HIGH-VOLTAGE FAST-AVALANCHE DIODE

Abstract

A high-voltage fast-avalanche diode, being a silicon-avalanche shaper or sharpener (SAS), has a thick active region above 300 microns in thickness.

Description

FIELD OF THE INVENTION

The present invention relates generally to high-voltage fast-avalanche diodes, such as a silicon-avalanche shaper or sharpener (SAS).

BACKGROUND OF THE INVENTION

The silicon-avalanche shaper or sharpener (SAS) is a fast closing semiconductor switch. For positive voltages, it operates in the negative, cathode to anode, direction. It receives a high-voltage pulse at its cathode, and, when turned on, the current through the device rises rapidly. Typical input voltages are in the order of a few kilovolts, with a rise-time of the order of nanosecond. The device turn on time is in the order of 100 ps. A similar operation is obtained with devices known as fast-ionization dynistors (FIDs), deep-layer dynistors (DLDs), delayed breakdown devices (DBDs), etc.

The current state-of-the-art knowledge shows that in order to get fast pulses, the device active region (e.g., the n in a p⁺-n-n⁺ structure) has to be thin, about 100 to 300 microns. Therefore in order to obtain a fast operation one has to use a thin active region. The tradeoff of a thin region is the lowering of the device voltage.

Fast high-voltage diodes such as the drift-step-recovery diode (DSRD) and the silicon-avalanche shaper (SAS) have been developed [references 1-4 and references therein].

Fast avalanche diodes can be used for various applications such as plasma discharge experiments [5] and for ultra-wideband pulse generation [6, 7]. The diode structure is typically p+-n-n+, where the n region is some 100 to 300 microns, N_D=10¹⁴ cm⁻³ [7-9]. For a given diode, the switching characteristics, such as rise-time, voltage, and residual voltage, depend mostly on the applied voltage and its rise-time [2, 7-11].

In [12], an impact ionization model shows that when a 8 kV/0.8 ns ramp is applied to a 100 micron p⁺-n-n⁺ structure, it results in a super-fast turn on from 8 kV to 10V in 15 ps, e.g 500 kV/ns.

In [13], a ramp of 180 kV/400 ps was applied to a stack of 44 series connected p⁺-p-n-n⁺ diodes, having a p-n junction depth of 180 microns. The load voltage was 150 kV with a 100-ps rise time.

In [14], a 2.5 kV/1 ns pulse was applied to a 100 micron p⁺-n-n⁺ structure, resulting in a 1 kV/100 ps voltage ramp applied to the load (e.g. 10 kV/ns). The experimental results were supported by a numerical simulation, showing electron concentration and the electric field along the structure at various times.

In all of the experimental papers, the input voltage per diode was between 2 and 4 kilo-volts in 0.4 to 1 ns, and the load voltage ramp was between 10 and 36 kV/ns. The active structure in these papers was less than 200 microns, thus deep diffusion of some tens of microns was required in order to achieve the desired active region.

SUMMARY

The present invention provides a high-voltage fast-avalanche diode, such as an SAS with a thick active region, above 300 microns, which can operate in the 100-ps range and obtain a higher voltage than the thin substrate. The condition for such operation is a high quality substrate with a high-resistivity, such as a float-zone wafer. Further advantages of the invention is that the manufacturing of such devices becomes substantially easier since it is easy to handle thick wafers without the risk of breaking, and there is no need for deep, 10-s of micron, diffusion in order to narrow the active region.

The invention provides a 7.9-kV, 100-ps rise-time silicon-avalanche shaper (SAS) diode. A high-voltage vertical p+-n₀-n⁺ structure was designed using Synopsys-TCAD. The substrate is a 525-μm float-zone, N-type Si wafer, N_D=10¹² cm⁻³. The p+-n₀ junction is fabricated using boron spin-on-dopant (SOD), whereas the n₀-n⁺ junction is fabricated using phosphorus SOD on the other side of the wafer. An electrostatic voltage of 3 kV was demonstrated without breakdown. The results are supported by a numerical simulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates SAS structure (not to scale), in accordance with an embodiment of the invention.

FIG. 2 illustrates an experimental setup circuitry for the device of the invention.

FIG. 3 shows the load voltage versus time, measured on a 50 Ω load.

DETAILED DESCRIPTION

Reference is now made to FIGS. 1 and 2.

The SAS diode may be prepared by the following process, in accordance with a non-limiting embodiment of the invention:

First implant, of the p⁺ layer:

1. A thermal oxide, SiO₂, is deposited on both sides of a 525 micron, 10 kΩ-cm, n-type, float zone wafer.

2. Selective opening in the “top” (boron) side is made via a mask which includes the p⁺-n₀ junction, as in FIG. 1.

3. A boron-SOD is applied to the structure top side. It is noted that this step can be replaced by a standard method such as ion-implantation, or diffusion using a solid or a gas source. Also, other acceptor materials such as aluminum can be used as well.

4. A drive-in of the boron for a depth of a few microns is made.

Second Implant, of the n+ Layer:

1. Etching of the oxide in the “bottom” (phosphorous) side of the wafer is made.

2. A phosphorous SOD is applied to the structure bottom side. It is noted that this step can be replaced by a standard method such as ion-implantation, or diffusion using a solid or a gas source. Also, other donor materials such as arsenic can be used as well.

3. A drive-in of the phosphorous for a depth of about a few microns is made.

Finally, metallic contacts are applied to the structure.

A diode with a junction cross section of 4 mm² may be thus prepared. Its leakage current is measured at a reverse voltage of 3 kV. A 72 μA current is measured without a breakdown. When two diodes are connected in series the leakage current is reduced by a factor of 2.

In order to test it dynamically, the inventors connected its cathode to the ground via a 1 pF peaking capacitor, and also to a 6-kV, 1-ns pulsed-power generator (measured on a 50-Ω load) via a 200 pF DC-block capacitor and a 20 nH coil, as shown in FIG. 2. The SAS anode was connected to a 50 Ω load and measured by a high-voltage attenuator and an 8-GHz oscilloscope. The inventors estimate that the maximum voltage obtained at the peaking capacitor before the SAS was turned on was about 14 kV.

Experimental Result

FIG. 3 shows the load voltage, measured on a 50 Ω load. A peak voltage of 7.9 kV was obtained. Following a pre-pulse of about 1.5 kV, a sharp, 100-ps rise time was obtained. The maximum voltage rise-rate at the load was 52 kV/ns. The experimental results are supported by a numerical simulation (not shown here).

Comparison

Table 1 shows a comparison of this invention to two other state-of-the-art sharpening devices from the literature (first column). The second column shows the width of the active region. It is noted that the width in this invention is substantially wider, which enables working with a thick wafer, and alleviates the need for deep diffusion. Therefore it simplifies the process and reduces the cost.

The third and fourth columns describe the input and output pulse, respectively, in terms of peak voltage and rise-time. The fifth column describes the maximum rise-rate of the load voltage. The sixth column refers to the number of diodes used, in case of a high-voltage stack [13], thus, the seventh column calculates the rise-rate per diode. The eighth column shows a figure of merit for the sharpening quality, in terms of output rise-rate to input voltage and its rise-time.

It is seen that in this invention the voltage rise rate (single diode) and the quality of sharpening is higher than the prior art.

				Max	# of		sharpening
	Active			dVo/dt	diodes,	(dVo/dt)/N	(dVo/dt)/
Ref.	region	V-in	V-out	[kV/ns]	N	[kV/ns]	(Vin/tr)
Lyubutin	180 μm	180 kV/	150 kV/100 ps	1580	44	36	3.5
(2010) [13]		400 ps
Brylevskiy	100 μm	2.2 kV/	1.25 kV/100 ps	10	1	10	3.2
(2019) [14]		700 ps
This	520 μm	10 kV/	7.9 kV/100 ps	48	1	52	5.2
invention		1 ns

REFERENCES

1  Kesar, A. S., et al. “6-kV, 130-ps rise-time pulsed-power
   circuit featuring cascaded compression by fast recovery and
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2  Merensky, Lev M., et al. “The driving conditions for obtaining
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3  Kesar, Amit S., et al. “Characterization of a drift-step-recovery
   diode based on all epi-Si growth.” IEEE Transactions on
   Plasma Science 44.10 (2016): 2424-2428, and references
   therein.
4  Kesar, Amit S. “A Compact, 10-kV, 2-ns risetime pulsed-power
   circuit based on off-the-shelf components.” IEEE Transactions
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5  Yatom, S., et al. “Spectroscopic study of plasma evolution in
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7  Kardo-Sysoev, A. F. “New power semiconductor devices for
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8  Kardo-Sysoev, A. F., and M. V. Popova. “Modeling of fast
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Claims

1. A diode device comprising: a high-voltage fast-avalanche diode comprising a silicon-avalanche shaper or sharpener (SAS) that has a thick active region above 300 microns in thickness.

2. The diode device according to claim 1, wherein said SAS has a rise time of 100 ps.

3. The diode device according to claim 1, wherein said SAS has an output voltage of 7.9 kV.

4. The diode device according to claim 1, wherein said SAS comprises a vertical p+-n0-n+ structure with a substrate comprising a float-zone, N-type Si wafer.

5. A method of making the diode device of claim 1 comprising: creating a p+ layer by depositing a thermal oxide on both sides of an at least 300 micron, 10 kΩ-cm, n-type, float zone wafer;selectively making on a top side of the wafer a p+-n0 junction;applying a boron SOD or other doping acceptors such as aluminum, and other methods such as an ion-implantation to the top side;making a drive-in of boron;etching the thermal oxide on the bottom side of the wafer;applying phosphorous SOD or other donors such as arsenic, and other methods such as ion-implantation to the bottom side;making a drive-in of phosphorous on the bottom side; andforming metallic contacts to the structure.