The infrared sensing MOSFET

Abstract

Experimental results are presented on the operation of gold-, indium-, and gallium-doped silicon MOSFET's as infrared photon detectors (IRFET's) C11. Photoionization of these impurity centers in the surface space charge depletion region of the MOSFET causes a modulation in the threshold voltage and the conductivity of the MOSFET. Previous results on gold-doped devices [2,31 have been extended to indium-doped devices C41 and preliminary results obtained on gallium-doped devices. The IRFET exhibits large gains and responsivities of 1.0 milliamp/microwatt (10 milliwatts/microjoule) are easily achieved with indium-doped devices. It is shown that shot noise or background limited rather than l/f noise limited operation can be achieved. By the use of these three dopants, the near, middle, and far infrared wavelength regions can be covered. DEVICE OPERATIONThe silicon infrared sensing n-channel MOSFET (IRFET) shown in Fig. 1, is doped with an impurity in addition to the normal boron doping employed in the p-type substrate. The device is an integrating photon detector and is operated at low temperatures. The IRFET is reset, or first preset, by applying a large negative gate voltage to accumulate the surface and fill all impurity centers with holes. The IRFET (MOSFET) is then turned on by applying a positive gate voltage in excess of the threshold voltage, if the temperature is low then the rate of thermal emission of holes from the impurity centers can be negligible and the impurity centers will retain the trapped hole, if infrared radiation is now incident upon the device the trapped holes can be released by photoionization. In this manner the space charge in the surface depletion region of the MOSFET can be modulated.Modulation of the space charge in the surface depletion region causes a modulation in the threshold voltage of the MOSFET since the threshold or turn-on voltage depends on this space charge. AIternatively, if it is assumed the MOSFET is operated in the linear region, where the drain-tosource voltage, V D~, is much less than the excess of gate-to-source voltage above the threshold voltage, VGS-VT, then operation of the device can be viewed on the basis of a charge neutrality requirement. If the gate-to-source voltage, V G~, i a fixed, then the positive charge on the metal gate must be balanced in part by negative charge in the space charge region due to boron and other centers and the negative charge associated mobile carriers in the channel, designated surface impurity with as QCH in Fig. 1. In the case of indium-and gallium-doped devices the impurity centers are in the neutral charge state following the reset operation, photoionization by the optical emission of a hole will result in a change to the negative charge state and increase in the amount of negative space charge in the surface depletion region. This increase in negative space charge means the number of free electrons in the channel of the device must decrease and the conductivity of the device decrease.