Enhancement MOSFET (E‒MOSFET)

Enhancement MOSFET (E‒MOSFET)

• 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

• Fig. 5.13.1 shows the basic construction 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 Fig. 5.13.1.

• 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 V_GS there is a measurable current flow between drain and source. This value of V_GS is called threshold voltage denoted by V_T.

• 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 Fig. 5.13.2.

• 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 V_GS = 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. 5.13.3 shows the drain characteristics of an n‒channel enhancement type MOSFET. Looking at Fig. 5.13.3 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 V_GS, the drain current reaches a saturation level.

Fig. 5.13.3 Drain characteristics of an n‒channel enhancement type MOSFET

• The value of V_DS at this point is known as pinch‒off voltage (V_P). The levelling off of I_D is due to a pinch‒off process. Fig. 5.13.4 shows pinch off process for n‒channel enhancement type MOSFET.

Fig. 5.13.4 Change in channel and depletion region with increasing level of V_DS for a fixed value of V_GS

• Fig. 5.13.5 shows the transfer characteristic for n‒channel enhancement type MOSFET.

• This characteristic is quite different from characteristic that we we obtained for JFET and depletion type MOSFET. For an n‒channel enhancement type MOSFET it is now totally in the positive V_GS 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

           ……….(5.13.1)

• 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

        ………(5.13.2)

• The parameter K is called conduction parameter. It is given by K = Wμ_nC_OX / 2L

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

C_OX = ε_ox / t_ox

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

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

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

Ex. 5.13.1: For N‒EMOSFET V_T = 0.7 V, W= 30 μm, L=5 μm, μ_n = 650 cm²/V‒S, t_ox = 450 A (450 × 10^‒8). ε_ox = 3.9 × 8.85 × 10^‒14 F/cm

Assume transistor is biased in saturation region:

i) Determine drain current when V_GS = 2V_T. ii) If t_ox is doubled, find new K.

Solution:

i) W = 30 μm = 30 × 10^‒6 m = 30 × 10^‒4 cm

L = 5 μm = 5 × 10^‒6 m = 5 × 10^‒4 cm

K = Wμ_nε_ox / 2Lt_ox

= [ (30×10^‒4) (650) (3.9×8.85×10^‒14) ] / [ 2(5×10^‒4)(450×10⁻⁸) ]

= 0.1495 mA/V²

V_GS = 2 V_T = 2 × 0.7 = 1.4 V

i_D = K (V_GS ‒ V_T)² = (0.1495) (1.4 ‒ 0.7)²

= 0.0732 mA

ii) For t_ox = 2 × 450 × 10^‒8 = 900 × 10^‒8

K = Wμ_nε_ox / 2Lt_ox

=  [ (30×10^‒4) (650) (3.9×8.85×10^‒14) ] / [ 2(5×10^‒4)(900×10^‒8) ]

= 0.07475 mA/V²

Ex. 5.13.2: Evaluate the body effect coefficient (γ) in a MOSFET with N_a = 3×10¹⁶/cm³, ε_r = 11.6 and ε₀ = 8.854×10^‒12 F/m and C_ox = 1.726×10^‒7 F/cm².

Solution::

where ε_S = ε_r × ε₀

= 11.6×8.854×10^‒12 F/m

= 11.6×8.854×10^‒14 F/cm

 γ = { √[2×(1.6×10^‒19)×(3×10¹⁶)×(11.6×8.854×10⁻¹⁴) ] / [ 1.726×10^‒7 ]

= 0.5753

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 Fig. 5.13.6.6

• As shown in Fig. 5.13.7 voltage polarities and current directions are reversed.

• The drain characteristics appear exactly as in the Fig. 5.13.7 but with V_DS with negative values, I_D in opposite direction and V_GS having opposite polarities as shown in Fig. 5.13.7.

• Fig. 5.13.8 shows the transfer characteristics of p‒channel enhancement type MOSFET.

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

E‒MOSFET symbols

Fig. 5.13.9 shows graphic symbols for n and p‒channel enhancement type MOSFET.

Review Questions

1. Define threshold voltage.

2. Define pinchoff voltage.

3. Sketch and explain the construction of N‒channel enhancement type MOSFET. Give also its symbol.

4. Explain the operation of an enhancement type MOSFET.

5. Describe, with suitable sketches, the operation and characteristics (transfer, drain) of the N‒channel enhancement type MOSFET.

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

7. Draw a circuit diagram of the cross section of a enhancement MOSFET. Also discuss the drain and transfer characteristics for EMOSFET.

8. Explain the concept of threshold voltage in a MOSFET.

9. Discuss the characteristics of MOSFET.

10. Describe the working and characteristics of MOSFET, D MOSFET and E MOSFET.

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