Characterisation and Reliability Testing
of THz Schottky Diodes
Chris Price University of Birmingham, UK
http://www.sr.bham.ac.uk/yr4pasr/project06/CP/schottkydiodes/mainpage.html
Physics
Background
• Characteristics of a Schottky diode can be derived from the
energy band diagram of the system
• The motivation for this project is to develop Schottky diodes which are
sensitive to the requirements of Europe: To do this the fabrication process
needs to be optimised and the reliability of the devices tested
• If a material is in equilibrium, its energy levels are constant,
Fig 3
• A Schottky diode is formed by the intimate contact between a metal and a
doped semiconductor
Fig 3: Energy band diagram of a metal and
semiconductor in equilibrium but not in contact
• Schottky diodes produced at RAL are planar devices, shown in Fig 1. The
Schottky junction is made from titanium and n type gallium – arsenide
 Fermi level, Ef, must remain constant throughout the system
 The electron affinity, χ, must remain constant
• Schottky diodes are specifically used for their non-linear current voltage
relationship, shown in Fig 2, as frequency mixers and multipliers
Fig 4: Energy band diagram of metal and
semiconductor in equilibrium and in contact
1.0
0.8
Current [mA]
 The free space energy level, E0, must remain continuous
•As the Fermi levels are different in the two materials the electrons
with the highest level spontaneously move to the lowest level. This
creates a depletion zone in one material as all of its free electrons
move
1.2
0.6
0.4
0.2
0.0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage [V]
Fig 1: Schematic of a planar Schottky Diode
• When the two materials are brought into contact and remain
in equilibrium, the energy band diagram changes subject to 3
laws, Fig 4:
Fig 2: A graph showing the non-linear
properties of a Schottky diode
Fig 5a: Energy band diagram when
a forward bias is applied
• These effects create a potential barrier at the junction the height of
which is the difference between the two work functions
• This barrier can be lowered or raised by applying a forward or
reverse bias voltage, Fig 5a & 5b
Derivation of the I-V relation
• Electron conduction across the junction occurs primarily by thermionic
emission
• Charge density is given by the Maxwell-Boltzmann distribution:
Fig 5b: Energy band diagram when
a reverse bias is applied
Evaluation of Schottky diodes
WHY - As with any product manufactured, the end product is never as good as the ideal design.
Therefore methods of measuring the quality of each device are used to ascertain how good or how
close to the ideal product each end product is
where
• Current across the junction is directly proportional to the change in charge
density from zero bias to forward bias conditions
Current voltage characteristics
IDEALITY - In real life, diodes do not behave in a completely ideal manner, so a parameter needs to
be added to compensate for their non-ideal behaviour
The ideality factor is a number around 1.0, typically between 1.1 and 1.2 for real diodes. As the
ideality factor increases, the non-linearity behaviour of the diode decreases changing the gradient of
its I – V plot, Fig 2; the effect of this is to reduce all aspects of a Schottky diode’s performance
whether it is being used for harmonic generation or RF mixing.
• It therefore follows:
where
Where n1 is the charge density under zero bias, n2 is the charge density under forward bias, nd is the doping density, kB
is Boltzmann constant, E is energy, T is the absolute temperature, q is the charge on an electron, Φbi is the built in
potential, Φm & Φs is the work function of the metal and semiconductor respectively, V is the voltage applied, I0 is the
reverse saturation current, A** is the Richardson constant and W is the junction area
Development
Progress
Ideality equation
Modified I-V equation
SERIES RESISTANCE - The series resistance within a Schottky diode structure is mainly formed from
highly doped semiconductor regions at semiconductor–metal interfaces. This affects the diodes’
performance when the voltage across this region is comparable to the voltage across the junction and
also varies as a function of current Fig 6
20
HOW – Measuring and comparing the ideality and series resistance properties
of each diode batch
Series resistance
equation
0
0
5
10
15
20
25
30
Fig 8: Table of the
currents used to
evaluate the DUT
S10
1.190-1.200
1.180-1.190
S9
1.170-1.180
Reliability can be defined simply as the quality over time
1.30
14
Repeatability
1.25
12
1.20
10
S8
6
• Tested by contacting a diode once and repeatedly
measuring its properties
• The result of this experiment can be seen in Fig 15
S4
S3
S2
Ideality
S1
1
2
3
4
5
6
7
8
9
10
RESULTS – Ideality across a wafer is fairly uniform with no obvious areas of an
inconsistent fabrication process
X
USES – Potential to test several fabrication condition across a wafer, saving
time and resources and see visually how the diodes’ properties change
Native oxide growth
Fig 13: A diagram showing
the partitioning technique
Left
Centre
Right
• The average ideality for the two sets of experiments can
be seen in Fig 17
To measure the effect of short term oxide
growth a wafer was split into 3 sections, Fig
13: left 0 mins, centre 5 mins, right 15mins
It can be seen that there is a threshold temperature at around
130°C after which the diodes begin to degrade
Fig 14: Surface map showing the increased
ideality as exposure to air increases
The most probable reason for this degradation is material
inter-diffusion at the anode
34
45
56
67
78
89
100
111
122
133
144
155
166
177
188
199
1.20
20
1.18
18
1.16
16
1.14
14
1.12
12
1.10
10
1.08
8
1.06
6
1.04
4
Ideality
Series Resistance [ohm]
1.02
Short term thermal reliability
• Several diodes with similar idealities were heated for 30
seconds, cooled and had their characteristics measured;
this was done over a temperature range between 80200°C and 90-190°C in 20°C steps
23
Fig 15: A graph showing how ideality and series
resistance vary with multiple measurements
The conclusion was that these effects are negligible and would
not bias the results
The most common method for integrating these devices into
circuits/waveguides is to use solder, however, little is known
about how this heating changes the diodes’ properties
12
Measurement Number
Ideality
Fig 12: Surface map of
how ideality varies across
a wafer (10 x 10 array)
0
1
• Repeatedly contacting the same pad may lead to inconsistent
connections
• The result of this experiment can be seen in Fig 16
2
1.00
2
1.00
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Number of times probed
Fig 16: A graph showing how ideality and series
resistance vary with multiple contacts
1.180
1.175
1.170
Ideality
S5
4
ideality
Spreading Resistance [Ohm]
S6
Y
1.10
1.05
• Tested by repeatedly contacting the same diode
S7
8
Resistance [ohm]
1.15
Ideality
• Repeatedly measuring the diode ideality and series resistance
may cause them to change due to a burn-in effect
1.160-1.170
1.150-1.160
16
Series resistance [ohms]
As the diodes are produced in an array, Fig 11, a measurement of each one
can be taken and plotted as a map, Fig 12.
RESULTS – The average ideality increased
from 1.171 (left) to 1.189 (centre) to 1.205
(right).
4
Reliability testing
RESULTS – bunching of data points shows uniformity in the fabrication process
and the motion towards the bottom left corner shows the improvement in
quality of the devices
Surface mapping
Oxide growth on the anode is a major cause
of degradation in a diodes’ performance. It
is formed simply by exposing the cleaned
anode to air
8
HOW - The calculations of the I – V characteristics are done by applying a current over several
decades and measuring the voltage across the diode
Fig 7: Simplified circuit
diagram of how the
voltage is measured
Fig 9 shows a scatter plot comparing ideality and series resistance of two
generations of diodes Fig 10 shows how 2 batches of diodes compare by
showing the average value and standard deviation
Fig 11: Picture of a wafer
produced at RAL
12
Current [mA]
Fig 9: Ideality against series
resistance scatter plot of diodes
produced at RAL
Fig 10: Average ideality against
series resistance plot for same
fabrication process
16
Series Resistance [Ohms]
WHY – optimise the fabrication process to produce the best quality devices;
as the smaller the anode size, the more sensitive the diodes are to the
fabrication process
Fig 6: A graph to show
the series resistance as a
function of measurement
current
1.165
1.160
1.155
1.150
0
20
40
60
80
100
120
140
160
Temperature [C]
Fig 17: A graph showing how ideality
changes with increasing temperature
180
200