Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Transistors
MISFET under different operating condition
Introduction:
This section describes the MISFET under different operating conditions in dfetails.
Different operating condition:
Consider an N channel MISFET & there is three operating conditions.
(a) linear region for VG > VT and VD < (VG- VT);
(b) onset of saturation at pinch-off, VG > VT and VD=(VG- VT,);
(c) strong saturation, VG> VT and VD > (VG- VT).
- The threshold voltage VT is the minimum gate voltage required to induce the chanriel. In general, the positive gate voltage of an n-channel device must be larger than some value VT before a conducting channel is induced.
- Similarly, a p-channel device (made on an n-type substrate with p-type source and drain implants or diffusions) requires a gate voltage more negative than some threshold value to induce the required positive charge (mobile holes) in the channel.
- For example, some n-channel devices have a channel already with zero gate voltage, and in fact a negative gate voltage is required to turn the device off.
- Such a "normally on" device is called a depletion-mode transistor, since gate voltage is used to deplete a channel which exists at equilibrium.
- The more common MOS transistor is "normally off" with zero gate voltage, and operates in the enhancement mode by applying gate voltage large enough to induce a conducting channel.
- An alternative view of a MOSFET is that it is a gate-controlled resistor. If the (positive) gate voltage exceeds the threshold voltage in an n-channel device, electrons are induced in the p-type substrate.
- Since this channel is connected to the n source and drain regions, the structure looks electrically like an induced n-type resistor. As the gate voltage increases, more electron charge is induced in the channel and, therefore, the channel becomes more conducting.
- The drain current initially increases linearly with the drain bias (the linear region).
- As more drain current flows in the channel, however, there is more ohmic voltage drop along the channel such that the channel potential varies from zero near the grounded source to whatever the applied drain potential is near the drain end of the channel.
- Hence, the voltage difference between the gate and the channel reduces from VG near the source to (VG-VD) near the drain end.
- Once the drain bias is increased to the point that (VG -VD) = VT, threshold is barely maintained near the drain end, and the channel is said to be pinched off.
- Increasing the drain bias beyond this point (VD (sat.)) causes the point at which the channel gets pinched off to move more and more into the channel, closer to the source end (Fig.c).