Compare BJT with FET and explain D MOSFET.
| Comparison of BJT & FET |
| Bipolar Junction Transistor (BJT) |
Field Effect Transistor (FET) |
| Bipolar device |
Unipolar device |
| Lower switching speed |
Higher switching speed |
| Current controlled device |
Voltage controlled device |
| Less input impedance |
High input impedance |
| Lesser thermal stability |
More thermal stability |
| In IC fabrication, BJTs are occupying more space |
FETs are easier to fabricate and occupying less space |
| Relatively more affected by radiations |
Less affected |
The Depletion-Mode MOSFET
- Fig. - 1 shows a depletion-mode MOSFET, a piece of n material with an insulated gate on the left and a p region on the right. The p region is called the substrate.
- Electrons flowing from source to drain must pass through the narrow channel between the gate and the p substrate. A thin layer of silicon dioxide () is deposited on the left side of the channel.
- Silicon dioxide is the same as glass, which is an insulator. In a MOSFET, the gate is metallic. Because the metallic gate is insulated from the channel, negligible gate current flows even when the gate voltage is positive.
- Fig. - 2(a) shows a depletion-mode MOSFET with a negative gate voltage. The VDD supply forces free electrons to flow from source to drain. These electrons flow through the narrow channel on the left of the p substrate.
- As with a JFET, the gate voltage controls the width of the channel. The more negative the gate voltage, the smaller the drain current. When the gate voltage is negative enough, the drain current is cut off. Therefore, the operation of a depletion-mode
- MOSFET is similar to that of a JFET when VGS is negative. Since the gate is insulated, we can also use a positive input voltage, as shown in Fig. - 2(b).
- The positive gate voltage increases the number of free electrons flowing through the channel. The more positive the gate voltage, the greater the conduction from source to drain.
