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.

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.

       Next read: Depletion MOSFET and negative logic- why is this not possible

Negative gate delay - is it possible

As discussed in our post ‘propagation delay’, the difference in time from the input reaching 50% of the final value of the transition to that of the output is termed as propagation delay. It seems a bit absurd to have negative value of propagation delay as it provides a misinterpretation of the effect happening before the cause. Common sense says that the output should only change after input. However, under certain special cases, it is possible to have negative delay. In most of such cases, we have one or more of the following conditions:

i)                    A high drive strength transistor

ii)                   Slow transition at the input

iii)                 Small load at the output

Under all of the above mentioned conditions, the output is expected to transition faster than the input signal, and can result in negative propagation delay. An example negative delay scenario is shown in the figure below. The output signal starts to change only after the input signal; however, the faster transition of the output signal causes it to attain 50% level before input signal, thus, resulting in negative propagation delay. In other words, negative delay is a relative concept.

Figure 1: Input and output transitions showing negative input delay

Also read:

• Race conditions
• Why clock gating checks are needed
• Worst slew propagation
• Can a net have negative propagation delay