Our cash and cash equivalents consist of cash and instruments with original maturities of less than three months. Our investments consist of instruments with original maturities of more than three months. As of September 30, 2021 and December 31, 2020, our cash, cash equivalents and debt investments are classified as follows (in thousands):

1.	Certificates of deposit with original maturities of less than three months.

2.	Certificates of deposit with original maturities of more than three months.

3.	Classified as “Short-term investments” in our condensed consolidated balance sheets.

4.	Classified as “Long-term investments” in our condensed consolidated balance sheets.

We manage our debt investments as a single portfolio of highly marketable securities that is intended to be available to meet our current cash requirements. Certificates of deposit and corporate bonds are typically held until maturity.

Historically, the gross unrealized losses related to our portfolio of available-for-sale debt securities were immaterial, and primarily due to normal market fluctuations and not due to increased credit risk or other valuation concerns. There was an insignificant amount of gross unrealized losses on our available-for-sale debt securities as of September 30, 2021, and historically, such gross unrealized losses have been temporary in nature and we believe that it is probable the principal and interest will be collected in accordance with the contractual terms. We review our debt investment portfolio at least quarterly, or when there are changes in credit risks or other potential valuation concerns, to identify and evaluate whether an allowance for credit losses or impairment would be necessary. Factors considered in determining whether a loss is temporary include the magnitude of the decline in market value, the length of time the market value has been below cost (or adjusted cost), credit quality, and our ability and intent to hold the securities for a period of time sufficient to allow for any anticipated recovery in market value.

A portion of our debt investments would generate a loss if we sold them on September 30, 2021. The following table summarizes the fair value and gross unrealized losses related to available-for-sale debt securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position as of September 30, 2021 (in thousands):


	In Loss Position				In Loss Position				Total In
	< 12 months				> 12 months				Loss Position
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
As of September 30, 2021	Value		(Losses)		Value		(Losses)		Value		(Losses)
Investments:
Certificates of deposit	$	959	$	(1)	$	—	$	—	$	959	$	(1)
Corporate bonds		5,216		(10)		—		—		5,216		(10)
Total in loss position	$	6,175	$	(11)	$	—	$	—	$	6,175	$	(11)

The following table summarizes the fair value and gross unrealized losses related to available-for-sale debt securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position as of December 31, 2020 (in thousands):


	In Loss Position				In Loss Position				Total In
	< 12 months				> 12 months				Loss Position
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
As of December 31, 2020	Value		(Loss)		Value		(Loss)		Value		(Loss)
Investments:
Corporate bonds		2,048		(2)		—		—		2,048		(2)
Total in loss position	$	2,048	$	(2)	$	—	$	—	$	2,048	$	(2)

Investments in Privately-held Raw Material Companies

We have made strategic investments in private companies located in China in order to gain access at a competitive cost to raw materials that are critical to our substrate business (see Note 7). The investment balances for the non-consolidated companies are accounted for under the equity method and included in “Other assets” in the condensed consolidated balance sheets and totaled $9.3 million and $6.4 million as of September 30, 2021 and December 31, 2020,

respectively. As of September 30, 2021, there were five companies accounted for under the equity method. There were no impairment charges in the three and nine months ended September 30, 2021 and 2020.

Fair Value Measurements

We invest primarily in money market accounts, certificates of deposits, corporate bonds and notes, and government securities. We review our debt investment portfolio for credit loss at least quarterly or when there are changes in credit risk or other potential valuation concerns. As of September 30, 2021 and December 31, 2020, the total unrealized loss, net of tax, included in accumulated other comprehensive income was immaterial. We believe it is probable the principal and interest will be collected in accordance with the contractual terms, and the unrealized loss on these securities was due to normal market fluctuations, and not due to increased credit risk or other valuation concerns. Accounting Standards Codification (“ASC”) Topic 820, Fair Value Measurements and Disclosures (“ASC 820”), establishes three levels of inputs that may be used to measure fair value. Level 1 instrument valuations are obtained from real-time quotes for transactions in active exchange markets of the asset or identical assets. Level 2 instrument valuations are obtained from readily-available, observable pricing sources for comparable instruments. Level 3 instrument valuations are obtained from unobservable inputs in which there is little or no market data, which require us to develop our own assumptions. On a recurring basis, we measure certain financial assets and liabilities at fair value, primarily consisting of our short-term and long-term debt investments.

The type of instrument valued based on quoted market prices in active markets include our money market funds, which are generally classified within Level 1 of the fair value hierarchy. We classify our available-for-sale debt securities including certificates of deposit and corporate bonds as having Level 2 inputs. The valuation techniques used to measure the fair value of these financial instruments having Level 2 inputs were derived from bank statements, quoted market prices, broker or dealer statements or quotations, or alternative pricing sources with reasonable levels of price transparency.

We place short-term foreign currency hedges that are intended to offset the potential cash exposure related to fluctuations in the exchange rate between the United States dollar and Japanese yen. We measure the fair value of these foreign currency hedges at each month end and quarter end using current exchange rates and in accordance with US GAAP. At quarter end, any foreign currency hedges not settled are netted in “Accrued liabilities” on the condensed consolidated balance sheet and classified as Level 3 assets and liabilities. As of September 30, 2021, the net change in fair value from the placement of the hedge to settlement at each month end during the quarter had a de minimis impact on the condensed consolidated results.

There were no changes in valuation techniques or related inputs in the three and nine months ended September 30, 2021. There have been no transfers between fair value measurements levels during the three and nine months ended September 30, 2021.

The following table summarizes our financial assets and liabilities measured at fair value on a recurring basis in accordance with ASC 820 as of September 30, 2021 (in thousands):


			Quoted Prices in				Significant
			Active Markets of		Significant Other		Unobservable
	Balance as of		Identical Assets		Observable Inputs		Inputs
	September 30, 2021		(Level 1)		(Level 2)		(Level 3)
Assets:
Cash equivalents and investments:
Certificates of deposit	$	4,762	$	—	$	4,762	$	—
Corporate bonds		7,599		—		7,599		—
Total	$	12,361	$	—	$	12,361	$	—

The following table summarizes our financial assets and liabilities measured at fair value on a recurring basis in accordance with ASC 820 as of December 31, 2020 (in thousands):


			Quoted Prices in				Significant
			Active Markets of		Significant Other		Unobservable
	Balance as of		Identical Assets		Observable Inputs		Inputs
	December 31, 2020		(Level 1)		(Level 2)		(Level 3)
Assets:
Cash equivalents and investments:
Certificates of deposit	$	2,885	$	—	$	2,885	$	—
Corporate bonds		3,081		—		3,081		—
Total	$	5,966	$	—	$	5,966	$	—

Items Measured at Fair Value on a Nonrecurring Basis

Certain assets that are subject to nonrecurring fair value measurements are not included in the table above. These assets include investments in privately-held companies accounted for by the equity or cost method (see Note 7). We did not record any other-than-temporary impairment charges for these investments during the three and nine months ended September 30, 2021 and 2020, respectively.