Power MOSFET

POWER MOSFET

Power MOSFET is a special type of MOSFET designed to handle high power levels.

Power MOSFET are commonly used power devices due to the following advantages.

1. Low gate drive power

2. Fast switching speed

3. Parallel operation

For high power applications, the standards MOSFET structure cannot be used because

1. To increase current capability of MOSFET, its width should be made large and its channel length L should be made as small as possible. But it reduces the breakdown voltage and can't handle high voltages in power transistor applications.

Therefore, MOSFET is designed with short channel fabrication (1 to 2 μm) with high breakdown voltages.

CONSTRUCTION

Double-diffused MOSFET (or) DMOS transistor is used as power MOSFET. The device is fabricated on a lightly doped n-type substrate with a heavily doped region at the bottom for the drain contact. Two diffusions are used one to form p-type body region and other to form the n-type source region.

In power MOSFETs, source and drain are constructed on opposite sides of a vertical structure in order to support higher current and voltage. The structure of power MOSFET is classified into horizontal and vertical type. Vertical type is further classified into planar type and trench type.

In vertical power MOSFET, the current flows in the vertical direction, the on- resistance per chip area is reduced due to miniaturization. Trench MOSFETs are used for voltage rating of < 200 V due to their higher channel density and lower on- resistance.

Planar MOSFETs are used for higher voltage ratings because on-resistance is dominated by epi-layer resistance and high cell density is not required.

OPERATION

When positive gate voltage VGS, greater than threshold voltage V_T (V_GS > V_T) is applied, a lateral n-channel is induced in the p-type body region under the gate oxide. The length of the resulting n-channel is short and denoted as L.

The electrons from source move towards the substrate through the induced channel and conducts current. Then the electrons move vertically down the substrate to the drain.

The breakdown voltage is very high (as high as 600V) (Inspite of having short channel) because the depletion region between the substrate and the body extends mostly in the lightly doped substrate and doesnot spread into the channel.

Thus MOS transistor has a high current capability (≈ 50A) and high breakdown voltage. The vertical structure of the device enables efficient utilization of the silicon area.

CHARACTERISTICS

Power MOSFETs has threshold voltages in the range of 2V to 4V. The drain current and V_GS are related by

The high electric field in the short channel increases the velocity of charge carriers to be maximum. This is known as velocity saturation. Thus the drain current is given by

where U_sat → Saturation velocity (5 x 10⁶ cm/s for electrons in Silicon)

The linear i_D - V_GS relationship implies a constant g_m in the velocity – saturation region.

g_m is proportional to W and is large for power devices. Thus, power MOSFETs have high transconductance values.