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The MOSFET, or Metal-Oxide-Semiconductor Field-Effect Transistor, is by far the most common field effect transistor in both digital and analog circuits. The MOSFET is composed of a channel of n-type or p-type semiconductor material (see article on semiconductor devices), and is accordingly called an NMOSFET or a PMOSFET. Usually the semiconductor of choice is silicon, but some chip manufacturers, most notably IBM, have begun to use a mixture of silicon and germanium (SiGe) in MOSFET channels. Unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide, do not form good gate oxides and thus are not suitable for MOSFETs.
The gate terminal is a layer of polysilicon (polycrystalline silicon; why polysilicon is used will be explained below) placed over the channel, but separated from the channel by a thin layer of insulating silicon dioxide. When a voltage is applied between the gate and source terminals, the electric field generated penetrates through the oxide and creates a so-called "inversion channel" in the channel underneath. The inversion channel is of the same type—p-type or n-type—as the source and drain, so it provides a conduit through which current can pass. Varying the voltage between the gate and body modulates the conductivity of this layer and makes it possible to control the current flow between drain and source.
The operation of a MOSFET can be seperated into three different modes, depending on the bias at the terminals. For the NMOSFET the modes are:
- 1. Cut-off or sub-threshold mode: When VGS < Vtn where Vtn is the threshold voltage of the device.
- Here the switch is turned off, and there is no conduction between drain and source. While the current between drain and source should ideally be 0 since the switch is turned off, there is a weak-inversion current, or subthreshold leakage. With MOSFET scaling subthreshold leakage composes a large percentage of total power consumption.
- 2. Triode or linar region: When VGS > Vtn and VDS < VGS - Vtn
- The switch is turned on, and a channel has been created which allows current to flow between the drain and souce. The MOSFET operates like a resistor, controlled by the gate bias. The current from drain to source is
- 3. Saturation: When VGS > Vtn and VDS > VGS - Vtn
- The switch is turned on, and a channel has been created which allows current to flow between the drain and source. Since the drain bias is higher than the gate bias, a portion of the channel is turned off. The onset of this region is also known as pinch-off. In first approximation the drain current is now independent of the drain bias, and the current is only controlled by the gate bias:
In digital circuits the transistors are only operated in cut-off and saturation mode. The linear mode is mainly relevant for analogue applications.
The primacy of MOSFETs
The growth of digital technologies like the microprocessors has provided the motivation to advance MOSFET technology faster than any other type of silicon-based transistor. The principal reason for the success of the MOSFET was the development of CMOS digital logic, which uses MOSFETs as building blocks. The great advantage of CMOS circuits is that they allow no current to flow (ideally), and thus no power to be consumed, except when the inputs to logic gates are being switched. CMOS accomplishes this by complementing every NMOSFET with a PMOSFET and wiring the same input to both in such a way that whenever one is conducting, the other is not (see article on CMOS). Not only does this arrangement conserve energy, but perhaps more importantly it prevents overheating that would cause chips to fail. Overheating is a major concern in integrated circuits, since millions of transistors are packed into small chips. Due to MOSFET scaling, subthreshold leakage power is consumed even when the circuit is not operating causing increased power consumption.
Another advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents any DC current from flowing through the gate, reducing power consumption. Even more importantly, this isolation between the gate and channel effectively isolates a MOSFET in one logic state from earlier and consequent stages, since the gate of one MOSFET is usually driven by the output from a previous logic stage. This isolation makes it easier for designers to design logic stages independently. As MOSFETs are scaled down, there is now a current between the gate and channel; see below for more info.
The MOSFET's strengths as the workhorse transistor in most digital circuits do not translate into supremacy in analog circuits, in which the bipolar junction transistor (BJT) has traditionally been seen as the transistor of choice, due largely to its high transconductance. Nevertheless, since it is both economically and operationally advantageous to incorporate digital and analog circuits onto the same chip, and since it is technologically difficult to fabricate BJTs and MOSFETs on the same chip, MOSFETs are widely relied upon for analog purposes as well. Ironically, the BJT has some advantages over the MOSFET in digital circuits, and some complex digital circuit designs incorporate BJTs to speed things up in critical locations. These mixed-transistor digital circuits are called BiCMOS (bipolar-CMOS) circuits.
For more on the BJT, which is not considered a field-effect transistor, see bipolar junction transistor.
Over the past decades, the MOSFET has continually been scaled down in size; typical MOSFET channel lengths were once several micrometres, but today's integrated circuits are incorporating MOSFETs with channel lengths of about a tenth of a micrometre. Until the late 1990s, this size reduction resulted in great improvement to MOSFET operation with no deleterious consequences. Historically, the difficulties with decreasing the size of the MOSFET have been associated with the semiconductor device fabrication process.
Reasons for MOSFET scaling
Smaller MOSFETs are desirable for two main reasons. First, smaller MOSFETs allow more current to pass. Conceptually, MOSFETs are like resistors in the on-state, and shorter resistors have less resistance. Second, smaller MOSFETs have smaller gates, and thus lower gate capacitance. These two factors contribute to lower switching times, and thus higher processing speeds.
A a third reason for MOSFETs scaling is reduced area leading to reduced cost; smaller MOSFETs can be packed more densely, resulting in either smaller chips or chips with more computing power in the same area. Since the cost of producing integrated circuits is highly related to the number of chips that can be produced per wafer, the price per chip is reduced.
Difficulties arising due to MOSFET scaling
Producing MOSFETs with channel lengths smaller than a micrometre is a tremendous challenge, and the difficulties of semiconductor device fabrication are always a limiting factor in advancing integrated circuit technology. Recently though, the small size of the MOSFET has created operational problems.
With the small MOSFET geometries the voltage that can be applied to the gate has to be reduced to maintain reliability ability. To maintain performance, the threshold voltage of the MOSFET has to reduced in step. With reduced threshold voltages, the transistor cannot be completely turned off, resulting in a weak-inversion layer which consumes power in the form of subthreshold leakage when the transistor is not conducting. Subthreshold leakage, which could have been ignored in the past, now can consume upwards of 50% of the total power consumption of the chip.
Traditionally switching time was roughly proportional to the gate capacitance of gates. However, with transistors becoming smaller and more transistors being placed on the chip, interconnect capacitance, the capacitance of the wires connecting different parts of the chip is becoming a larger percentage of capacitance as signals have to travel across the chip, leading to increased delay and lower performance.
The ever-increasing density of MOSFETs on an integrated circuit is creating problems of substantial localized heat generation that can impair circuit operation. Circuits operate slower at high temperatures, and have a reduced reliability. Heat sinks and other cooling methods are now required for many integrated circuits including microprocessors.
Gate Oxide Leakage
The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to allow for increased current when the transistor is on leading to increased performance and reduced subthreshold leakage when the transistor is off. However, with current gate oxides with a thickness of around 2 nanometer (comprised of 5 atoms) a tunneling leakage develops between the gate and channel leading to increased chip power consumptions.
Insulators that have a larger dielectric constant than silicon dioxide, such as hafnium oxide , are now being researched to reduce the gate leakage . Increasing the dielectric constant of the gate oxide material allows a thicker layer while maintaining a high capacitance. The higher thickness reduces the tunneling current between the gate and the channel. An important consideration is the barrier height of the new gate oxide; the difference in conduction band energy between the semiconductor and the oxide (and the corresponding difference in valence band energy) will also affect the leakage current level. For the traditional gate oxide, silicon dioxide, the former barrier is approximately 3 eV. For many alternative dielectrics the value is significantly lower, somewhat negating the advantage of higher dielectric constant.
With MOSFETS becoming smaller, the number of atoms in the silicon that produce many of the transistor's properties is becoming smaller (100s of atoms). During chip manufacturing, the number of atoms used to make the transistor can vary significantly affecting the transistors's characterics, which are no longer deterministic, but statistical. This statistical characteristic makes design very difficult.
Other MOSFET Issues
The primary criterion for the gate material is that it is a good conductor. Highly-doped poly-crystalline silicon is an acceptable, but certainly not ideal conductor, and it also suffers from some more technical deficiencies in its role as the standard gate material. So why use polysilicon instead of a metal like aluminum? There are a few reasons, namely 1) The threshold voltage (and consequently the drain to source on-current) is determined by the work function difference between the gate material and channel. In the past, when gate voltages were high (in the order of 3V to 5V), the high threshold voltage (Vt), caused by the work function difference between a metal gate and silicon channel could still be overcome by the applied gate voltage (i.e. |Vg - Vt| > 0). However, as transistor sizes were scaled down, the applied voltages were also brought down for various reasons (such as to avoid gate oxide breakdown, hot-electron reduction, power consumption reduction, etc) and a device with such a high threshold voltage will not be operational. Thus, poly-crystalline silicon (industrial short-form: polysilicon) was used as gate material since it is essentially of the same chemical composition as the silicon channel beneath the gate oxide. Thus, at inversion, the work-function difference becomes close to zero, ensuring the transistor can be turned on at a lower voltage and the on-current is high. 2) in the MOSFET IC fabrication process, the gate material must be deposited prior to high-temperature steps that would melt metals. And thus a high melting point material such as poly-crystalline silicon is preferable to metal, as gate material. However polysilicon is highly resistive (in the order of 1000X more resistive than metal),which in turn will reduce the signal propagation speed through the material (and increase the time the transistor takes to switch from 'off' to 'on' or vice versa). Thus, metal is blended into the top of the polysilicon to decrease the resistivity. Such a blended material is called silicide. Silicide - polysilicon layered material has better electrical properties than polysilicon alone and still does not melt in subsequent processing. Also the threshold voltage is not significantly higher than polysilicon alone, because the material above the silicon channel, after the gate oxide, still remains to be doped polysilicon alone.
Depletion mode MOSFETs and non-CMOS logic
There are also depletion mode MOSFET devices, which are less commonly used than the standard "enhancement mode" devices already described. These are MOSFET devices which are doped so that a channel exists even without any voltage applied to the gate. When one then applies a voltage to the gate, the channel is depleted, which reduces the current flow through the device. In essence the depletion mode device is equivalent to a normally closed switch, while the enhancement mode device is equivalent to a normally open switch.
Historically, n-channel MOSFETs tended to be smaller and therefore cheaper to produce. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusively. However, NMOS logic consumes power even when no switching is taking place, unlike CMOS logic which combines n-channel and p-channel MOSFETs on a single chip. With advances in technology, CMOS logic displaced NMOS in the 1980s to become the preferred choice for digital chips.
DMOS stands for Double Diffused MOS. Types are the Lateral Double-diffused MOS (LDMOS) and the Vertical Double-diffused MOS (VDMOS) transistor.
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