8. STOCK- BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Stock-based compensation expense by line item
Stock-based compensation expense	$ 542	$ 478
Other information
Income tax benefit recognized from allocation of share-based compensation expense	0
Windfall tax benefits realized	0	0
Unrecognized compensation costs	$ 3,800
Weighted average period of recognition of unrecognized compensation costs	2 years 1 month 10 days
Cost of revenues
Stock-based compensation expense by line item
Stock-based compensation expense	$ 53	47
Research and development
Stock-based compensation expense by line item
Stock-based compensation expense	323	267
Selling, general and administrative
Stock-based compensation expense by line item
Stock-based compensation expense	$ 166	$ 164

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
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Amount of deferred tax expense from write-off of the deferred tax asset related to deductible stock options at exercise.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Unrecognized cost of unvested options awarded to employees as compensation.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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