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| 型號： | AN211A |
| 廠商： | Motorola, Inc. |
| 英文描述： | FIFELD EFFECT TRANSISTORS IN THEORY AND PRACTICE |
| 中文描述： | FIFELD場效應(yīng)晶體管理論與實(shí)踐 |
| 文件頁數(shù)： | 3/12頁 |
| 文件大?。?/td> | 340K |
| 代理商： | AN211A |
3
MOTOROLA SEMICONDUCTOR APPLICATION INFORMATION
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Figure 5. Channel Enhancement. Application of
Positive Gate Voltage Causes Redistribution of Minority
Carriers in the Substrate and Results in the Formation
of a Conductive Channel Between Source and Drain
An equivalent circuit for the MOSFET is shown in Figure
6. Here, C
g(ch)
is the distributed gate-to-channel capacitance
representing the nitride-oxide capacitance. C
gs
is the
gate-source capacitance of the metal gate area overlapping
the source, while C
gd
is the gate-drain capacitance of the
metal gate area overlapping the drain. C
d(sub)
and C
s(sub)
are junction capacitances from drain to substrate and source
to substrate. Y
fs
is the transadmittance between drain current
and gate-source voltage. The modulated channel resistance
is r
ds
. R
D
and R
S
are the bulk resistances of the drain and
source.
The input resistance of the MOSFET is exceptionally high
because the gate behaves as a capacitor with very low
leakage (r
in
10
14
). The output impedance is a function
of r
ds
(which is related to the gate voltage) and the drain
and source bulk resistances (R
D
and R
S
).
To turn the MOSFET “on”, the gate-channel capacitance,
C
g(ch)
, and the Miller capacitance, C
gd
, must be charged.
In turning “on”, the drain-substrate capacitance, C
d(sub)
, must
be discharged. The resistance of the substrate determines
the peak discharge current for this capacitance.
The FET just described is called an enhancement-type
MOSFET. A depletion-type MOSFET can be made in the
following manner: Starting with the basic structure of Figure
4, a moderate resistivity n-channel is diffused between the
source and drain so that drain current can flow when the
gate potential is at zero volts (Figure 7). In this manner, the
MOSFET can be made to exhibit depletion characteristics.
For positive gate voltages, the structure enhances in the
same manner as the device of Figure 4. With negative gate
voltage, the enhancement process is reversed and the
channel begins to deplete of carriers as seen in Figure 8.
As with the JFET, drain-current flow depletes the channel
area nearest the drain first.
The structure of Figure 7, therefore, is both a
depletion-mode and an enhancement-mode device.
MODES OF OPERATION
There are two basic modes of operation of FET’s —
depletion and enhancement. Depletion mode, as previously
mentioned, refers to the decrease of carriers in the channel
due to variation in gate voltage. Enhancement mode refers
to the increase of carriers in the channel due to application
of gate voltage. A third type of FET that can operate in both
the depletion and the enhancement modes has also been
described.
The basic differences between these modes can most
easily be understood by examining the transfer
characteristics of Figure 9. The depletion-mode device has
considerable drain-current flow for zero gate voltage. Drain
current is reduced by applying a reverse voltage to the gate
terminal. The depletion-type FET is not characterized with
forward gate voltage.
The depletion/enhancement mode type device also has
considerable drain current with zero gate voltage. This type
device is defined in the forward region and may have usable
forward characteristics for quite large gate voltages. Notice
that for the junction FET, drain current may be enhanced
by forward gate voltage only until the gate-source p-n
junction becomes forward biased.
The third type of FET operates only in the enhancement
mode. This FET has extremely low drain current flow for zero
gate-source voltage. Drain current conduction occurs for a
V
GS
greater than some threshold value, V
GS(th)
. For gate
voltages greater than the threshold, the transfer
characteristics are similar to the depletion/enhancement
mode FET.
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Figure 6. Equivalent Circuit of Enhancement-
Mode MOSFET
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Figure 7. Depletion Mode MOSFET Structure.
This Type of Device May Be Designed to Operate in
Both the Enhancement and Depletion Modes
F
Freescale Semiconductor, Inc.
Go to: www.freescale.com
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