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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

Depletion MOSFET N-channel type

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 VDS at VGS 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 charge induced in it. This will result into depletion of majority carriers(i.e. electrons) and hence reduction in conductivity. 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.

Depletion MOSFET Drain characteristics

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

Enhancement MOSFET

Enhancement MOSFET N-channel type

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) from P-type bulk into the concentrated layer.

Enhancement MOSFET Drain characteristics

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 keep the 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 while depletion 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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