Depletion MOSFET (D-MOSFET)

Depletion MOSFET (D‒MOSFET)

1. Construction of n‒channel MOSFET

• Fig. 5.12.1 shows the basic construction of n‒channel depletion type MOSFET.

   Fig. 5.12.1 n‒channel depletion‒type MOSFET

• Two highly doped n‒regions are diffused into a lightly doped p‒type substrate.

• These two highly doped n‒regions represent source and drain. Usually substrate is internally connected to the source terminal.

• The source and drain terminals are connected through metallic contacts to n‒doped regions linked by an n‒channel as shown in Fig. 5.12.1.

• The gate is also connected to a metal contact surface but remains insulated from the n‒channel by a very thin layer of dielectric material, silicon dioxide (SiO₂).

• Thus, there is no direct electrical connection between the gate terminal and the channel of a MOSFET, increasing the input impedance of the device.

2. Operation, Characteristics and Parameters of n‒channel MOSFET

• On the application of drain to source voltage, V_DS and keeping gate to source voltage to zero by directly connecting gate terminal to the source terminal, free electrons from the n‒channel are attracted towards positive potential of drain terminal.

• This establishes current through the channel to be denoted as I_DSS at V_GS = 0 V, as shown in Fig. 5.12.2.

Fig. 5.12.2 n‒channel depletion type MOSFET with V_GS= 0 V and an applied voltage V_DD

• If we apply negative gate voltage, the negative charges on the gate repel conduction electrons from the channel, and attract holes from the p‒type substrate.

• This initiates recombination of repelled electrons and attracted holes as shown in Fig. 5.12.3.

• The level of recombination between electrons and holes depends on the magnitude of the negative voltage applied at the gate.

• This recombination reduces the number of free electrons in the n‒channel for the conduction, reducing the drain current.

• In other words we can say that, due to recombinations, n‒channel is depleted of some of its electrons, thus decreasing the channel conductivity.

• The greater the negative voltage applied at the gate, the greater the depletion of n‒channel electrons.

• The level of drain current will reduce with increasing negative bias for V_GS as shown in the transfer characteristics of depletion type MOSFET (Fig. 5.12.4).

Fig. 5.12.4 Transfer characteristics for an n‒channel depletion type MOSFET

• For positive values of V_GS the positive gate will draw additional electrons from the p‒type substrate due to reverse leakage current and establish new carriers through the collisions between accelerating particles. Because of this, as gate to source voltage increases in positive direction, the drain current also increases as shown in Fig. 5.12.4.

• The application of a positive gate to source voltage has "enhanced" the level of free carriers in the channel compared to that encountered with V_GS = 0 V. For this reason the region of positive gate voltages on the drain or transfer characteristics is referred to as enhancement region and the region between cut‒off and the saturation levels of I_DSS referred to as depletion region.

• Fig. 5.12.5 shows drain characteristics for n‒channel depletion type MOSFET. It is similar to that of JFET. The only difference is that it has positive part of V_GS.

Transconductance (g_m)

• It is defined as the ratio of change in drain current to the corresponding change in gate to source voltage, at a constant of drain to source voltage

Important Concept

1. I_D = 0 corresponds to V_GS(off), V_GS(off) = − V_P.

2. V_GS = 0 corresponds to I_DSS.

3. Both positive and negative values of V_GS can be used to bias D‒MOSFET.

3. p‒Channel Depletion Type MOSFET

• The construction of the p‒channel depletion type MOSFET is exactly opposite of that of n‒channel depletion type MOSFET.

• Here, the substrate is of n‒type, and regions and channels are of p‒type as shown in Fig. 5.12.6 (a).

• As shown in Fig. 5.12.6 (a) voltage polarities and current directions are reversed.

• The drain characteristics appear exactly as in Fig. 5.12.6 (b) but V_DS with negative values, I_D in the opposite direction and V_GS having opposite polarities as shown in Fig. 5.12.6 (b).

• Fig. 5.12.6 (c) shows the transfer characteristics of p‒channel depletion type MOSFET.

• In the p‒channel depletion type MOSFET, the transfer characteristics is a mirror image about the I_D axis (Y axis) of the transfer characteristic of n‒channel depletion type MOSFET, since the V_GS is positive in p‒channel depletion region.

4. D‒MOSFET Symbols

Fig. 5.12.7 shows graphic symbols for a n and p‒channel depletion type MOSFET.

Review Questions

1. Explain the construction, operation and characteristics of n‒channel depletion type MOSFET.

2. Define transconductance.

3. With the help of suitable diagrams explain the working of different types of MOSFET.

4. Sketch the graph symbol for n ‒ channel and p channel MOSFET.

5. Discuss your understanding on MOSFET detailing the types, construction and characteristics.

6. Describe the output and transfer characteristics of MOSFET.