Temperature Dependent and Self Heating Characteristics of Pseudo Vertical AlN Diodes on Bulk AlN Substrates

Pseudo vertical AlN Schottky barrier diodes were fabricated on bulk AlN substrates. The epitaxial stack grown using MOCVD consisted of 2 μm thick n++ Al_0.8Ga_0.2N, 100 nm n++ graded AlGaN (x= 0.8 to 1) and 1 μm AlN drift layer (N_D= 1x10¹⁸ cm^-3). Ni Schottky contacts with diameter of 50 – 150 μm were fabricated on AlN surface and Ti/Al/Ni/Au ohmic contacts were deposited on 80% n++ AlGaN contact layer. The diodes exhibited a turn-on voltage of ~ 3.0 V, an ideality factor greater than 2.5, and an on/off ratio exceeding 10⁹. A maximum forward current density above 2 kA/cm² was achieved at 10 V (50 μm devices). The DC forward current in AlN SBDs was found to be limited by self-heating and current crowding effects arising from high sheet resistance of n++ Al_0.8Ga_0.2N contact layer. These effects were modeled by using TCAD device simulations to match the experimentally measured I-V characteristics. The self-heating effects were further confirmed using pulsed I-V characterization using sub microsecond (<1 μs) pulse widths. The capacitance-voltage profiling showed a net donor (N_D-N_A) concentration of 5.0x10¹⁷ cm^-3 at 300 K suggesting significant compensation. However, the N_D-N_A increased with temperature approaching ~ 1x10¹⁸ cm^-3 for temperatures exceeding 373 K. The AlN devices exhibited stable operation up to 300℃ with an increasing trend in current density and decreasing trend for the ideality factor. The diodes displayed a non-destructive surface breakdown field of ~ 6.4 MV/cm with repeatable reverse I-V characteristics.