Question

a) Although buried in the same semiconductor substrate, why the source and drain of a transistor is electrically isolated when no external voltage is applied?

b) Why is it essential to connect the p-type substrate of a NMOS transistor to the least potential (usually ground) and the n-type substrate of a PMOS transistor to the highest potential (usually supply) for the MOSFET operation?

c) What is a threshold voltage? What happens to the drain current below threshold?

d) What isa pinch-off voltage? What happens to the drain current at pinch off?

e) A MOSFET is usually called a voltage controlled current source because the output current is controlled by voltage(s).

Identify the voltage (s) which control the flow of the drain current in the channel of a MOSFET?

Answer 1

(a) Source and drain of the transistor is electrically isolated when no external voltage is applied:

• It is the electrical separation between various functional circuits in a system such that there is no direct conduction path available between them
• isolation may be necessary for functional purposes
• It provides protection against any fault on the high voltage side as the isolation barrier blocks electrical power from reaching the user in spite of component damage or failure.

(b) connect the p-type substrate of a NMOS transistor to the least potential (usually ground) and the n-type substrate of a PMOS transistor to the highest potential (usually supply) for the MOSFET operation :

• Cross section of NMOS and PMOS as shown below,
• 

• The first thing to notice is that the all of the substrate is p-doped. Beside the NMOS there is a terminal labeled B, with a p+ doping.

• We understand p+ as a heavily p-doped semiconductor, which increases its conductivity. On the PMOS side, what you have is a n-doped well “n-well”), with a bias terminal connected.

• When the correct voltage is applied to the gate, charge accumulation constructs a channel between the source and the drain.

  That B terminal must be connected in such way to make a reverse diodes with the sources and drains of both transistors.

• That means that in the NMOS, it will be connected to the lowest available potential, and in the PMOS, to the highest available potential.

• These connections avoid the those drain and source terminals to conduct directly to the substrate, which would make the transistor act as a fixed resistor.

(c) threshold voltage:

• Threshold voltage is the voltage applied between gate and source of a MOSFET that is needed to turn the device on for linear and saturation regions of operation.
• drain current below threshold:
  • the channel is "pinched off" so that no current can flow, even though there are charge carriers available.

(d) pinch-off voltage:

• 
• the channel behaves as a semiconductor resistance, therefore it follows ohm's law. The region is called ohmic region, with increasing current, the ohmic voltage drop between the source and the channel region reverse biased the junction, the conducting portion of the channel begins to constrict and I_D begins to level off until a specific value of V_DS is reached, called the pinch of voltage V_P
• the drain current reaches at the pinch-off potential, and saturates which can be shown in above figure.