A high-density neural network array. In an exemplary embodiment, an apparatus includes a three-dimensional (3D) structure having a plurality of layers forming a neural network. Each layer comprises one or more conductors forming neurons with each neuron having neuron inputs and neuron outputs. The apparatus also includes synapse elements coupled between the neurons outputs and the neuron inputs of neurons in adjacent layers. Each synapse element comprises a material that applies a selected weight to signals passing between neurons connected to that synapse element.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. An apparatus, comprising: a three-dimensional (3D) structure having a plurality of layers forming a neural network, wherein each layer comprises one or more conductors forming neurons with each neuron having neuron inputs and neuron outputs; and synapse elements coupled between the neurons outputs and the neuron inputs of neurons in adjacent layers, wherein each synapse element comprises a material that applies a selected weight to signals passing between neurons connected to that synapse element. 2. The apparatus of claim 1, wherein the plurality of layers forms an input layer, one or more hidden layers, and an output layer. 3. The apparatus of claim 1, wherein the conductor of each neuron forms one or more neuron inputs. 4. The apparatus of claim 1, wherein each neuron output comprises a threshold material coupled between the conductor and a synapse element of a neuron in an adjacent layer, wherein the threshold material performs a threshold function. 5. The apparatus of claim 4, wherein the threshold material comprises a material selected from a set of materials comprising diode material, Schottky diode material, NbOx material, TaOx material or VCrOx material. 6. The apparatus of claim 1, wherein the material of each synapse element is programmable to provide a plurality of selectable weights. 7. The apparatus of claim 6, wherein the material of each synapse element comprises a material selected from a set of materials comprising resistive material, phase change material, ferroelectric material, and magnetic material. 8. The apparatus of claim 2, further comprising a reference circuit coupled to the neurons of the one more hidden layers, wherein the reference circuit biases the neurons of the one or more hidden layers. 9. The apparatus of claim 1, further comprising a programming circuit coupled to the plurality of layers, wherein the programming circuit programs the material of each synapse element. 10. The apparatus of claim 1, wherein the plurality of layers of the three-dimensional (3D) structure are partitioned to form multiple neural networks. 11. A method for programming a three-dimensional (3D) structure having a plurality of layers forming a neural network, wherein each layer comprises one or more conductors forming neurons, and wherein synapse elements are coupled between neurons of adjacent layers, the method comprising: applying input voltages to an input layer of the neural network; measuring output voltages at an output layer of the neural network; determining an error value as a function of the input voltages and the output voltages; and adjusting weights associated with the synapse elements if the error value is greater than an error threshold. 12. The method of claim 11, further comprising applying references voltages to the neural network before applying the input voltages. 13. The method of claim 11, wherein the operation of adjusting comprises programming each synapse element to have a selected weight value. 14. The method of claim 11, wherein the operation of adjusting comprises programming each synapse element to have one of eight selectable weight values. 15. The method of claim 11, further comprising repeating the operations of applying, measuring, determining, and adjusting until the error value is less than an error threshold. 16. The method of claim 11, further comprising storing the adjusted weights. 17. A three-dimensional (3D) neural network structure, comprising: an input layer having at least one input conductor forming an input neuron; one or more hidden layers, each hidden layer having at least one hidden conductor forming hidden neurons; an output layer having at least one output conductor forming an output neuron; threshold material coupled to each of the input, hidden, and output conductors; and synapse elements coupled between the threshold material associated with a selected layer and the conductors of an adjacent layer, wherein each synapse element comprises a material that applies a selected weight to signals passing through that synapse element. 18. The apparatus of claim 17, wherein the threshold material comprises a material selected from a set of materials comprising diode material, Schottky diode material, NbOx material, TaOx material or VCrOx material. 19. The apparatus of claim 17, wherein the material of each synapse element comprises a material selected from a set of materials comprising resistive material, phase change material, ferroelectric material, and magnetic material. 20. The apparatus of claim 17, further comprising a reference circuit coupled to the hidden conductors.