Introduction
Metal-Oxide-Semiconductor FETs
Electronics – Field-effect transistors
Prof. Márta Rencz, Gergely Nagy
BME DED
October 7, 2013
Other transistor types
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The basics of field-effect transistors
The operation of field-effect transistors is based on ability of
controlling the current that flows through a given
volume by an external electric field.
Reminder: in a BJT transistor the current of a pn-junction
(base-emitter junction) controls the current of another junction
(base-collector junction).
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The properties of field-effect transistors
There are two types:
1
2
MOS: Metal-Oxide Semiconductor,
JFET: Junction Field-effect Transistor.
Common properties:
almost zero input current (!) – they can be controlled by
with very little power consumption,
they consume much less space – they are more suitable for
integrated circuits,
they are unipolar – their outputs (source and drain) are
interchangeable.
Their basic behavior can be modelled with a voltage
controlled current source.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Introduction
MOS FETs were named after the material layers that make up
the device:
metal: in the early days aluminium was used, then poli-Si
became the standard material for this role and nowadays
metals are used again,
oxide: the oxide of the semiconductor (SiO2 ) – aka. quartz,
semiconductor: silicon.
Historical overview:
1957: the first MOS was manufactured,
1970: first IC manufactured in large volumes (1 kbit RAM
consisting of 3-transistor cells by Intel),
nowadays: 4 billion MOS FETs/chip (NVIDIA GF100 GPU) –
it is the leading technology today.
In 2005 more transistors were manufactured than rice grains
grown.
The operation of MOS FETs is based on the MOS
capacitance.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
MOS capacitance
One of the conducting plates of the
capacitor is the semiconducting material.
When voltage is applied on the structure:
a space-charge region is generated in the
semiconductor at the proximity of the
oxide,
2 when the electric-field exceeds a certain
value, charge carriers appear at the
vicinity of the oxide (inversion charge).
1
At the region where the inversion charge accumulates, the
semiconductor behaves as if it was doped to the opposite type.
The voltage that is needed to create the inversion layer is the
theshold voltage (VT ).
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Charge-coupled device – CCD I.
MOS capacitance can also be used to
move charges around.
This is the basis of the operation of
charge-coupled devices (CCD).
When the device is exposed to light,
charges are generated.
When the device is exposed to light,
charges are generated. The number of
the generated charges is proportional
to the intensity of the light.
The number of the generated charges
is proportional to the intensity of the
light.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Charge-coupled device – CCD II.
V2 > VT > V1 ≈ V3 : the charges stay
in capacitance no. 2,
V3 > V2 > VT > V1 : charges move
over to capacitance no. 3,
V3 > VT > V1 ≈ V2 : the charges stay
in capacitance no. 3.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The structure of MOS transistors I.
The MOS transistor is an MOS capacitance complemented
with a source and a drain electrode.
n-type MOS: the substrate is p-type, electrons make up the
inversion layer and form the channel between the source and
the drain,
p-type MOS: the substrate is n-type, holes make up the
inversion layer and form the channel between the source and
the drain.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The structure of MOS transistors II.
Important dimensions:
Tox : oxide thickness (few
nm)
L: the length of the
channel (20 nm to a few
µm)
W : width of the channel
(20 nm to a few µm)
The device operates as a relay – when the control voltage on
the gate is above the threshold voltage, a conducting channel
connects the source and the drain, otherwise it behaves as an
open circuit.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The structure of MOS transistors III.
In fact the device has four terminals but we assume that the
substrate (bulk) is connected to
the most negative potential in the circuit in case of n-type
devices,
the most positive potential in the circuit in case of p-type
devices.
If this wasn’t the case, the source-bulk
and drain-bulk diodes would open and
large currents would flow accross the
device.
If a large voltage is applied to a MOS
transistor in the opposite direction, it
can be demaged.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The operation of MOS transistors I.
If VGS > VT an inversion layer of
electrons is generated at the
Si − SiO2 interface.
The source region (n+ ): ensures that the inversion layer is
generated quickly (electrons arrive from here to the proximity of the
Si − SiO2 interface).
The drain region (n+ ): is where the charge carriers flow when a
positive VGS voltage is applied to the channel.
Due to VDS the space-charge region is wider at the drain.
The number of charge carriers in the channel is controlled by VGS .
As a result of the voltage drop accross the channel, the channel gets
narrower towards the drain.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The operation of MOS transistors II.
The characteristic equation of MOS transistors
2 VDS
W µn εox
ID =
(VGS − VT ) · VDS −
L tox
2
if VGS ≥ VT and
VDS ≤ VGS − VT .
At a given drain voltage (VDSsat –
saturation voltage) the channel gets
pinched off at the drain:
VDSsat = VGS − VT
as in this case no inversion layer is
created at the drain, thus channel can
not exist there.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The characteristic curves of MOS FETs I.
The output characteristic curve: ID = f (VDS ) (parameter:
VGS ).
When VDS > VDSsat the transistor enters the saturation
region.
Drain current [arbitary unit]
50
VGS-VTH=7 V
40
linear region
6V
30
5V
20
saturation region
4V
3V
10
0
2V
1V
0
2
4
6
8
10
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The characteristic curves of MOS FETs II.
The transfer characteristic equation:
MOS transistors are usually used in saturation.
In saturation the characteristic equation of MOS FETs:
ID =
W µn εox
KW
2
2
(VGS − VT ) =
(VGS − VT )
L 2 · tox
2 L
where
W is the width of the gate,
L is the length of the gate,
εox /tox is the oxide capacitance per unit area,
µn is the mobility of the charge carriers in the channel,
VGS is the gate-source voltage,
VT is the threshold voltage of the device.
Designers can only alter the value of W and L.
Introduction
Metal-Oxide-Semiconductor FETs
The characteristic curves of MOS FETs III.
The transfer characteristic equation:
Other transistor types
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Non-idealities in small MOS FETs I.
At the moment the smallest MOS FET manufactured is 32 nm
and the smallest under development is already below 20 nm.
If the charge carriers moving in the channel reach their
saturation velocity, the transfer characteristic curve becomes
linear.
The threshold voltage has a strong dependence on the
channel length and VDS .
Reasons for leakage current:
the current that flows below the threshold voltage,
current can flow through the gate.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Non-idealities in small MOS FETs II.
In a 65 nm transistor the speed of charge carriers becomes a
linear function of the applied voltage as the velocity of the
carriers saturates.
the characteristic equation becomes linear:
ID =
W µn εox
(VGS − VT )α
L 2 · tox
where α ≈ 1.2 − 1.3.
This model is based on measurements.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Leakage current I.
As VT is small in small MOS FETs, the current at VGS = 0 is
significant – D-S leakage.
Charge-carriers get across the gate oxide with the
tunelling-effect as the gate layer is only a few atoms wide in
modern transistors.
Introduction
Metal-Oxide-Semiconductor FETs
Leakage current II.
Solutions
In manufactoring:
Strained silicon
High K oxide materials
insulators
SOI
In a research phase:
FinFETs
MEMS + MOS
We will learn more about
these topics at the end of
the semester.
Other transistor types
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Other transistor types
JFET
Junction Field Effect Transistor,
used in analog electronics as the input stage of amplifiers.
Power electronics
Power MOS FETs
IGBT – Insulated Gate Bipolar Transistor
Thin film transistors
they control LCD displays,
TFT – Thin-Film Transistor.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The structure of JFETs I.
The current of the majority charge carriers flow from the source
towards the drain.
The extent of the current can be controlled by the voltage
applied to the gate.
The gate and the bulk of the semiconductor form a
pn-junction which is operated in the reverse direction.
The amplitude of the reverse voltage determines the width of
the space-charge region.
If the reverse voltage is increased, the width of the space-charge
region increases thus the cross-section of the conducting region
decreases and so does the current of the transistor.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The structure of JFETs II.
The name of the terminals:
source: the charge carriers flow from here towards the drain,
drain: the charge carriers flow in the direction of the drain,
gate: it is used to control the current flow.
There are n and p type devices.
In the normal range of operation in case of an n-type device:
VDS > 0: if it was negative the drain would act as the source
and the source as the drain as the device is unipolar
VGS < 0: the pn-junction is reverse biased.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
The structure of JFETs III.
The characteristic equation of JFETs is similar to that of the
MOS transistors:
2
I ∼ VGS
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Power MOS HEXFET I.
It is used to switch large currents at a
frequency of n × 100 kHz.
BJTs can not be used for such tasks as at large currents
in saturation, the current gain is around 20, so 100 A
would have to be switch with a 5 A current!
In power electronics 10-100 A currents need to be switched
(e.g. DC-DC converters (power supply), inverters (AC→DC)).
A modern processor also consumes ∼100 A – the current flows
through the parallel connection of many transistors (logic
gates).
The same idea is used in HEXFETs: a parallel structure of
MOS transistors are created on a large chip.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Power MOS HEXFET II.
The MOS transistor used in HEXFETs has a special
structure optimized for large currents.
The package of such elements always includes large cooling
surfaces or is attachable to heatsinks.
The operation of a power MOSFET is identical to that
of ordinary MOS transistors:
VGS < VT : the transistor is an open circuit,
VGS > VT : the transistor provides a current path between the
source and the drain with a resistance of a few mΩ.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
Power MOS HEXFET III.
Special structure: the bulk is the drain contact.
The hexagonal metal layer transports the source current.
The position of the channel is shown with the red ellipse.
Introduction
Metal-Oxide-Semiconductor FETs
Other transistor types
IGBT – Insulated Gate Bipolar Transistor
Used in power electronics.
For circuits where the currents are in the range of 100 -1000 A
and the voltage is in the range of a few kVs.
E.g. in trains and electric cars.
A power MOS transistor is supplemented with a bipolar
transistor.
The output characteristic equation is similar to that of
BJTs.
Introduction
Metal-Oxide-Semiconductor FETs
Thin-film transistors
It is made with thin-film
technology.
In LCDs it is used to switch liquid
crystal cells on and off.
It is a transparent MOS
transistor
Amorphous and policrystalline Si is used on a glass or plastic
substrate.
Every layer is very thin – this is why they are transparent.
Other transistor types