High-Power High-Temperature Heterobipolar TransistorWith Gallium Nitride Emitter
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 39
High-Power High-Temperature Heterobipolar TransistorWith Gallium Nitride Emitter J. I. Pankove, M. Leksono, S. S. Chang, C. Walker Astralux Inc. B. Van Zeghbroeck University of Colorado This article was received on August 15, 1996 and accepted on November 28, 1996.
Abstract A new heterobipolar transistor was made with the wide bandgap semicon-ductors gallium nitride (GaN) and silicon carbide (SiC). The heterojunction allows high injection efficiency, even at elevated temperatures. A record current gain of ten million was obtained at room temperature, decreasing to 100 at 535°C. An Arrhenius plot of current gain vs 1/T yields an activation energy of 0.43 eV that corresponds to the valence band barrier blocking the escape of holes from the base to the emitter. This activation energy is approximately equal to the difference of energy gaps between emitter and base. This Transistor can operate at high power without cooling. A power density of 30 KW/cm2 was sustained.
1. Principle of Operation A heterobipolar transistor was made by growing a GaN emitter on a commercial SiC pn junction as shown in Figure 1. [1] [2] The reason for this structure is evident from Figure 2 that shows a band diagram of this transistor while biased to amplify a signal. The electrons injected at low voltage by the n-type emitter traverse the SiC base and are collected at high voltage in the Collector. This represents a high power gain. The injection efficiency is very high because all the emitter current consists of electrons injected by the emitter and none involving holes from the base because the holes are blocked by a valence band barrier at the emitter. Furthermore, SiC being an indirect gap semiconductor, the injected electrons have a long lifetime in the base. Hence this transistor is endowed with an extremely high amplification factor. The current gain begins to degrade at high temperature when holes in the Boltzmann tail at the base can overcome the barrier and escape into the emitter.
2. Device Fabrication To fabricate this structure, a layer of n-type GaN is grown on the p-type surface of the SiC using an MOCVD reactor. The emitter is patterned and reactively etched. Then the surfaces are metallized with chromium and aluminum. After patterning to define the emitter and base electrodes, an etching step is used. The collector contact of Cr/Al is applied over the back of the waver. The next step consists in etching a trench around each base to define each transistor and avoid having all the collector-base junctions on the chip in parallel. Figure 3 shows a cross section of the processed device. While Figure 4 is an SEM view of part of a chip. The largest emitter (lower row) measures 250 x 250 µm2 while the smallest transistor (next row above) has a 10 x 20 µm2 emitter surrounded by the base. Note that both emitter and base are connected to bonding pads.
3. Device Characteristics The common base operating characteristics of the new HBT at room temperature are
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