Supplimentary Material for Manuscript (MS #L14-09947)
“A Comparison between Bipolar Transistor and Nanowire FET Biosensors” by Sufi Zafar et al.
Fig. s1: Measured collector current (IC) and base current (IB) as a function of applied voltage. (A)
Standard HBT device with no solution, reference electrode and TiN sensing surface. (B) HBT
sensor measured in pH 6 solution of 10 mM and 100 mM concentrations. (C) HBT sensor
measured in 100 mM and pH 4 solution. (D) HBT sensor measured in 100 mM and pH 8 solution.
As shown in the figures, IB  2x10-9 A over the entire sensing range. Also from (C), IC curves are
observed to be independent of solution concentrations, thereby indicating (IBR) is negligible.
1
IC (A)
1.5
pH 8, 100 mM
pH 4, 100 mM
pH 6, 10 mM
pH 6, 100 mM
HBT (no sol.)
1.0
0.5
10
-10
IB (A)
10
-9
Fig. s2: (A) Measured dependence of IC on the base current IB for the standard HBT device (solid
line) and HBT sensors with solution of different concentrations and pH values (symbols). The
observed dependence for sensors with different solutions is indistinguishable from that for the
standard device (with no solution). From equation 1, this implies that (IB ∙ R) is negligible.
2
Discussion on the sensitivity definition:
As discussed in the manuscript, a sensor has two components: device and sensing surface.
The relative change in the sensing signal (I /I) can be written as: I /I = (gm/I) whereand
(gm/I) measure sensing surface and device contributions, respectively. The goal of the present
study is to demonstrate the enhancement in the sensor performance metrics due to the device
component of a sensor: i.e. bipolar transistor versus nanowire FET. In Fig. 2 (C), the sensitivity of
bipolar transistor sensors are compared with that for nanowire FET sensors. These sensors have
different sensing surfaces, and therefore it is important that the sensing surface contributions are
excluded so as to make a fair comparison between sensors with different device components. Since
the definition of I /I per volt measures only the contribution of the device component, use of this
sensitivity definition enables us to unambiguously demonstrate the superior sensing characteristics
of bipolar transistor based sensors in comparison to nanowire FET based sensors. Furthermore,
use of this definition in Fig. 2 (C) enables us to verify the prediction of equation 3 which predicts
that I /I per volt = q/kT, irrespective of bipolar transistor device fabrication details.
An alternate sensitivity definition is I /I per pH. This definition includes the contributions
of both sensing surface and device components and is therefore not appropriate for the present
study. However, it is of interest to re-plot the Fig 2. (C) in accordance with the definition of I /I
per pH. The re-plotted data is shown in Fig. s3.
3
I / I ( pH
-1
)
Bipolar Transistor Sensors
2
Nanowire FET sensors
[6]
[19]
[4]
0
-10
-8
10
10
-6
10
Sensing Current (A)
Fig. s3: A comparison between bipolar transistor and nanowire FET sensors using alternate
sensitivity definition of I /I per pH.
4
-7
VBE (V)
10
-10
2
SI (A /Hz)
10
0.7
0.8
0.9
1.0
pH 6,100 mM
-13
10
-16
IC (A)
-10
1x10
-9
4x10
-7
2x10
-6
7x10
10
-19
10
-22
10
1/f
-25
10
-28
10
0.1
1
f (Hz)
Fig. s4: Typical collector current noise power spectra (SI) for the extended gate SiGe HBT sensor
with 100 mM phosphate buffer solution at pH 6 at different IC values corresponding to different
VBE values. SI is observed to have a power law dependence on frequency (f) with exponent of ~1
at lower VBE = 0.7 V and ~1.8 at higher VBE = 1.0 V. This implies that that 1/f noise dominates at
lower VBE whereas random telegraphic noise starts to dominate at higher VBE values. Similar SI
dependence is also observed for a HBT sensor with 10 mM buffer solution and the HBT device
only (i.e. without the extended base consisting of TiN sensing surface, solution and reference
electrode).
5
1080
pH 7.01
IC (pA)
1077
1038
pH 7.03
pH 7.03
1036
0
30
60
time (s)
90
Fig. s5: pH sensing measurements to investigate the limit of detection for HBT sensor; measurements
show detection of pH = 0.02 with a measured signal to noise ratio of ~ 80.
Discussion on Fig. s5
We further investigate limit of detection (LOD) using pH sensing measurements. The
measured SNR of 2x105 V-1 for the HBT biosensor (Fig. 3(c)) translates to a minimum detectable
voltage of ~15 V (for a typical sensing measurement: sampling rate is ~ 10 Hz and the total
measurement time of ~ 30 minutes which gives BW =3). Using the measured pH sensitivity of
55 mV/pH and voltage ~15 V, the LOD is ~ 0.0003 pH. Fig. s4 shows the detection measurements
for pH change of ~ 0.02 with signal to noise ratio of 80 which corresponds to measured LOD of
~ 0.00025 pH, consistent with the deduced LOD from SNR measurements. This measured LOD ~
0.00025 pH is significantly less than the reported value of 0.01 pH for nanowire FET sensors.16
6
10
-9
(a)
70
(b)
o
Tthermocouple ( C) SS(mV/dec)
10
60
59.0
61.2
63.1
66.1
68.2
o
25
35
45
60
70
THBT ( C)
I c (A)
10
-10
-11
THBT= 0.6 + Tthermocouple
50
40
100 mM
HBT only
30
10
100 mM, pH 6
-12
0.5
20
20
0.6
VBE (V)
30
40
50
Tthermocouple
60
70
o
( C)
Fig. s6: (a) Dependence of IC on VBE measured at different temperatures for extended base SiGe
HBT sensor with 100 mM buffer solution at pH 6. Symbols are measurements and solid lines are
fits in accordance with eq. (1b); Tthermocouple is the temperature measured using thermocouple and
SS is the sub-threshold swing estimated from the fits. (b) Comparison between THBT and
Tthermocouple; THBT is temperature estimated from the measured sub-threshold swing (SS) of the HBT
sensor.
7