Circuit construction for protective biasing
This application is a file-wrapper continuation of application Ser. No. 08/222,316, filed Apr. 1, 1994, which is a continuation application of Ser. No. 07/848,754, filed Mar. 10, 1992; both now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains to a circuit for protecting circuit elements from reverse voltage. More particularly, the present invention pertains to a circuit for preventing damage to circuit elements, such as integrated circuits, when a reverse voltage appears at a power supply terminal. 
     BACKGROUND OF THE INVENTION 
     In electronic circuits designed to receive supply voltages at two input terminals herein called V+ and V-, the V+ terminal is intended to receive a higher (i.e., more positive) potential than the V- terminal in normal operation. Generally, the V+ terminal receives a positive voltage, relative to a reference ground, and the V- terminal receives a negative voltage; or both voltages may be of the same polarity but different in magnitude. In some cases, however, it is possible for the supply voltage connections to be reversed, for instance, by the attachment of a supply connector in reverse. Unfortunately, when the supply voltage to an electronic circuit is reversed, the electronic circuit may be damaged. Many types of circuits may be damaged by supply reversal, especially junction isolated integrated circuits. 
     FIG. 1 shows a partly isometric, partly cross-sectional view of a part of a typical prior art integrated circuit, which includes a p-type substrate 10, a buried layer 12 of n+ type material, an n-type epitaxial layer 14, an n+ type diffusion 20 formed in the epitaxial layer 14 to act as a contact, and p-type isolation bands 16. The buried layer 12, epitaxial layer 14 (enclosed within junction isolation bands 16) and diffusion 20 are commonly referred to as the &#34;pocket&#34;. The pocket is typically biased as close to V+ as possible while the p-type isolation bands are biased at V-. The p-n junction formed between the pocket and the isolation bands 16 is thus reverse biased. If supply reversal occurs, this junction becomes forward biased and the circuit will be damaged. To prevent forward biasing of this junction in this case, the pocket is often connected to the V+ supply via a diode (not shown). When supply reversal occurs, the diode disconnects the pocket from the power supply and this junction is not forward biased. Unfortunately, some circuit elements formed in such n-type pockets require biasing the pocket closer to the V+ supply than a diode allows. Two examples of such circuit elements are thin film resistors and diffused resistors. 
     Also shown in FIG. 1 is a thin film resistor 18 formed on an oxide layer (not shown) on the upper surface of the epitaxial layer 14. With such a thin film resistor 18, if the pocket is biased more negative than the resistor, for example, by a diode offset, the electrical field generated from the voltage differential may cause long term corrosion of the thin film resistor 18. Therefore, a diode cannot be used to connect the pocket to the V+ supply. However, a direct connection between the pocket and the V+ supply will result in damage to the circuit when supply reversal occurs. 
     FIG. 2 shows a partly isometric, partly cross-sectional view of a typical prior art integrated circuit including a p-type diffusion resistor 22. The construction of this integrated circuit is otherwise generally similar to that of FIG. 1. With such a construction, the resulting p-n junction between the resistor 22 and the n-type pocket must remain reverse biased. Because a voltage differential of a diode offset would be sufficient to forward bias this junction (assuming the resistor is connected to the V+ supply), a diode cannot be used to connect the pocket to the V+ supply. However, a direct connection between the pocket and the V+ supply will result in damage to the circuit when supply reversal occurs. 
     Connection of n-type pockets directly to a positive (V+) supply terminal ordinarily is not a problem. However, for automotive circuits or other products which require protection against negative voltages on the positive supply terminal, connection of n-type pockets to the positive (V+) supply terminal can create problems. A reverse voltage causes the junction formed between the p-type isolation bands and substrate and the n-type pocket to be forward biased. This results in clamping of the positive supply terminal to within less than a volt of the negative supply terminal. In the case of automotive circuits, large currents are available during field decay events and when a battery is connected in reverse. The integrated circuit, or more likely the bond wires, will be completely destroyed by such a reverse connection of a battery or during field decay events. 
     Other types of integrated circuits can also be damaged by supply reversal. The present invention may be readily applied to such circuits as well. 
     Accordingly, it is an object of the present invention to provide a circuit construction for protective biasing of integrated circuit elements. 
     Another object of the invention is to provide a circuit means which provides transistor isolation and prevents injection of unwanted carriers when a negative voltage is applied to the positive supply terminal. 
     Yet another object of the invention is to provide a circuit means which protects integrated circuit elements from reverse supply connection, and in particular, protects automotive circuits from reverse connection of a battery. 
     SUMMARY OF THE INVENTION 
     The foregoing and other objects of the present invention are achieved by using the collector voltage of a saturated pnp transistor, which has its emitter connected to the more positive supply voltage V+, to bias the pocket enclosing a circuit element close to the V+ supply. When the supply voltage is reversed, the emitter-base junction of the transistor becomes reverse biased, allowing its collector-base junction to turn off. As a consequence, the pocket enclosing a circuit element becomes disconnected from the V+ supply. Therefore, the p-n junction between the n-type pocket and the p-type junction isolation bands and substrate does not become forward biased. Moreover, for p-type diffused resistors in an n-type pocket, the resistor can be driven negative. The pnp transistor which biases the pocket may also be provided with an additional collector to handle leakage of carriers to the isolation bands during saturation. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing, 
     FIG. 1 is an isometric, cross-sectional view of a typical thin-film resistor formed in a typical prior art integrated circuit; 
     FIG. 2 is an isometric, cross-sectional view of a typical diffused resistor formed in a typical prior art integrated circuit; 
     FIG. 3 is an isometric, cross-sectional view of a lateral pnp transistor to be used in accordance with the present invention; 
     FIG. 4 is a schematic diagram of an exemplary circuit according to the present invention for use in protecting a resistor; 
     FIG. 5 is an isometric, cross-sectional view of a lateral pnp transistor incorporating an additional ring of material; and 
     FIG. 6 is a schematic diagram of an exemplary circuit according to the present invention connected to the drain of an epi-FET. 
    
    
     DETAILED DESCRIPTION 
     An embodiment of the invention will now be described in detail in connection with FIGS. 3-6. Similar reference numbers in the figures indicate similar structures. 
     FIG. 3 shows a lateral pnp transistor having a base defined by n-type regions 12, 14, and 24, a collector 26 and an emitter 28. The n+ type region 24 provides a connector to the base. 
     Referring now to FIG. 4, the pnp transistor 40 schematically represents the transistor shown in FIG. 3. The emitter 28 of this transistor is connected to a supply rail 32 which, in normal operation, receives a V+ supply voltage from a supply 42. A resistor 36 is connected between the base 24 and supply rail 38 which, in normal operation, receives a V- supply voltage. The resistor 36 is used as a drive to cause the transistor 40 to saturate. The collector 26 of transistor 40 is connected to a pocket enclosing a circuit element (a diffused resistor 43, for example) to bias the pocket with the saturation voltage V CESAT . Resistor 43 is connected between the V+ supply rail 32 and a circuit 45 (via conductive traces, not shown). The details of circuit 45 are irrelevant to this discussion since numerous circuits may be employed. If the resistor 43 is a p-type diffused resistor (as shown in FIG. 2), it forms a diode 46 with the pocket. P-type substrate 10 and isolation diffusion regions 16 of the circuit, such as shown in FIG. 2, form a diode 44 with the pocket defined by regions 12, 14 and 20. These p-type regions 10 and 16 are normally connected to the V- supply rail 38, so that the p-n junction between p-type regions 10 and 16 and the pocket (n-type regions 12, 14 and 20) is reversed biased. 
     When the supply voltage is reversed, for example in an automotive circuit when an automobile battery is improperly connected with its terminals reversed, the lower potential appears on rail 32, and the emitter-base junction of transistor 40 becomes reverse biased, causing its collector-base junction to turn off. As a consequence, the pocket enclosing resistor 43 is disconnected from the voltage on rail 32. Therefore, the p-n junction between the n-type pocket and p-type isolation bands 16 and region 10 is not forward biased, thus protecting the pocket from damage. Moreover, the p-type resistor 43 can be driven negative without damage by the reversed voltage appearing on rail 32. Because the pnp transistor 40 (FIG. 4) operates in saturation, an undesirable current occurs due to the leakage of carriers to the p-type regions 10 and 16, as represented at arrow A in FIG. 3. To eliminate the undesirable current, a ring of material 30 (FIG. 5) of the same type of material (e.g., p-type) as collector 26 surrounds the collector 26 to intercept carriers and return the corresponding current to the base 24 by an appropriate interconnection (such as via conductive traces, not shown). This interconnection is shown schematically in FIG. 6 at node 34. The interception of carriers by the ring of material 30 diminishes the drive current to the transistor base 24, so as to minimize excess emitter current. The emitter current thus consists mostly of the drive current from the drive resistor 36 (FIG. 4), or other current-limited drive which may be used, and any load current delivered by the collector. Without the ring of material 30, a large current would be drawn at the emitter 28 and delivered to the substrate 10. If the extra emitter current does not create a problem, the ring of material 30 may be disconnected from the base and used to provide current to operate other circuits at lower voltage, or simply as an indication that the pnp collector is saturated. 
     Referring now to FIG. 6, the pnp transistor 40 may also be used to provide functions other than the disconnect function described above. The base current used to bias the pnp transistor 40 in an &#34;on&#34; state, can also be used internally in the circuit. Furthermore, the disconnect feature can be used to protect other structures, such as an FET 48 and/or a control circuit 50. In FIG. 6, the resistor 36 of FIG. 1 has been replaced by an epi-FET 48 in series with control circuit 50. The drain of an epi-FET, used for startup or some other purpose, cannot be connected directly to the supply voltage V+, since it would be forward biased to the substrate by a reverse transient. By combining the epi-FET 48 with the pnp transistor 40, the startup current is allowed to bias the pnp transistor 40 while the disconnect feature protects the epi-FET 48. The control circuit 50 may be any circuit which can be damaged by a reverse voltage appearing at the positive terminal. 
     It should be understood that the foregoing description of the invention is intended merely to be illustrative thereof, having been presented by way of example only. Numerous other embodiments, modification and equivalents may be apparent to those skilled in the art without departing from its spirit. For instance, an alternative embodiment of the present invention may incorporate an npn transistor in place of the pnp transistor 10, and connect it to the negative supply voltage V- to protect integrated circuits from the appearance of a positive voltage across the negative supply terminal. Having thus described the invention, what I claim is: