Gate bias circuit for MOS Charge Coupled Devices
A biasing circuit for use with a Charged Coupled Device (CCD) that creates a gate bias voltage by maintaining a model or surrogate representation of the surface potentials within the CCD storage and barrier regions. In one embodiment the invention is a bias circuit that includes at least a first and second model transistor for modeling the two regions. The first model transistor is connected to a supply voltage to provide a first reference voltage at a first node, and models the first charge storage region. A resistive circuit element is coupled between the first node N 1 and a second node N 2 in order to allow a step voltage to be developed. The second model transistor is in turn connected to the second node N 2 and provides the bias voltage at an output portion that can be used to control the gate of the barrier region. The model circuit therefore allows a proper bias voltage to be maintained through process and operating condition variations.
1. A charge transfer device comprising:
a semiconductor substrate;
a first charge transfer region formed in said semiconductor substrate, said first charge transfer region having a first input source, a first gate electrode, and a first output portion;
a second charge transfer region coupled to the first charged transfer region and also formed in said semiconductor substrate, said second charge transfer region having a second input source coupled to said first output portion, a second gate input electrode in a second output portion; and
a bias circuit for accepting a control voltage reference signal applied to the first charge transfer region, and for developing at least a first reference voltage φ A , the reference voltage corresponding to a surface potential of at least a portion of the first charge transfer region, and also developing therefrom a second reference voltage, φ B , the second reference voltage representing a surface potential of the second charge transfer region, and for further deriving a bias voltage to be applied to control the second charge transfer region.
2. A device as in claim 1 wherein the bias circuit additionally comprises:
a first semiconductor model transistor connected to provide the first reference voltage φ A at a first node N 1 , the first semiconductor transistor representing a model of the first charge transfer region such that the reference voltage φ A at node N 1 corresponds to surface potential under the first charge transfer region;
a resistive circuit element, R, coupled between the first node N 1 and a second node N 2 , such that a step voltage Vstep is maintained therebetween;
a second semiconductor model transistor connected to the second node N 2 and providing the bias voltage Vbias, such that the second semiconductor transistor represents a model of the second charge region, and so that the node N 2 is maintained at a voltage indicative of the surface potential under the second charge transfer region; and
wherein the bias voltage Vbias is in turn applied to control the second gate electrode of the second charge transfer region.
3. A device as in claim 1 wherein the bias circuit uses metal oxide semiconductor transistors.
4. A device as in claim 1 wherein the bias circuit is formed on the same substrate as first and second charge transfer regions.
5. A device as in claim 1 wherein the first transistor is a Metal Oxide Semiconductor (MOS) device with a gate terminal connected to a source terminal, and so that a bias voltage Vgsa developed across the gate and drain thereof corresponds to a threshold voltage of the first charge transfer region.
6. A device as in claim 1 wherein at least one of the model transistors is formed on the same substrate as at least one of the charge transfer regions.
7. A device as in claim 5 wherein the gate and source terminal of the first model transistor are connected to a supply voltage reference Vdd.
8. A device as in claim 7 wherein a bias current source maintains a step voltage Vstep across resistor R.
9. A device as in claim 1 wherein the drain of second model transistor is connected to a second node N 2 , and the gate and source terminal of the second model transistor are coupled together to provide the bias voltage, Vbias.
10. A device as in claim 1 wherein the first model transistor is biased at a threshold value by a current source, such that its gate source voltage Vgs is the same as a threshold voltage of the first charge transfer device.
11. A device as in claim 9 wherein the voltage provided at the output of the first model transistor at node N 1 is a representation of the surface potential beneath the first charge transfer region.
12. A device as in claim 1 wherein the voltage at node N 2 is a representation of the surface potential beneath the second charge transfer region.
13. A device as in claim 10 wherein the second model transistor is biased at a threshold point to establish a potential increase in voltage from node N 2 to the bias voltage Vbias.
14. A device as in claim 1 wherein the first charge transfer region is a charge storage device.
15. A device as in claim 1 wherein the second charge transfer region is a barrier device.