US20140197865A1 - On-chip randomness generation - Google Patents

Abstract

An on-chip true noise generator including an embedded noise source with a low-voltage, high-noise zener diode(s), and an in-situ close-loop zener diode power control circuit. The present invention proposes the use of heavily doped polysilicon and silicon p-n diode(s) structures to minimize the breakdown voltage, increasing noise level and improving reliability. The present invention also proposes an in-situ close-loop zener diode control circuit to safe-guard the zener diode from catastrophic burn-out.

Description

FIELD OF THE INVENTION
• The present invention relates to random number generation. More specifically, the present invention relates to generating random noise/numbers embedded in a semiconductor chip with a reduced operating voltage.
BACKGROUND
• Good cryptography requires good random numbers. Almost all cryptographic protocols require the generation and use of secret values that must be unknown to attackers. For example, random number generators are required to generate public/private key pairs for asymmetric (public key) algorithms including RSA, DSA, and Diffie-Hellman. Keys for symmetric and hybrid cryptosystems are also generated randomly. RNGs are also used to create challenges, nonces (salts), padding bytes, and blinding values. The one time pad—the only provably-secure encryption system—uses as much key material as cipher text and requires that the keystream be generated from a truly random process.
• Generating real random numbers has become increasingly vital to information securities, particularly for recent developments in cloud computing, because of its unpredictable pattern suited for the encryption security application. Thus, random number generators have been widely employed from large stationary servers to small mobile devices. However, traditional pseudo-random numbers generated by digital circuits no longer meet the security requirements due to the increasing availability of powerful deciphering computing systems.
• It is well known that a zener diode or an avalanche PN junction is a commonly used as the white noise source in discrete circuits. However, at least two challenges must be overcome for the noise source to be implemented in the integrated circuits. First, the breakdown voltage of a discrete zener is quite high, typically about 6V, which is much higher than the maximum operation voltage of the advanced technologies. Second, when the diode is operated in the avalanche mode close to the breakdown condition, the current-voltage curve is very steep. If the voltage is too low, the diode may not enter the avalanche mode. If the voltage is too high, the current becomes very large and the diode could be damaged by breakdown. Although the immediate solution for the discrete zener diode is to enlarge the diode size, it is not feasible in integrated circuits because of the parasitic capacitance concern that will deteriorate circuit performance, cost considerations and reliability concerns.
• In prior random noise/number generators, the noise sources are always presented as a block and require external physical noise sources for the circuits. Some noise generators rely on the noise based on physical phenomenon like the thermal noise of resistors.
• It is known in the art that a zener diode is a very strong noise source due to the physical nature of its avalanche phenomenon close to the breakdown condition. However, the zener voltage of conventional zener diodes used for random noise generation is about 6V, which exceeds the operation voltage of typical advanced CMOS technologies, for example 1V-2.5V. As zener diodes are p-n junction diodes, increasing p-n junction doping level and abruptness will theoretically result in lower breakdown voltages.
• Differential noise pair circuits have been explored previously to cancel out the common cause of variability, such as temperature fluctuation, in order to achieve true white noise sources. Previously used circuits employ one differential circuit for two noise generating blocks. However, they use amplifier circuits in each noise generating block which will distort the noise spectrum because of the limited bandwidth of the amplifiers. Other circuits employ differential circuits on two sources of random noise and then amplify the resulting noise to the level required by voltage comparator. The main drawback of this approach is that the required gain level of amplification is very high (several orders of magnitude) and the resulting reduction in the bandwidth of the amplifier (note that because the product of the gain and the bandwidth of an amplifier is about constant). Such reduction in amplifier bandwidth increases the signal correlation and reduces the randomness of the generated noise.
SUMMARY OF THE INVENTION
• The present invention provides solutions including a Noise Generating Unit (NGU) comprising a highly doped diode to make a low breakdown voltage surface zener diode and an automatic avalanche current control loop, allowing the entire noise source to be embedded into and compatible with other random noise generator circuits. Further, the present invention will utilize the high noise signal of the abovementioned NGU and construct a Stochastic Noise Amplification apparatus comprising multi-NGU structures that is capable of not only neutralizing effects of common causes of variability (such as local thermal effects), but also of naturally amplifying the magnitude of noise to the level where only minimal gain of amplification is needed, if at all. The Stochastic Noise Amplification apparatus allows the output noise level to be amplified by a factor of 1.414 (square root of 2) when two identical NGU's pass through one differential amplifier. If the noise level is amplified by multiple stages, n, the noise level can be increased by the factor of 1.414ⁿ, while keeping the same total magnitude of amplification.
• The present invention provides a true noise generator including a low breakdown voltage surface zener diode, an automatic avalanche current control loop, an algorithm for automatic voltage regulation, and an apparatus to increase noise level with multiple differential amplification stages. The present invention concerns designing an on-chip true noise generator including an embedded noise source with a low-voltage, high-noise zener diode(s), and an in-situ close-loop zener diode power control circuit for optimization between performance and reliability. In order to reduce operating voltage so that it can be used in the ASIC library, the present invention proposes the use of heavily doped polysilicon and silicon p-n diode(s) structures for a surface zener diode that minimizes the breakdown voltage, increase noise level and improves reliability. The present invention also proposes an in-situ close-loop zener diode control circuit to optimize performance while also safe-guard the zener diode from catastrophic burn-out. The present invention further proposes an algorithm or methodology to teach the procedure for optimizing between the noise generating performance and the reliability of the zener diode. Furthermore, the present invention also proposes a Stochastic Noise Amplification apparatus to amplify the noise level and at the same time neutralize effects of common causes of variability (such as local thermal effects).
• The present invention forms an on-chip physical noise source for random noise generation, which can be integrated and fabricated in any standard CMOS or BiCMOS circuits. Furthermore, the present invention embeds a noise source having control and protection circuits which facilitate stable noise output and long operating lifetime. The present invention also generates white noises directly from one pair of differential embedded noise sources. Moreover, the present invention generates true random noise from multiple parallel pairs of signal noise amplification.
• In a first aspect of the invention, there is an on-chip semiconductor structure forming a low breakdown voltage surface zener diode with either single- or multi-finger configuration as the embedded noise source.
• In a second aspect of the invention, there is a an automatic avalanche current control loop to regulate the supply voltage for the low breakdown surface zener diode for optimizing between noise generating performance and device reliability.
• In a third aspect of the invention, there is a methodology to conduct supply voltage regulation for automatic avalanche current control in order to optimize the performance and reliability of the noise generating source.
• In a fourth aspect of the invention, there is a stochastic noise amplification apparatus with paired noise generating source to amplify the noise level and at the same time neutralize the effects of common causes of variability.
BRIEF DESCRIPTION OF THE DRAWINGS
• The features and elements of the present invention are set forth with respect to the appended claims and illustrated in the drawings.
• FIG. 1A illustrates top view of a heavily doped polysilicon and silicon p-n zener diode.
• FIG. 1B illustrates a cross section of a heavily doped polysilicon and silicon p-n zener diode.
• FIG. 2 illustrates the voltage characteristics of a heavily doped polysilicon and silicon p-n zener.
• FIG. 3 illustrates a block diagram of a noise generating unit or NGU.
• FIG. 4 illustrates a preferred embodiment of an adjustable voltage source according to aspects of the present invention.
• FIG. 5 illustrates the circuit of a current probe according to aspects of the present invention.
• FIG. 6 illustrates a current monitor according to aspects of the present invention.
• FIG. 7 illustrates a first embodiment of a stochastic noise amplification apparatus to increase the noise source according to aspects of the present invention.
• FIG. 8 illustrates a second embodiment of a stochastic noise amplification apparatus to increase the noise source according to aspects of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The present invention provides a true noise generator including a differential zener diode pair and a stochastic noise amplifier. The present invention concerns designing an on-chip true noise generator including an embedded noise source with a low-voltage, high-noise zener diode(s), and an in-situ close-loop zener diode current control circuit. In order to reduce operating voltage so that it can be used in the ASIC library, the present invention proposes the use of heavily doped polysilicon and silicon p-n diode(s) structures to minimize the breakdown voltage, increasing noise level and improving reliability. The present invention also proposes an in-situ close-loop zener diode current control circuit to safe-guard the zener diode from catastrophic burn-out.
• The present invention forms an on-chip physical noise source for random noise generation, which can be integrated and fabricated in any standard CMOS or BiCMOS circuits. Furthermore, the present invention embeds a noise source having control and protection circuits which facilitate stable noise output and long operating lifetime. The present invention also generates white noises directly from one pair of differential embedded noise sources. Moreover, the present invention generates true random noise from multiple stage signal noise amplification.
• The following describes embodiments of the present invention with reference to the drawings. The embodiments are illustrations of the invention, which can be embodied in various forms. The present invention is not limited to the embodiments described below, rather representative for teaching one skilled in the art how to make and use it. Some aspects of the drawings repeat from one drawing to the next. The aspects retain their same numbering from their first appearance throughout each of the preceding drawings.
• FIG. 1A illustrates top view of a heavily doped polysilicon and siliconp-n zener diode300 fabricated on asubstrate310.Zener diode300, is formed on asubstrate310 and comprises a heavily doped silicon layer320 (e.g. heavily P-type doped), with a heavily doped (e.g. heavily N-type doped)polysilicon layer330 periodically formed on thesilicon layer320. Thezener diode300 is a surface diode, which is well known to be noisier than typical buried zener diode. The low breakdown voltage of the p-n junction is due to the heavily doped polysilicon widely employed in the polysilicon emitter of bipolar transistors in recent BiCMOS technologies. The proposed low breakdown voltagesurface zener diode300 has a PN junction on the primary surface of a P-dopedsilicon substrate320, where the diode is formed between a heavily P-doped (>10¹⁸cm⁻³)silicon layer320 on the surface and a heavily N-doped (>10¹⁸cm⁻³)polysilicon feature330 on the surface of thesilicon layer320. The P and N region are both heavily doped in order to lower the breakdown voltage for the application. Both thesilicon layer320 and the heavily N-doped polysilicon (poly)layer330 have silicidecontacts340.
• FIG. 1B illustrates a cross section of a heavily doped polysilicon and siliconp-n zener diode300. The cross section is from sections X to X′ ofFIG. 1A. As shown the N-dopedpolysilicon layer330 is located above and in contact with thesilicon layer320.Silicon layer320 is heavily doped with p-type material such as boron (B).N Poly layer330 is heavily doped with n-type materials such as arsenic (As). Aspacer350 is located around N-dopedpoly layer330. Thespacer350 may be a protective silicon oxide ornitride spacer350 and may be formed to protect the sidewall of the raisedpolysilicon feature330 and to further increase noise. Silicide regions (340) (such as Ti silicide, Co silicide, etc.) are formed to provide electrical contacts for anode (+) and cathode (−). Note that each discrete surface zener diode comprises a pair of anode/cathode, and it can also be contemplated that a “multi-finger” structure with multiple discrete diodes formed together to increase current and noise level, as shown inFIG. 1A.
• FIG. 2 illustrates the voltage characteristics of a heavily doped polysilicon and silicon p-n zener. The y axis illustrates the current flow throughzener diode300 and the x axis illustrates the voltage across thezener diode300. As can be seen from the graph the avalanche range of thezener diode300 is approximately 1.5 volts before reaching the breakdown condition. The inventors have determined that by operating thezener diode300 in the avalanche range close to breakdown condition, it will produce a noise signal upon the dc voltage across thezener diode300. The inventors have also determined that by maintaining a current range across thezener diode300 in the range of 100 to 300 nAmps, the zener diode will stay in the avalanche range. As can be seen the avalanche range is well within the operating voltages of a modern semiconductor.
• The block diagram shown inFIG. 3 illustrates a Noise Generating Unit10 (NGU) including anadjustable voltage source100, acurrent probe200, azener diode300 and acurrent monitor400. TheNGU10 operates to control the voltage across thezener diode300 and ensures that thezener diode300 is only biased at the avalanche current range discussed above. When thezener diode300 is placed in the avalanche region before the breakdown occurs, thezener diode300 introduces significantly higher noise signal onto the voltage. The close-loop control system avoids the catastrophic failure normally seen in a single voltage controlled noise sources because of possible thermal run-away if the device is controlled by voltage when temperature rises. The voltage at the input to the zener diode is provided to a capacitor C₁which provides the output noise signal Vnoise forNGU10.
• FIG. 4 shows a preferred embodiment of theadjustable voltage source100.Operational amplifier110 andresistors106,108,112 and114 form a voltage adder.Resistors112 and114 may have the same resistance andresistors106 and108 may have the same resistance such that the output voltage is V_supply=2*(V₀+V₁). Wherein V₀is a fixed voltage, which is selected such that 2V₀does not cause breakdown inzener diode300. V₁is an adjustable voltage , which is directly related to the current throughzener diode300. V₁is the output voltage of digital to analog converter (DAC)104. The input ofDAC104 is the output ofcounter102; thus V₁is determined by the content ofcounter102.Counter102 operates in the increment or decrement mode when a rising edge of logic low to logic high is applied to the input of either increase INC or decrease DEC. The content ofcounter102 is increased by one when a rising edge of logic high is applied to the input of increase INC. When a rising edge of logic high is applied to the input of decrease DEC, the content ofcounter102 is decreased by one.Operational amplifier110 should have the capability to drive the avalanche current of thezener diode300. As an avalanche current has quite a steep slope versus the voltage applied on thezener diode300, V_supply is the sum of V₀and V₁to reach the high adjustment resolution for a given bit of DAC and to avoid the possible oscillation. Setting V_supply depends on the real device. As illustrated the zener diode breaks down at 1.5V with safe operating range from 1.4V to 1.6V. Therefore can set V₀at 0.6V, with V₁having a range of operation from 0.1V to 0.2V range.
• In another embodiment as shown inFIG. 4, an additional input tooperational amplifier110 may be an output from a NGU15 from a different NGU. The NGU15 is input tooperational amplifier110 through resistor116. The addition of the input from NGU15 adds a noise signal to V_supply which will ultimately add to the unpredictability of the noise output ofzener diode300.
• The circuit ofcurrent probe200 is shown inFIG. 5. Two p-type metal-oxide-semiconductor field-effect transistors (MOSFETs)202 and204 form a current minor that minors the current ofzener diode300 and provides current tocurrent monitor400.Resistor206 is the load of the noise generated byzener diode300. Terminal a is connected tozener diode300 and terminal b is connected tocurrent monitor400.
• Current monitor400 is shown inFIG. 6, whereinoperational amplifier404 andresistor402 form a trans-impedance amplifier, which converts the input current to the output voltage V₂. V_bis the positive input bias voltage, which sets a positive bias output voltage of V₂. The output voltage V₂decreases with increasing sink current fromcurrent probe200.Voltage comparators406 and408 have predefined threshold voltages V_th1and V_th2and also receive V₂as input. ANDgates410 and412 receive outputs c and d fromvoltage comparators406 and408 respectively. V_th1is the corresponding threshold voltage of the upper limit of the avalanche current. When the avalanche current is increased, the trans-impedance amplifier output voltage V₂becomes lower than V_th1, and the output ofvoltage comparator406 becomes logic high. Consequently, ANDgate410 outputs the clock pulse p− to reduce the voltage V_supply in order to protect thezener diode300 from damage. V_th2is the corresponding threshold voltage of the lower limit of the avalanche current when the avalanche current is decreased, V₂becomes higher than V_th2. The output ofvoltage comparator408 becomes logic high and ANDgate412 outputs the clock pulse p+ to increase the voltage V_supply in order to keepzener diode300 within the avalanche condition. When V₂is between V_th1and V_th2, there is no pulse from either ANDgates410 or412, and thus V_supply stays unchanged. Vb is usually selected as the half value of the power supply voltage of404,406,408. The operation ofcurrent monitor400 is shown in Table 1 below:

• TABLE 1
  Vth1< Vth2

  Case 1: V1< Vth1→ c = H → p− → decrease V_supply

  Case 2: V1> Vth2→ d = H → p+ → increase V_supply

  Case 3: Vth1< V1< Vth2→ c = L & d = L → V_supply no change

• FIG. 7 illustrates a first embodiment of a stochasticnoise amplification apparatus700 to increase the noise source according to aspects of the present invention. Stochasticnoise amplification apparatus700 receives an input from two NGU's10 and20. The two inputs are provided tooperational amplifier710.NGU10 may be input into the positive input ofoperational amplifier710.NGU20 may be input into the negative input ofoperational amplifier710. The resulting noise signal through this single-stage differential amplification will be more non-deterministic, sporadic, and categorically not intermittent (i.e. random). The inventors have determined that the addition of two NGU signals increased the randomness of the signal by the square root two (√{square root over (2)}=1.414) times.
• FIG. 8 illustrates a three-stage differential amplification as an example of a second embodiment of a stochastic noise amplification apparatus800 to further increase the noise source according to aspects of the present invention. The inputs to apparatus800 are eight NGU inputs from NGU's10,20,30,40,50,60,70 and80.NGU10 may be input into the positive input ofoperational amplifier810.NGU20 may be input into the negative input ofoperational amplifier810.NGU30 may be input into the positive input ofoperational amplifier820.NGU40 may be input into the negative input ofoperational amplifier820.NGU50 may be input into the positive input ofoperational amplifier830.NGU60 may be input into the negative input ofoperational amplifier830.NGU70 may be input into the positive input ofoperational amplifier840.NGU80 may be input into the negative input ofoperational amplifier840.
• The output ofoperational amplifier810 may be input into the positive input ofoperational amplifier850. The output ofoperational amplifier820 may be input into the negative input ofoperational amplifier850. The output ofoperational amplifier830 may be input into the positive input ofoperational amplifier860. The output ofoperational amplifier840 may be input into the negative input ofoperational amplifier860. The output ofoperational amplifier850 may be input into the positive input ofoperational amplifier870. The output ofoperational amplifier860 may be input into the negative input ofoperational amplifier870. Finally the output ofamplifier870 incorporates the noise signal from each of the NGU inputs. The resultant noise signal through this exemplary three-stage differential amplification is increased by a factor of 2.8 ((√{square root over (2)})³=1.414³) from the noise signal through a single NGU, and the power of the noise signal is increased by a factor of 8 (2³) from that through a single NGU. Furthermore, this embodiment can be generalized to include an N-stage differential amplification, where the resultant noise signal through an N-stage differential amplification is increased by a factor of (√{square root over (2)})^N=1.414^Ncompared with a single NGU, and its power is increased by a factor of 2^N.
• The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.

Claims (20)

What is claimed is:

1. An apparatus comprising:

a zener diode located on a substrate;

a voltage source located on the substrate to provide a supply voltage to the zener diode; and

a current monitor located on the substrate to monitor the current through the zener diode and adjust the supply voltage provide by the voltage source to maintain the zener diode in the avalanche zone close to the breakdown condition, wherein the zener diode provides a random noise output.

2. The apparatus ofclaim 1 further comprising a current probe, wherein the current probe receives the supply voltage and provides a voltage to the zener diode, the current probe providing a current mirroring the current through the zener diode to the current monitor.

3. The apparatus ofclaim 1, wherein the zener diode is a heavily doped pn junction zener diode.

4. The apparatus ofclaim 1, wherein voltage source comprises a counter and a digital to analog converter wherein the digital to analog converter provides a threshold voltage to an amplifier, the amplifier providing the supply voltage.

5. The apparatus ofclaim 3, wherein the zener diode comprises a multi-finger structure.

6. The apparatus ofclaim 1, wherein the avalanche zone is around 1.5 volts.

7. The apparatus ofclaim 2, wherein the current monitor comprises a trans-impedance amplifier to monitor the current from the current probe and provide an output voltage, wherein the current monitor provides a increment signal to the voltage source when the output voltage is below a first threshold, and a decrement signal to the voltage source when the output voltage is above a second threshold.

8. The apparatus ofclaim 4, wherein a noise signal is provided to amplifier in the voltage source.

9. An apparatus comprising:

a first and a second noise generating unit; and

a single-stage differential stochastic amplifier, wherein the first noise generating unit is input into a negative input of the amplifier, and the second noise generating unit is input into the positive input of the amplifier.

10. The apparatus ofclaim 9, wherein each of the noise generating unit comprises:

a zener diode located on a substrate;

a voltage source located on the substrate to provide a supply voltage to the zener diode; and

a current monitor located on the substrate to monitor the current through the zener diode and adjust the supply voltage provide by the voltage source to maintain the zener diode in the avalanchezone, wherein the zener diode provides a random noise output.

11. The apparatus ofclaim 10, wherein the zener diode is a heavily doped pn junction zener diode.

12. The apparatus ofclaim 11, wherein voltage source comprises a counter and a digital to analog converter wherein the digital to analog converter provides a threshold voltage to an amplifier, the amplifier providing the supply voltage.

13. The apparatus ofclaim 12, wherein the zener diode comprises a multi-finger structure.

14. The apparatus ofclaim 11, wherein the avalanche zone is around 1.5 volts.

15. An apparatus comprising:

A plurality of noise generating unit pairs, each of said pair of noise generating units having a first and a second noise generating units;

a first set of single-stage differential stochastic amplifiers, wherein each of said noise generating units pairs is input into one of the first set of single-stage differential stochastic amplifiers the first noise generating unit is input into a negative input of one of the first set of single-stage differential stochastic amplifiers, and the second noise generating unit is input into the positive input of one of the first set of single-stage differential stochastic amplifiers; and

at least a second single-stage differential stochastic amplifier receiving an input from two of the first set of single-stage differential stochastic amplifiers.

16. The apparatus ofclaim 15 wherein each of the noise generating unit comprises:

a zener diode located on a substrate;

a voltage source located on the substrate to provide a supply voltage to the zener diode; and

a current monitor located on the substrate to monitor the current through the zener diode and adjust the supply voltage provide by the voltage source to maintain the zener diode in the avalanche zone, wherein the zener diode provides a random noise output.

17. The apparatus ofclaim 16, wherein the zener diode is a heavily doped pn junction zener diode.

18. The apparatus ofclaim 17, wherein voltage source comprises a counter and a digital to analog converter wherein the digital to analog converter provides a threshold voltage to an amplifier, the amplifier providing the supply voltage.

19. The apparatus ofclaim 18, wherein the zener diode comprises a multi-finger structure.

20. The apparatus ofclaim 17, wherein the avalanche zone is around 1.5 volts.

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