Class No 09 Zunaib Ali
DIFFERENCE BETWEEN FET & BJT ON THE BASIS OF
Bipolar Junction Transistor Field Effect Transistor
1) A bipolar transistor requires a small
amount of current flowing to keep the
transistor on. While the current for one
transistor may be negligible, it adds up
when millions are switching
simultaneously. The heat dissipated on
bipolar limits the total number of
transistors that can be built on the chip
2) A BJT will consume more power in the
on- state.it cannot switch with less than
a 0.3V voltage drop.
3) BJT’s function as regulators of currents
as small current is regulating a large
4) The bipolar transistor is liable for
thermal runway(over heating) due to a
negative temperature co-efficient.
5) BJTs are preferred for low current
6) BJTs have low-medium input
impedance(~1k -3k ohms).
7) BJTs are used where we need high
gain& fast response.
8) BJT's have a higher cutoff frequency
and a higher maximum current then
1) Once the gate terminal on an FET has
been charged, no more current is
needed to keep that transistor on
(closed) for the duration of time
2) FETs are preferred in Wide line or load
variations& have low power
3) FET function as voltage regulators as
applied voltage on gate control the
4) FET have a positive temperature co-
efficient, stopping thermal runway.
5) FETs are preferred in Low-voltage
applications ( less than 250V).
6) no current flows through the gate, the
input impedance of the FET is
extremely large (in the range of
Ω). the large input impedance
of the FET makes them an excellent
choice for amplifier inputs.
7) FETs have low-medium gain.
8) FETs are preferred in High frequency
applications ( greater than 200kHz).
Class No 09 Zunaib Ali
9) To operate BJTs at high switching
frequencies & high current, we have to
prevent the devices from going into
haerd saturation as this will increase
storage times( making it difficult to
switch off quickly) but then cause the
device to dissipate more power due to
10) BJTs are relatively greater in size than
FET of same rating.
11) BJT is temperature sensitive at higher
12) BJTs have high switching speed but
they are noisy also.
13) BJT Have small duty cycles.
9) FETs are low switching devices.FET is
therefore used for power switch design
and high power functions( less than
500W output power).
10) FET are smaller in size.so area
consumption of FET is less so Icsmade
by FETs provide higher packing
density as compared to BJTs.
11) FET is more stable to temperature &
therefore it is used in high temperature
12) As FET introduce lower noise level to
the system so where sensitivity to the
noise is very critcal and cannot be
neglected,JFET amplifiers are preferred
13) JFET used for micro wave
communication such as VHF, UHF
14) FET's are easy to fabricate in large scale
and have higher element density the BJT's.
15) JFET is mostly used in digital circuits.
16) Switch mode power supplies (SMPS):
Hard switching above 200kHz
17) Switch mode power supplies (SMPS): ZVS
below 1000 watts