nMOS Transistor — how does it work as a switch
nMOS transistor is an important component in CMOS digital electronic circuits. In this article, we are going to learn its basic electrical characteristics as a switch.
The circuit symbol for an n-channel MOSFET (nMOS) is shown below:
The MOSFET is a four terminal device — the terminals are Gate, Source, Drain and Bulk.
Currents and voltages in an nMOS are shown below:
The gate is the control terminal which controls whether the drain current I_Dn flows through the device from drain to source or not. The value of this current is controlled by the gate-source voltage V_GSn. Actually the value of I_Dn is determined by both V_GSn and the drain-source voltage V_DSn. The source-bulk voltage V_SBn also determines the value of this current, but to a lesser degree.
The cross sectional view of an nMOS is shown below:
A specific region in this cross sectional view is boxed as shown below:
This boxed region is called the Metal-Oxide-Semiconductor (MOS) subsystem. It has a conducting layer of metal (M) called gate. This metal is on top of an insulating layer of Silicon Dioxide (O). This oxide layer is on top of the semiconductor layer of p-type. From its physical structure it can be understood that this MOS subsystem is a capacitor. This subsystem is coupled to the two n+ regions to its left and right sides. These two regions are called source and drain. The I-V characteristics of the nMOS are a result of the MOS subsystem coupled with the two n+ regions on its left and right. The p-type substrate is called the bulk terminal.
The distance between the two n+ regions is called the channel length L of the nMOS. This length is also called the electrical channel length. This is one of the critical dimensions on which the electrical characteristics of the device depends on.
The top view of the nMOS is shown below:
W is the width of the channel between the two n+ regions. The ratio W/L is called the aspect ratio which is an important parameter of the device.
There is a voltage in the I-V characteristics of the device called the threshold voltage V_T. The physics of the device determines V_T. The MOS transistors used in CMOS circuits are Enhancement mode (E-mode) transistors. The meaning of this is that the gate voltage enhances the conduction between the drain and source. An E-mode nMOS has a positive threshold voltage V_T > 0 (typical value of V_T is between 0.5 and 0.9 V).
When the gate-source voltage V_GSn is less than V_T i.e. V_GSn < V_T, the nMOS is on cutoff mode of operation and the current I_Dn is 0. This means the device doesn’t conduct any current.
When V_GSn > V_T, the nMOS is in the active mode of operation and it conducts I_Dn.
So, the gate-source voltage V_GSn determines if the nMOS is ON or OFF
ON -> I_Dn = 0
OFF -> I_Dn > 0
This is how nMOS works as an electrical switch.
