Negative Differential Resistivity in GaN Metal-Semiconductor-Metal Photoconductors
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ABSTRACT We have observed a negative differential resistivity (NDR) in metal-semiconductormetal (M-S-M) photoconductor made from unintentionally doped GaN grown by metalorganic chemical vapor deposition. The threshold field for the onset of NDR was found to be independent of the spacing of M-S-M fingers, and was measured to be 1.91x10 5 V/cm for GaN with an n-type carrier concentration of 1014 cm"3 . We believe that the observed NDR is due to transferred electron effect in GaN. The threshold field value is very close to the value obtained from the theoretical simulation. This observation, to the best of our knowledge, is the first experimental evidence of transferred-electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn-effect devices using GaN materials. INTRODUCTION GaN has received extensive interest in recent years due to its successful application in blue light emitting diodes (LEDs)1 and its great potential in high power and high temperature electronic devices. It is also predicted that GaN can be used for high frequency devices such as transferred electron devices (TED) due to its large longitudinal optical phonon energy and large energy separation between the central valley (F) of the conduction band and the next-lowestenergy minimum 2' 3. In 1975, Littlejohn et al. 4 first predicted a negative differential resistivity (NDR) in GaN under high electric field. Recently, Gelmont et al. 5 showed through an ensemble Monte Carlo simulation that intervalley electron transfer plays a dominant role in GaN in high electric fields leading to a strongly inverted electron distribution and to a large negative differential resistance. The mechanism responsible for the NDR is a field-induced transfer of conduction-band electrons from a low-energy, high-mobility valley to high-energy, low-mobility satellite valleys. The NDR is important for microwave device applications. However, no direct evidence of NDR phenomena has been reported so far in GaN materials or related devices. The direct measurement of the velocity of electrons as a function of the electric field is usually difficult in semiconductors due to oscillations in high conductivity materials and nonuniformity of electric field in low conductivity materials, and becomes even more difficult in GaN due to its large energy band gap which requires a large threshold field. In this letter, we report the first observation of the negative differential resistance in GaN material through a metalsemiconductor-metal (M-S-M) system. The threshold field was found to be 1.91x105 V/cm in GaN with a background concentration of 1014 cm-3 .
789 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
EXPERIMENT The GaN material used in this letter was grown on (0001) sapphire by metalorganic chemical vapor deposition. It is unintentionally doped and has a carrier concentration of 1014 cm 3 at room temperature as obtained from capacitance-voltage measurement. The M-S-M finger-type patterns, shown in Fig. 1,
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