Method and circuit for compensating MOSFET capacitance variations in integrated circuits

• US 20030184315A1
• Filed: 04/10/2002
• Published: 10/02/2003
• Est. Priority Date: 04/02/2002
• Status: Active Grant

• Current Assignee: Dialog Semiconductor GmbH (Renesas Electronics Corporation)
• Original Assignee: Dialog Semiconductor GmbH (Renesas Electronics Corporation)
• Inventors: Eberlein, Matthias
• Granted Patent: US 6,774,644 B2
• Time in Patent Office: Days
• US Class Current: 324/658
• CPC Class Codes: H03K 3/011 Modifications of generator to compensate for variations in physical values, e.g. voltage, temperature to maintain energy constant H03K3/015
• CPC Class Codes: H03K 3/0231 Astable circuits H03K3/0315 takes precedence

Abstract

A method for tracking the MOS oxide thickness by the native threshold voltage of a “native” MOS transistor without channel implantation for the purpose of compensating MOS capacitance variations is achieved. The invention makes use of the fact that in MOS devices the threshold voltage is proportionally correlated to the oxide thickness of said MOS device. Said threshold voltage can therefore be used to build a reference voltage Vx+Vth which accurately tracks the MOS capacitance variations in integrated circuits. Circuits are achieved to create a frequency reference and a capacitance reference using said method. Additionally a method is introduced to create a capacitance reference in integrated circuits using said MOSFET capacitors.

Claims

1. A method for tracking the MOS oxide thickness by the native threshold voltage Vth of a “

native”

MOS transistor for the purpose of compensating MOS capacitance variations comprising;

provide a MOS transistor having a native threshold voltage Vth;

sense the native threshold voltage Vth of a native MOS transistor;

build a suitable reference voltage level by adding by a circuit means another constant voltage Vx;

charge a MOS capacitor built out of a native MOS transistor up to the reference level Vref=Vx+Vth; and

measure a capacitance related value in a suitable configuration for oscillators which depends now no more on the oxide thickness.

2. The method of claim 1 wherein said MOS transistor is of NMOS type.

3. The method of claim 1 wherein said MOS transistor is of PMOS type

4. The method of claim 1 wherein said capacitance related value measured is the charging time of said MOS capacitor.

5. The method of claim 1 wherein said capacitance related value measured is the resulting frequency.

6. The method of claim 1 wherein said suitable configuration for oscillators is a feedback configuration.

7. The method of claim 1 wherein said suitable configuration for oscillators is a repeating configuration.

8. A circuit for a frequency reference in an integrated circuit using a MOSFET as a stable capacitor, comprising

a constant current source;

switching means charging and discharging said MOSFET capacitor being activated periodically by the output of comparing means and charging and discharging said MOSFET capacitance;

comparing means having an input and an output wherein the input is the voltage of said capacitor and a voltage exceeding the threshold voltage of said MOSFET capacitor to influence the frequency and the amplitude of the output of said comparing means and the output is a periodical pulse;

a MOSFET capacitor being charged/discharged periodically by said constant current source and its voltage level being the input to said comparing means and its threshold voltage is used to compensate the variations of its capacitance;

means to provide a voltage exceeding the threshold voltage of said MOSFET to influence the frequency and the amplitude of the output of said comparing means; and

means to transform the periodical pulse into an alternating clock signal to drive said switching means and being the output of said frequency reference circuit.

9. The circuit of claim 8 wherein said switching means is performed by transistors.

10. The circuit of claim 8 wherein said stable current source is using a reference voltage and an resistor external to the integrated circuit.

11. The circuit of claim 8 wherein said comparing means is a pair of transistors.

12. The circuit of claim 8 wherein said MOSFET capacitor is a pair of MOSFETs which are being alternating charged and uncharged.

13. The circuit of claim 8 wherein said means to provide a voltage exceeding the threshold voltage of said MOSFET capacitor to influence the frequency and the amplitude of the output of said comparing means are a voltage divider and an amplifier having an input and an output wherein the input is the output of said voltage divider and a reference voltage and the output is said voltage as input of said comparing means.

14. The circuit of claim 8 wherein a pulse former and inverters are used to transform the output pulses of the comparing means into alternating clock signals being the input to said switching means.

15. A circuit to use a MOSFET capacitor as a reference capacitor in an integrated circuit comprising:

an RC-oscillator circuit having an input and an output wherein the input is a current source and a signal from a phase detector regulating the capacitance of said RC oscillator and the output is a periodical signal;

a frequency reference circuit having an input and an output wherein the input is a current source and the output is a periodical signal;

a phase detector circuit having an input and an output wherein the input are the periodical output signals from said RC oscillator and from said frequency reference and the output is a signal to regulate the capacitance of the RC oscillator and the capacitance of the capacitor to become a reference capacitor; and

said capacitor to become a reference capacitor.

16. The circuit of claim 15 wherein said frequency reference circuit comprises:

a constant current source;

switching means charging and discharging said MOSFET capacitors being activated periodically by the output of comparing means and charging and discharging said MOSFET capacitor;

comparing means having an input and an output wherein the input is the voltage of said MOSFET capacitor and a voltage exceeding the threshold voltage of said MOSFET to influence the frequency and the amplitude of the output of said comparing means and the output is a periodical pulse;

a MOSFET capacitor being charged and discharged periodically by said constant current source and its voltage level being the input to said comparing means and its threshold voltage is used to compensate the variations of its capacitance;

means to provide a voltage exceeding the threshold voltage of said MOSFET to influence the frequency and the amplitude of the output of said comparing means; and

means to transform the periodical pulse into an alternating clock signal to drive said switching means and being the output of said frequency reference circuit.

17. The circuit of claim 16 wherein said switching means are transistors.

18. The circuit of claim 16 wherein said stable current source is using a reference voltage and an resistor external to the integrated circuit.

19. The circuit of claim 16 wherein said comparing means are a pair of transistors.

20. The circuit of claim 16 wherein said MOSFET capacitor is a pair of MOSFETs which are alternating charged and discharged.

21. The circuit of claim 16 wherein said means to provide a voltage exceeding the threshold voltage of said MOSFET to influence the frequency and the amplitude of the output of said comparing means are a voltage divider and an amplifier having an input and an output wherein the input is the output of said voltage divider and a reference voltage and the output is said voltage as input of said comparing means.

22. The circuit of claim 16 wherein the said capacitor is a pair of MOSFETs.

23. The circuit of claim 16 wherein a flip-flop circuit and inverters are used to transform the output pulses of the comparing means into alternating clock signals being the input to said switching means forming the output of said frequency reference circuit.

24. A method for creating a capacitor reference in integrated circuits comprising:

providing an RC oscillator circuit, a constant current source, a frequency reference, a phase detector and a capacitor to become a reference capacitor;

generate with said RC oscillator circuit a periodic signal with frequency f₁;

generate with said frequency reference circuit a signal with frequency f₂;

compare said frequency f₁ and frequency f₂ and regulate the capacitance of the capacitor of said RC oscillator until f₁ equals f₂;

calculate the capacitance of the reference capacitor using an equation; and

use the calculated capacitance as reference capacitance.

25. The method of claim 24 wherein the equation to calculate the capacitance of said capacitor is: