Depletion MOSFET vs. Enhancement MOSFET: Differences Explained

This article highlights the differences between Depletion MOSFETs and Enhancement MOSFETs.

Depletion MOSFET

Figure 1 illustrates the construction of a Depletion-type MOSFET and its N-channel circuit symbol.

Figure 1: Depletion MOSFET N-channel type

Depletion MOSFETs offer very high input resistance, typically in the range of 10¹⁰ to 10¹⁵ Ohms. A significant current flows for a given V_DS at a gate-source voltage (V_GS) of 0 volts.

When the gate is made positive (acting as one plate of a capacitor), the channel (the other plate) will have a positive charge induced in it. This results in the depletion of majority carriers (electrons), reducing conductivity. This behavior is similar to that of a JFET. The symbol indicates that the gate is insulated from the channel. For a P-channel type MOSFET symbol, the arrow direction is reversed.

Figure 2: Depletion MOSFET Drain characteristics

Figure 2 shows the drain characteristics and transfer curve of a Depletion-type MOSFET (N-channel).

Enhancement MOSFET

Figure 3 depicts the construction of an Enhancement-type MOSFET and its N-channel circuit symbol.

Figure 3: Enhancement MOSFET N-channel type

In contrast to Depletion MOSFETs, Enhancement MOSFETs do not have a continuous channel between the source and drain. As a result, no current flows when the gate voltage is zero. The symbol reflects this by depicting a broken channel between the ‘S’ and ‘D’ terminals.

Applying a positive voltage to the gate induces a channel by drawing minority carriers (electrons) from the P-type bulk into the concentrated layer.

Figure 4: Enhancement MOSFET Drain characteristics

Figure 4 illustrates the drain characteristics and transfer curve of an Enhancement-type MOSFET (N-channel).

A minimum threshold voltage is required for the drain current to start flowing. This type of FET is well-suited for switching applications because no gate voltage is needed to keep the device in the ‘off’ state. The device can be powered ON with the application of the same polarity as the drain terminal.

Key Comparison Features

Here’s a comparison between Depletion and Enhancement MOSFETs:

• Conduction at 0V: Enhancement MOSFETs do not conduct at 0 volts because there is no existing channel. Depletion MOSFETs, on the other hand, do conduct at 0 volts.
• Cut-off Gate Voltage: Applying a positive cut-off gate voltage to a Depletion MOSFET is possible, making them less preferred in some situations.
• Leakage Currents: Depletion MOSFETs typically do not have leakage currents like gate oxide and sub-threshold types.
• Logic Operations: Depletion MOSFET logic operations are the opposite of those performed by Enhancement MOSFETs.
• Diffusion Current: Diffusion current (sub-threshold leakage current) exists in Enhancement MOSFETs, while Depletion MOSFETs generally do not have this.