Showing posts with label Depletion mode MOSFET. Show all posts
Showing posts with label Depletion mode MOSFET. Show all posts

Enhancement and depletion MOSFETs


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.


A MOSFET has 4 terminals, source, drain, gate and body (bulk)
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.

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.