MOSFET Basics: Symbol and Structure

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. MOSFET transistor as an electronic component has four terminals source, drain, gate and body. Body is always grounded, so MOSFET transistor is referred to as a three terminal device electronic component. When compared to BJT in terms of an electronic component, MOSFET transistor is smaller in size. Therefore more MOSFET transistor can be included in a single chip. Along with this, other advantages like high frequency operation, less noise, less leakage and high reliability, make MOSFET transistor a better replacement over BJTs in integrated circuits.

Symbol of both N-channel and P-channel MOSFET transistor electronic component  is shown in the figure below. In N-channel MOSFET transistor (left side) arrow is towards the gate and in the P-channel MOSFET transistor arrow is away from the gate.


                                                                                                MOSFET Basics: Symbol 

There are two ways in which MOSFET transistor can function, that is depletion mode and enhancement mode. Not just the functioning but the construction itself is different in enhancement and depletion type MOSFET transistor. In depletion mode, N-channel is formed by doping the region between the source and drain. The conductance of the channel is maximum when voltage is not applied at the gate terminal and conductivity reduces when voltage is increased either positively or negatively (depends on N-channel or P-channel). In enhancement type MOSFET transistor, the channel conductivity is close to zero, when voltage is not applied to the gate terminal and channel conductivity is increased, when the voltage is increased either positively or negatively (depends on N-channel or P-channel).

Following figure shows the structure and working of an N-channel enhancement type MOSFET transistor. The channel is in between the source and drain region of the MOSFET transistor. Width of the channel can be varied by applying voltage at the gate terminal of the transistor.



                                                                                                  MOSFET Basics : Structure

When a positive voltage is applied, holes will reach the gate end of the MOSFET transistor. Positive charge thus developed at the gate terminal will attract electrons in the silicon substrate. The two regions are separated by insulator (metal oxide). Electrons move towards the gate region and align themselves at the top of the substrate. Electrons rich region thus formed between source and drain will act like a conductive material. Flow of electrons from drain to source continues until the gate terminal is grounded or negative potential is applied at the gate.

MOSFET Basics - MOSFET Operation

MOSFET operation can be explained by considering an N-channel, enhancement mode MOSFET transistor.

Case 1: No voltage

When no voltage is applied at the gate terminal, depletion layer formed at the P-N junction will act like an insulator barrier between source and drain. During this condition, no current flows between source and drain and this condition is called cut-off mode. Following figure shows depletion region of a MOSFET transistor during cut-off mode.


                                                                                         Case1: MOSFET Operation

Case 2: Postive Voltage Applied

When a positive voltage is applied to the gate terminal, holes are equally distributed at the gate region. Electrons in the P-substrates are attracted towards the region, under the gate, due to electrostatic force of attraction. Because of the oxide layer between gate and P-substrate, these electrons will remain in the depletion layer and a channel of free electrons is formed between source and drain. When the positive voltage applied to the gate terminal is increased, more electrons move to this region thereby increasing the conductivity of the channel. If the gate voltage is continuously increased, at a particular voltage, the charge carrier density of the layer formed will be the same as that of the N-region in the MOSFET transistor. This voltage is called threshold voltage. To form a conductive channel between source and drain, the gate voltage should be greater than the threshold voltage. Figure shown below, shows N-channel developed between source and drain. Current flows through the channel till drain voltage is less than gate voltage by threshold value.

Linear Region

                                                                                      Case2 : MOSFET Operation

When positive voltage applied at the drain region is increased, distribution of holes at the gate region is affected. Due to electrostatic force of repulsion between like charges, hole density at the drain end of the gate is low. Channel formation also gets modulated by this effect. Electron density at the drain end is also reduced when drain voltage is increased.

Pinch off Region

                                                                                                               Saturation mode MOSFET Operation

The above figure shows pinch-off condition. When drain voltage is increased, electron density is decreased such that, the entire channel is pinched-off at drain end. This is, when the voltage across gate and drain is less than threshold voltage.

Saturated Mode

When drain voltage is further increased, channel length is reduced as shown in the figure.


                                                                                      MOSFET Basics: Characteristics

Graph shows the relationship between Drain-Source voltage and drain current for different Gate-Source voltages.

 (i). Cut-off mode MOSFET Operation

When the voltage across the gate and source terminal is less than the threshold voltage, no channel is formed between source and drain terminals. Resistance is so high that no current can flow through it. As shown in the graph, drain current remains zero even if drain-source voltage is increased.

(ii). Triode Mode MOSFET Operation

If the voltage at the gate terminal is greater than the threshold voltage, a channel is formed. Conductivity of the channel is proportional to the applied voltage at the gate. Now the drain current depends on voltage across both drain-source and gate-source terminals. As shown in the above figure, in triode region, drain current is proportional to drain-source voltage. And conductivity of the channel (slope of the graph) is proportional to gate-source voltage. This region is also called linear region, since, current increases linearly with the increase in applied voltage.

(iii). Saturation Mode MOSFET Operation

When the drain voltage is increased further, at a particular voltage, the gate-drain voltage (that is, the difference between gate and drain voltage) will be less than threshold voltage. Now depletion layer is developed at the drain end, there by pinching off the channel. Drain voltage at which channel pinch off takes place is called pinch off voltage.  From this point, even if we increase the drain voltage, current remains constant.

One might think that when depletion layer is created, it will cut off the channel, so drain current will reduce to zero. This is not true, because when depletion layer is formed, current reduction is only for a small moment. Now the voltage across the layer is only the gate voltage (since the effective drain voltage is zero), which is greater than the threshold voltage.  Therefore, current flow resumes but soon voltage across the gate-drain terminal goes below the threshold voltage. Turn on and off occurs so fast and therefore, the current appears to remain constant after the pinch-off voltage.

MOSFET Parameters

  • Maximum Drain-Source Voltage

VDS is the maximum instantaneous operating voltage.

  • Maximum Gate Source Voltage, VGS

VGS is the voltage given  between gate and source.

  • Continuous Drain Current, ID

ID is the current the MOSFET transistor can carry at a particular temperature.

  • Gate Threshold Voltage

VT is the gate voltage at which the transistor will turn ON.

  • Maximum Pulsed Drain Current, IDM

 IDM is greater than drain current and specified for particular pulse width and duty cycle.

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