In the last post, we saw MOSFET biasing and some equations. I hope now you are very much comfortable in MOSFET. In this post, we will understand the key differences between D-MOSFET and E-MOSFET. Although we had covered almost all of the key differences in the previous posts here we will revise a few points and at the end, we will conclude that which MOSFET is better. Remember I'm considering N-type MOSFET as it is much more efficient than P-type MOSFET.
1) On basis of Symbol:-
In the case of N-type MOSFET remember that the arrow will always point towards the gate terminal (<---) or to the oxide layer and for P-type MOSFETs, the arrow will point in opposite direction to the oxide layer (--->) or you can say that it is pointing away from the oxide layer. Since there is an oxide layer present below the gate terminal which acts as an insulator therefore the small gap which is shown in the symbol represents that oxide layer.
In D-MOSFET, the two regions that are drain and source are connected to an extra channel present below an oxide layer therefore in the symbolic form you can see that drain and source are connected while in the case of E-MOSFET, there isn't any additional channel present between the drain and the source region therefore in the symbolic representation you can see that the drain, source terminal are separated by a small gap.
2) Construction:-
As we had studied in the previous posts of MOSFET that the major difference between the D-MOSFET and E-MOSFET is that the D-MOSFET has one extra channel present in between the oxide layer and the substrate. In the case of E-MOSFET, this channel is absent. Thus from this, we can also conclude that in D-MOSFET the oxide layer is present in between the gate terminal and an extra channel but in the case of E-MOSFET, the oxide layer is present in between the gate and substrate terminal. Below you will get to see the actual difference in the construction part between both the MOSFETs. Due to this reason, D-MOSFET can also be used as an E-MOSFET but E-MOSFET cannot be used as D-MOSFET below I had explained this point taking the characteristic curve.
3) Characteristics and Working:-
To operate D-MOSFET, the input voltage Vgs should be negative. Output current or drain current Id decreases as Vgs becomes more negative, on contrary, it increases as Vgs becomes more positive. As Vgs=0v, Id = Id(max). For D-MOSFET, we generally try to avoid providing positive input voltage as the output current would be much greater, due to which MOSFET may heat and may burn completely therefore we provide negative voltage to D-MOSFET to avoid heating issues.
To operate E-MOSFET, the input voltage Vgs should be positive. Output or drain current increases as Vgs increases on contrary it decreases as Vgs decreases. For E-MOSFET, if Vgs = 0v then Id = 0mA thus at zero input voltage, it acts as an open switch.
4) Current Equation:-
This is just a short trick or a pointer from my side to you all guys to remember the formula, we know that for D-MOSFET to operate the input voltage (Vgs) should be negative therefore in the above formula we can see in the current equation of D-MOSFET there is a negative sign before Vgs. Similarly, to operate E-MOSFET, the input voltage (Vgs) should be positive therefore in the formula of E-MOSFET you can see a positive sign before Vgs.
@ How D-MOSFET can be operated as an E-MOSFET?
Thus from the above diagram, we can easily conclude that the D-MOSFET can be operated as an E-MOSFET but E-MOSFET cannot be operated as D-MOSFET.
Conclusion:-
You might have heard or you might have read in different books that E-MOSFET is preferred over D-MOSFET because when the input voltage (Vgs) is zero there won't be any current flowing in E-MOSFET whereas in the case of D-MOSFET maximum current flows when Vgs is zero and even though when Vgs becomes more negative, a little current flows, which we call as leakage current and this current is due to the minority charge carriers.
Therefore when we want to use MOSFET as a switch we prefer using E-MOSFET as it acts as a pure open switch when the input voltage is zero and acts as a closed switch when there is sufficient input voltage i.e. Vgs > Vth. On the contrary, D-MOSFET fails to do so therefore E-MOSFET is always preferred over D-MOSFET.
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