MOSFET Basics.pdf

November 2,1999

AN9010

MOSFET Basics

April 1999

R & D 2 Group

Fairchild Korea Semiconductor

CONTENTS

1. History of Power MOSFETs........................................................................................................ 2

2. FETs ........................................................................................................................................... 2

1) JFET....................................................................................................................................... 2

2) MOSFET ................................................................................................................................ 3

3. The structure of MOSFET .......................................................................................................... 4

1) Lateral Channel Structure ..................................................................................................... 4

2) Vertical Channel Structure ..................................................................................................... 4

4. The characteristics of MOSFET .................................................................................................5

1) Advantages ............................................................................................................................ 5

2) Disadvantage ......................................................................................................................... 6

3) Basic Characteristics.............................................................................................................. 6

5. Characteristics of MOSFET’s ON , OFF..................................................................................... 9

1) Off State ................................................................................................................................. 9

2) Turn – on Transient ................................................................................................................ 9

3) On State ............................................................................................................................... 14

4) Turn – off Transient .............................................................................................................. 15

6. User’s Manual .......................................................................................................................... 16

1) Characteristics of Capacitance ............................................................................................ 16

2) Characteristics of the Gate Charge ...................................................................................... 18

3) Drain – source On Resistance ............................................................................................. 22

4) Threshold Voltage ................................................................................................................ 24

5) Transconductance................................................................................................................ 24

6) Drain – source Breakdown Voltage

Breakdown Voltage Temp. Coeff. ......................................................................................... 25

7) Drain – to – source Leakage Current ................................................................................... 26

8) Gate – to – source Voltage...................................................................................................26

9) Gate – source Leakage , Forward / Reverse ....................................................................... 26

10) Switching Characteristics ...................................................................................................26

11) Single – pulsed Avalanche Energy..................................................................................... 27

12) Repetitive Avalanche Rating .............................................................................................. 29

13) Drain – to – source dv / dt Ratings ..................................................................................... 29

14) Thermal Characteristics .....................................................................................................34

15) Continuous Drain Current, Drain Current – pulsed ............................................................ 37

16) Total Power Dissipation, Linear Derating Factor ................................................................ 37

17) Safe Operating Areas......................................................................................................... 38

Rev C, November 1999

The Bipolar Power Transistor as a switching device for a power application had few disadvantages

and this led to the development of the power MOSFET ( Metal Oxide Semiconductor Field Effect

Transistor ). Power MOSFET is being used in many applications such as SMPS, computer periph-

erals, automotive, motor control, and etc. in place of BJT, and continuous research led its charac-

teristics to become ideal.

This application note describes general description of power MOSFET and detailed presentation of

items of FSC’s data book specification.

1. History of Power MOSFETs

The theory of Field Effect Transistor had been advent around 1920~1930 which is 20 years before

the Bipolar Junction Transistor has been invented, which is from 1940’s and through early 1950s.

At that time J.E. Lilienfeld of America suggested a transistor model having two metal contact at

each side with metallic plate (Aluminum) on top of the semiconductor. The electric field at the semi-

conductor surface formed by the voltage supplied at the metallic plate enabled the control of the

current flow between the metal contacts, and this was the initial conception of the Field Effect Tran-

sistor. But due to the immature semiconductor materials and the technology, the progress of the

development was very sluggish. In 1952, W. Shockely introduced JFET (Junction Field Effect Tran-

sistors), in 1953, Dacey and Ross materiallized it. In JFET, the metallic plate of Lilienfeld structure

was replaced by pn junction, and named the metal contact as source and drain, and also named

the field effect electrode as gate. Even though there were continuous research of small-signal

MOSFET after that, there was no prominent result for the power MOSFET, and the commercially

available products started to come out by 1970s.

History of FSC’s Enhancement Type Power MOSFET

In March 1986, FSC formed up a TFT with 9 people, and started the research on the power MOS-

FET. They started with 60~700V level n-ch power MOSFET development, and in 1987, they suc-

cessfully developed p-ch power MOSFET. In 1990, 60~200V level logic-level FET and 50~60V

level low voltage, low R DS(on) device were developed. In 1991, 800V level, and in 1993, 900V level

high voltage MOSFET, and in 1992 current sense FET, and in 1995, 800V level 3~10A sense FET,

and in 1996, 600V level 6A low charge MOSFET were developed sequentially. Finally, in 1999 the

leading technology of FSC has led to develop Q-FET series.

2. FETs

JFET, MOSFET

1) JFET (Junction Field Effect Transistors)

There are two kinds of JFETs. One is n-channel type and the other is p-channel type. They both

control the drain-to-source current by the voltage supplied to the gate. As shown in the Fig. 1 (a), if

the bias is not supplied at the gate, the current flows from the drain to the source, and when the

bias is supplied at the gate, depletion region begin to grow and reduces the current as shown in

Fig. 1 (b). And the reason why the depletion region of the drain is wider than the depletion region of

the source is because the reverse bias of the gate and the drain V DG (=V GS +V DS ) is higher than the

V GS (bias between the gate and the source).

Rev C, November 1999

Drain

Depletion

region

V DS

Gate

V GS

Source

(a)

(b)

Fig. 1. The structure of JFET and its operation

(a) When V GS (Gate-source voltage) has not been supplied

(b) When V GS (Gate-source voltage) has been supplied

2) MOSFET ( Metal Oxide Semiconductor Field Effect Transistors )

There are depletion type and enhancement type, and each has n / p – channel type. The depletion

type is normally on, and operates as JFET (Refer to Fig. 2). And the enhancement type is normally

off, which means that the drain – t o – source current increases as the voltage at the gate

increases. And no current flows when there are no voltage supplied at the gate (Refer to Fig.3).

Fig. 2. The structure of depletion type MOSFET and its operation

(a) When V GS (Gate-source voltage) has not been supplied

(b) When V GS (Gate-source voltage) has been supplied

Rev C, November 1999

Fig. 3. The structure of enhancement type MOSFET and its operation

(a) When V GS (Gate-source voltage) has not been supplied

(b) When V GS (Gate-source voltage) has been supplied

3. The structure of MOSFET

1) Lateral Channel Structure

All the drain, gate, and the source terminal are placed on the surface of a silicon wafer, and it is

suitable for the integration but not for obtaining high power ratings as the length between the

source region and the drain region must be far away from each other to obtain better voltage block-

ing capability, and as the drain-to-source current is inversely proportional to the length.

2) Vertical Channel Structure

The drain and the source are placed in the opposite side of the wafer, and it is suitable for a power

device as more space could be used as source region, and as the length between the source

region and the drain region is reduced, it is possible to increase the drain-to-source current rating,

and it could also increase voltage blocking capability by growing the epitaxial layer (drain drift

region).

1. The VMOSFET Structure

As shown in Fig. 4 (a), this structure has V-groove at the gate region and it is the first commer-

cialized structure. But as there was stability problem in manufacturing, and the high electric

field at the tip of V-groove, this VMOSFET structure was pushed out by the DMOSFET struc-

ture.

2. The DMOSFET Structure

As shown in Fig. 4 (b), it has double-diffusion structure having P-base region and N + source

region, and it is the most commercially successful structure.

Rev C, November 1999

3. The UMOSFET Structure

As shown in Fig 4 (c), this structure has U-groove at the gate region. This structure has higher

channel density so that it can reduce on-resistance compared to the VMOSFET and the

DMOSFET. UMOSFET structure using trench etching technique was commercialized in

1990s.

Source

Gate

N +

Gate

N +

P-body

N — epitaxial layer

N + substrate

Drain

(a)

(b)

Source

N +

P-body

Gate

P-body

N — epitaxial layer

N + substrate

Drain

(c)

Fig. 4. Vertical Channel Structure

(a) The VMOSFET Structure

(b) The DMOSFET Structure

4. The characteristics of MOSFET

1) Advantages

1. High input impedance, Voltage controlled device, Easy to drive.

To maintain on-state, base drive current which is 1/5 or 1/10 of collector current is required,

and larger reverse base drive current is needed for the high speed turn-off for the current con-

trolled device, BJT. Due to these characteristics base drive circuit design becomes compli-

Rev C, November 1999

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