12.17 : MOSFET: Enhancement Mode

Enhancement-mode MOSFETs are pivotal components in electronics, distinguished by their capacity to act as highly efficient switches. They are part of the larger family of metal-oxide Semiconductor Field-Effect Transistors (MOSFETs). They are available in two types: p-channel and n-channel, each tailored to specific polarity operations.

In their basic form, enhancement-mode MOSFETs are typically non-conductive when the gate-source voltage (V_gs) is zero. This default 'off' state means no current flows between the drain and source terminals unless a positive V_gs is applied. When this voltage is applied, it generates an electric field that attracts electrons (for n-channel) or holes (for p-channel) towards the oxide layer, effectively creating what is known as an inversion layer. This layer forms a conductive channel between the source and drain, allowing current to flow.

The unique feature of enhancement-mode MOSFETs is their ability to control the magnitude of drain current (i_d) by adjusting V_gs. Increasing V_gs enhances the conductivity of the channel, thereby allowing more current to pass through. This relationship between V_gs and i_d makes these devices excellent for precision control applications, such as adjusting the brightness of LED lights through a dimmer switch. Here, rotating the dimmer's knob varies V_gs; at zero voltage, the LEDs are off, and increasing the voltage progressively brightens the LEDs.

Furthermore, enhancement-mode MOSFETs are ideal for power-switching circuits and creating CMOS-type logic gates in integrated circuits due to their high input and low ON resistance. These characteristics enable efficient, high-speed switching with minimal power loss, making enhancement-mode MOSFETs essential in modern electronic design.