The depletion type of MOSFET can operate in the A depletion mode only B from ELECTRICAL 2015sa at University Of the City of Manila (Pamantasan ng Lungsod ng Maynila).

MOSFET

1. In this video, the Enhancement-Type MOSFET, its Construction, Working and, drain and transfer Characteristics have been explained.By watching this video you.
2. What is depletion mode, and why you might want it. Enhancement MOSFET: normally off (the common stuff). Depletion MOSFET: normally on. To switch off an n-channel depletion MOSFET, you should apply negative voltage to gate terminal. That is, it behaves exactly the same as an enhancement n-channel, but it has negative gate threshold voltage.

MOSFET Basics: Symbol and Structure

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. MOSFETtransistor 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 BJTin 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

Enhancement Mode And Depletion Mode Mosfet Definition

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.

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.

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.

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.

difference between Depletion MOSFET vs Enhancement MOSFET

This page on Depletion MOSFET vs Enhancement MOSFET mentions difference between Depletion MOSFET and Enhancement MOSFET.

Depletion MOSFET

Figure-1 depicts construction of depletion type MOSFET. It also mentions circuit symbol of N-channel MOSFET of depletion type.Due to its construction if offers very high input resistance (about 10¹⁰ to 10¹⁵).Significant current flows for given V_DS at V_GS of 0 volt.

When gate(i.e. one plate of capacitor) is made positive, the channel((i.e. the other plate of capacitor) will have positive chargeinduced in it. This will result into depletion of majority carriers(i.e. electrons) and hence reduction inconductivity. Hence the curve similarto JFET is obtained as shown in the figure-2.

As shown in the symbol here gate is insulated from the channel.For P-channel type MOSFET symbol, arrow will be reversed.

Figure-2 depicts drain characteristics and transfer curve of depletion type of MOSFET(N-channel).

Enhancement MOSFET

Figure-3 depicts construction of enhancement type MOSFET. It also mentions circuit symbol of N-channel MOSFET of enhancement type.Here continuous channel does not exist from source to drain. Hence no current flows at zero gate voltage.Symbol depicts broken channel between 'S' to 'D' terminals.

When positive voltage is applied to the gate, it will induce a channel by flowing minority carriers(i.e. electrons) fromP-type bulk into the concentrated layer.

Figure-4 depicts drain characteristics and transfer curve of enhancement type of MOSFET(N-channel).As shown in the figure-4 minimum threshold voltage is needed for the flow of drain current to start.

This type of FET is ideal for switching application. This is due to the fact that no gate voltage is needed to keepthe device in 'off' state. Moreover the device can be powered ON with the application of same polarity as drain terminal.

Following are the important comparison features between Depletion and Enhancement MOSFET types:
• Enhancement MOSFET does not conduct at 0 volt, as there is no channel in this type to conduct.Depletion MOSFET conducts at 0 volt. Moreover when positive cut-off gate voltage is applied to depletion MOSFET,hence it is less preferred.
• The depletion MOSFET does not have any kind of leakage currents such as gate oxide and sub threshold type.
• Depletion MOSFET logic operations are opposite to enhancement type of MOSFETs.
• Diffusion current(i.e. sub-threshold leakage current) exists in enhancement MOSFET whiledepletion MOSFET do not have any diffusion current.

Comparison between NMOS and PMOS types

Refer NMOS vs PMOS which mentions comparison between NMOS and PMOS type of MOSFETs.

Difference between JFET and MOSFET

Refer JUGFET vs MOSFET which mentions difference between JFET and MOSFET.

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