Why Can't we swap PMOS & NMOS in CMOS Inverter Circuit ? | Physical Design | Interview Question | VLSI

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Why Can't We Swap PMOS & NMOS in a CMOS Inverter Circuit?

The CMOS inverter is a fundamental component in VLSI (Very-Large-Scale Integration) circuits. It serves as the building block for many other circuits. In a standard CMOS inverter, the PMOS transistor is placed at the top with its source connected to the power supply (Vdd), while the NMOS transistor is placed at the bottom with its source connected to ground (GND). The drains of both transistors are connected together to form the output node.


Before discussing why swapping the PMOS and NMOS transistors is not feasible, let's first understand the operation of a standard CMOS inverter.


Standard CMOS Inverter Operation

Consider a CMOS inverter with a threshold voltage (Vth) of 0.7V and a power supply voltage (Vdd) of 5V. The threshold voltage is the minimum voltage required to turn on the transistor. Let's analyze the inverter's behavior under two conditions: when the input voltage (Vin) is low (0V) and when it is high (5V).

Condition 1: Vin = 0V

  • PMOS Operation:

    • For the PMOS to be ON, the source-to-gate voltage (Vsg) must be greater than the magnitude of the threshold voltage (|Vth|).
    • Since Vsg = Vdd - Vin = 5V - 0V = 5V, and 5V > |0.7V|, the PMOS turns ON.
    • When the PMOS is ON, it charges the output capacitor, resulting in an output voltage (Vout) of 5V.

  • NMOS Operation:

    • For the NMOS to be ON, the gate-to-source voltage (Vgs) must be greater than Vth.
    • Since Vgs = Vin - 0V = 0V, and 0V < 0.7V, the NMOS remains OFF.

Thus, when Vin is 0V, the PMOS is ON, and the NMOS is OFF, leading to an output voltage of 5V.

Condition 2: Vin = 5V


  • PMOS Operation:

    • For the PMOS to be ON, Vsg must be greater than |Vth|.
    • Since Vsg = Vdd - Vin = 5V - 5V = 0V, and 0V < |0.7V|, the PMOS remains OFF.

  • NMOS Operation:

    • For the NMOS to be ON, Vgs must be greater than Vth.
    • Since Vgs = Vin - 0V = 5V, and 5V > 0.7V, the NMOS turns ON.
    • When the NMOS is ON, it discharges the output capacitor, resulting in an output voltage (Vout) of 0V.

Thus, when Vin is 5V, the NMOS is ON, and the PMOS is OFF, leading to an output voltage of 0V.


Swapped Configuration of CMOS Inverter:

In the swapped configuration, the NMOS transistor is placed at the top connected to Vdd, while the PMOS transistor is placed at the bottom connected to GND. Let's analyze this configuration under the same conditions.

Condition 1: Vin = 5V

  • NMOS Operation:
    • For the NMOS to be ON, Vgs must be greater than Vth. Here, the source voltage is connected to the output node (initially at 0V).
    • Since Vgs = Vin - Vout = 5V - 0V = 5V, and 5V > 0.7V, the NMOS turns ON.
    • When the NMOS is ON, it attempts to charge the output. However, as Vout rises, Vgs decreases.
    • When Vout reaches approximately 4.3V (5V - 0.7V), Vgs becomes 0.7V, and the NMOS turns OFF.
    • Thus, Vout does not fully reach 5V, stopping at around 4.3V.

Condition 2: Vin = 0V

  • PMOS Operation:
    • For the PMOS to be ON, Vsg must be greater than |Vth|.
    • Since Vsg = Vout - Vin = 4.3V - 0V = 4.3V, and 4.3V > 0.7V, the PMOS turns ON.
    • When the PMOS is ON, it attempts to discharge the output.
    • As Vout decreases, Vsg also decreases. When Vout reaches approximately 0.6V, Vsg becomes 0.6V, and the PMOS turns OFF.
    • Thus, Vout does not fully discharge to 0V, stopping at around 0.6V.

Conclusion

The swapped configuration creates a "weak buffer" rather than a strong inverter. The output voltage levels do not reach the desired logic levels (0V and Vdd), leading to improper operation. Additionally, this configuration can result in higher power consumption due to simultaneous conduction paths.

In summary, swapping the PMOS and NMOS transistors in a CMOS inverter results in a non-functional circuit with potential issues like improper logic levels and high power consumption. This is why the standard placement of PMOS at the top and NMOS at the bottom is essential for the proper operation of a CMOS inverter.


Interview Question:

Why is it essential to place the PMOS transistor at the top and the NMOS transistor at the bottom in a CMOS inverter?

What happens to the output voltage and overall circuit behavior when these transistors are swapped?

How do the threshold voltage and power supply impact the operation of a standard CMOS inverter, and why does the swapped configuration result in a weak buffer with improper logic levels and potential high power consumption?


#scripting #coding #Tcl #pd #physicaldesign #physicalverification #pv #vlsi #design #india #semiconductor #freshers #professional #hardware #floorplanning #motivation #leadership #magnet #bound #placement #synopsys #cadence #interview #vlsi #question #power #irdrop #powerswitches #rushcurrent #lowpower #staticpower #dynamicpower #wifi #bluetooth #leakage #current #cmos #buffer #inverter



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