Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2003-10-10
2004-11-30
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S329000
Reexamination Certificate
active
06825537
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor element having a parallel p-n junction layer with an arrangement of alternately joining a plurality of drift regions made up of a first conduction type semiconductor and a plurality of partition regions made up of a second conduction type semiconductor. The drift regions extend from a first principal surface side of a semiconductor substrate toward a second principal surface side thereof, the partition regions extend in the same way as the drift regions, and both regions are joined in a direction crossing the direction in which the regions extend. The parallel p-n junction layer becomes a drift layer that allows a current to flow when the semiconductor element is in a turned-on state and becomes depleted when in a turned-off state. The invention particularly relates to a MOSFET (Insulated-gate field effect transistor), an IGBT (Insulated-gate bipolar transistor) and a semiconductor which is a applicable to a bipolar transistor etc., and can be provided with compatibility between a high breakdown voltage capability and a high current capacity capability.
2. Description of the Related Art
Semiconductor elements may be generally classified into lateral elements and vertical elements. A lateral element is provided with electrodes on one of surfaces of a semiconductor substrate to allow a current to flow in a direction in parallel with a principal surface. A vertical element is provided with electrodes on both surfaces of a semiconductor substrate to allow a current to flow in a direction perpendicular to the principal surface. In the vertical semiconductor element, a direction in which a drift current flows when the element is made turned-on is the same as a direction in which a depletion layer is extended by a reverse bias voltage applied when the element is made turned-off. For example, in an ordinary planer n-channel vertical MOSFET, a section of an n
−
-drift layer with high resistance operates as a region of allowing a drift current to flow in the vertical direction when the MOSFET is in a turned-on state and becomes a depletion region when the MOSFET is in a turned-off state to increase the breakdown voltage.
To shorten a current path in the n
−
-drift layer with high resistance is to lower drift resistance to a current. This leads to an effect of reducing substantial on-resistance of the MOSFET. However, the expanding width of a depletion layer between a drain and a base, traveling from the p-n junction between a p-base region and the n
−
-drift region, is adversely narrow. This makes electric field strength in the depletion layer quickly reach the critical electric field strength of silicon to reduce the breakdown voltage. Conversely, in a semiconductor device with a high breakdown voltage, the n
−
-drift layer is provided as being thick, by which on-resistance is inevitably made increased to result in increased loss. That is, there is a tradeoff between the on-resistance and the breakdown voltage.
It is known that the same tradeoff holds also for such semiconductor elements as IGBTs, bipolar transistors and diodes. Moreover, the problem is common to a lateral semiconductor element in which the flowing direction of a drift current when the element is turned-on differs from the travelling direction of a depletion layer expanded by a reverse bias voltage applied when the element is turned-off. As measures for solving the problem, structures of semiconductor devices are disclosed in, for example, EP-B 0 053 854, U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215 and JP-A-9-266311. In each of the disclosed structures, a drift layer is arranged with a parallel p-n junction layer in which highly doped n-type regions and p-type regions are alternately disposed. The parallel p-n junction layer becomes a depletion layer when the device is in a turned-off state so as to bear a voltage to withstand.
The structural difference between the above semiconductor device and an ordinary planer n-channel vertical MOSFET is that the drift layer is not made up of a layer with a uniform and single conduction type, but of the above-described parallel p-n junction layer. In the parallel p-n junction layer, when the element is in a turned-off state, from each of the p-n junction aligned in the vertical direction of the parallel p-n structure, a depletion layer expands in the lateral direction on both sides of the p-n junction even though an impurity concentration is high. This brings the whole drift region to become a depletion region to allow the device to have a high breakdown voltage. In the specification, the semiconductor element provided with a drift section with such a parallel p-n junction structure is to be referred to as a super junction semiconductor element.
Incidentally, on-resistance of a planer super junction MOSFET (Ron·A) is generally expressed approximately by the following expression (1), where resistance of a source layer is denoted by R
s
, channel resistance is denoted by R
ch
, resistance of an accumulation layer is denoted by R
acc
, resistance by a junction FET (JFET) effect is denoted by R
JFET
, drift resistance is denoted by R
drift
, resistance of a drain layer is denoted by R
d
and an area of a region causing the on-resistance is denoted by A:
Ron·
A
=(
R
s
+R
ch
+R
acc
+R
JFET
+R
drift
+R
d
)·
A.
(1)
In the super junction semiconductor element, the drift resistance R
drift
is given by the following expression (2). Therefore, even though a breakdown voltage is increased, only the drift resistance increases in proportion to the breakdown voltage. This allows dramatic reduction in the on-resistance compared with related MOSFETs. Furthermore, for the same breakdown voltage, by reducing a width d of the n-drift region in the parallel p-n junction layer, the on-resistance can be further reduced. In the expression (2), &mgr; is a mobility of electrons, &egr;
o
is the permittivity in vacuum, &egr;
s
is the specific permittivity of silicon, Ec is critical electric field strength and Vb is a breakdown voltage:
R
drift
·A=
(4·
d·vb
)/(&mgr;·&egr;
o
·&egr;
s
·Ec
2
) (2)
However, while the drift resistance R
drift
is dramatically reduced, resistance components other than the drift resistance in the expression (1) become significant. In particular, the proportion of the resistance R
JFET
in the JFET effect is large in the on-resistance. For improving this, application of a so-called trench MOSFET is proposed in which a gate electrode fills each of trenches dug from the substrate surface for inducing a channel in a section on the side wall of the trench. About a trench super junction semiconductor element, there is a disclosure in, for example, JP-A-2002-76339.
However, also in the case of the trench MOSFET, a voltage withstanding structure section is provided in the same way as in the planer MOSFET. Therefore, when the MOSFET has stripe-like trenches, an end portion of each of the trenches is sometimes formed in a region where the structure of the region changes to that of the voltage withstanding structure section. In such a case, the end portion of the trench formed in a shape of a three-dimensional curved surface causes electric field concentration in a region at the end portion of the trench to bring about possible reduction in a breakdown voltage.
In addition, in a transition stage of being shifted from a turned-on state to a turned-off state, the depletion layer is quickly expanded in the parallel p-n junction structure. This prevents accumulated carriers from escaping to cause the discharged carriers to encounter a strong electric field due to electric field concentration, which makes the carriers easily injected into a gate insulator film as hot carriers. Thus, the gate insulator film is degraded to cause such possible lowering in reliability of the gate insulator film as to bring about reduction in a threshold voltage. The applicant discloses in JP-A-2001-313391 a structure of a pla
Iwamoto Susumu
Nagaoka Tatsuji
Onishi Yasuhiko
Sato Takahiro
Farahani Dana
Fuji Electric Device Technology Co. Ltd.
Pham Long
Rossi & Associates
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