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MOSFET Operating Regions

NOTE: The images in the article are taken from Behzad Razavi’s “Design of Analog CMOS Integrated Circuits” Book.

MOSFET type transistors consist of four terminals in total, namely gate, drain, source and body terminals. Unlike the base current of BJT transistors, MOSFETs have no gate current.

There are two types of MOSFETs, N and P type. In N-type transistors, after the threshold voltage (Vth) is applied between the gate and source terminals of the transistor, the excess electrons in the N-doped region to which the source terminal is connected are attracted to form N-type channels on the P-type substrate/body and the channel is reversed (Inversion Layer). ). The reversal of the channel is called inversion. In P-type transistors, similar to the situation in N-type transistors, a reverse voltage of the threshold voltage Vth is applied between the gate and source terminals and the N-type substrate is converted to the P-type channel.

In N-type transistors, a positive voltage is applied between the gate and source, and in P-type transistors, a negative voltage is applied, that is, the source voltage must be higher than the gate voltage. Vth threshold voltages of N and P type transistors differ in MOSFET technologies. In N-type transistors, the current is carried out by electrons, and in P-type transistors, with positively charged carriers called holes. The fluency and mobility of electrons is higher than the holes. MOSFET transistors have a total of three working regions: Cut-Off, Triode (Linear, Ohmic) and Saturation (Saturation).

Cut Off (Closed)

In the cut-off region, the gate-source voltage (VGS) of the N-type transistor is less than the Vth voltage. This causes the so-called inversion layer to fail to occur. Therefore, as long as VGS < VTH, the transistor is in the off state and the drain or source current is close to zero.

Triode Mode (Linear)

In the triode region, the gate-source voltage of the transistor is higher than the threshold voltage and the drain-source voltage is less than the overdrive voltage. In this region, the drain current increases linearly (ohmically) depending on the drain-source voltage. Therefore, the transistor can be thought of as a voltage-controlled resistor between the drain-source terminals.

Saturation

In the saturation region, the drain current of the ideal MOSFET transistor no longer changes due to VDS, the drain current is in a saturated state. This is because the voltage applied to the gate no longer has an effect on the inversion layer (channel). So the channel zone no longer has room to expand any further. This condition is also referred to as the “channel pinch-off” condition.

In design tools and books, the drain-source voltage required to keep the transistor at saturation can be expressed as VDS,sat or VD,sat.
If the transistor is to be used as an amplifier, it must be operated in the saturation region. In the saturation region, the small signal voltage applied to the gate of the transistor creates a small signal drain current. The ratio of small signal current and voltage is called Transconductance. Transconductance is zero in the cut-off and triode regions. Transconductance can be expressed as in the following equations.

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