The metal-oxide-semiconductor GET (SAMOSET) is similar in many respects to its JFET counterpart, in that both have drain, gate, and source terminals, and both are devices whose channel conductivity is controlled by a gate-to-source voltage. The principal feature that distinguishes a from a JFET is the fact that the gate terminal in a is insulated from its channel region. For this reason. a is often called an insulated-!’ fET. or IOfET. Then! arc two kinds of: the depletion type and the I type. also referred to ode and enhancement-. These names are derived from
the two different ways that the conductivity of the channel can be altered by variations in Vcs, as we shall see.
Figure 7–27 shows the structure of an Channel, depletion-type SAMOSET. A block of high-resistance, P-type silicon forms a substrate, in which are embedded two heavily doped N-type wells, or pockets, labeled N+. A thin layer of silicon dioxide (Si02), which is an insulating material, is deposited along the surface. Metal contacts penetrate the silicon dioxide layer at the two N’ weJls and become the drain and source terminals. Between the two N+ welIs is a more lightly doped region of N material that forms the channel. Metal (aluminum) is deposited on ! ‘le silicon dioxide opposite the channel and becomes t.le gate terminal. Note that the silicon dioxide insulates the gate from the channel. Going from gate to channel, we encounter metal, oxide, and semiconductor, in that sequence, which accounts for the name MOSFET. Notice that there is no PN junction formed between gate and channel, as there is in a JFET. Figure 7-2R shows the normal mode of operation of a depletion-type, N-challllcl MOSFET. A voltage Vvs is connected between drain and source to make the drain positive with respect to the source. The substrate is usually connected to the source, as shown in the figure. When the gate is made negative with resp •.ct to the source by Ves, the electric field it produces in the channel drives electrons away from a portion of the channel near the Si02 layer. This portion is thus depleted of carriers and the channel width is effectively narrowed. The narrower the channel, the greater its resistance and the smaller the current flow from drain t source. Thus, the dC1 rce behaves very much like an N-channel JFET, the principal difference being that the channel width is controlled by the action of the electric field rather than by the size of the depletion region of a PN junction. Since there is no PN junction, the voltage Ves can be made positive without any concern for the consequences of forward biasing a junction. In fact, making Ves positive attracts more electrons into kennel and increases, or enhances, voltage. in a depletion-type can be varied through both positive and negative voltages and the device can operate in both depletion and enhancement modes. For this reason, the depletion-type is also called a depletion-enhancement type. Although there is a PN junction between the N material and the P substrate, this junction is always reverse biased and very little substrate current flows. The substrate has little bearing on the operation of the device and we will hereafter :gn~~o its presence, other than to show its usual connee ion to the source. The rcsistauce lOoK1I1ginto the gate is extremely large, on t hI’ nrdl’r of thousands of lr’ gohms, because there is no PN junction and no path for current to flow through the insulating layer separating the gate and the channel. Because of the similarity of a depletion-type MOSFET to a JFET, we would expect it to have similar parameters and operating characteristics. This is indeed “1e case, as shown by the drain characteristics in Figure 7-29. Note that current It linearly with increasing VDS until a pinch-off condition is reached. Beyond pinch-off, the drain current remains constant at a saturation value depending on V(i~. More negative values of VGS cause pinch-off to be reached sooner and result in smaller values of saturation current. If = 0, the drain current saturates at loss when VDS = – Vp volts. If is made sufficiently negative to deplete the entire Drain characteristics of nil Ni channel depletion-type MOSES~ showing operation in the depletion enhancement modes channel, the drain current is completely cut off. The value of Vc;s at which this occurs is the gate-to-source cutoff voltage, VGS(Cl/ = Vp. Note that the characteristics in Figure 7-29 also show operation in the enhancement mode. where V(,s is positive. Figure 7-30 shows the drain characteristics of a Channel. Placation-type SAMOSET. Notice that depletion occurs in this device when is positive and enhancement when Vc;s is negative. Figure 7-31 shows the thematic symbols for N l-channel depletion-type Marmosets. The arrow on each symbol is drawn on the -prostrate terminal and its direction indicates whether the device is N-channel OJ P channel. It points in device for an Channel and outward for a P-channel. “NIce dICIt tile gate terminal