DE2227339A1 - Electrical protection circuit - Google Patents

Description

THE NATIONAL CASH REGISTER COMPANY Dayton, Ohio (V.St.A.)

Patent application: Our reference number: 1392 / Germany ELECTRICAL CIRCUIT

The invention relates to an electrical protection circuit for a field effect transistor with an insulated gate electrode made from a field effect semiconductor element.

As is well known, overvoltages occur e.g. through static charges when handling the high-eared ones Input circuits of field effect transistors. Static charges can destroy the transistors will. From US Pat. No. 3,395,290 an electrical protection circuit is already known in which one to be protected transistor, the source-drain path of a field effect transistor is connected upstream, via which to protective transistor the control voltage is supplied. This protective circuit is only functional if on the upstream transistor a corresponding control voltage is available. In addition, from this patent specification is also a protection circuit with a transistor connected as a diode, which is connected to the control electrode of the to be protected Transistor is connected. With this circuit it is possible that the transistor to be protected is destroyed, before the transistor, which is connected as a diode, becomes effective in order to divert an overvoltage to ground.

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It is the object of the invention to show an electrical protection circuit which is a better one Provides protection against destruction as a result of high voltages occurring, than the known aforementioned electrical Protection circuits.

The invention is characterized in that the field effect semiconductor element consists of a semiconductor carrier with a first conductivity, in which a source electrode region and a drain electrode region with a second conductivity are provided, wherein a Electrode area forms a path, the beginning of which via a drain connection terminal with an input terminal and which End via a drain junction with the gate electrcode of the transistor to be protected are connected, the other electrode area being connected to a reference potential and that between the gate electrode, which is connected to the drain connection terminal, and the semiconductor substrate insulating layer lying thicker than one lying between the gate electrode and the source-drain electrode region Is the insulating layer of the transistor.

If in the protective circuit according to the invention a Voltage occurs that exceeds a predetermined value is over a large range that at the input of the circuit overvoltage that occurs is transferred from the drain electrode region to the source electrode region. By the arrangement a drain electrode region between the input terminal and the gate electrode of the transistor to be protected according to FIG effective protection is achieved according to the invention.

The invention is illustrated below using an exemplary embodiment described in detail with the help of drawings. In this shows:

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Fig. 1 is a plan view of an electrical protective circuit according to the invention,

Fig. Z is a sectional view along the lines 2-2 in: Fig. 1,

Fig. 3 is a graph to explain the operation of the protection circuit according to FIGS. 1 and

FIG. 4 shows a protective circuit shown in FIG. 1 equivalent circuit diagram.

In Fig. 1, a field effect semiconductor element for protecting a field effect transistor 24 between a negative voltage source 50 and the gate electrode of the transistor 24 is provided. The transistor 24 is an enhancement type field effect transistor. The field effect semiconductor element 16 contains a large source electrode region and a large drain electrode region 18. These regions 14 and 18 are formed as p-regions in a silicon substrate 17. The silicon carrier 17 is η-doped. The field effect semiconductor element 16 also has a gate electrode 10 which is separated from a n-doped channel 13 located between the regions 14 and 18 by an insulation layer 12,000 Angstroms thick. A U-shaped region 20 of the drain electrode region 18 has a length of more than 400 μm. The areas 14 and 18 can either be straight or, for example, in the shape shown in FIG. 1. The field effect semiconductor element 16 is also of the enhancement type and has a threshold voltage of approximately 40 volts. A voltage pulse V _g of 5000 volts with a duration of a few nanoseconds can be excited via a resistor 52 to the gate electrode 10 without the thick oxide field effect semiconductor element 16 being destroyed. The gate electrode 10 is with an am

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the left part of the drain electrode region 18 connected terminal 21 (FIG. 2). The source electrode area 14 is connected to ground,

A negative voltage V _g can be applied to the gate electrode 10 via the terminal 21 and to the drain electrode region 18 of the field effect semiconductor element 16 via the terminal 19. If the voltage V is not more negative than the threshold level of about -40 volts, electrons can pass from the drain electrode region 18 to the gate electrode 30 of the transistor 24 so that it is controlled. The transistor 24 is thus activated in normal operation by the voltage V _g.

If there is a negative potential of a few 1000 volts, e.g. as a result of the static Charges was generated at the terminal 21 of the field effect semiconductor element 16 is applied, the field effect semiconductor element 16 becomes conductive, dJi. the electrons now flow from the drain electrode region 18 to the source electrode region 17 and thus to ground. The flow of electrons takes place over the entire region of the channel 13 between the drain electrode region 18 and the source electrode region 14 instead.

The right end of the gate electrode 30 of the transistor 24 is connected to the drain electrode connection point 22 connected to the drain electrode 18 of the field effect semiconductor element. The transistor 24 is a metal oxide semiconductor transistor of the enhancement type and has a p-channel. The transistor 24 has a p-doped source electrode region 32 and a drain electrode region of the same type. These regions are arranged in the n-doped silicon carrier 17. The drain electrode region 34 is connected to a negative voltage source 64. The source

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Electrode area 32 is connected to ground, via the source and drain electrode regions 33 and 34 are an oxide insulating layer 38 provided. The insulation layer 38 of the transistor 24 is 1200 angstroms thick. Of the The threshold level of this transistor is only -4 volts. The voltage at which this transistor is destroyed is around 100 volts. The right part of the drain electrode area 18 at the drain connection point 22 has a resistance of approximately 2000 ohms and is also used as a protective resistor between the terminal 21 and the gate electrode 30 of the transistor 24. The U-shaped Part 20 of the drain electrode area 18 has a resistance of about 6000 ohms. The resistor 52 is used as a current limiting resistor and is about 20,000 ohms.

Since the insulation layer 38 of the transistor 24 is only 1200 angstroms thick, a Voltage of over -100 volts between the gate electrode 30 and the drain electrode region 32 for the time of a few Destruction of the transistor can take place within a few nanoseconds. The insulation layer 38 must therefore before a Voltage V of over -100 volts must be protected. In the The arrangement shown in FIG. 1 provides this protection.

FIG. 2 shows part of that shown in FIG Arrangement shown in section along line 2-2. The source electrode region 14 and the drain electrode region 18 are arranged within the silicon carrier 17, as can be seen from FIG. The drain terminal 19 and the drain connection point 22 at the two ends of the drain region 18 are also shown in FIG of the parts shown in FIG. 2 are identical to the reference numerals used in FIG. The oxide insulation layer 23 covers most of the source-drain regions 14 and 18 and, as already stated, has one Strength of 12,000 angstroms. The gate electrode 10 is vapor deposited on the oxide insulation layer 23 and is with the Drain connection terminal 19 of the drain electrode region 18 connected. The drain junction 22 of the drain elec-

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electrode area 18 is connected to the gate electrode 30 of the Transistor 24 connected, as can be seen from the right part of FIG. The transistor 24 is in the n-doped silicon carrier 17 is integrated. It could however, a transistor which is not connected to the silicon substrate 17 can also be used.

The maximum voltage which can reach the gate electrode 30, is -60 volts, as can be seen from Fig. 3, since a large part of the applied to the input voltage V _$ over the entire length of the channel 13 of the field effect type semiconductor element is neben_geschlossen sixteenth This effectively protects the transistor 24.

The mode of operation of the electrical protective circuit shown in FIG. 1 is described below with reference to FIG. A resistor 20 * and a transistor 16 'represent a first part of the field effect semiconductor element shown in FIG. 1. A resistor 20 ″ and a transistor 16 ″ represent a second section of the field effect semiconductor element 16. Resistors 20 ⁿ and transistors 16 ⁿ represent the the remaining part of the field effect semiconductor element, where η is a large number of equally sized sections into which the field effect semiconductor element 16 can be divided. The sum of the resistances 20 * to 20 ⁿ represents the total resistance of the U-shaped part 20 in the electrode region 18. The resistor 22 ¹ represents the resistance of the drain junction 22 in FIG.

As can be seen from FIG. 4 in conjunction with FIG. 1, the parallel connection divides a voltage V _{s present} at the input in the field effect semiconductor element 16 via the resistors 20 * to 20 ⁿ and the transistors 16 * to 16 ⁿ . As a result, the transistor 24 becomes effective against an excessively high voltage occurring at the input

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protected. Resistor 52 serves as a current limiting resistor. With the aid of a voltmeter 62, the voltage occurring at the gate electrode 30 of the transistor 24 can be measured in relation to ground. In Fig. 4 diodes 13 * to 13 ⁿ are shown. These diodes are formed within the field effect semiconductor element 16. They become conductive when there is a good connection between the drain electrode region 18 and the source electrode region 14, and thus to ground. This is the case when a voltage of −80 volts occurs along the channel 13 between the drain electrode region 18 and the source electrode region 14 and the distance is 400 μm.

In Fig. 3, voltage V is plotted on a vertical axis and voltage V is plotted on a horizontal axis. When the voltage V _c occurring at the input becomes about -40 volts, the field effect semiconductor element 16 becomes effective and generates a shunt path to ground. When the input voltage V -150 volts is large, the gate electrode 30 of the transistor 24 is a maximum of -60 volts. The remaining -90 volts are diverted to ground via the resistors 20 * to 20 ⁿ and the transistors 16 'to 16 ^{n shown in FIG.}

The diode breakdown voltage is reached at a voltage V _{g of approximately -150 volts.} The diode breakdown occurs as a result of the reverse biasing of the pn junction of the diodes 13 'between the drain connection terminal. 19 of the p-doped drain electrode region 18 in FIG. 2 and of the n-doped silicon carrier 17. The pn junction of the diode 19 * works like an avalanche diode. The diode breakdown results in improved conduction for negative charges via the silicon substrate 17 to ground. When this diode breakdown occurs, the voltage V on the electrode 30 is limited to the value of minus 40 volts, although the input voltage V _s on the terminal 21 can be several thousand volts.

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The protection circuit shown in FIG

also offers protection against excessive positive voltages, since the p-doped drain electrode region 18 in this case in relation to the n-doped silicon carrier 17 in Forward direction would be biased.

. The field effect semiconductor element 16 in FIG. 1 and the transistor 24 can also alternate with an n-channel being constructed. If the circuit according to FIG. 1 is constructed with an η-channel, a field effect transistor can be used with protected against high positive overvoltages via an η-channel. The voltage generated by the voltage source 64 is then rather positive. The control voltage appearing at terminal 21 must be positive in order to use the η-channel of a transistor 24 of the enrichment type to be able to control.

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Claims (1)

Patent claims: Electrical protective circuit for a field effect transistor with an insulated gate electrode made of a field effect semiconductor element, characterized in that the field effect semiconductor element (16) consists of a semiconductor carrier (17) with a first conductivity, in which a source electrode region (14) and a drain electrode region (18) with a second conductivity are provided, wherein an electrode region (z. B. 18) forms a path (20), the beginning of which via a drain connection terminal (19) with an input terminal (21) and the end of which via a drain connection point (22) to the gate electrode (30) of the to be protected transistor (24) are connected, wherein the other electrode area (z. B. 14) is connected to a reference potential (z. B. ground), and that between the gate electrode, which is connected to the drain connection terminal (19), and the insulating layer (23) lying on the semiconductor substrate (17) is thicker than one between the gate electrode (30) and the source-drain electrode region ch lying insulating layer (38) of the transistor (24). 2. Circuit according to claim 1, characterized in that the source electrode region (14) and the drain electrode region (18) are finger-shaped and intermesh in a comb-like manner. 3. Circuit according to claim 1 or 2, characterized in that the thickness of the insulating layer (23) of the semiconductor element (16), which covers at least the channel (13), several times greater than the thickness of the insulating layer (38) of transistor (24) is. 5.6.72 209857 / Π977 4. Circuit according to one of the preceding claims, characterized in that the transition between the Drain electrode region (18) and the semiconductor carrier (17) is designed so that an avalanche breakdown effect in the vicinity of the drain terminal (19). 5. Circuit according to one of the preceding claims, characterized in that the distance between the source Electrode area (14) and the drain electrode area (18) is so large that an enlarged contact area is produced when a compared to the threshold level is applied to the terminal (21) of the transistor (24) high voltage is applied. 6. Circuit according to one of the preceding claims, characterized in that the field effect semiconductor element (16) and the transistor (24) are contained on a semiconductor carrier (17) as an integrated circuit. 5.6.72 209852/0977 Blank page