In this post I will talk about another form of Field Effect Transistor (FET) that is not the Junction Field Effect Transistor (JFET) that you are probably familiar with.
This device is known as the Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and is classified as Insulated Gate Field Effect Transistors (IGFET).
The MOSFET is an extraordinary semiconductor device because its gate input is electrically separated from the primary current-carrying channel, which can provide a significant benefit in certain applications.
The MOSFET is widely used in many electronic circuits due to its unique construction. It is in fact, the most prevalent form of insulated gate FET, primarily because of its versatility and effectiveness in a variety of scenarios.
Inside a MOSFET the Gate is separated from the main n-channel or p-channel by an ultra-thin layer of insulating material, typically silicon dioxide which you may also know as glass.
This layer provides electrical insulation, forming what we refer to as a “Metal Oxide” Gate electrode.
We can consider this extremely thin insulated metal Gate electrode as functioning similarly to one plate of a capacitor.
This design allows the controlling Gate to be electrically isolated, giving the MOSFET an exceptionally high input resistance, often reaching the Mega-ohms (MΩ) range, and which makes it virtually near infinity.
This feature is integral to the MOSFET’s operation, distinguishing it from other transistors like the JFET and allowing it to act as a voltage-controlled field effect transistor with a highly responsive, electrically insulated Gate.
As we know, the gate of the MOSFET is electrically isolated from the primary current-carrying pathway that exists between the drain and source.
Hence we observe that “No current is able to flow or enter into the gate.” Similar to the Junction Field Effect Transistor (JFET), we realize that the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) operates as a voltage-controlled resistor.
This means that the current traversing through the principal channel connecting the Drain and Source is directly proportional to the input voltage applied.
Furthermore, similar to JFETs, we see that MOSFETs exhibits an exceptionally high input resistance which allows them to accumulate substantial amounts of static charge.
This characteristic causes MOSFETs to be susceptible to damage if they are not handled with care or adequately protected.
As we learned in our previous discussion about JFETs, we note that MOSFETs are also three-terminal devices comprising a Gate, Drain, and Source.
In this category we find that both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available for various applications, as required by us.
But we find that there exists a fundamental difference between these devices, as MOSFETs can be categorized into two primary types:
Depletion Type: This type of transistor necessitates a specific Gate-Source voltage (VGS) to effectively switch the device “OFF.”
Basically, we can compare the depletion mode MOSFET to a “Normally Closed” switch.
Enhancement Type: Conversely, this type of transistor requires a Gate-Source voltage (VGS) in order to activate and switch the device “ON.” Thus we can compare the enhancement mode MOSFET to a “Normally Open” switch.
Below are the symbols and fundamental construction for both the MOSFET variants.
Upon examining the four MOSFET symbols shown above, we see an additional terminal known as the Substrate. It is important to note that this terminal is not typically utilized as either an input or an output connection.
Instead, its primary function is to serve as a grounding point for the substrate.
This terminal establishes a connection to the main semiconductive channel via a diode junction that links to the body or metal tab of the MOSFET.
In most discrete-type MOSFETs, we find that this substrate lead is internally connected to the source terminal. When this internal connection exists, particularly in enhancement-type MOSFETs, we often omit the substrate from the symbol for the sake of clarity and simplicity.
Furthermore within the MOSFET symbol, the line that runs between the drain (D) and source (S) connections represents the semiconductive channel of the transistor.
When this channel line appears as a solid, unbroken line, it signifies a “Depletion” mode (normally-ON) MOSFET indicating that drain current can flow even when there is no gate biasing potential applied.
Conversely if we observe that the channel line is depicted as a dotted or broken line, this indicates that an “Enhancement” mode (normally-OFF) MOSFET which means that zero drain current will flow when there is no gate potential applied.
Also, we should pay attention to the direction of the arrow pointing toward this channel line, as it serves to indicate that whether the conductive channel is made up of a P-type or an N-type semiconductor material.
Fundamental Structure and Symbol of a MOSFET
When we investigate the Metal Oxide Semiconductor Field Effect Transistor (MOSFET), we see that it differs greatly from the Junction Field Effect Transistor (JFET).
In both Depletion and Enhancement MOSFETs, we influence the flow of charge carriers using an electric field created by a gate voltage.
In particular, the electrons are involved in n-channel MOSFETs whereas holes are involved in p-channel MOSFETs. The change in state takes place inside the semiconductive channel that links the drain and source terminals.
A MOSFETs design consists of a gate electrode sitting on top of an incredibly thin insulating layer. Underneath this layer, we find two tiny n-type areas just beneath the drain and source electrodes.
This structural arrangement is critical to the device’s functionality.
In our last tutorial, we explained how to bias the gate of a Junction Field Effect Transistor (JFET) so that it reverse-biases the pn-junction.
But similar limits do not apply to insulated gate MOSFET devices. This flexibility enables us to bias the gate of a MOSFET with either positive (+ve) or negative (-ve) voltages, increasing its versatility.
This characteristic causes the MOSFETs to be ideal for use as electronic switches or in the fabrication of logic gates. In particular when no bias is imposed, these devices often do not conduct.
Furthermore, because MOSFETs are primarily voltage-controlled devices, their high gate input resistance requires very little, if any, control current to function properly.
In addition, p-channel and n-channel MOSFETs are offered in two major configurations: enhancement and depletion. This variety allows us to choose the best kind for our unique electronic applications, demonstrating the versatility and applicability of MOSFET technology in modern electronics.
Understanding the Depletion-mode MOSFET
When we analyze the MOSFETs we come across the Depletion-mode MOSFET, which, although less common than its Enhancement-mode siblings, has unique qualities which set it apart.
This particular kind of MOSFET is normally in a “ON” state, which means it is conducting, with no gate bias voltage required.
In this scenario, we can see that the channel continues to be conductive when the gate-source voltage (VGS) is zero, indicating that it is a “normally-closed” device. The circuit symbol for a depletion-mode MOS transistor, as shown above, has a solid channel line which represents the normally closed conductive channel.
In the case of the n-channel depletion MOS transistor, we discovered that supplying a negative gate-source voltage (-VGS) depletes the conductive channel of its free electrons, a mechanism that is properly represented in its nomenclature, resulting in the transistor getting turned “OFF.”
In a similar way when we study the p-channel depletion MOS transistor, we see that a positive gate-source voltage (+VGS) depletes the free holes of the channel, causing it to be switched “OFF.”
To put it another way, in an n-channel depletion-mode MOSFET, if we supply a positive gate-source voltage (+VGS), it increases the amount of electrons readily available in the channel, allowing for more current to flow.
On the other hand when we apply a negative gate-source voltage (-VGS), it reduces the quantity of electrons, resulting in a drop in current flow. This relationship also applies to p-channel types but in the reverse manner.
Finally, we may compare the depletion-mode MOSFET to a “normally-closed” switch, emphasizing its distinct operating properties in electrical circuits.
Circuit Symbols of Depletion-mode N-Channel MOSFET
The construction of the depletion-mode MOSFET is similar to that of JFET transistors, with the drain-source channel already having conduction due to the electrons and holes in the n-type or p-type channel. The doping in the channel creates a pathway with low resistance between the Drain and Source, even when there is no Gate bias.
Understanding the Enhancement-mode MOSFET
As we investigate MOSFET technology, we come across the more widely used Enhancement-mode MOSFET, also known as the eMOSFET. This MOSFET acts in a fundamentally opposite manner from the depletion-mode variety.
More precisely, the conducting channel in an enhancement-mode device is either weakly doped or fully undoped, making it non-conductive in normal conditions.
As a result, we observe that the device remains “OFF” (non-conducting) when the gate bias voltage (VGS) is zero. The circuit symbol for an enhanced MOS transistor, as seen above, has a broken channel line, implying that the channel is generally open and non-conductive.
In the case of the n-channel enhancement MOS transistor, we see that drain current begins to flow as soon as a gate voltage (VGS) greater than a particular threshold voltage (VTH) is applied to the gate terminal. This threshold voltage marks the point at which conductance starts off, identifying the device as a transconductance device.
Now, the moment we introduce a positive (+ve) gate voltage to an n-type eMOSFET, it draws more electrons to the oxide layer around the gate.
As this happens, the channel’s thickness increases (or is enhanced), allowing for a higher flow of current.
This phenomena is precisely why we call this type of transistor an enhancement-mode device, in which the application of gate voltage significantly improves the channel’s conductive characteristics, which results in an improved efficiency in electronic applications.
When we increase the positive gate voltage provided to an n-channel enhancement-mode MOSFET, we find that the channel resistance decreases. This decrease in resistance then causes an increase in the drain current (ID) that flows through the channel.
Simply said, in case of an n-channel enhancement-mode MOSFET, a positive gate-source voltage (+VGS) effectively toggles the transistor “ON,” but a gate-source voltage of zero or a negative value (-VGS) causes the transistor to remain turned “OFF.”
In this respect, we can easily compare the enhancement-mode MOSFET to a “normally-open” switch since it calls for a certain gate voltage to start its conduction.
But oppositely, if we look at the p-channel enhancement MOS transistor, we discover that its operation is very very different. Here, whenever the gate-source voltage (VGS) is zero, the device stays in the “OFF” state, with the channel open.
The moment we supply a negative gate voltage (-VGS) to a p-type eMOSFET, we increase the channel’s conductivity which causes the transistor to switch “ON.”
Hence with an p-channel enhancement-mode MOSFET, a positive gate-source voltage (+VGS) turns the transistor “OFF,” and a negative gate-source voltage (-VGS) turns it “ON.” This specific operational property demonstrates the versatile nature and utility of enhancement-mode MOSFETs in electronic circuits.
Circuit Symbols of Enhancement-mode N-Channel
When we consider the application of Enhancement-mode MOSFETs, it becomes clear that they work like very efficient electronic switches.
This is primarily due to their remarkably low “ON” resistance combined with their extraordinarily high “OFF” resistance. Additionally we cannot overlook the fact that these devices possess an infinitely high input resistance, which is a direct result of their isolated gate structure.
Enhancement-mode MOSFETs become very useful components of integrated circuits in real-world applications, especially when it comes to the development of CMOS (Complementary Metal-Oxide-Semiconductor) type logic gates.
In this design, we use both PMOS (P-channel) and NMOS (N-channel) gates to perform the necessary logical operations.
The word CMOS refers for Complementary MOS, which means that these logic devices use both PMOS and NMOS transistors in their architectural architecture. This complimentary configuration not only improves the circuits’ performance and efficiency but also increases their adaptability in a variety of electronic applications.
Understanding The MOSFET as an Amplifier
In the same way as the Junction Field Effect Transistor (JFET), we can effectively employ MOSFETs to construct single-stage Class “A” amplifier circuits.
Among these, the enhancement-mode n-channel MOSFET common source amplifier works most efficiently and is the most widely utilized configuration. Even though the depletion-mode MOSFET amplifiers share many similarities with their JFET counterparts, it is important to note that MOSFETs inherently possess a significantly higher input impedance.
This increased input impedance is primarily regulated by the gate biasing resistive network which consists of resistors R1 and R2.
Furthermore it may be important to highlight that the output signal produced by the enhancement-mode common source MOSFET amplifier exhibits an inversion characteristic.
Specifically when the gate voltage (VG) is low, the transistor transitions into an “OFF” state, resulting in a high output voltage (VD or Vout). But on the other hand, when the gate voltage (VG) is increased, the transistor switches “ON,” leading to a low output voltage (VD or Vout).
Analyzing an Enhancement-mode N-Channel Amplifier Circuit
- VG = [R2/(R1 + R2)]VDD
- ID = VS/RS
When we look at the DC biasing of the common source (CS) MOSFET amplifier circuit, we find that it closely resembles the biasing approach utilized in JFET amplifiers.
In this configuration, we achieve the biasing in Class A mode through the implementation of a voltage divider network which is constructed using resistors R1 and R2. This arrangement allows us to establish the necessary operating point for the amplifier.
Furthermore we can determine that the AC input resistance of this common source MOSFET amplifier is represented as RIN = RG = 1 MΩ. This high input resistance is advantageous because it minimizes the loading effects on preceding stages in a circuit.
In terms of their fundamental characteristics, Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are classified as three-terminal active devices.
These devices are fabricated from various semiconductor materials which grant them the unique ability to function either as insulators or conductors depending on the application of a small signal voltage.
The MOSFETs’ unique ability to shift between two separate states enables them to carry out two basic functions: “switching” in digital electronics and “amplification” in analog electronics. MOSFETs may perform these functions in three distinct regions, each with its own set of voltage conditions:
Cut-off Region – In this region when the gate-source voltage (VGS) is less than the threshold voltage (Vthreshold), we find that the gate-source voltage is significantly lower than the transistor’s threshold voltage.
As a result the MOSFET is effectively switched “fully-OFF,” leading to a situation where the drain current (ID) equals zero. In this state, the transistor behaves like an open switch, regardless of the value of the drain-source voltage (VDS).
Linear (Ohmic) area – When VGS exceeds Vthreshold but VDS is smaller than VGS, the transistor approaches its linear or constant resistance area. In this situation the MOSFET works as a voltage-controlled resistor, with the resistive value set by the gate voltage (VGS). This provides fine control over current flow.
Saturation region – Once VGS reaches Vthreshold and VDS rises above VGS, the transistor gets into the saturation area and works as a continuous current source. The transistor is “fully-ON” in this condition, and the drain current (ID) achieves the highest possible value. Here, the MOSFET acts as a closed switch, which enables optimum current flow.
Conclusions
The Metal Oxide Semiconductor Field Effect Transistor, or MOSFET, is known for its extremely high input gate resistance. This feature allows the gate voltage to efficiently regulate the current flowing across the channel connecting the source and drain.
But we must exercise caution since MOSFETs’ high input impedance and gain characteristic render them extremely sensitive to static electricity damage if not handled or protected properly.
Due to their advantageous properties, MOSFETs are ideally suited for use as electronic switches or as common-source amplifiers, primarily because their power consumption is remarkably low. Typical applications for these versatile devices include microprocessors, memory storage systems, calculators, and CMOS logic gates, among others.
When we examine the symbols used to represent different types of MOSFETs, we notice that a dotted or broken line within the symbol signifies a normally “OFF” enhancement type. This indicates that when zero gate-source voltage (VGS) is applied, there is “NO” current flowing through the channel.
In contrast, a continuous unbroken line within the symbol denotes a normally “ON” depletion type which shows that current “CAN” flow through the channel even when there is zero gate voltage applied.
For p-channel MOSFETs, it is important to note that the symbols are identical for both types, but in this case, the arrow points outward. This distinction helps us summarize their operational characteristics in a straightforward switching table format, as shown below:
| MOSFET type | VGS = +ve | VGS = 0 | VGS = -ve |
| N-Channel Depletion | ON | ON | OFF |
| N-Channel Enhancement | ON | OFF | OFF |
| P-Channel Depletion | OFF | ON | ON |
| P-Channel Enhancement | OFF | OFF | ON |
In the case of n-type enhancement-mode MOSFETs, we find that applying a positive gate voltage effectively turns the transistor “ON” allowing current to flow through the channel. Conversely when we apply a zero voltage at the gate, the transistor remains in a switched “OFF” state, which prevents any current from passing through the device.
In the same way for p-channel enhancement-mode MOSFETs also we observe that a negative gate voltage is required to switch the transistor “ON” but a zero gate voltage results in the transistor to get switched “OFF.” The specific point at which the MOSFET begins to conduct current through its channel is defined by the threshold voltage (VTH) of the device which is a critical parameter in its operation.
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