Depletion MOSFET and negative logic. Why it is not possible?

As we know, depletion MOSFET conducts current even with gate and source at same voltage level. To cut-off the current in depletion MOSFET, a voltage has to be applied at gate so as to exhaust the already existing carriers inside the channel. On the other hand, enhancement type MOSFET is cut-off when gate and source are at same voltage.

Taking the example of NMOS, for a depletion MOS, with source and gate at same level, there is still a channel available, hence, it conducts electric current. To bring it to cut-off, a negative potential is needed to be applied at gate (considering source at ‘X’ potential). Thus, with source at ‘X’ potential and gate at ‘X’ potential, drain attains the potential of source. Since, to cut-off the device, gate has to be given a voltage less than ‘X’, so we can say “when Gate is 1 and source is 1, then drain is 1”.  On the other hand, when source is 1 and gate is 0, drain attains ‘high impedance’. The reverse is true for PMOS.

Similarly, with the same logic, for an enhancement NMOS, “When Gate is 1 and source is 0, drain attains 0 potential”; similarly, “When Gate is 0 and source is 0, drain is 0”. The reverse is true for PMOS.

Source voltage	Gate voltage	Drain voltage for enhancement NMOS	Drain voltage for enhancement PMOS	Drain voltage for depletion NMOS	Drain voltage for depletion PMOS
0	0	Z	Z	0	0
0	1	0	Z	0	Z
1	0	Z	1	Z	1
1	1	Z	Z	1	1

Thus, we can say that it is due to the inherent properties of NMOS and PMOS that that they cannot be used to create negative level logic.

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.

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