Characteristics of FETs

Figure 4.24 shows the different types of FETs that can be fabricated today. A cross section of an n-channel junction FET is shown in Figure 4.28 alongside the symbols used to denote the n-channel and p -channel depletion types.

94 STANDARD MICROELECTRONIC TECHNOLOGIES Gate (G)

Source (S)

JFET depletion type (b)

Figure 4.28 (a) Cross section of an n-channel JFET and (b) symbols for an n-channel and p-channel JFET

The more commonly used small-signal n-channel MOSFET is shown in Figure 4.29.

MOSFETs can not only be n-channel or p-channel but also be of the depleted type or the enhanced type. Figure 4.29 shows the symbols used to represent the four basic types of MOSFETs.

FETs can be used to make a number of different types of microsensors. For example, a FET can be used to make an ion-selective or gas-sensitive chemical sensor by modifying its gate and exposing it to the local environment (see Section 8.6). For this reason, it is useful to provide the basic properties of an MOSFET, especially for the less experienced reader who can see the transfer characteristics of such a device. Figure 4.30 shows both the output characteristics ID-VDS and transfer characteristic ID-VGS of a typical n-channel enhancement-mode DMOSFET.

The drain current I^D just starts to flow when the gate-source voltage reaches the device threshold voltage VT (or off voltage for the depletion-type devices). When operating the

Gate (G)

Source (S) Drain (D)

S O S MOSFET depletion type

Body (U)

n-channel negative voltage (a)

s 'l.

MOSFET enhancement type (b)

Figure 4.29 (a) Cross section of an n-channel MOSFET and (b) symbols for n -channel and p-channel depletion-type and enhancement-type MOSFETs

MONOLITHIC PROCESSING 95 device in the linear region (i.e. VDs < VGs - VT, VGs > VT), the drain current is given by

ID = Kn [2 (VGS - VT) VDS - V2DS] (4.24) where Kn is the device constant and, for an n-type MOSFET, is related to the channel length L, width W, electron mobility un, gate oxide capacitance C'o by

(4.25) In the saturated region of operation (i.e. VDS > VGs - VT), the device is switched on and the drain current simplifies to

n

= ~ (V^GS - V^T)² (4.26)

as shown by the transfer characteristic illustrated in Figure 4.30. Pinch-off occurs when VGS is less that VT, and, ideally, the drain current is zero when the device is switched off.

The basic dynamic properties of an FET device in a common-source configuration can be characterised by the low-frequency equivalent circuit11 shown in Figure 4.31 in which the main small-signal conductances12 are shown. The low-frequency gate-source, gate-drain, and drain-source conductances are defined as

dIG

d V^GS

dIG

GS 'GD

and gds = d ID

(4.27)

DS

Saturation region

(V) (a)

4

-(V) 10

(b)

Figure 4.30 Typical characteristics of an n-channel MOSFET (enhancement-type): (a) drain (output) characteristic, with dotted line separating ohmic and saturated regions of operation and (b) transfer (input-output) characteristic in the saturated region

11 Leakage current and reactive components are ignored.

12 The subscripts used are gate g, drain d, source s, and forward f.

96 STANDARD MICROELECTRONIC TECHNOLOGIES

Voltage in

Output

External load O—I O

RL

Figure 4.31 Low-frequency, small-signal equivalent circuit of an FET showing the principal conductances. An ideal source voltage (no internal impedance) and ideal load (no reactance) are also shown

An important parameter for both transistors and microsensors is the small-signal for-ward transconductance gfs that is a measure of the transfer characteristic or sensitivity of a device. The forward transconductance g^fs is defined by

d ID

(4.28) Therefore, in the case of an MOSFET, the forward transconductance may be found from Equations (4.24) and (4.26) and is

gfs = Kn [2 (VGS - VT) ~ 2VDS] (ohmic region) gk = Kn (VGs - W) (saturated region)

(4.29) Clearly, the transconductance is a function of the gate-source voltage and can be deter-mined in the saturation (S) region from Equations (4.27) and (4.26), where

gfs s = 2

(VGS - VT) (4.30)

The low-frequency input conductance g-ls (when RL is large) is simply the sum of the gate-source and gate-drain conductances,

gis = ggs + (4.31)

The output or channel conductance gds is a function of the gate-source voltage and thus varies with the type of FET. Figure 4.32 shows the variation of channel conductance for n-channel and p-channel FETs.

The channel conductance of an FET that is turned on is low, and this corresponds to VDS being low as well. For an n-channel depletion-type FET, the on-resistance rds(on) is related to the forward transconductance and is given by, when V^GS > V^T,

rds(on) — gfs —

1

Kn (VGS - VT) ^(4.32)

MONOLITHIC PROCESSING 97

p-channel

Enhanced

n-channel

Enhanced

GS (off) GS (th) o v,GS (th) V,_{OS (off)} Gate-source voltage VGS

Figure 4.32 Variation of the channel conductance gds with gate-source voltage for the various types of FETs when the drain-source voltage VDS is set to zero

Generally, the channel conductance is related to the drain-source voltage in the linear (1) region from Equations (4.27) and (4.24), and is given by

/D (4.33)

In the practical use of an FET, the various static and dynamic properties will be affected when a load resistor RL is applied across the drain and source (see Figure 4.31) to create a common-source voltage amplifier. However, when the output conductance is low, the gain is related simply to the transconductance as follows:

(4.34) The input capacitance of the FET transistor is an important parameter, and Figure 4.33 shows the principal capacitances within a transistor. A low-input capacitance is desirable because, when coupled with a low on-resistance rds(on), the switching time is very fast.

Short-channel transistors, such as those produced by the DMOS process, have very fast (i.e.

nanosecond) switching times and so are used in high-speed circuitry. The output capacitance C^ds is mainly determined by the n-p junction capacitance and is inversely proportional to the square root of the drain-source voltage. However, the other capacitances depend on both gate and drain voltages, threshold voltage, and parasitic capacitances. In all these cases, it should be remembered that the device capacitances are in the picofarad range, so care must be taken when designing and interfacing ICs and also while using transistors as either sensing or actuating devices. Any stray capacitance will act as a charge divider and reduce the voltage signals accordingly in a capacitive microtransducer.