GB2259809A

GB2259809A - PIN Diode

Info

Publication number: GB2259809A
Application number: GB9219228A
Authority: GB
Inventors: Suresh Chand Jain; Kenneth John Rawlings; David Harold John Totterdell
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1991-09-20
Filing date: 1992-09-11
Publication date: 1993-03-24
Anticipated expiration: 2012-09-11
Also published as: GB9120109D0; GB2259809B; GB9219228D0

Abstract

A PIN diode (10) suitable for use as a neutron dosimeter, and with greater sensitivity than previous dosimeters, comprises a long rod or bar of high resistivity silicon with p and n-doped regions (12, 20) at its ends. The mean carrier lifetime within the high resistivity base region (30) is at least 200 microseconds, while the length of the base region (30) is at least three times the mean carrier diffusion length. All the lateral surfaces (28) of the rod are oxidised, so minimizing the rate of carrier recombination at the surface. <IMAGE>

Description

PIN Diode This invention relates to a PIN diode (i.e. p-type, intrinsic, n-type) which is suitable for use as a fast neutron detector and dosimeter.

It is known to use PIN diodes to detect fast neutrons, the forward voltage drop across such a diode for constant current increasing with fast neutron damage to the base region of the diode, and so being related to the neutron dose. For example US patent number 4 163 240 (Swinehart et al) describes a PIN diode which can be sensitive to an absorbed dose as low as 1 mGy (0.1 rad), and with a sensitivity at least 1 V/Gy (10 mV/rad). The diode comprises a bar or cylinder of high purity silicon of resistivity in the range 50 ohm cm to 1000 ohm cm, in which the effective minority carrier lifetime is in the range 250 to 750 microseconds. Dopants (phosphorus and boron) are diffused into the two ends to form p and n-type regions.

After diffusion, the effective bulk carrier lifetime is said to be preferably greater than about 100 microseconds, and can be measured by an open circuit voltage decay technique. The length of the intrinsic zone, referred to as the "base width", is greater than the width or diameter of the bar, and is said to be typically between 0.75 mm and 3.2 mm. The peak sensitivity indicated for such a PIN diode is about 7 V/Gy. A diode which could be sensitive to smaller doses, and of greater sensitivity, would be very advantageous.

According to the present invention there is provided a PIN diode comprising a base region of high resistivity silicon between a p-type doped region and an n-type doped region, wherein the mean carrier lifetime within the base region is at least 200 microseconds, wherein the length of the base region is at least three times the mean carrier diffusion length in the base region, and wherein all the external surfaces of the base region are oxidised.

Desirably the diode is fabricated from float zone silicon of initial resistivity at least 10 kohm cm, preferably 40 kohm cm. Desirably the p-type region and the n-type region are both thin, desirably less than 10 micrometres thick and preferably between 0.5 and 5 micrometres, most preferably about 1 micrometre thick, as this thin layer can be made without a prolonged heat treatment, so that consequential increases in the conductivity of the base region are minimized.

The mean carrier diffusion length is given by the square root of the product of the diffusion coefficient and the mean carrier lifetime. For high resistivity silicon the diffusion coefficient is typically in the range 50 to 80 cm2/sec. Hence if the mean lifetime is 400 microseconds, the diffusion length will be between 1.4 and 1.8 mm. The length of the base region is desirably at least three times the diffusion length for the material of which it is made, preferably five or even ten times the diffusion length.

The invention will now be further described, by way of example only, and with reference to the accompanying drawing which shows a diagrammatic perspective view, not to scale, of a PIN diode 10. The diode 10 comprises a bar of silicon of length 7 mm, of square cross-section with a side of 5 mm. It is fabricated from high resistivity float zone silicon, initially n-type and of resistivity 40 kohm cm.

The initial carrier lifetime in this material is greater than 1 ms.

At one end the bar is highly doped with phosphorus to create an n-type region 12 of low resistivity (about 0.001 ohm cm) to a depth of about 1 micron, so there is low impedance junction 14 at that end. The end surface is coated with aluminium 16 of thickness 1 micron, to which a contact wire 18 is soldered. At the opposite end the bar is highly doped with boron to create a low resistivity p-type region 20 to a depth of about 1 micron, so there is a rectifying junction 22 at that end. The end surface is also coated with aluminium 24 of thickness 1 micron, to which a contact wire 26 is soldered. All four side surfaces 28 of the bar are oxidised.The shallow depth of the highly doped regions 12 and 20 can be achieved with only a short high temperature diffusion treatment, so minimizing the changes to the properties of the intrinsic, high-resistivity region 30 between the doped regions 12 and 20, which is referred to as the base; after manufacture, the carrier lifetime in the intrinsic region 30 (or base) is several hundred microseconds, typically about 500 microseconds.

In use the PIN diode 10 is connected to a constant current source, and the potential difference between its terminals 18, 26 is measured. It can then be disconnected from the circuit. Exposure of the diode 10 to fast neutrons causes damage to the crystal lattice and leads to the creation of an acceptor level just above the valence band; consequently the recombination rate for carriers in the intrinsic region or base 30 is increased, and the electrical resistance of the base 30 is increased. After exposure, the diode 10 is again connected to the constant current source, and the potential difference between its terminals 18, 26 measured. The increase in this potential difference is a measure of the total dose of fast neutrons received by the diode 10.

Because the lateral surfaces 28 are oxidised, the density of recombinant sites, and so the surface recombination rate, is minimized. It will be appreciated that since the mean carrier diffusion length in the base region 30 of the diode 10 is about 2 mm and the width of the silicon bar is only 5 mm the carriers frequently diffuse to the lateral surfaces 28, so that minimizing the rate of recombination at those surfaces 28 is consequently of considerable importance.

In principle, as large a current as possible should be passed through the diode 10, though in practice considerations of power supply and of diode heating limit the value.

The diode 20 has been found to have a response to neutron irradiation about a hundred times greater than that of known PIN diodes used as dosimeters, with a sensitivity of about 100 V/Gy.

Claims

1. A PIN diode comprising a base region of high resistivity silicon between a p-type doped region and an n-type doped region, wherein the mean carrier lifetime within the base region is at least 200 microseconds, wherein the length of the base region is at least three times the mean carrier diffusion length in the base region, and wherein all the external surfaces of the base region are oxidised.

2. A PIN diode as claimed in Claim 1 fabricated from silicon of initial resistivity at least 10 kohm cm.

3. A PIN diode as claimed in Claim 2 wherein the initial resistivity is at least 40 kohm cm.

4. A PIN diode as claimed in any one of the preceding Claims wherein the p-type doped region and the n-type doped region are both less than 10 micrometres thick.

5. A PIN diode as claimed in Claim 4 wherein both the said doped regions are of thickness between 0.5 and 5 micrometres.

6. A PIN diode as claimed in any one of the preceding Claims wherein the base region is of length between five and ten times the diffusion length.

7. A PIN diode substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.