MOSFET Regions of Operation
4.4.2 MOSFET Regions of Operation
SiO 2
Most of the MOSFET devices used in power electronics n C + applications are of the n-channel, enhancement-type like that
which is shown in Fig. 4.6a. For the MOSFET to carry drain C ds p
gs
C gd n −
current, a channel between the drain and the source must be created. This occurs when the gate-to-source voltage exceeds
the device threshold voltage, V Th . For v GS > V Th , the device can be either in the triode region, which is also called “con- stant resistance” region, or in the saturation region, depending
D on the value of v DS . For given v GS , with small v DS (v DS <
(b)
v GS −V Th ), the device operates in the triode region(saturation region in the BJT), and for larger v DS (v DS > v GS
Th ), FIGURE 4.9 (a) Equivalent MOSFET representation including junction
−V capacitances and (b) representation of this physical location.
the device enters the saturation region (active region in the BJT). For v GS < V Th , the device turns off, with drain current almost equals zero. Under both regions of operation, the gate of the gate control circuit must take into consideration the current is almost zero. This is why the MOSFET is known variation in this capacitance (Fig. 4.9b). The largest variation as a voltage-driven device, and therefore, requires simple gate occurs in the gate-to-drain capacitance as the drain-to-gate control circuit. voltage varies. The MOSFET parasitic capacitance are given in
The characteristic curves in Fig. 4.6b show that there are terms of the device data sheet parameters C iss ,C oss , and C rss three distinct regions of operation labeled as triode region, as follows,
saturation region, and cut-off-region. When used as a switch- ing device, only triode and cut-off regions are used, whereas,
C gd =C rss
when it is used as an amplifier, the MOSFET must operate in the saturation region, which corresponds to the active region
C gs =C iss −C rss
in the BJT.
The device operates in the cut-off region (off-state) when
C ds =C oss −C rss v GS < V Th , resulting in no induced channel. In order to oper- ate the MOSFET in either the triode or saturation region, a
where C rss = small-signal reverse transfer capacitance. channel must first be induced. This can be accomplished by
C iss = small-signal input capacitance with the applying gate-to-source voltage that exceeds V Th , i.e. drain and source terminals are shorted.
C oss = small-signal output capacitance with the gate and source terminals are shorted.
v GS > V Th
The MOSFET capacitances C gs ,C gd , and C ds are non-linear Once the channel is induced, the MOSFET can either operate and function of the dc bias voltage. The variations in C oss and in the triode region (when the channel is continuous with no
C iss are significant as the drain-to-source and gate-to-source pinch-off, resulting in the drain current proportioned to the voltages cross zero, respectively. The objective of the drive channel resistance) or in the saturation region (the channel circuit is to charge and discharge the gate-to-source and gate- pinches off, resulting in constant I D ). The gate-to-drain bias to-drain parasitic capacitance to turn on and off the device, voltage (v GD ) determines whether the induced channel enter respectively.
pinch-off or not. This is subject to the following restriction.
54 I. Batarseh For triode mode of operation, we have
v GD > V Th v GD < V Th
And for the saturation region of operation,
Pinch-off occurs when v GD =V Th .
In terms of v DS , the above inequalities may be expressed as v GS follows:
V Th
FIGURE 4.10 Input transfer characteristics for a MOSFET device when
1. For triode region of operation
operating in the saturation region.
v DS < v GS −V Th and v GS > V Th
2. For saturation region of operation D
DS > v GS −V Th and v GS > V Th
3. For cut-off region of operation
It can be shown that drain current, i D , can be mathematically
approximated as follows:
2 FIGURE 4.11 Large signal equivalent circuit model.
i D = K [2(v GS −V Th )v DS −v DS ] Triode Region
D = K (v GS −V Th ) Saturation Region (4.7) is shown in Fig. 4.11. The drain current is represented by a current source as the function of V Th and v GS .
1 If we assume that once the channel is pinched-off, the drain– where,
source current will no longer be constant but rather depends μ n = electron mobility
K = μ n C OX
on the value of v DS as shown in Fig. 4.12. The increased
C OX = oxide capacitance per unit area value of v DS results in reduced channel length, resulting in L
a phenomenon known as channel-length modulation [3, 4]. If W = width of the channel.
= length of the channel
the v DS –i D lines are extended as shown in Fig. 4.12, they all intercept the v DS -axis at a single point labeled –1/λ, where λ
Typical values for the above parameters are given in the is a positive constant MOSFET parameter. The term (1 +λ PSPICE model discussed later. At the boundary between the v DS ) is added to the i D equation in order to account for the saturation (active) and triode regions, we have,
increase in i D due to the channel-length modulation, i.e. i D is
given by,
Th ) (1 + λv DS ) Saturation Region (4.10)
Resulting in the following equation for i D ,
From the definition of the r o given in Eq. (4.1), it is easy to
D = kv DS
(4.9) show the MOSFET output resistance which can be expressed
as follows:
The input transfer characteristics curve for i D and v S .v GS is
when the device is operating in the saturation region shown
(4.11) λ k(v GS −V Th )
in Fig. 4.10. The large signal equivalent circuit model for a n-channel
If we assume the MOSFET is operating under small signal enhancement-type MOSFET operating in the saturation mode condition, i.e. the variation in v GS on i D vs v GS is in the
4 The Power MOSFET 55
v GS
Slope = 1 Increasing
0 v DS
FIGURE 4.12 MOSFET characteristics curve including output resistance.
neighborhood of the dc operating point Q at i D and v GS as
shown in Fig. 4.13. As a result, the i D current source can be
represented by the product of the slope g m and v GS as shown in Fig. 4.14.
vGS
gmvgs rO