A MOSFET (Metal
Oxide Semiconductor Field Effect Transistor) is a 4-terminal device with
Source, Drain, Gate and Body as its terminals. It is used for amplification or
switching of electronic signals and is the most common transistor in both
digital and analog integrated circuits. The generic structure of a MOSFET is
shown in figure 1. The source and drain terminals are separated by a channel.
The conduction of the channel is determined by the carrier density in the
channel which is a function of voltage applied at the gate terminal. The body
terminal is normally connected to the source so as to allow only minimal
leakage current to flow.
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| Figure 1: A MOSFET |
MOSFETs are categorized into two
categories based upon the nature of channel:
1)
Enhancement
mode MOSFETs: In an enhancement MOSFET, the channel is devoid of carriers. The
channel has to be created by creating a suitable voltage difference between
gate and source terminals. With gate and source at same potential, only minimal
current flows. However, when a positive potential difference is applied which
is greater than threshold voltage for the MOSFET, a channel is created. Thus,
the current will now flow between source and drain if there is a potential
difference between them. Figure 2 below shows how a channel is formed on applying a voltage between source and gate terminals.
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| Figure 2: Channel formation in Enhancement MOSFET |
2)
Depletion
mode MOSFETs: In a depletion mode MOSFET, the channel is already present
with the help of ion-implantation. Even
with gate and source at same voltage, it will conduct current. The channel has
to be depleted by applying suitable potential.