Revenue Recognition The Company recognizes revenue when it satisfies the performance obligation of transferring control over a product or service to a customer. Revenue is measured based on the consideration specified in a contract with a customer, which consideration is paid in exchange for a product or service. The Foundry Services Group of the Company manufactures products, which the Company refers to as foundry products, based on customers specific product designs. The Company recognizes revenue over time for foundry products that do not have an alternative use when the Company has an enforceable right to payment. Revenue recognized over time is in proportion of wafer manufacturing costs incurred relative to total estimated costs for completion. However, in certain circumstances, pursuant to a customer contract or an individual purchase order, the Company may not have an enforceable right to payment for services performed at a given time. In this situation, the Company recognizes revenue at the time when a customer obtains control of the product, which is generally upon product shipment, delivery at the customers location or upon customer acceptance, depending on the terms of the arrangement. The Standards Products Group of the Company sells products manufactured based on the Companys design. The Standard Products Groups products are either standardized with an alternative use or the Company does not have an enforceable right to payment for the related manufacturing services completed to date. Therefore, revenue for the Standards Products Groups products is recognized when a customer obtains control of the product, which is generally upon product shipment, delivery at the customers location or upon customer acceptance, depending on the terms of the arrangement. A portion of the Companys sales are made through distributors for which the Company applies the same revenue recognition guidance described above. The Company defers the recognition of revenue when it receives consideration from the customers prior to the fulfillment of performance obligations. These amounts are classified as deferred revenue on the consolidated balance sheets. Of the recorded deferred revenue of $6,477 thousand as of December 31, 2018, $1,885 thousand w as recognized as revenue during the year ended December 31, 2019 and $ 4,428 thousand was refunded to a certain customer as the relevant advance payment agreement was terminated. Of the recorded deferred revenue of $8,335 thousand as of January 1, 2018, $3,496 thousand was recognized as revenue during the year ended December 31, 2018, and $4,671 thousand was reclassified to other current liabilities as the relevant advance payment agreement with a certain customer was terminated. In accordance with revenue recognition guidance, any tax assessed by a governmental authority that is both imposed on and concurrent with a specific revenue-producing transaction, and that is collected by the Company from a customer, is excluded from revenue and related revenue is presented in the statements of operations on a net basis. The Company provides warranties under which customers can return defective products. The Company also provides allowances for additional products that may have to be provided free of charge to compensate customers for not meeting previously agreed upon yield criteria, which the Company refers to as the low yield compensation reserve. The Company estimates the costs related to warranty claims, repair or replacements and low yield compensation reserves, and records them as components of cost of sales. In addition, the Company offers sales returns (other than those that relate to defective products under warranty), cash discounts for early payments, sales incentives including discounts and volume rebates, and certain allowances to the Companys customers, including the Companys distributors. The Company records reserves for those returns, discounts, incentives and allowances as a deduction from sales, based on historical experience and other quantitative and qualitative factors. Substantially all of the Companys contracts are one year or less in duration. The standard payment terms with customers are generally thirty to sixty days from the time of shipment, product delivery to the customers location or customer acceptance, depending on the terms of the related arrangement. Unbilled accounts receivable represents the Companys contractual right to consideration for manufacturing work performed on a customer contract or an individual purchase order that has not been invoiced to the customer. As of December 31, 2019 and 2018, the Company recorded unbilled accounts receivable of $17,094, net of an allowance of $627 thousand, and $ 38,181 thousand, net of an allowance of nil, respectively. Of the recorded unbilled accounts receivable of $38,181 thousand as of December 31, 2018, $34,910 thousand were billed to customers upon shipment, upon product delivery or upon customer acceptance, depending on the terms of the related arrangement, during the year ended December 31, 2019. All amounts billed to a customer related to shipping and handling are classified as sales while all costs incurred by the Company for shipping and handling are classified as selling, general and administrative expenses. The amounts charged to selling, general and administrative expenses were $ 1,769 thousand, $ 1,861 thousand, and $ 1,652 thousand for the years ended December 31, 2019, 2018 and 2017, respectively.