Question

Explain briefly drain characteristics of N-channel enhancement MOSFET

Answer 1

Enhancement MOSFET, or eMOSFET, can be classed as normally-off (non-conducting) devices, that is they only conduct when a suitable gate-to-source positive voltage is applied, unlike Depletion type mosfets which are normally-on devices conducting when the gate voltage is zero.

However, due to the construction and physics of an enhancement type mosfet, there is a minimum gate-to-source voltage, called the threshold voltage VTH that must be applied to the gate before it starts to conduct allowing drain current to flow.

In other words, an enhancement mosfet does not conduct when the gate-source voltage, VGS is less than the threshold voltage, VTHbut as the gates forward bias increases, the drain current, ID (also known as drain-source current IDS) will also increase, similar to a bipolar transistor, making the eMOSFET ideal for use in mosfet amplifier circuits.

The characteristics of the MOS conductive channel can be thought of as a variable resistor that is controlled by the gate. The amount of drain current that flows through this n-channel therefore depends on the gate-source voltage and one of the many measurements we can take using a mosfet is to plot a transfer characteristics graph to show the i-v relationship between the drain current and the gate voltage as shown.

N-channel eMOSFET I-V Characteristics



With a fixed VDS drain-source voltage connected across the eMOSFET we can plot the values of drain current, ID with varying values of VGS to obtain a graph of the mosfets forward DC characteristics. These characteristics give the transconductance, gmof the transistor.

This transconductance relates the output current to the input voltage representing the gain of the transistor. The slope of the transconductance curve at any point along it is therefore given as: gm = ID/VGS for a constant value of VDS.

So for example, assume a MOS transistor passes a drain current of 2mA when VGS = 3v and a drain current of 14mA when VGS = 7v. Then:



This ratio is called the transistors static or DC transconductance which is short for “transfer conductance” and is given the unit of Siemens (S), as its amps per volt. Voltage gain of a mosfet amplifier is directly proportional to the transconductance and to the value of the drain resistor.

At VGS = 0, no current flows through the MOS transistors channel because the field effect around the gate is insufficient to create or “open” the n-type channel. Then the transistor is in its cut-off region acting as an open switch. In other words, with zero gate voltage applied the n-channel eMOSFET is said to be normally-off and this “OFF” condition is represented by the broken channel line in the eMOSFET symbol (unlike the depletion types that have a continuous channel line).

As we now gradually increase the positive gate-source voltage VGS , the field effect begins to enhance the channel regions conductivity and there becomes a point where the channel starts to to conduct. This point is known as the threshold voltage VTH. As we increase VGS more positive, the conductive channel becomes wider (less resistance) with the amount of drain current, ID increases as a result. Remember that the gate never conducts any current as its electrical isolated from the channel giving a mosfet amplifier an extremely high input impedance.

Therefore the n-channel enhancement mosfet will be in its cut-off mode when the gate-source voltage, VGS is less than its threshold voltage level, VTH and its channel conducts or saturates when VGS is above this threshold level. When the eMOS transistor is operating in the saturation region the drain current, ID is given by:

eMOSFET Drain Current



Note that the values of k (conduction parameter) and VTH (threshold voltage) vary from one eMOSFET to the next and can not be physically changed. This is because they are specific specification relating to the material and device geometry which are in-built during the fabrication of the transistor.

The static transfer characteristics curve on the right is generally parabolic (square law) in shape and then linear. The increase in drain current, ID for a given increase in gate-source voltage, VGS determines the slope or gradient of the curve for constant values of VDS.

Then we can see that turning an enhancement MOS transistor “ON” is a gradual process and in order for us to use the MOSFET as an amplifier we must bias its gate terminal at some point above its threshold level.

There are many different ways we can do this from using two separate voltage supplies, to drain feedback biasing, to zener diode biasing, etc, etc. But whichever biasing method we use, we must make sure that the gate voltage is more positive than the source by an amount greater than VTH. In this mosfet amplifier tutorial we will use the now familiar universal voltage divider biasing circuit.