Enhancement MOSFET (E-MOSFET)

Enhancement MOSFET (E-MOSFET)

AU : May-13, 14, 15, 16. 17. Dec.-13, 15

• This type of MOSFET operates only in the enhancement mode and has no depletion mode. It differs in construction from the depletion MOSFET in that it has no physical channel.

1. Construction of n-Channel E-MOSFET

• The Fig. 4.3.1 shows the physical structure of n-channel enhancement type MOSFET.

• Like, depletion type MOSFET, two highly doped n-regions are diffused into a lightly doped p-type substrate.

• The source and drain are taken out through metallic contacts to n-doped regions as shown in the Fig. 4.3.1.

• But the channel between two n-regions is absent in the enhancement type MOSFET.

• The SiO₂ layer is still present to isolate the gate metallic platform from the region between the drain and source, but now it is simply separated from a section of the p-type material.

2. Operation, Characteristics and Parameters of n-Channel E-MOSFET

• On application of drain to source voltage V_DS and keeping gate to source voltage zero by directly connecting gate terminal to the source terminal, practically zero current flows-quite different from the depletion type MOSFET and JFET.

• If we increase magnitude of V_GS in the positive direction, the concentration of electrons near the SiO₂ surface increases.

• At a particular value of VGS there is a measurable current flow between drain and source. This value of VGS is called threshold voltage denoted by VT.

• Thus, we can say that in an enhancement type n-channel MOSFET, a positive gate voltage above a threshold value induces a channel and hence the drain current by creating a thin layer of negative charges in the substrate region adjacent to the SiO2 layer, as shown in the Fig. 4.3.2.

• The conductivity of the channel is enhanced by increasing the gate to source voltage and thus pulling more electrons into the channel. 

• For any voltage below the threshold value, there is no channel.

• Since the channel does not exist with VGS = 0 V and "enhanced" by the application of a positive gate to source voltage, this type of MOSFET is called an enhancement type MOSFET.

• Fig. 4.3.3 shows the drain characteristics of an n-channel enhancement type MOSFET. Looking at Fig. 4.3.3 we can say that as V_GS increases beyond the threshold level, the density of free carriers (electrons) in the induced channel increases, increasing the drain current. However, at some point of V_DS, for constant VGS, the drain current reaches a saturation level.

 • The value of V_DS at this point is known as pinch-off voltage (VP). The levelling off of ID is due to a pinch-off process. Fig. 4.3.4 shows pinch off process for n-channel enhancement type MOSFET.

• Fig. 4.3.5 shows the transfer characteristic for n-channel enhancement type MOSFET.

• This characteristic is quite different from characteristic that we obtained for JFET and depletion type MOSFET. For an n-channel enhancement type MOSFET it is now totally in the positive VGS region and as we know I_D does not flow until V_GS = V_T

• For V_GS > V_T the relationship between drain current and V_GS is nonlinear and it is given as

I_D = K (V_GS-V_T)²   ...(4.3.1)

where V_GS - V_T is a gate to source overdrive voltage denoted by Vo_V’

• The K term is a constant that is a function of the construction of the device. The value of K can be determined from equation,

K = I_{D (ON)} / (V_GS(ON) – V_T)² ...(4.3.2)

• The parameter K is called conduction parameter. It is given by

K = W_µnC_ox / 2L

• Where C_ox is the oxide capacitance per unit area. The capacitance is given by

C_ox = Ɛ_ox / t_ox

where t_ox is the oxide thickness and Ɛ_ox is the oxide permittivity.

• The parameter µ_n is the mobility of the electrons in the inversion layer.

K = K w/2L

• The µ_n C_ox is a constant known as process transconductance parameter. It determines the value of MOSFET transconductance, denoted by k_n and has dimensions of A/V².

• The parameter W is the channel width and parameter L is the channel length.

Ex. 4.3.1 For N-EMOSFET V_T = 0.7 V, W = 30 µm L = 5 µm µn = 650 cm²/V - S, tox = 450 A (450 x 10~8\ eox = 3.9 x 8.85 x 10-14 F/ cm

Assume transistor is biased in saturation region :

i) Determine drain current when V_GS = 2V_T.

ii) If tox is doubled, find new K.

Sol. :

3. p-Channel Enhancement Type MOSFET

• The construction of the p-channel enhancement type MOSFET is exactly opposite to that of n-channel enhancement type MOSFET. Here, the substrate is of n-type and regions are of p-type as shown in the Fig. 4.3.6.

• As shown in the Fig. 4.3.7 voltage polarities and current directions are reversed.

• The drain characteristics appear exactly as in the Fig. 4.3.7 but with V_DS with negative values, I_D in opposite direction and V_GS having opposite polarities as shown in the Fig. 4.3.7.

• In the p-channel enhancement type MOSFET, the transfer characteristic is a mirror image about the ID axis (y axis) of the transfer characteristics of n-channel depletion type MOSFET, since the VGS is negative.

E-MOSFET symbols

• Fig. 4.3.9 shows graphic symbols for n and p-channel enhancement type MOSFET.

4. Channel Length Modulation

• The Fig. 4.3.10 shows induced channel at different levels of V_DS- In the figure, the thickness of the induced channel layer qualitatively indicates the relative charge density.

• In Fig. 4.3.10 (a), applied V_DS small and for this case the relative charge density is constant along the entire channel length.

• The Fig. 4.3.10 (b) shows the situation when VDS increases. As the drain voltage increases, the voltage drop across the oxide near the drain terminal decreases, which means that the induced inversion charge density near the drain also decreases. The incremental conductance of the channel at the drain then decreases, which causes the slope of the I_D versus V_DS curve to decrease. This effect is shown in the I_D versus curve in the figure.

• As V_DS increases to the point where the potential difference across the oxide at the drain terminal is equal to VT, the induced inversion charge density at the drain terminal is zero. This is illustrated in Fig. 4.3.10 (c).

• When V_DS becomes larger than V_{DS (sat)}the point in the channel at which the inversion charge is just zero moves towards the source terminal. In this case, electrons enter the channel at the source, travel through the channel towards the drain, and then at the point where the charge goes to zero, are injected into the depletion region, where they are swept by the E-field to the drain contact.

• It is observed that as VDS increases beyond VDS (sat effective channel length decreases, producing the phenomenon called channel length modulation.

Important Concepts

• The MOSFET will have a smaller threshold voltage V_TH if the channel length is reduced.

• Reducing the channel length increases the drain current and hence the transconductance of the MOSFET.

Review Questions

1. Explain the construction, operation and characteristics of n-channel enhancement type

AU : Dec.-13, 15, May-14, Marks 8

2. With the help of suitable diagram, explain the working of N-channel enhancement MOSFET.

AU : May-13, 15, Marks 16

3. Define threshold voltage.

4. Define pinch-off voltage.

5. Explain the channel length modulation.

6. State true or false : The transconductance increases if channel length is reduced.

7. Elaborately discuss the drain current characteristics and transfer characteristics of MOSFET.

8. Explain the drain and transfer characteristics of Enhancement type MOSFET.

AU : May-17, Marks 7