back to
Backing.
v. Water supply is turned on, and then the Diffusion Pump is switched on.
vi. In around 20 minutes, the pressure in the backing region also falls to 0.005
mbar.
vii. CV is changed to Roughing for 1-2 minutes and again brought back to Backing.
viii. When the pressure reaches 10-4 mbar, HV is opened.
ix. Liquid N2 is poured into the cold trap.
x. The pressure is allowed to stabilize at <10-5 mbar for 30 minutes.
Figure 1 - High Vacuum Generator and Thin Film Depositor
Now current is passed through the electrode for heating up the Au to evaporating
temperatures. The current is increased at the rate of 2 Amp/minute, till deposition temperature is
reached. The DTM was not functioning properly, so it was used only for getting a rough estimate,
by seeing the rate of deposition from time to time. This is carried out over a period of 30 minutes,
and the current rate was reduced again at a steady rate of 2 Amp/minute. It was estimated that
800nm – 1200 nm of Gold had been deposited on the Silicon surface.
Figure 2 – Gold sputtering in process in 10-5
mbar pressure
Page 7 of 22
After 10 minutes, HV is closed, and the DP is switched off. After around 20 more minutes,
CV is closed and Rotary pump and water supply are also switched off.
The air admittance valve is opened for fast release of the vacuum. The sample is
collected out. The gold coating on it is visible.
It is found that gold had been deposited at the edges of the Silicon piece, which can cause
unexpected characteristics. So the edges with depositions are chipped off. Indium is used to
solder a thin copper wire onto the rough surface of the Silicon (for the Ohmic contact). On the
other surface, Silver paste is used to attach a thin copper wire on to the Gold.
For ease of use, the sample thus produced is mounted on a PCB. The Au coated Si piece is
vertically attached to the PCB using insulating glue. Thicker wires are soldered on-to the PCB
and connected to the two thin wires of the diode. These two terminals can now be directly
plugged into bread-boards for electrical characterization.
Figure 4 – Design schematic of Fabricated diode
Figure 3 – The fabricated diode mounted on a PCB with connections
Page 8 of 22
Electrical Characterization
1) Current Voltage Characteristics
The following circuit was implemented for the normal Voltage Current characteristics:
Figure 5 - Circuit Diagram A
The circuit with the diode A was made on a breadboard. A Voltage source, 2 Multimeters, and
Wires were the other requirements
Figure 6 - Circuit Set-up A
Page 9 of 22
Readings for Forward Bias:
Voltage in mV Current in µA
Voltage in mV Current in µA
Voltage in mV Current in µA
10.6 0.04
319.0 17.9
903 105
31.7 0.15
339.0 20.3
959 116
41.3 0.22
361.6 23.1
1039 133
62.0 0.41
386.4 26.3
1078 141
82.8 0.70
406.9 28.9
1152 156
99.4 1.03
432.1 32.1
1215 177
131.2 1.97
451.2 34.5
1294 196
178.2 4.33
487.5 39.2
1405 220
198.5 5.76
546.8 46.8
1720 290
221.8 7.64
570.1 49.9
2090 401
259.2 11.2
656.0 64.4
2410 469
291.0 14.6
728.0 75.4
3450 743
301.0 15.8
764.0 81.1
3920 941
310.2 16.9
824.0 91.2
5700 2064
Readings for Reverse Bias:
Voltage in mV Current in µA
Voltage in mV Current in µA
Voltage in mV Current in µA
-33.8 -0.116
-137.2 -0.303
-384.0 -0.627
-53.6 -0.159
-173.9 -0.367
-445.5 -0.717
-71.4 -0.185
-197.5 -0.401
-501.2 -0.824
-83.9 -0.206
-208.1 -0.418
-594.2 -1.058
-95.4 -0.227
-245.1 -0.465
-722.5 -1.68
-109.5 -0.253
-294.2 -0.516
-856.9 -2.62
-114.8 -0.264
-358.5 -0.586
-1015.6 -3.80
Page 10 of 22
Figure 7. Current vs Voltage characteristics in both forward reverse regions
• The I vs V characteristic shows that current flow in one direction is definitely
favoured over the other.
• In the reverse bias, there is flow of current, but substantially low (<5 µA at 1 V)
• However, it can be seen that the positive region does not have a very steep slope
as expected from normal diodes. This is indicative of an inbuilt Resistance in series,
possibly at one of the junction of the diodes, or also because of the thick layer of
Silicon.
• We can roughly say that sans the series resistance the Voltage drop across the diode
would have been 0.19 V.
• The resistance changes with temperature. So at higher voltages when there is
substantial heating, the current reading gradually goes up, for (>5 minutes at 2 V
forward bias) indicating that the resistance is lowered. If the current is switched
off for some time, the resistance returns to its room temperature value. So when
switched on again, the current returns to its low starting value.
It was found that above 500 mV, the heating effects caused too unstable
current values, and hence the readings taken in that region were not very reliable,
and hence has been excluded from the plot (Figure 7).
Page 11 of 22
The V-I equation for an ideal diode is :
−= 1exp
nkT
qVII oD
Correction for resistance:
−
−= 1
V
)IRV(expII
0
o where V0 = ����
i.e. V = �� � ���
+ 1� + ��
Figure 8 - Plot of Forward biased region after curve fitting
Using OriginLab 7.5, Curve fitting was carried out for the above equation and found the
coefficients as:
R = 6.2 + 0.1 kΩ
I0 = 0.134 + 0.005 µA
V0 = 42.7 + 0.5 mV
It is noted that the resistance of 6200 Ω is quite high in comparison to normal diodes.
Page 12 of 22
2) Current variation with time due to heating
The rate of change of resistance value is quite high as the following dataset comprising of
current values noted down at intervals of 10 seconds at a voltage of 2.0V indicates. These
characteristics were repeated if the circuit was given sufficient time to cool down.
Figure 9 – Plot of Diode Current against time at Voltage = 2.0 V
The possibility of this effect being caused by a capacitor discharging was also considered, but
given the high time period it is highly unlikely of a capacitor of capacitance ~ 0.025F to be
present alongside the diode. Besides, the results of the following section show that there is
capacitance but of the order of nanoFarads.
Time in secs Current in mA
Time in secs Current in mA
Time in secs Current in mA
0 0.3342
145 0.5679
285 0.695
15 0.3629
155 0.5796
295 0.7029
25 0.3836
165 0.5914
305 0.7099
35 0.4022
175 0.6021
315 0.7163
45 0.4244
185 0.612
325 0.7227
55 0.4427
195 0.6222
335 0.7279
65 0.4599
205 0.6298
345 0.7338
75 0.4757
215 0.6381
355 0.7395
85 0.4903
225 0.6463
365 0.746
95 0.504
235 0.6559
375 0.751
105 0.5192
245 0.6656
395 0.7645
115 0.534
255 0.6739
405 0.768
125 0.5444
265 0.6808
425 0.7794
135 0.5573
275 0.6884
Page 13 of 22
3) Voltage characteristics on application of square wave pulse:
The following circuit was implemented, with the use of a Function Generator at 1kHz and the
characteristics were studied by using a Oscilloscope:
Figure 10 - Circuit Diagram B
Figure 11 – Circuit Set-up B
The experiment was once carried out for -3V to 3V and for 0V to 6V using both the sample
made and a normal LED. The data were collected on a PC and analyzed separately.
Page 14 of 22
Vin = -2.96 V to +2.96 V
LED:
Figure 12 – Plot of V(in) and V(out) for LED and V(in) changing from -3 V to +3V
Voltage across LED in forward bias= 2.96 – 1.16 = 1.80 V
Diode A:
Figure 13 - Plot of V(in) and V(out) for Diode A and V(in) changing from -3 V to +3V
Page 15 of 22
Voltage drop across the diode = 2.96 V – 0.56 V = 2.4 V.
Resistance of Diode in forward bias = (2.4/0.56)*1 = 4.3 kΩ
• The sample prepared follows the first requirement of a diode. It allows current to flow
in the forward direction and prevents it from flowing in the reverse direction.
• There is substantial capacitance in the diode, because of which we get a curve like that
of a differentiator circuit. The charge stored in the capacitor continues to discharge
even after the voltage is reversed.
• In the reverse direction, the slope of the curve changes twice in the course of the
value returning to zero. Initially it was thought that it is indicative of the fact that there
are 2 capacitors at work, the effect of one of which changes with time. But on closer
inspection it was noticed that the input voltage had developed a kink in that region.
Somehow, due to momentary heavy current drawing, the Voltage source fails to keep
the same negative Voltage for a period of about 66 µs. It was further found that this
effect reduced and became indiscernible, as the Voltage amplitude was brought down
to zero. Because of this analysis of the negative regime would largely prove to be futile.
Figure 14 – Magnified view of region of voltage direction changing
It is to be noted that these magnified values were recorded after some heating had
already taken place in the circuit and hence the resistance being lower, voltage drop
across the diode is lesser and that across the 1kΩ resistor is more.
Page 16 of 22
The capacitance in the forward direction was obtained by curve fitting of an exponentially
decaying curve using OriginLab.
In the curve, y = V0 + A*exp(-t/τ) , subjected to the constraint V0 + A = 5.9 (theoretical
Voltage value from which capacitor starts discharging), the coefficients obtained by
Levenberg-Marquardt iterations are as follows:
V0 = 0.96+ 0.07 V
A = 4.9 + 0.1 V
τ = (4.8 + 0.1 )x 10-7
s
From equation, at t = ∞, Voltage drop across diode = 2.96 – 0.96 = 2.00V
Thus, diode resistance in forward bias R = (2/0.96)*1 kΩ = 2.1 kΩ
Hence from the following model I of
circuit (in forward biased region), we
get
By circuit analysis, Equivalent
resistance = 3.1 kΩ
Capacitance C = τ/R = 1.6 nF
Another model II of circuit (in
forward biased region), is given
alongside.
From this,
By circuit analysis, Equivalent
resistance = (1*2.1)/(1+2.1) = 0.68
kΩ
Capacitance C = τ/R = 7.1 nF
Figure 15 – Possible simple equivalent circuit A
Figure 16 – Possible simple equivalent circuit B
Page 17 of 22
(i) Vin = 0 V to +5.92 V
LED:
Figure 17 – Plot of V(in) and V(out) for LED and V(in) changing from -3 V to +3V
Voltage across LED in forward bias= 5.92 – 3.92 = 2.00 V
Diode A:
Figure 18 – Plot of V(in) and V(out) for Diode A and V(in) changing from 0 V to +6V
Page 18 of 22
Voltage drop across diode = (5.92 – 1.52) V = 4.40 V
Resistance of Diode in forward bias = (4.4/1.52)*1 = 2.9 kΩ
It is evident from the curves that the slope in the forward and reverse biasing are
different. Thus it indicates that the Time constant during the 2 cases are different. If we
model the diode as having only one capacitance, it must be having a resistance in parallel
to it, that becomes infinite on reverse biasing. This might be in addition to another series
resistance within the diode. So we may model the diode as follows:
Figure 19 - Model for diode circuit that can explain the results of both the forward and reverse biases
Figure 20 – Magnified view of region of voltage direction changing for forward bias, V(in) changing from 0 V to +6V
Page 19 of 22
Note: The input voltage is a square pulse from 0.08 V to 5.80 V.
The forward bias region of the curve, was fitted on to an exponentially decaying polynomial
curve, y = V0 + A*exp(-t/τ) , subjected to the constraint V0 + A = 5.8 (theoretical
Voltage value from which capacitor starts discharging), the coefficients obtained by
Levenberg-Marquardt iterations are as follows:
V0 = 1.4 + 0.1 V
A = 4.4 + 0.1 V
τ1 = (5.6 + 0.1 )x 10-7
s
From equation, at t = ∞, Voltage drop across diode = 5.8 – 1.4 = 4.4 V
Thus, diode resistance in forward bias = (4.4/1.4)*1 kΩ = 3.1 kΩ
From Figure 19, R1 + R2 = 3.1 kΩ …(1)
Equivalent resistance as seen from capacitor = (R1*(R2 +1))/(R1 + R2 +1)
Thus Time constant τ1 = 5.6 x 10-7
= C(R1*(R2 +1))/4.1 … (2)
Figure 21 – Magnified view of region of voltage direction changing for reverse bias, V(in) changing from 0 V to +6V
The reverse bias region of the curve, was also fitted on to an exponentially decaying
polynomial curve, y = V0 + A*exp(-t/τ) , subjected to the constraint V0 + A = 5.8
(theoretical Voltage value from which capacitor starts discharging), the coefficients
obtained by Levenberg-Marquardt iterations are as follows:
Page 20 of 22
V0 = 0.1 + 0.1 V
A = 5.7 + 0.1 V
τ2 = (2.7 + 0.3 )x 10-7
s
Now from Figure 19, the circuit is only a discharging circuit having a capacitor C and
resistors R2 and 1 kΩ. So τ2 = 2.7 x 10-6
= C*(R2 +1) …(3)
From (1), (2) and (3),
R1 = 0.85 kΩ, R2 = 2.25 kΩ, C = 0.8 nF
Figure 22 - Model for diode circuit that can explain the results of both the forward and reverse biases
Page 21 of 22
Discussion and Scopes of Improvement
The diode that was fabricated acted like a diode but it had many defects most visibly the high
resistance and capacitance. None of the results regarding the calculation of the resistances
and capacitances are very conclusive as no two results exactly support each other. However
we can say that that the diode has a resistance of the order of kΩ and a capacitance of the
order of nF.
A better analysis could have been possible had the diode been studied for a few more ranges
of square waves.
The capacitance and high resistance can be arising from :
a. Formation of oxide layer on the silicon wafer. This possibility could have been
removed by treating the Silicon wafer with HF before using it as a substrate.
b. An intrinsic property of the silicon wafer, caused during the formation of the
gradient in the doping concentration
Also there is substantial effect of temperature on the resistance of the diode, which again
makes the results dependent on how long the experimentation is on. This effect can be
reduced by using some mechanism of dissipating away the heat generated, in the thick Si
layer.
Since the voltage sources are not very sharp in the transition from +ve to –ve Voltage, it was
not possible to exactly measure the reverse recovery time, which is supposed to zero for
Schottky diodes.
Page 22 of 22
References
Papers:
• Metal-semiconductor Contacts for Schottky Diode Fabrication – Mark D. Barlow
• Comparison of Current-Voltage Characteristics of n- and p-Type 6H-SiC Schottky Diodes
– Q. Zhang, V. Madangarli, M. Tarplee, and T.S. Sudarshan
• Schottky Contact Barrier Height Enhancement on p-Type Silicon by Wet Chemical Etching
– G. A. Adegboyega, A. Poggi, E. Susi, A. Castaldini, and A. Cavallini
Web-sites:
• http://en.wikipedia.org/wiki/Schottky_diode
• http://www.radio-electronics.com/info/data/semicond/schottky_diode/schottky_barrier_diode.php
• http://journals.iop.org/
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