1. What is a MOSFET? Why is it called Field Effect Transistor?
A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of transistor used to amplify or switch electronic signals in various electronic devices.
It is a semiconductor device with four terminals – Source, Drain, Gate, Body.
Source: The terminal through which carriers (electrons for n-channel MOSFETs or holes for p-channel MOSFETs) enter the channel.
Drain: The terminal through which carriers exit the channel.
Gate: The terminal that modulates the conductivity of the channel or controls the flow of carriers in the channel.
Body: The bulk of the semiconductor material on which the MOSFET is built. It is usually connected to the lowest potential in the circuit (ground for n-channel, VDD for p-channel).
In MOSFET, Current that flows between Drain and Source terminal is controlled by an electric field which is perpendicular to the direction of flow of current and this is why it is called Field Effect Transistor [FET].
NOTE:
- It is voltage controlled current source.
- As there is a SiO2 layer present at Gate terminal, which is a perfect insulator. Therefore, no current (negligible current) flows through gate terminal of MOSFET.
2. How do you decide which terminal is which in a given MOSFET?
A MOSFET is a four-terminal device – Gate, Drain, Source, Body. Gate and Body of a MOSFET are fixed and cannot be changed but there is no fixed Drain or Source. It completely depends on what is the voltage applied to the terminal.
- nMOS – Source is connected to lowest voltage in the circuit. Generally, ground.
- pMOS – Source is connected to the highest voltage in the circuit. Generally, VDD.
3. What are the types of MOSFET?
Based on Operation Mode:
1. Enhancement Mode MOSFET (E-MOSFET):
The channel is "enhanced" or created when a sufficient gate voltage is applied.
2. Depletion Mode MOSFET (D-MOSFET):
A conductive channel is already formed at VGS = 0, or we can say that a channel is formed during its manufacturing process
Based on Channel Type:
N-Channel MOSFET
- The current carriers are electrons.
- Generally, N-Channel MOSFETs have higher mobility for electrons, leading to better performance.
P-Channel MOSFET
- The current carriers are holes.
- P-Channel MOSFETs typically have higher on-resistance and are slower than N-Channel MOSFETs.
4. What is threshold Voltage?
Threshold voltage (Vth) is a critical parameter in the operation of a MOSFET. It is defined as the minimum gate-to-source voltage (VGS) required to create a conductive channel between the source and drain terminals, allowing current to flow through the MOSFET.
5. What is the effect of temperature on the threshold voltage of the MOSFET?
As the temperature increases, the threshold voltage of the MOSFET decreases. The mobility of charge carriers (electrons in NMOS, holes in PMOS) decreases with increasing temperature due to increased phonon scattering. This affects the current flow and indirectly influences Vth.
Qualitative Description over a Range of Voltages
- At low temperatures, carrier mobility is higher, and intrinsic carrier concentration is low.
- Vth is relatively higher because fewer thermal carriers are available to invert the channel.
- As temperature increases, carrier mobility decreases slightly, and intrinsic carrier concentration starts to increase.
- Vth decreases approximately linearly with temperature within this range, typically a decrease of 1-3 mV/°C.
- At high temperatures, the intrinsic carrier concentration becomes significant.
- Vth decreases more rapidly with temperature due to the increased number of thermal carriers and enhanced scattering effects.
6. Explain the working of a MOSFET.
The working of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) involves controlling the flow of current between the drain and source terminals by applying a voltage to the gate terminal.
Consider an nMOS device.
Cutoff Region
Condition: VGS < Vth
Behaviour: The MOSFET is off, and there is no conduction between the drain and source.
Explanation: When the gate-source voltage (VGS) is less than the threshold voltage (Vth), the MOSFET channel is not formed. Thus, no current flows from the drain to the source (ID ≈ 0).
Triode (Linear) Region
Condition: VGS > Vth and VDS < VGS - Vth
Behaviour: The MOSFET operates like a variable resistor, and the current flow is proportional to VDS.
Explanation: When VGS is greater than Vth, a conductive channel forms between the drain and source. As VDS increases but remains less than VGS - Vth, the channel is enhanced, allowing current to flow.
The current ID is given by:
Where µCox is the process transconductance parameter, W is the channel width, and L is the
channel length.
Saturation (Active) Region
Condition: VGS > Vth and VDS ≥ VGS - Vth
Behaviour: The MOSFET operates as a constant current source; the current is relatively independent of VDS.
Explanation: When VDS increases to VDS ≥ VGS - Vth, the channel near the drain end becomes pinched off. Beyond this point, further increases in VDS do not significantly increase the current.
The current ID is given by:
In this region, ID is primarily controlled by VGS and is relatively unaffected by VDS.
7. In which regions of operation, does the MOSFET works as a switch?
A MOSFET can function as a switch in the Cutoff and Triode (Linear) regions. Here is a detailed explanation of how it works in each region:
1. Cutoff Region
Condition: VGS < Vth
Behavior as a Switch: The MOSFET is OFF.
Explanation: When the gate-source voltage (VGS) is less than the threshold voltage (Vth), no conductive channel forms between the drain and source. Consequently, the MOSFET is in a non-conducting state, and the current (ID) is effectively zero. This condition is equivalent to an open switch.
2. Triode (Linear) Region
Condition: VGS > Vth and VDS < VGS - Vth
Behavior as a Switch: The MOSFET is ON.
Explanation: When VGS is greater than Vth and VDS is small, a conductive channel forms between the drain and source, allowing current to flow. In this region, the MOSFET behaves like a low-resistance path, akin to a closed switch. The drain-source voltage (VDS) is small, and the MOSFET can pass significant current with minimal voltage drop, making it suitable for applications requiring a switch in the ON state.
8. Which is faster, BJTs or MOSFETs? Why?
BJTs are faster than MOSFETs because the junction capacitances of the BJTs are less when compared to that of MOSFETs. So, it takes less time to charge and discharge the junction capacitances for BJTs than MOSFETs. But MOSFETs are preferred over BJTs because BJTs are power hungry due to their less ohmic resistances.
9. Draw the transfer characteristics of a MOSFET. Show the regions of operation.
Drain current in a MOSFET is given by
From the above current equations, the transfer characteristics can be drawn as below.
As VGS increases, the drain current increases and it is constant after VDS = VGS - Vth.
ID independent of VDS: ID = ID,sat
10. Draw the ID vs VGS graph for a MOSFET. Show the regions of operation.
When VGS increases, the drain current increases for a given VDS.
Initially, the drain current is 0 as the device is in cutoff (VGS < Vth).
As it increases, for a particular VDS, the device is in saturation as VDS > VGS - Vth. The ID vs VGS relation is quadratic in nature.
When VGS is such that VDS < VGS - Vth, the device enters linear or ohmic region and the ID vs VGS relation is linear.
11. What are second order effects of a MOSFET?
The second-order effects in MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) refer to phenomena that deviate from the ideal behavior of the transistor and affect its performance in practical circuits. These effects become significant in advanced technology nodes and high-precision applications. Here are some key second-order effects:
1. Body Effect.
In a CMOS technology, generally body of an nMOS is tied to ground and body of a pMOS to VDD. The threshold voltage Vth of a MOS depends on the potential between source and the body by the relation:
As VSB increases, the threshold voltage of a MOS increases. This effect is called Body Effect.
2. Channel Length Modulation
In the channel of a MOS, the effective voltage is VGS – Vth - VDS. For given VGS when VDS increases, the effective voltage in the channel approaches to 0 and the channel terminates there instead of at the drain. Due to this the effective channel length decreases and the current increases. The charge carriers in the channel rush from the channel to the drain because of the high electric field and the current gradually increases instead of being constant. This effect is called Channel Length Modulation. The current equation considering the channel length modulation is
λ is a constant value which is called channel length modulation coefficient.
3. Velocity Saturation.
The drift velocity of the charge carriers is proportional to the electric field across the channel. But this behaviour is not seen at higher electric fields. In short channel devices, the electric field is generally very high. The drift velocity of the charge carriers will not increase, and the drain current saturates before it enters the saturation region. This effect is called Velocity Saturation.
4. Subthreshold Conduction
Ideally when VGS < Vth, there is no drain current. But practically there is a weak inversion layer present which generates a small current in the channel even in cutoff region. This current is proportional to VGS. and is given by
This is called Subthreshold Conduction. Typically, to increase a current by a decade, then VGS should increase by 80mV.
5. Drain-Induced Barrier Leakage (DIBL)
In an nMOS, when VDS is positive, the drain and body junction is reverse biased. This creates a depletion region around the drain reducing the available accumulated ions to deplete. Consequently, the threshold voltage reduces with increasing VDS by the relation Vth = Vtho - ηVDS
This is called Drain-Induced Barrier Leakage (DIBL). If the VDS is further increased, the depletion region around drain will further increase and at one point, it can even short with the source. This is called punch-through. When this happens, the device ceases to work as MOSFET.
12. What is unified current model of a MOSFET?
Considering all the second order effects, the current equation of a MOSFET can be generalised as below.
Where VGS > Vth and Vmin = min ((VGS - Vth), VDS, VDsat). VDsat is VDS at which the device enters velocity saturation.
13. What is CMOS Latch up?
The CMOS processes sometimes create n-p-n-p structures in the design unintentionally. These thyristor-like devices can cause the shorting of VDD and VSS. Consider the below example.
This structure will result to back-to-back connected BJTs. When one of the BJT is forward biased, it feeds the base of the other transistor, which will lead to high current and burning up of the device. To avoid this, Rnwell and Rpsubs should be reduced by inserting more well and substrate contacts, placed close to the source connections of nMOS and pMOS.
14. What are pass transistors? Explain the working.
Pass transistors are fundamental components in digital circuits, often used for creating multiplexers, switches, and transmission gates. They work by allowing or blocking the flow of electrical signals through them based on a control signal.
NMOS Pass Transistor
Structure: An NMOS pass transistor consists of an n-type MOSFET with the source, drain, gate, and body terminals.
Operation:
- Control Signal (Gate): The gate voltage VG controls the conductivity of the NMOS transistor.
- Source-Drain Path: The source (S) and drain (D) terminals are the path through which the signal passes.
Working:
- ON State: When the gate voltage VG is high (typically VDD), the NMOS transistor turns on. For an NMOS to conduct well, VG should be much higher than the source voltage VS. The source-drain path becomes a low-resistance channel, allowing the signal to pass from source to drain.
- OFF State: When the gate voltage VG is low (typically 0V), the NMOS transistor turns off. The source-drain path becomes a high-resistance channel, blocking the signal.
Example:
If VDD = 5V and the gate is at 5V, the NMOS pass transistor can pass a voltage from source to drain. However, the maximum voltage it can pass is slightly less than VDD (around VDD −Vth, where Vth is the threshold voltage of the NMOS).
PMOS Pass Transistor
Structure: A PMOS pass transistor consists of a p-type MOSFET with the source, drain, gate, and body terminals.
Operation:
- Control Signal (Gate): The gate voltage VG controls the conductivity of the PMOS transistor.
- Source-Drain Path: The source (S) and drain (D) terminals are the path through which the signal passes.
Working:
- ON State: When the gate voltage VG is low (typically 0V), the PMOS transistor turns on. For a PMOS to conduct well, VG should be much lower than the source voltage VS. The source-drain path becomes a low-resistance channel, allowing the signal to pass from source to drain.
- OFF State: When the gate voltage VG is high (typically VDD, the PMOS transistor turns off. The source-drain path becomes a high-resistance channel, blocking the signal.
Example:
Suppose VDD = 5V and Vth = −0.7V for the PMOS transistor.
When the gate is at 0V, the maximum voltage that can be passed to the drain is:
Vmax = VDD − ∣Vth∣ = 5V − 0.7V = 4.3V
This means that if the source is at 5V and the gate is at 0V, the drain voltage can rise up to 4.3V. Beyond this point, the PMOS transistor will not fully conduct, and the voltage drop across the transistor will prevent the drain voltage from reaching the source voltage entirely.
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