Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-03-09
2003-03-25
Wilson, Allan R. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S296000, C257S327000, C257S334000, C257S335000, C257S337000, C257S338000, C257S339000
Reexamination Certificate
active
06538279
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to power drive circuits using active semiconductor devices.
Background: “MOS” Transistors
One of the basic types of transistors is the insulated-gate field-effect transistor, which is commonly also referred to as a “MOS” transistor or “MOSFET”. In any MOS transistor, the voltage on the “gate” terminal controls the density of majority carriers (electrons or holes) in a semiconducting “channel” region which is part of the conduction path. By controlling the number of majority carriers in the channel, the flow of current from the “source” through the channel and to the “drain” is controlled. Thus a MOS transistor operates as a switch which is controlled (at least partially) by the voltage on its gate.
A “MOSFET” may be configured to be a lateral, vertical, or grooved vertical structure. There are four regions in a MOSFET: source, drain, body, and gate. The source is the region where carriers originate when current is flowing in the transistor in normal operating conditions. The drain is the region where the carriers terminate when current is flowing in the transistor in normal operating conditions. The body is the region whose surface is inverted to form a “channel” when carriers flow in the transistor in normal operating conditions. The gate is an electrode that covers the surface of the body region between the source and the drain regions, and controls the carrier flow between the source and the drain of the transistor in normal operating conditions.
Background: N-Channel Versus P-Channel
MOSFETs are either n-channel devices (in which the majority carriers are electrons) or p-channel devices (in which the majority carriers are holes). The relative performance of equivalent n-channel and p-channel MOSFETs is determined primarily by the “mobility” of the electrons and holes in the inverted “channel” region at the surface of the body region. Electrons have a higher mobility, so the resistance of an n-channel MOSFET is lower than a p-channel MOSFET with the same geometry. (For example, in silicon at room temperature the electron mobility is about 2 to 3 times the hole mobility, and the ratio of mobilities is even larger in gallium arsenide; but in some semiconductors, such as germanium or gallium phosphide, the hole mobility is more nearly equal to the electron mobility.
N-channel transistors also differ from P-channel transistors in some other ways which can be important for power devices. For example, the transient behavior at turn-on and turn-off can be slightly different. This difference can be important for power devices, where mismatch during turn-on (or during turn-off of an inductive load) may lead to disastrous side-effects. In any case, N-channel and P-channel MOS-gated devices rarely have the same switching characteristics for the same geometry.
Background: Overdrive
An N-channel MOS transistor begins to turn “on” when the gate voltage (measured with respect to source voltage) exceeds the threshold voltage (V
T
). The amount by which the gate voltage exceeds the threshold voltage will be referred to as the “overdrive” voltage. As the overdrive voltage increases, the transistor passes more and more current, up to a “saturation” current value. If the gate voltage is increased further, the current increases much more slowly. However, with power transistors it is common to increase the gate voltage beyond the point where saturation occurs. This gate voltage increase is used to decrease the on-resistance of the transistor as much as possible.
Background: DMOS
A “DMOS” transistor is a particular type of MOS transistor which is commonly used for high-voltage applications. A DMOS device uses a short channel which is defined by differential diffusion, in combination with a drift region which is interposed between the channel and the drain. (The drift region helps to stand off high source-drain voltages when the channel has not been turned “on.”) For high-power applications, one commonly used form of a vertical DMOS is shown in FIG.
1
. In the solid structure shown, majority carriers (electrons in this example) flow laterally from source regions
12
through channel regions
14
into drift regions
16
, within which the carriers pass vertically to drain
18
on the backside of the semiconductor chip.
MOS-gated devices with their high input impedance were viewed as nearly ideal switches. However, it was the use of the DMOS structure to fabricate power transistors that began a period of significant development in MOS-gated power devices. This evolution is covered in several books and articles, such as “Trends in Power Semiconductor Devices” by B. Jayant Baliga (
IEEE Transactions on Electron Devices
, Vol. 43, No. 10, October 1996, pp., 1717-1731), Chapter 1 titled “Power Semiconductor Devices for Variable Frequency Drives” written by B. Jayant Baliga in the book,
Power Electronics and Variable Frequency Drives
edited by Bismal K. Bose (IEEE Press, 1996) and
Power Semiconductor Devices
by B. Jayant Baliga (PWS Publishing Company, 1996). Other, newer MOS-gated power devices based on DMOS technology such as the insulated-gate thyristors or IGTH (J. S. Ajit and D. M. Kinzer, “1200V, 150A Insulated-Gate Thyristors,”
Proceedings of
1995
International Symposium on Power Semiconductor Devices & I.C.s.
) and the base open/short-controlled thyristor or BOSCHT have also been developed. (J. S. Ajit, “A New Three-Terminal Thyristor-Based High-Power Switching Configuration with High-Voltage Current Saturation,”
IEEE Electron Device Letters, Vol.,
18, No., 7, July, 1997.)
Double-diffused MOS or DMOS transistors are MOSFET with a structure that eliminates many of the on-resistance and voltage limitations of conventional MOSFETs. DMOS transistors use the difference in the diffusion of sequentially introduced body and source impurities from a common edge to determine the channel length. Control of both the channel length and the peak dopant concentration in the body are obtained, as in double-diffused bipolar technology, by control of the amount of dopant introduced at the body and source doping step, and by the subsequent diffusion cycle. A DMOS transistor differs from a conventional MOS transistor in two distinct ways:
1. Because the channel Length L is determined by the difference between two sequential diffusions moving in the same direction from a common point of origin, L can be reproducibly controlled to values in the 0.5- to 2-micrometer range.
2. The body region is more heavily doped than the N-drain region, resulting in a junction that depletes further into the drain region than into the body region when a reverse bias is placed across the drain-to-body junction. This difference allows significantly higher voltages to be placed across the body-to-drain junction without markedly affecting the electrical channel length of the transistor.
These two differences in device structure result in MOS transistors with both a short channel length and the ability to withstand high drain-to-source voltages due to the separation of the active or channel region of the device from the region of the device that sustains the drain-to-source voltage. The DMOS transistor structure is somewhat analogous to that used in double-diffused bipolar transistors for many years. (In bipolar transistors, a narrow, moderately doped base region controls device electrical characteristics. A lightly doped N-collector region is used to support applied potentials.)
Background: Depletion-Mode MOS Devices
An N-channel MOS transistor begins to conduct when its gate voltage (measured with respect to its source voltage) rises above a certain “threshold” voltage (which is determined by the compositions, dopings, hand dimensions of any particular device). If the threshold voltage is positive (as is normal), the transistor is normally “off”, and is referred to as an “enhancement-mode” device. Most power MOS transistors have generally been enhancement mode (normally “off”) devices.
However, if the threshold voltage of an N-channel transistor is below zero,
Ortiz Edgardo
Robinson Richard K.
Wilson Allan R.
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