Note 3. Balance Sheet Components.

Accounts Receivable, Net.

Accounts receivable, net consisted of the following (in thousands):


	March 31,		December 31,
	2020		2019
Accounts receivable	$	32,541	$	35,757
Accounts receivable, unbilled		5,593		4,795
Allowance for doubtful accounts		(60)		(60)
Total accounts receivable, net	$	38,074	$	40,492

Inventories.

Inventories consisted of the following (in thousands):


	March 31,		December 31,
	2020		2019
Raw materials	$	1,843	$	6,081
Work-in-process		10,051		6,715
Finished goods		10,316		7,227
Total inventories	$	22,210	$	20,023

For the three months ended March 31, 2020, we did not record any additional write-downs related to excess inventory. For the three months ended March 31, 2019, we recorded a write-down of $0.3 million related to excess inventory.

Inventory write-downs are primarily associated with products built in excess of customer demand which resulted in excess inventory levels, legacy products for which no demand exists, lower of cost or net realizable value write-downs associated with products for which costs exceeded net realizable value, and write-downs associated with the closing of the Echelon lighting business.

Property and Equipment, Net.

Property and equipment, net consisted of the following (in thousands):


	March 31,		December 31,
	2020		2019
Machinery and equipment	$	17,057	$	17,522
Leasehold improvements		4,444		4,445
Computer software		4,791		4,142
Furniture and fixtures		744		733
Construction in progress		299		115
Property and equipment, at cost		27,335		26,957
Accumulated depreciation and amortization		(19,537)		(18,844)
Property and equipment, net	$	7,798	$	8,113

The Company incurs costs for the fabrication of masks used by its foundry partners to manufacture its products. Beginning the first fiscal quarter of 2017, the Company capitalizes mask costs that are expected to be utilized in production manufacturing as the Company’s product development process has become more predictable and thus supports capitalization of the mask. The capitalized mask costs begin depreciating to cost of revenue once the products go into production. Depreciation is computed using the straight-line method over a three- year period which is the expected useful life of the mask. Previously mask sets were expensed to research and development.

Depreciation and amortization expense of property and equipment for the three months ended March 31, 2020 and 2019 was $1.0 million and $0.6 million, respectively.

Accrued Expenses and Other Current Liabilities.

Accrued expenses and other current liabilities consisted of the following (in thousands):


	March 31,		December 31,
	2020		2019
Accrued sales commission payable	$	258	$	389
Accrued manufacturing expenses		1,686		1,106
Liabilities to certain customers		2,242		4,144
Warranty reserve		504		513
Income tax payable		513		132
Deferred revenue, current portion		224		415
Interest payable		171		950
Other accrued liabilities		1,508		1,719
Total accrued expenses and other current liabilities	$	7,106	$	9,368