STUDY OF FIELD EFFECT TRANSISTORS ON LOADING WITH BIOLOGICAL
MATERIAL IN ELECTROMAGNETIC FIELD
Sarah Rizvi (1), Pallavi Purwar(2), V.R. Singh(3)
National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India
(1)
rizvis@mail.nplindia.ernet.in, (2) purwar_pallavi@mail.nplindia.ernet.in, (3) vrsingh@mail.nplindia.ernet.in
ABSTRACT
The solid-state devices are very important in the instrumentation systems for various industrial, scientific and biomedical
applications. With the ongoing miniaturisation trends, new devices of micro and nano sizes are now in use. Nanotechnology
for the development of devices for new applications has progressed at a rapid speed in the recent past. In biomedical devices
also, new types of microchips, biochips and sensing systems have been developed. Recently, the biological materials have
been used in the devices like diodes, op amps, FETs. Some basic studies have been made on the physical, electrical and
other properties for such devices. Acoustic stress effect and electromagnetic interference effect on the performance of solidstate devices have been reported. However, a systematic and detailed study is required to be made in biological
environments.
In the present work, study of solid-state devices with biological loading in RF field is pursued. FETs are chosen here, as
these are very important in digital systems. The effect of loading the FET device with a biological material, in this case,
bone material, is studied, the FETs use a gate element that when charged creates an electromagnetic field that changes the
conductivity of a silicon channel and turns the transistor on or off. Investigations have been carried out by some of the
researchers, on possible biological effects of exposure of devices to radio frequency and/or microwave radiation.
The study on the influence of RF field on the performance of the devices has been of assistance to the scientific and
industrial community, for making necessary improvement in the design of such devices. The performance characteristics of
the device like current gain, power output, transconductance, pinch off voltage, saturated drain to source current, breakdown
etc., have been analysed both under normal and under the influence of high frequency sources. The current-voltage
parameters of the FETs are measured where the output drain current/voltage relationship is plotted at discrete gate voltage.
The transfer characteristics that depict the variation in drain voltage due to variation in gate voltage, at some fixed drain
voltage due in the saturation region are also measured. The study would assist in making necessary corrections in the device
parameters under biological environment, particularly in precision instruments.
INTRODUCTION
Rapid advances in science and technology have provided a variety of semiconductor devices with highly controlled and
unique properties, which enabled the fabrication of various functional devices. FETs and other devices coupled with
biological materials are now useful in the development of biochips and micro-sensors. The integration of biomaterial with
semiconductor devices greatly expands the impact of bioelectronics, and hence scientists have recently begun to extensively
study these devices and to apply them to a variety of applications ranging from diagnosis of disease to gene therapies. A
significant step has been taken in the direction of the study of these devices under biological influence. The radio frequency
performance of a large variety of silicon devices has already been extensively studied for the realisation of high
performance devices [1]. A variety of transistors and FETs etc. with performance adequate for practical applications at radio
frequency have been reported [2,3]. The effect of RF power on biological tissues has also been widely investigated and
documented [4]. However, a systematic and detailed study is required to be made to study the performance of these devices
under the collective influence of a biological material and RF field. Here, the study of FETs on loading with biological
material is made to ascertain the device response with and without RF field.
1
MATERIALS AND METHOD
A basic n-channel depletion-mode field effect transistor with configuration, BFW 11, has been used in the present study.
The in vitro animal bones are thoroughly washed and kept in saline solution for at least two days. Measurements have been
made using finely ground powder and square shaped bone pieces (0.01x0.01 m2 approx.), after drying the solid samples for
10 days. In the present work, the effect of bone samples on the device performance has been studied in the frequency
range 1 KHz – 15 MHz. The radio frequency has special biological significance since it can readily be transmitted through,
absorbed by and reflected at biological sample boundaries in varying degrees. The FETs have been analysed using standard
transistor characteristics and transfer characteristics (Fig.1) and also by connecting the device in the amplification mode
(Fig.2). The investigation has been carried out by constructing a cell with parallel electrodes to apply the bone samples
(moist in saline solution) as load at the gate terminal of the biasing circuit and a common source amplifier. The whole set-up
is placed in field strength of 1.43x102 V/m, by constructing a condenser with aluminium conducting plates to which RF was
applied. The temperature of the set-up is maintained in the range 18°-20°.
2
RESULTS AND DISCUSSION
The absorption in the solid bone samples, below 20 MHz, is poor due to severe reflection and low electrical
conductivity [5]. No change in device output is thus observed when the bone pieces are introduced at the gate terminal, both
with and without field as shown in table 1, however bone powder shows considerable conductivity for the same conditions.
Hence, the FETs have been analysed after loading with bone powder for frequencies less than 20 MHz. Since the gate
terminal acts as the control element, the study is conducted by applying the load at the gate.
A frequency dependent behaviour of the output parameter is observed for fixed values of drain and gate voltages, the
current being maximum (0.38 mA for 40.2% w/v) at 7.7MHz (see Fig.3), as opposed to the conventional response of the
device under similar conditions [6]. The way in which RF energy interacts with an object depends mainly on the field
distributions within the material [4]. Hence, this response is explained by the interaction of RF with the bone sample, which
results in rotation of dipole molecules at the frequency of applied electric field and hence affects the device characteristics.
It has also been seen that after loading with different concentrations of bone powder, an increase in the concentration of the
sample corresponds to increase in the value of current (Fig.4). This behaviour is because of change in the displacement
current through the medium, which occurs as a result of change in physical properties of sample. The physical properties of
bone depend on factors of physical geometry and dimensions, density, rigidity, hardness, mineral content, position and
orientation of the fibres and hydration [7].
In the amplification mode, initially the device response has been computed as a function of frequency, without loading the
FET. The gain of the amplifier reduces to zero when frequency is increased. However, when the sample is introduced at the
gate as well as at the drain terminal of the device, the gain attains a constant value for frequencies beyond a particular value,
depending upon sample w/v percentage, up to 15 MHz. A distinctive behaviour is also observed on loading with bone
sample (> 37.5% w/v) at the drain terminal for 1KHz – 100 KHz as shown in the Fig.5.
0.40
-------> Drain Current (mA)
0.35
0.30
Table 1. Response of output voltage to change
in weight of sample in amplification mode
(without field)
w = 1.0048 g
Vds = 0.5 V
Vgs = -3.0 V
0.25
0.20
2
Applied Field (AC) = 1.01x10 V/m
0.15
0.10
0.05
0.00
5
6
7
8
9
10
-------> Frquency (MHz)
Fig.3. Frequency dependent behaviour of drain current with RF field
3
Weight of
sample
0
0.939
1.005
1.124
1.5398
Output voltage (Vo), mV
Bone Pieces
Bone Powder
69.286
65.044
69.286
71.407
69.286
70.700
69.286
85.406
69.286
69.569
120
2.0
Vds = 0.5 v
110
120
2
Applied field = 1.01x10 V/m
Frequency = 8 MHz
1.8
1.6
100
90
80
100
70
60
w = 1.0048 g
80
1.2
1.0
V0 (V) --------->
Idf/Ido -------->
1.4
w = 0.9385 g
0.8
0.6
at gate
50
at drain
0
20
40
60
80
100
60
40
20
w = 1.5398 g
0.4
0
0.2
0.0
-0.0
-0.5
-1.0
-1.5
-2.0
-2.5
100
-3.0
1000
10000
Frequency (KHz) -------->
Fig.5. Output voltage of JFET amplifier on loading with bone powder
at gate and drain terminal
Vgs (V) ------->
Fig.4. Idf/Ido vs. Vgs graphs for different weights
CONCLUSION
The radio frequency (RF) voltage gain and the drain current efficiency of field effect transistor (FET) devices have been
improved by introducing the bone powder at the gate terminal of the FET circuit. The various output parameters of the
FETs are studied under the combined influence of RF field and biological material, applied as load to the gate terminal of
the circuit. The device with improved performance can be utilized in biomedical instrumentation as well as in various
scientific applications. The present study would assist in understanding of interaction of biological material and/or RF field
with the device.
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