Light detection device

Details Light detection device Light detection device

1999-07-29

2001-05-15

Allen, Stephone B. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C327S514000

Reexamination Certificate

active

06232591

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to light detection devices and particularly but not exclusively to light detection devices for use in tamper detection applications.
BACKGROUND OF THE INVENTION
In a semiconductor integrated circuit (IC), such as a banking smartcard, the IC is vulnerable to a breach of security if it falls into the hands of a dishonest person. The IC may be reverse engineered in order to reveal or modify functions and confidential data contained therein. It is known that such IC's have been decapsulated and have even undergone depassivation of the upper protective layer.
U.S. Pat. No. 4,952,796 describes a circuit which comprises a current generator delivering current which flows into a reversed biased transistor junction. If subject to light, the reverse current in the junction increases, and the voltage at the junction terminals drops.
A problem with this arrangement is that incident light will generate reverse currents in transistors
11
and
2
of
FIG.1
, and this may affect the voltage drop detected at the output. Furthermore, the biasing and current generation functions of the above circuit are more susceptible to manufacturing process variations.
Also current drain, particularly in a smartcard, should be kept to a minimum, and the reverse current adversely affects the current consumption of the device. Lastly, the above circuit takes up much valuable semiconductor area, and again this is disadvantageous in a smartcard, where demand for space is at a premium.
This invention seeks to provide a light detection device which mitigates the above mentioned disadvantages.
SUMMARY OF THE INVENTION
According to the present invention there is provided a light detection device comprising: a biasing transistor, arranged to provide a bias current; a reverse biased transistor having a control electrode arranged to be reverse biased by the bias current and having a conducting electrode; and, a resistor coupled between a supply voltage and the conducting electrode of the reverse biased transistor; wherein incident visible light is detected by a voltage drop at the conducting electrode of the reverse biased transistor.
In this way a light detection device is provided which does not generate parasitic reverse currents, and is less susceptible to manufacturing process variations.


REFERENCES:
patent: 4910707 (1990-03-01), Schrenk
patent: 4952796 (1990-08-01), Fruhauf et al.
patent: 0437307A2 (1991-07-01), None
patent: 2074788A (1981-11-01), None

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